Glossary of Terms¶

This glossary defines every key concept in the book in plain language, with an example and a link to the chapter where it is taught.

A · B · C · D · E · F · G · H · I · J · K · L · M · N · O · P · Q · R · S · T · U · V · W · X · Y · Z

A¶

Acetylcholine¶

A neurotransmitter that activates skeletal muscles at the neuromuscular junction and also helps control attention, memory, and many automatic body functions.

Example: Myasthenia gravis disrupts acetylcholine signaling at muscles, causing weakness.

See also: Neuromuscular Junction

Taught in Chapter 3.

Action Potential¶

A brief, self-renewing electrical pulse in which the membrane potential rapidly rises and then falls, traveling along an axon to carry a signal.

Example: Each action potential lasts only about one to two thousandths of a second, so a neuron can fire many of them each second.

See also: Threshold, Refractory Period

Taught in Chapter 2.

Addiction¶

A treatable, long-lasting condition in which a person keeps using a substance or doing a behavior despite harm, linked to changes in brain circuits for reward, motivation, and self-control.

Example: Medicines combined with counseling help many people with an opioid use disorder recover.

Taught in Chapter 15.

Adolescent Brain¶

The brain during the teenage years, when it is heavily remodeled through synaptic pruning, increasing myelination, and changes in reward and control circuits.

Example: The adolescent brain's high plasticity makes the teen years a strong time to learn new languages, instruments, and sports.

Taught in Chapter 8.

Agonist and Antagonist¶

An agonist is a substance that activates a receptor, copying the natural signal; an antagonist binds a receptor and blocks it without activating it.

Example: Morphine is an opioid agonist, and naloxone is an opioid antagonist that can reverse an overdose.

See also: Receptor, Naloxone

Taught in Chapter 3.

All-or-None Principle¶

The property of an action potential that it either fires at full size or does not fire at all, no matter how far a stimulus goes past threshold.

Example: A gentle tap and a hard tap produce action potentials of the same height; the hard tap instead produces more of them per second.

See also: Firing Rate Coding

Taught in Chapter 2.

Alzheimer's Disease¶

A progressive neurodegenerative disease marked by amyloid plaques and tau tangles in the brain that usually begins with trouble forming new memories.

Example: An early sign may be asking the same question again and again within a short time.

Taught in Chapter 13.

Amnesia¶

A serious loss of memory caused by brain injury, disease, or psychological trauma, affecting either the formation of new memories (anterograde) or recall of earlier ones (retrograde).

Example: After a concussion, a player may not remember the play that caused the injury.

See also: Patient H.M.

Taught in Chapter 9.

Amygdala¶

An almond-shaped cluster of neurons deep in each temporal lobe that detects threats and other emotionally important events and helps form emotional memories.

Example: The amygdala responds to a fearful face even when it is flashed too quickly for a person to notice consciously.

See also: Fear Conditioning

Taught in Chapter 11.

Amyloid Plaque¶

A clump of the protein fragment amyloid-beta that builds up between neurons and is a hallmark of Alzheimer's disease.

Example: A special PET scan can show amyloid plaques in a living person's brain.

See also: Tau Tangle

Taught in Chapter 13.

Amyotrophic Lateral Sclerosis (ALS)¶

A progressive neurodegenerative disease that destroys both upper and lower motor neurons, causing increasing muscle weakness and eventually paralysis.

Example: Most cases of ALS are not inherited, but some are caused by changes in genes such as SOD1.

Taught in Chapter 13.

Anatomical Directions¶

Standard words that describe where body or brain parts sit relative to one another, such as anterior (front), posterior (back), superior (top), inferior (bottom), medial (toward the middle), and lateral (toward the side).

Example: The frontal lobe is anterior to the parietal lobe.

See also: Brain Planes and Sections

Taught in Chapter 4.

Animal Models¶

Animals such as mice, rats, fruit flies, zebrafish, or sea slugs that are studied to understand biological processes or diseases that also occur in humans.

Example: Mice carrying the human gene change that causes Huntington's disease are used to test possible new treatments.

See also: Aplysia Model, Research Ethics

Taught in Chapter 9.

Anti-Amyloid Antibody¶

A lab-made antibody medicine that binds amyloid-beta and helps the immune system clear amyloid plaques from the brain.

Example: Lecanemab (approved in 2023) and donanemab (approved in 2024) are FDA-approved for early Alzheimer's disease; they modestly slow decline and require MRI monitoring for brain swelling or bleeding.

See also: Amyloid Plaque, Biomarker

Taught in Chapter 13.

Antisense Oligonucleotide¶

A short, lab-made strand of genetic material designed to stick to a specific RNA message and block or change how a protein is made from it.

Example: Tofersen, an antisense oligonucleotide that lowers SOD1 protein, received FDA accelerated approval in 2023 for ALS caused by SOD1 gene changes.

Taught in Chapter 13.

Anxiety Disorders¶

A group of mental health conditions in which fear or worry is intense, long-lasting, and out of proportion to the situation, interfering with daily life.

Example: Generalized anxiety disorder, panic disorder, and social anxiety disorder are all anxiety disorders.

See also: Amygdala

Taught in Chapter 15.

Aphasia¶

A language disorder caused by brain damage, often from a stroke, that affects speaking, understanding, reading, or writing, while general intelligence is often spared.

Example: A grandmother with aphasia after a stroke may know exactly what she wants to say but be unable to find the words.

See also: Broca's Area, Wernicke's Area

Taught in Chapter 12.

Aplysia Model¶

The use of the sea slug Aplysia californica, which has relatively few and unusually large neurons, to study how learning changes synapses.

Example: Eric Kandel used Aplysia to show that simple memories involve changes in synaptic strength, work recognized with a Nobel Prize in 2000.

Taught in Chapter 9.

Astrocyte¶

A star-shaped glial cell that supplies neurons with nutrients, controls the chemical surroundings of synapses, and helps form the blood-brain barrier.

Example: After a brain injury, astrocytes multiply and form a scar that walls off the damaged area.

See also: Blood-Brain Barrier

Taught in Chapter 1.

Attention¶

The brain's ability to select some information for focused processing while ignoring the rest.

Example: In a noisy cafeteria, attention lets you follow your friend's voice and tune out other conversations.

See also: Executive Function

Taught in Chapter 12.

Attention-Deficit/Hyperactivity Disorder (ADHD)¶

A developmental condition marked by ongoing patterns of inattention, hyperactivity, or impulsivity that interfere with daily life.

Example: A student with ADHD might lose track of assignments and find it hard to stay seated, and may be helped by behavioral strategies, school supports, or medication.

See also: Attention, Executive Function

Taught in Chapter 15.

Auditory Pathway¶

The chain of connections that carries sound information from the cochlea through the brainstem and thalamus to the auditory cortex in the temporal lobe.

Example: Because the auditory pathway sends information from each ear to both sides of the brain, a stroke on one side rarely causes deafness in one ear.

See also: Cochlea and Hair Cells

Taught in Chapter 6.

Autism Spectrum Disorder (ASD)¶

A developmental condition involving differences in social communication and interaction along with focused interests or repetitive behaviors, which vary widely from person to person.

Example: One autistic person may need daily support while another excels in a specialized career; both are on the autism spectrum.

See also: Nature and Nurture

Taught in Chapter 15.

Autonomic Nervous System¶

The part of the nervous system that controls automatic body functions, such as heart rate, digestion, sweating, and pupil size, without conscious effort.

Example: You do not have to think about digesting lunch, because your autonomic nervous system handles it.

See also: Sympathetic vs Parasympathetic

Taught in Chapter 4.

Axon¶

A long, thin extension of a neuron that carries electrical signals away from the cell body toward other cells.

Example: A numbing shot at the dentist blocks signals traveling along the axons of pain-sensing neurons in your gums.

See also: Dendrite, Myelin Sheath

Taught in Chapter 1.

Axon Terminal¶

The swollen tip at the end of an axon branch, where a neuron releases chemical messengers onto another cell.

Example: Botulinum toxin (the active ingredient in Botox) acts at axon terminals, stopping them from releasing acetylcholine onto muscles.

See also: Synaptic Vesicle

Taught in Chapter 1.

B¶

Basal Ganglia¶

A group of structures deep in the brain that help select which movements to make and which to hold back, and that also take part in habits and reward.

Example: Parkinson's disease and Huntington's disease both disrupt the basal ganglia.

See also: Direct and Indirect Pathways

Taught in Chapter 7.

BE-FAST Warning Signs¶

A memory aid for spotting a possible stroke: Balance loss, Eyes (vision changes), Face drooping, Arm weakness, Speech difficulty, and Time to call emergency services.

Example: If a grandparent suddenly has slurred speech and a drooping smile, BE-FAST tells you to call 911 right away.

See also: Stroke

Taught in Chapter 14.

Biomarker¶

A measurable sign in the body, such as a substance in blood or spinal fluid or a feature on a scan, that indicates a normal process, a disease, or a response to treatment.

Example: Neurofilament light, a protein released when nerve cells are damaged, can be measured in blood or spinal fluid as a biomarker.

Taught in Chapter 13.

Blood-Brain Barrier¶

A tightly sealed lining of the brain's blood vessels that blocks many substances in the blood from entering brain tissue while letting needed nutrients through.

Example: Dopamine pills would not help Parkinson's disease because dopamine cannot cross the blood-brain barrier, so doctors give levodopa, which can.

See also: Astrocyte, Levodopa

Taught in Chapter 1.

Brain Planes and Sections¶

The standard ways of slicing the brain for viewing: sagittal (separating left from right), coronal (separating front from back), and horizontal or axial (separating top from bottom).

Example: CT scans of the head are usually viewed as a stack of axial slices.

See also: Anatomical Directions

Taught in Chapter 4.

Brain Waves¶

Rhythmic patterns of electrical activity produced by large groups of neurons firing together, described by their speed (frequency), such as delta, theta, alpha, beta, and gamma.

Example: Slow, large delta waves dominate deep NREM sleep.

See also: Electroencephalogram (EEG)

Taught in Chapter 10.

Brain-Computer Interface (BCI)¶

A system that records brain activity and translates it into commands for an outside device, such as a computer cursor or robotic arm.

Example: In clinical trials, implanted brain-computer interfaces have let some people with paralysis type or move a cursor using brain signals alone. As of late 2026, no implanted brain-computer interface has FDA approval for long-term use, so these devices are still experimental.

See also: Deep Brain Stimulation

Taught in Chapter 16.

Brainstem¶

The stalk of the brain that connects the cerebrum to the spinal cord, made of the midbrain, pons, and medulla, and controls breathing, heart rate, alertness, and many reflexes.

Example: Doctors check whether the pupils react to light to test brainstem function in an unresponsive patient.

See also: Cranial Nerves

Taught in Chapter 4.

Broca's Area¶

A region in the lower part of the frontal lobe, usually on the left, that is important for producing speech and building sentences.

Example: Damage to Broca's area can make speech slow, effortful, and missing small words like "the" and "is."

See also: Wernicke's Area, Aphasia

Taught in Chapter 12.

C¶

Cell Body (Soma)¶

The main part of a neuron that contains the nucleus and most of the machinery for making proteins and keeping the cell alive.

Example: In ALS, the cell bodies of motor neurons in the spinal cord die, so the muscles they control grow weak.

Taught in Chapter 1.

Cell Membrane¶

A thin, flexible double layer of fat molecules that surrounds every cell and controls which substances enter and leave it.

Example: A neuron's membrane keeps most sodium outside the cell, which sets up the electrical charge that neurons use to send signals.

See also: Ion Channel, Concentration Gradient

Taught in Chapter 1.

Central Nervous System¶

The brain and spinal cord, which together process information and make decisions for the body.

Example: Multiple sclerosis is called a central nervous system disease because its damage is in the brain, spinal cord, and optic nerves.

See also: Peripheral Nervous System

Taught in Chapter 1.

Cerebellar Coordination¶

The cerebellum's job of comparing intended movements with actual movements and correcting their timing, force, and accuracy.

Example: A person with cerebellar damage may overshoot when reaching for a glass of water.

See also: Cerebellum, Motor Learning

Taught in Chapter 7.

Cerebellum¶

A structure at the back of the brain, beneath the occipital lobes, that coordinates the timing and accuracy of movement, supports balance, and helps learn motor skills.

Example: Alcohol impairs the cerebellum, which is why roadside sobriety tests ask a person to walk in a straight line.

See also: Cerebellar Coordination

Taught in Chapter 4.

Cerebral Blood Supply¶

The network of arteries that delivers oxygen and glucose to the brain, coming mainly from two carotid arteries and two vertebral arteries that join in a ring at the base of the brain.

Example: A blockage in the middle cerebral artery, a common stroke site, can weaken the face and arm on the opposite side of the body.

See also: Stroke

Taught in Chapter 4.

Cerebral Cortex¶

The thin, folded outer layer of gray matter covering the two cerebral hemispheres, responsible for perception, voluntary movement, language, and thinking.

Example: The folds of the cortex pack a large surface area into the limited space of the skull.

See also: Frontal Lobe, Localization of Function

Taught in Chapter 4.

Chemical Synapse¶

A synapse where the sending cell releases neurotransmitter molecules across a tiny gap to bind receptors on the receiving cell.

Example: Most synapses in the human brain are chemical synapses, which is why so many medicines and drugs act on them.

See also: Electrical Synapse

Taught in Chapter 3.

Chronic Stress Effects¶

The harmful changes that can result when the stress response stays switched on for weeks or months, affecting sleep, mood, memory, blood pressure, and immune function.

Example: In animal studies, long-lasting stress shrinks dendrites in the hippocampus.

See also: Cortisol

Taught in Chapter 11.

Chronic Traumatic Encephalopathy (CTE)¶

A progressive brain disease linked to repeated head impacts, identified by a specific pattern of tau buildup in the brain that can currently be confirmed only after death.

Example: CTE has been found in the brains of some former football players and other contact-sport athletes.

See also: Tau Tangle, Concussion

Taught in Chapter 14.

Circadian Rhythm¶

A roughly 24-hour cycle in body functions such as sleepiness, body temperature, and hormone levels, driven by an internal clock and reset by light.

Example: Jet lag happens when your circadian rhythm is out of step with the local time.

See also: Suprachiasmatic Nucleus, Melatonin

Taught in Chapter 10.

Clot-Busting Treatment¶

Medicine given through a vein to dissolve a blood clot that is causing an ischemic stroke, such as alteplase or tenecteplase, which helps most when given within a few hours of symptom onset.

Example: Because "time is brain," stroke teams track how many minutes pass between a patient's arrival and the start of clot-busting treatment.

Taught in Chapter 14.

Cochlea and Hair Cells¶

The cochlea is the snail-shaped, fluid-filled part of the inner ear that turns sound vibrations into nerve signals; hair cells are its sensory cells, whose tiny bundles bend when the fluid vibrates.

Example: Loud concerts and earbuds at high volume can permanently damage hair cells and cause hearing loss.

See also: Auditory Pathway

Taught in Chapter 6.

Cognitive Reserve¶

The brain's ability to keep working well despite damage or disease, thought to be built through education, mentally challenging activities, and social connection.

Example: Two people with similar Alzheimer's changes on a brain scan may have very different symptoms, partly because of differences in cognitive reserve.

See also: Normal Brain Aging

Taught in Chapter 13.

Color Vision¶

The ability to tell apart different wavelengths of light, produced by comparing signals from three types of cones tuned to short, medium, and long wavelengths.

Example: In the most common form of color blindness, a person has trouble telling red from green because one type of cone is missing or altered.

See also: Rods and Cones

Taught in Chapter 5.

Computed Tomography (CT)¶

An imaging test that combines many X-ray images taken from different angles into cross-sectional pictures of the body or brain.

Example: A CT scan is usually the first test for a suspected stroke because it quickly shows whether there is bleeding.

See also: Magnetic Resonance Imaging (MRI)

Taught in Chapter 16.

Concentration Gradient¶

A difference in how much of a substance is present in one area compared with a nearby area, which causes the substance to drift from the crowded side toward the less crowded side.

Example: Potassium is much more concentrated inside a neuron than outside, so potassium tends to flow out whenever its channels open.

See also: Sodium-Potassium Pump

Taught in Chapter 1.

Concussion¶

A mild traumatic brain injury in which a blow or jolt makes the brain move rapidly inside the skull, temporarily disrupting how it works.

Example: A soccer player who feels dizzy and confused after a head-to-head collision may have a concussion and should be taken out of play and checked.

Taught in Chapter 14.

Conduction Velocity¶

The speed at which an action potential travels along an axon, which increases with myelin and with a wider axon.

Example: Doctors use nerve conduction studies, which measure conduction velocity, to find nerves damaged by disease.

See also: Saltatory Conduction

Taught in Chapter 2.

Consciousness¶

The state of being awake and aware of yourself and your surroundings.

Example: In the emergency room, doctors judge a patient's level of consciousness by whether the patient opens their eyes, speaks, and follows commands.

See also: Brainstem

Taught in Chapter 10.

Corpus Callosum¶

A thick band of white matter that connects the left and right cerebral hemispheres and lets them share information.

Example: In split-brain surgery, the corpus callosum is cut to stop severe seizures from spreading from one hemisphere to the other.

See also: Split-Brain Studies

Taught in Chapter 4.

Correlation vs Causation¶

The distinction between two things that tend to occur together (correlation) and one thing actually producing the other (causation).

Example: Teens who sleep less may get lower grades, but that pattern alone does not prove that short sleep causes the lower grades.

Taught in Chapter 16.

Corticospinal Tract¶

The main pathway carrying voluntary movement commands from the motor cortex down to the spinal cord, with most fibers crossing to the opposite side in the lower brainstem.

Example: Because the corticospinal tract crosses, damage to the right side of the brain weakens the left side of the body.

See also: Primary Motor Cortex

Taught in Chapter 7.

Cortisol¶

A steroid hormone released by the adrenal glands that raises blood sugar, adjusts immune activity, and helps the body cope with stress.

Example: Cortisol levels naturally peak shortly after a person wakes up in the morning.

Taught in Chapter 11.

Cranial Nerve Exam¶

A set of quick bedside tests of the twelve cranial nerves, such as checking pupil responses, eye movements, facial movement, hearing, and tongue movement.

Example: Asking a patient to smile and raise their eyebrows tests the facial nerve.

See also: Cranial Nerves

Taught in Chapter 16.

Cranial Nerves¶

Twelve pairs of nerves that leave directly from the brain and brainstem and mainly serve the head and neck, carrying smell, vision, eye and face movement, hearing, swallowing, and more.

Example: The seventh cranial nerve (the facial nerve) controls facial expression, so damage to it can make one side of the face droop.

See also: Cranial Nerve Exam

Taught in Chapter 4.

Critical Period¶

A window in development when the brain is especially sensitive to certain experiences, which are needed for some abilities to develop normally.

Example: If a baby's cloudy lens (cataract) is not treated early, that eye may have lasting vision problems even after later surgery.

See also: Neuroplasticity

Taught in Chapter 8.

D¶

Decision-Making¶

The process of choosing among options by weighing their likely outcomes, risks, and value.

Example: Choosing whether to study tonight or go to a movie involves decision-making circuits in the prefrontal cortex.

See also: Reward Prediction Error

Taught in Chapter 12.

Declarative Memory¶

Memory for facts and events that a person can consciously recall and put into words.

Example: Knowing that Paris is the capital of France and remembering your last birthday party are declarative memories.

See also: Procedural Memory, Hippocampus

Taught in Chapter 9.

Deep Brain Stimulation¶

A treatment in which thin electrodes are placed in specific deep brain areas and connected to a pacemaker-like device that sends steady electrical pulses.

Example: Deep brain stimulation can reduce tremor and stiffness in some people with Parkinson's disease when medicines alone are not enough.

See also: Parkinson's Disease

Taught in Chapter 7.

Dementia¶

A decline in memory, thinking, or other mental abilities severe enough to interfere with daily life, which can be caused by several different brain diseases.

Example: Alzheimer's disease is the most common cause of dementia.

See also: Alzheimer's Disease, Normal Brain Aging

Taught in Chapter 13.

Dendrite¶

A branching extension of a neuron that receives signals from other cells and carries them toward the cell body.

Example: A Purkinje cell in the cerebellum has a huge, fan-shaped tree of dendrites that collects input from many thousands of other neurons.

See also: Axon

Taught in Chapter 1.

Depolarization¶

A change in membrane potential that makes the inside of a cell less negative, moving it toward zero or above.

Example: When positively charged sodium ions rush into a neuron, the cell depolarizes.

See also: Repolarization, Hyperpolarization

Taught in Chapter 2.

Depression¶

A common mood disorder involving persistent sadness or loss of interest in activities for at least two weeks, often with changes in sleep, appetite, energy, and concentration.

Example: A teen with depression might stop enjoying a favorite sport, sleep much more than usual, and find it hard to focus in class.

See also: Serotonin, Psychiatric Medication

Taught in Chapter 15.

Dermatome¶

An area of skin whose sensation is supplied by a single spinal nerve root.

Example: Shingles causes a painful rash in a band that follows one dermatome.

See also: Somatosensory System

Taught in Chapter 6.

Differential Diagnosis¶

A list of possible conditions that could explain a patient's symptoms, narrowed down using the history, exam, and tests.

Example: For sudden weakness on one side of the body, the differential diagnosis includes stroke, weakness after a seizure, and very low blood sugar.

See also: Lesion Localization

Taught in Chapter 16.

Direct and Indirect Pathways¶

Two circuits through the basal ganglia: the direct pathway helps release wanted movements, and the indirect pathway helps hold back unwanted ones.

Example: Loss of dopamine in Parkinson's disease weakens the direct pathway and strengthens the indirect pathway, so movements become slow and hard to start.

See also: Basal Ganglia

Taught in Chapter 7.

Dopamine¶

A neurotransmitter involved in movement, motivation, and learning from rewards.

Example: Parkinson's disease happens when dopamine-making neurons in the substantia nigra die.

See also: Reward Pathway, Substantia Nigra

Taught in Chapter 3.

E¶

Electrical Synapse¶

A synapse where two cells are directly linked by protein channels called gap junctions, letting electrical current pass straight from one cell to the next.

Example: Electrical synapses help groups of neurons in the brainstem fire in sync, as in circuits that control breathing rhythms.

See also: Chemical Synapse

Taught in Chapter 3.

Electroencephalogram (EEG)¶

A test that records the brain's electrical activity through small electrodes placed on the scalp.

Example: An EEG can show the spike patterns that help doctors diagnose epilepsy.

See also: Brain Waves

Taught in Chapter 10.

Emotion¶

A brief, coordinated response to a meaningful event that includes feelings, body changes, and urges to act.

Example: The racing heart, sweaty palms, and urge to step back when you see a snake are all parts of an emotion.

See also: Limbic System, Amygdala

Taught in Chapter 11.

Emotion Regulation¶

The ways people influence which emotions they have and how strongly they feel and express them.

Example: Viewing a bad test grade as a clue about how to study better, rather than as proof of failure, is a form of emotion regulation.

See also: Prefrontal Cortex

Taught in Chapter 11.

Epilepsy¶

A brain condition in which a person has a lasting tendency to have repeated, unprovoked seizures.

Example: Many people with epilepsy control their seizures well with daily medication and live full, active lives.

See also: Seizure, Electroencephalogram (EEG)

Taught in Chapter 14.

Excitatory Postsynaptic Potential (EPSP)¶

A small, temporary depolarization in a receiving neuron that makes it more likely to fire an action potential.

Example: Glutamate binding to receptors on a dendrite usually produces an EPSP.

Taught in Chapter 3.

Executive Function¶

A set of mental skills, including planning, working memory, flexible thinking, and self-control, that help a person manage goals and behavior.

Example: Breaking a big science project into smaller tasks with deadlines uses executive function.

See also: Prefrontal Cortex, Working Memory

Taught in Chapter 12.

Eye and Retina¶

The eye is the organ that focuses light; the retina is the thin layer of neural tissue lining the back of the eye that detects light and begins processing images.

Example: Eye doctors look at the retina through the pupil to spot damage caused by diabetes or high blood pressure.

See also: Photoreceptor

Taught in Chapter 5.

F¶

Fear Conditioning¶

A form of learning in which a neutral cue becomes linked with something unpleasant, so the cue alone comes to trigger fear.

Example: After being bitten by a dog, a person might feel anxious just hearing a bark.

Taught in Chapter 11.

Fetal Alcohol Spectrum Disorders (FASD)¶

A range of lifelong physical, learning, and behavioral differences that can occur in a person who was exposed to alcohol before birth.

Example: A child with FASD may have trouble with attention, memory, and math, and benefits from early support at school.

See also: Teratogen

Taught in Chapter 8.

Fight or Flight¶

The rapid, automatic reaction to danger in which sympathetic nerves and the hormone adrenaline speed the heart, open the airways, and send more blood to the muscles.

Example: Jumping back when a car suddenly swerves toward you is a fight-or-flight reaction.

Taught in Chapter 11.

Firing Rate Coding¶

A way neurons represent the strength of a stimulus by how many action potentials they fire per second.

Example: A light touch might make a skin sensor fire a few times per second, while firm pressure makes it fire much faster.

See also: All-or-None Principle

Taught in Chapter 2.

Frontal Lobe¶

The front region of each cerebral hemisphere, involved in planning, decision-making, personality, voluntary movement, and speech production.

Example: Phineas Gage's personality changed after an iron rod damaged his frontal lobe in 1848.

See also: Prefrontal Cortex

Taught in Chapter 4.

Functional MRI (fMRI)¶

An MRI technique that tracks changes in blood oxygen levels to estimate which brain areas are more active during a task.

Example: Researchers use fMRI to see which brain regions become more active when people look at faces.

Taught in Chapter 16.

G¶

GABA (Gamma-Aminobutyric Acid)¶

The main inhibitory neurotransmitter of the brain, which makes receiving neurons less likely to fire.

Example: Benzodiazepine medicines, used for some seizures and anxiety, work by boosting the effect of GABA.

See also: Glutamate

Taught in Chapter 3.

Gate Control Theory¶

The idea that nerve circuits in the spinal cord can "close a gate" on pain signals when non-painful touch signals arrive at the same time.

Example: Rubbing a bumped elbow makes it hurt less, as gate control theory predicts.

See also: Pain Pathway

Taught in Chapter 6.

Gene Therapy¶

A treatment that adds, replaces, edits, or silences genes in a patient's cells to treat or prevent disease.

Example: Onasemnogene abeparvovec (Zolgensma), FDA-approved in 2019, delivers a working copy of the SMN1 gene to young children with spinal muscular atrophy.

See also: Antisense Oligonucleotide

Taught in Chapter 13.

Glial Cell¶

A cell of the nervous system that is not a neuron and that supports, protects, insulates, or cleans up after neurons.

Example: Gliomas, a common type of tumor that starts in the brain, grow from glial cells rather than from neurons.

See also: Astrocyte, Microglia

Taught in Chapter 1.

Glutamate¶

The main excitatory neurotransmitter of the brain, which makes receiving neurons more likely to fire.

Example: After a stroke, too much glutamate can overexcite and damage nearby neurons.

See also: GABA (Gamma-Aminobutyric Acid)

Taught in Chapter 3.

Gray Matter¶

Brain and spinal cord tissue made mostly of neuron cell bodies, dendrites, and synapses, where most information processing happens.

Example: The cerebral cortex is a thin outer layer of gray matter.

See also: White Matter

Taught in Chapter 4.

Gustation¶

The sense of taste, in which taste receptor cells on the tongue detect sweet, salty, sour, bitter, and savory (umami) chemicals.

Example: Food seems bland when you have a cold because much of what we call "flavor" actually comes from smell.

See also: Olfaction

Taught in Chapter 6.

H¶

Habituation and Sensitization¶

Habituation is a shrinking response to a repeated, harmless stimulus; sensitization is a stronger response to many stimuli after a strong or harmful one.

Example: You stop noticing the hum of a refrigerator (habituation), but after a loud crash you jump at every small sound (sensitization).

See also: Aplysia Model

Taught in Chapter 9.

Hebbian Learning¶

The principle that when one neuron repeatedly helps make another neuron fire, the connection between them grows stronger, often summarized as "cells that fire together wire together."

Example: Repeatedly hearing a new friend's name while seeing her face links the two in your memory.

See also: Long-Term Potentiation

Taught in Chapter 9.

Hemispheric Lateralization¶

The tendency for some brain functions to depend more on one side (hemisphere) of the brain than on the other.

Example: Language is usually handled mostly by the left hemisphere, while recognizing faces relies more on the right.

See also: Split-Brain Studies

Taught in Chapter 12.

Hemorrhagic Stroke¶

A stroke caused by a blood vessel bursting and bleeding into or around the brain.

Example: A sudden, extremely severe headache can be a warning sign of a hemorrhagic stroke.

See also: Ischemic Stroke

Taught in Chapter 14.

Hippocampus¶

A curved structure inside each temporal lobe that is essential for forming new memories of facts and events and for finding your way through space.

Example: Patient H.M. could not form new memories of facts and events after surgery removed much of the hippocampus on both sides.

See also: Patient H.M., Memory Consolidation

Taught in Chapter 9.

Homunculus¶

A map of the body laid out across the somatosensory or motor cortex, often drawn as a distorted figure whose body parts are sized by how much cortex they use.

Example: The homunculus has huge hands and lips because these body parts have many receptors and fine control.

Taught in Chapter 6.

HPA Axis (Hypothalamic-Pituitary-Adrenal Axis)¶

A hormone chain in which the hypothalamus signals the pituitary gland, which in turn signals the adrenal glands to release cortisol during stress.

Example: Rising cortisol signals back to the hypothalamus and pituitary to turn the HPA axis down, like a thermostat.

See also: Cortisol

Taught in Chapter 11.

Huntington's Disease¶

An inherited neurodegenerative disease caused by an expanded, repeated stretch of DNA in the HTT gene, leading to uncontrolled movements, mood changes, and declining thinking skills, usually starting in adulthood.

Example: Each child of a parent with Huntington's disease has a 50 percent chance of inheriting the disease-causing gene.

See also: Basal Ganglia

Taught in Chapter 13.

Hyperpolarization¶

A change in membrane potential that makes the inside of a cell more negative than its usual resting level.

Example: The neurotransmitter GABA often hyperpolarizes neurons, making them less likely to fire.

See also: Depolarization

Taught in Chapter 2.

Hypothalamus¶

A small region below the thalamus that controls hunger, thirst, body temperature, daily sleep cycles, and hormone release by the pituitary gland.

Example: During a fever, the hypothalamus raises the body's temperature set point.

Taught in Chapter 4.

I¶

Inhibitory Postsynaptic Potential (IPSP)¶

A small, temporary change in a receiving neuron, usually a hyperpolarization, that makes it less likely to fire an action potential.

Example: GABA released by an inhibitory interneuron typically produces an IPSP.

Taught in Chapter 3.

Interneuron¶

A neuron that connects other neurons within the brain or spinal cord, passing and shaping signals between incoming and outgoing pathways.

Example: During a knee-jerk reflex, spinal interneurons relax the hamstring muscle so the thigh muscle can straighten the leg.

See also: Sensory Neuron, Motor Neuron

Taught in Chapter 1.

Ion¶

An atom or small molecule that carries an electrical charge because it has gained or lost one or more electrons.

Example: Sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), and chloride (Cl⁻) are the main ions that let neurons fire.

See also: Ion Channel

Taught in Chapter 1.

Ion Channel¶

A protein that forms a pore through the cell membrane, letting particular ions flow in or out when it is open.

Example: Many anticonvulsant medicines work by changing how sodium or calcium ion channels open in overactive neurons.

See also: Voltage-Gated Channel

Taught in Chapter 2.

Ionotropic Receptor¶

A receptor that is itself an ion channel, opening directly when its neurotransmitter binds and producing a fast response.

Example: The acetylcholine receptors on skeletal muscle are ionotropic receptors.

See also: Metabotropic Receptor

Taught in Chapter 3.

Ischemic Stroke¶

A stroke caused by a blocked artery that cuts off blood flow to part of the brain.

Example: Most strokes are ischemic, often caused by a blood clot that travels from the heart or forms in a narrowed artery.

See also: Clot-Busting Treatment

Taught in Chapter 14.

L¶

Language Areas¶

The brain regions, usually in the left hemisphere, that support understanding and producing language.

Example: In most right-handed people, the language areas are in the left hemisphere.

See also: Broca's Area, Wernicke's Area

Taught in Chapter 12.

Learning¶

The process by which experience produces a lasting change in knowledge or behavior.

Example: After touching a cactus once, a young child avoids touching it again.

See also: Memory, Neuroplasticity

Taught in Chapter 9.

Lesion Localization¶

The reasoning doctors use to work out where in the nervous system an area of damage (a lesion) is, based on the pattern of symptoms and exam findings.

Example: Weakness of the right face, arm, and leg together with trouble speaking localizes the lesion to the left side of the brain.

Taught in Chapter 16.

Levodopa¶

A medicine for Parkinson's disease that crosses the blood-brain barrier and is converted into dopamine inside the brain.

Example: Levodopa is usually combined with carbidopa, which keeps it from turning into dopamine before it reaches the brain.

See also: Parkinson's Disease

Taught in Chapter 7.

Limbic System¶

A loosely defined group of connected brain structures, including the amygdala, hippocampus, and parts of the hypothalamus and cortex, involved in emotion, motivation, and memory.

Example: The limbic system links your memory of a place with the feelings you had there.

See also: Amygdala

Taught in Chapter 11.

Local Anesthetic¶

A drug that temporarily blocks feeling in one part of the body by blocking voltage-gated sodium channels in nerve axons.

Example: Lidocaine injected before stitches numbs the skin while the patient stays fully awake.

See also: Voltage-Gated Channel

Taught in Chapter 2.

Localization of Function¶

The finding that specific brain regions are specialized for specific jobs.

Example: Damage to Broca's area affects producing speech but usually spares understanding it, which shows localization of function.

See also: Lesion Localization

Taught in Chapter 4.

Long-Term Memory¶

Memory that lasts from hours to a lifetime and has a very large capacity.

Example: Your memory of your first day of high school is stored in long-term memory.

See also: Declarative Memory, Procedural Memory

Taught in Chapter 9.

Long-Term Potentiation¶

A lasting increase in the strength of a synapse after it has been activated strongly or repeatedly, thought to be a key cellular basis of learning and memory.

Example: Long-term potentiation was first described in the hippocampus of rabbits in the early 1970s.

See also: Hebbian Learning, Neuroplasticity

Taught in Chapter 9.

Lumbar Puncture¶

A procedure in which a thin needle is placed between bones of the lower back to collect cerebrospinal fluid for testing; it is also called a spinal tap.

Example: A lumbar puncture can detect signs of meningitis, such as white blood cells and bacteria in the fluid.

See also: Ventricles and CSF, Meninges

Taught in Chapter 16.

M¶

Magnetic Resonance Imaging (MRI)¶

An imaging method that uses a strong magnet and radio waves, without X-rays, to make detailed pictures of soft tissues such as the brain.

Example: MRI shows the patches of damaged myelin in multiple sclerosis much more clearly than CT does.

Taught in Chapter 16.

Mechanoreceptor¶

A sensory receptor that responds to mechanical force such as pressure, stretch, or vibration.

Example: Mechanoreceptors in your fingertips let you feel the texture of a phone case.

See also: Somatosensory System

Taught in Chapter 6.

Melatonin¶

A hormone released by the pineal gland, mainly in darkness, that signals nighttime to the body and helps time sleep.

Example: Bright screen light late in the evening can delay melatonin release and make it harder to fall asleep.

See also: Circadian Rhythm

Taught in Chapter 10.

Membrane Potential¶

The difference in electrical charge between the inside and the outside of a cell, measured across its membrane in millivolts.

Example: When a neuron is excited, its membrane potential becomes less negative.

See also: Resting Potential

Taught in Chapter 2.

Memory¶

The ability to store information gained from experience and retrieve it later.

Example: Remembering where you parked and knowing how to ride a bike are both forms of memory.

See also: Learning, Long-Term Memory

Taught in Chapter 9.

Memory Consolidation¶

The process by which new, fragile memories become stable and long-lasting, continuing for hours to years after learning.

Example: Sleeping after studying aids consolidation, so the material is easier to recall on test day.

See also: Sleep and Memory

Taught in Chapter 9.

Meninges¶

Three protective membranes, the dura mater, arachnoid mater, and pia mater, that wrap the brain and spinal cord.

Example: Meningitis is an infection that inflames the meninges, often causing headache, fever, and a stiff neck.

Taught in Chapter 4.

Mental Illness¶

A health condition involving major changes in thinking, emotion, or behavior that cause distress or make it hard to function at school, work, or in relationships.

Example: Depression, anxiety disorders, and schizophrenia are mental illnesses, and like other medical conditions, they can be treated.

See also: Stigma and Treatment Access

Taught in Chapter 15.

Mental Status Exam¶

The part of a neurological or psychiatric assessment that evaluates alertness, orientation, attention, memory, language, mood, and thought.

Example: Asking a patient for today's date and to remember three words for later are parts of a mental status exam.

See also: Neurological Exam

Taught in Chapter 16.

Metabotropic Receptor¶

A receptor that does not form a channel but instead starts a chain of chemical steps inside the cell, producing slower and longer-lasting effects.

Example: Most dopamine and serotonin receptors are metabotropic receptors.

See also: Ionotropic Receptor

Taught in Chapter 3.

Microglia¶

Small immune cells of the brain and spinal cord that patrol for injury or infection, swallow debris, and help trim unneeded synapses.

Example: In Alzheimer's disease, microglia gather around amyloid plaques and try to clear them away.

See also: Glial Cell

Taught in Chapter 1.

Motor Learning¶

The process of making a movement faster, more accurate, and smoother through practice until it becomes more automatic.

Example: Learning to dribble a soccer ball without looking down is motor learning.

See also: Procedural Memory

Taught in Chapter 7.

Motor Neuron¶

A neuron that carries commands from the brain or spinal cord out to muscles or glands.

Example: When Maya decides to kick a soccer ball, motor neurons in her spinal cord fire and make her leg muscles contract.

Taught in Chapter 1.

Motor Unit¶

One motor neuron together with all of the muscle fibers it controls.

Example: Muscles that move the eyes have small motor units for precise control, while large leg muscles have much bigger ones.

See also: Motor Neuron

Taught in Chapter 7.

Multiple Sclerosis¶

An autoimmune disease in which the body's immune system attacks myelin in the brain, spinal cord, and optic nerves, slowing or blocking nerve signals.

Example: A young adult with multiple sclerosis might have an episode of blurred vision in one eye that improves over several weeks.

Taught in Chapter 1.

Myasthenia Gravis¶

An autoimmune disease in which antibodies attack acetylcholine receptors or related proteins at the neuromuscular junction, causing muscle weakness that worsens with use.

Example: A person with myasthenia gravis may have drooping eyelids and double vision that get worse by the end of the day.

See also: Neuromuscular Junction

Taught in Chapter 3.

Myelin Sheath¶

A fatty, insulating wrapping around many axons, made by glial cells, that makes electrical signals travel much faster.

Example: In multiple sclerosis, the immune system damages myelin in the brain and spinal cord, so signals slow down or get blocked.

Taught in Chapter 1.

Myelination Over Time¶

The gradual addition of myelin to axons from before birth into early adulthood, which speeds communication between brain regions.

Example: Connections of the prefrontal cortex are among the last to finish myelinating, a process that continues into a person's twenties.

See also: Myelin Sheath, Prefrontal Maturation

Taught in Chapter 8.

N¶

Naloxone¶

A medicine that rapidly reverses an opioid overdose by blocking opioid receptors.

Example: Naloxone nasal spray has been sold without a prescription in the United States since 2023.

See also: Opioids, Agonist and Antagonist

Taught in Chapter 15.

Narcolepsy¶

A long-lasting sleep disorder that causes overwhelming daytime sleepiness and sudden sleep episodes, often with cataplexy, a brief loss of muscle tone triggered by strong emotions.

Example: A student with narcolepsy may fall asleep in class even after a full night of sleep, or feel their knees buckle when laughing.

See also: Orexin (Hypocretin)

Taught in Chapter 10.

Nature and Nurture¶

The combined influence of genes (nature) and of life experiences and environment (nurture) on how the brain and behavior develop.

Example: A person may inherit genes that raise the risk of depression, but stress and support in their life also shape whether it develops.

Taught in Chapter 8.

Neglect Syndrome¶

A condition, usually after damage to the right parietal lobe, in which a person fails to notice or respond to things on one side of space, typically the left.

Example: A patient with neglect may eat only the food on the right half of the plate.

See also: Parietal Lobe

Taught in Chapter 12.

Nervous System¶

The body's network of cells that collects information, processes it, and sends out commands, made up of the brain, spinal cord, and nerves.

Example: When you touch a hot pan, your nervous system detects the heat and pulls your hand away in a fraction of a second.

Taught in Chapter 1.

Neural Tube¶

The hollow tube of tissue in an early embryo that develops into the brain and spinal cord.

Example: Getting enough folic acid before and during early pregnancy lowers the risk of neural tube defects such as spina bifida.

Taught in Chapter 8.

Neurodegeneration¶

The progressive loss of structure or function of neurons, often ending in their death, which worsens over time.

Example: Alzheimer's disease, Parkinson's disease, Huntington's disease, and ALS are all neurodegenerative diseases.

See also: Protein Misfolding

Taught in Chapter 13.

Neurogenesis¶

The creation of new neurons from dividing stem cells.

Example: Most neurogenesis in humans happens before birth, which is why the developing brain is so sensitive to harm during pregnancy.

See also: Neuronal Migration

Taught in Chapter 8.

Neurological Exam¶

A step-by-step bedside check of the nervous system that tests mental status, cranial nerves, strength, reflexes, sensation, coordination, and walking.

Example: Asking a patient to walk heel to toe in a straight line tests balance and the cerebellum during a neurological exam.

Taught in Chapter 16.

Neuromuscular Junction¶

The synapse between a motor neuron and a skeletal muscle fiber, where acetylcholine triggers the muscle fiber to contract.

Example: Botulinum toxin blocks the neuromuscular junction, which is how Botox injections relax facial muscles.

See also: Acetylcholine, Myasthenia Gravis

Taught in Chapter 3.

Neuron¶

A cell specialized to receive, process, and send information using electrical and chemical signals.

Example: The neurons that let you wiggle your toes have axons that run all the way from the lower spinal cord to the foot.

See also: Glial Cell

Taught in Chapter 1.

Neuronal Migration¶

The movement of newly formed neurons from where they are born to their final positions in the developing brain.

Example: When neuronal migration goes wrong, the cortex can form with abnormal folds, which can lead to seizures.

See also: Neurogenesis

Taught in Chapter 8.

Neuroplasticity¶

The nervous system's ability to change its structure and function in response to experience, learning, or injury.

Example: After a stroke, rehabilitation helps healthy brain areas take over some of the lost functions.

See also: Long-Term Potentiation

Taught in Chapter 9.

Neuroscience Careers¶

Jobs that involve studying, treating, or applying knowledge of the nervous system, such as research neuroscientist, neurologist, neurosurgeon, psychiatrist, neuropsychologist, and neuroscience nurse.

Example: A neurologist diagnoses and treats brain disorders mainly with medicines, while a neurosurgeon treats them with operations.

See also: Pre-Med Pathway

Taught in Appendix: Neuroscience Careers.

Neurotoxin¶

A substance that damages nerve cells or disrupts the way they signal.

Example: Tetrodotoxin, found in pufferfish, blocks sodium channels and can cause paralysis.

See also: Local Anesthetic

Taught in Chapter 2.

Neurotransmitter¶

A chemical messenger that a neuron releases at a synapse to excite, inhibit, or otherwise change the activity of a target cell.

Example: Glutamate, GABA, dopamine, and serotonin are all neurotransmitters.

See also: Receptor

Taught in Chapter 3.

Nicotine and Vaping¶

Nicotine is an addictive chemical found in tobacco and most e-cigarettes; vaping is inhaling the aerosol an e-cigarette produces, which often delivers nicotine.

Example: Because the teen brain is still developing, nicotine from vaping can harm attention and learning and raise the risk of addiction.

See also: Addiction, Adolescent Brain

Taught in Chapter 15.

Nociception¶

The detection of harmful or possibly harmful stimuli, such as extreme heat, sharp pressure, or chemicals released by damaged tissue, by specialized sensory neurons.

Example: Nociception can trigger a withdrawal reflex before you consciously feel any pain.

See also: Pain Pathway

Taught in Chapter 6.

Node of Ranvier¶

A short, uncovered gap between sections of myelin along an axon, where ion channels are packed together and the electrical signal is renewed.

Example: In a myelinated axon, the signal "jumps" from one node of Ranvier to the next instead of creeping along every bit of membrane.

See also: Saltatory Conduction

Taught in Chapter 1.

Norepinephrine¶

A neurotransmitter and hormone that increases alertness, attention, heart rate, and blood pressure, especially during stress.

Example: Norepinephrine released by nerves to the heart makes it pound before a big test.

See also: Fight or Flight

Taught in Chapter 3.

Normal Brain Aging¶

The gradual brain changes that come with age in healthy people, such as slower processing speed and some loss of brain volume, while knowledge and vocabulary are often kept.

Example: An older adult may take longer to recall a name but still knows the meanings of more words than most teenagers.

See also: Dementia, Cognitive Reserve

Taught in Chapter 13.

NREM Sleep (Non-Rapid Eye Movement Sleep)¶

The part of sleep without rapid eye movements, ranging from light sleep to deep slow-wave sleep, during which brain activity slows down.

Example: Sleepwalking usually happens during deep NREM sleep.

Taught in Chapter 10.

O¶

Occipital Lobe¶

The back region of each cerebral hemisphere, devoted mainly to processing vision.

Example: A hard hit to the back of the head can make a person "see stars" because the occipital lobe is jolted.

See also: Primary Visual Cortex

Taught in Chapter 4.

Olfaction¶

The sense of smell, in which odor molecules bind receptors in the nose and signals travel to the olfactory bulb and then the brain.

Example: A reduced sense of smell can appear years before movement symptoms in some people who later develop Parkinson's disease.

See also: Gustation

Taught in Chapter 6.

Oligodendrocyte¶

A glial cell in the brain and spinal cord that wraps myelin around segments of several nearby axons at once.

Example: Multiple sclerosis damages myelin made by oligodendrocytes, which is why it affects the brain and spinal cord but not the nerves in your arms.

See also: Schwann Cell

Taught in Chapter 1.

Opioids¶

A class of drugs, including prescription pain relievers, heroin, and fentanyl, that act on opioid receptors to relieve pain and can slow breathing to dangerous levels.

Example: Illegally made fentanyl is involved in many overdose deaths in the United States.

See also: Naloxone, Agonist and Antagonist

Taught in Chapter 15.

Optic Chiasm¶

The X-shaped crossing point beneath the brain where nerve fibers from the inner (nose-side) half of each retina cross to the opposite side.

Example: A tumor pressing on the optic chiasm can cause loss of the outer half of vision in both eyes.

See also: Pituitary Tumor, Visual Field Defect

Taught in Chapter 5.

Optic Nerve¶

The cranial nerve that carries visual signals from the retina of each eye toward the brain.

Example: Glaucoma slowly damages the optic nerve, usually taking away side vision first.

See also: Optic Chiasm

Taught in Chapter 5.

Orexin (Hypocretin)¶

A chemical messenger made by neurons in the hypothalamus that promotes wakefulness and keeps the switch between sleep and waking stable.

Example: Most people with narcolepsy type 1 have lost the neurons that make orexin.

See also: Narcolepsy

Taught in Chapter 10.

P¶

Pain Pathway¶

The route that signals from damage-sensing neurons take into the spinal cord, up through the brainstem and thalamus, to brain areas that create the experience of pain.

Example: Opioid medicines reduce pain by acting at several points along the pain pathway.

See also: Nociception, Gate Control Theory

Taught in Chapter 6.

Parietal Lobe¶

The upper back region of each cerebral hemisphere, which processes touch and body position and helps track where things are in space.

Example: Damage to the right parietal lobe can make a person ignore the left side of space.

See also: Neglect Syndrome

Taught in Chapter 4.

Parkinson's Disease¶

A progressive brain disorder caused mainly by the loss of dopamine-making neurons in the substantia nigra, leading to tremor at rest, stiffness, and slow movement.

Example: An early sign can be a slight tremor in one hand and handwriting that grows smaller across the page.

See also: Levodopa, Deep Brain Stimulation

Taught in Chapter 7.

Patient H.M.¶

Henry Molaison, a man who had parts of both medial temporal lobes, including much of the hippocampus, removed in 1953 to treat severe epilepsy and who afterward could not form new declarative memories.

Example: H.M. got better at a mirror-drawing task over several days without remembering that he had practiced, showing that procedural memory relies on different brain systems.

See also: Hippocampus, Amnesia

Taught in Chapter 9.

Peripheral Nervous System¶

All the nerves and nerve cells outside the brain and spinal cord, which link them to the skin, muscles, and internal organs.

Example: Numbness and tingling in the feet caused by long-term diabetes is a peripheral nerve problem, not a brain problem.

See also: Central Nervous System

Taught in Chapter 1.

PET Scan (Positron Emission Tomography)¶

An imaging test that detects a weakly radioactive tracer injected into the blood to show metabolism, blood flow, or specific molecules in the brain.

Example: An amyloid PET scan can show whether amyloid plaques are present before a patient starts anti-amyloid treatment.

See also: Amyloid Plaque, Biomarker

Taught in Chapter 16.

Phantom Limb Pain¶

Pain that a person feels as if it comes from a body part that has been amputated.

Example: Mirror therapy, in which a person watches the reflection of their remaining limb move, eases phantom limb pain for some people.

See also: Neuroplasticity

Taught in Chapter 6.

Photoreceptor¶

A light-sensing cell in the retina that absorbs light and converts it into an electrical signal.

Example: Rods and cones are the two types of photoreceptors used for vision in the human eye.

See also: Rods and Cones

Taught in Chapter 5.

Pituitary Tumor¶

An abnormal growth, usually not cancer, in the pituitary gland at the base of the brain that can disturb hormone levels or press on nearby structures.

Example: A large pituitary tumor can press on the optic chiasm and take away side vision in both eyes.

See also: Optic Chiasm

Taught in Chapter 5.

Post-Traumatic Stress Disorder (PTSD)¶

A mental health condition that can develop after a terrifying event, involving unwanted memories or flashbacks, avoidance of reminders, negative changes in mood, and feeling constantly on alert.

Example: A person with PTSD after a car crash may avoid driving and startle easily at loud noises; therapy and medication can help.

See also: Fear Conditioning, Amygdala

Taught in Chapter 11.

Pre-Med Pathway¶

The usual route to medical school in the United States: a bachelor's degree that includes required science courses, clinical and research experience, and the MCAT admissions exam.

Example: A pre-med student might major in neuroscience, volunteer at a hospital, and take the MCAT before applying to medical school.

See also: Neuroscience Careers

Taught in Appendix: Neuroscience Careers.

Prefrontal Cortex¶

The front-most part of the frontal lobe, which supports planning, judgment, impulse control, working memory, and social behavior.

Example: The prefrontal cortex helps you resist checking your phone in the middle of an exam.

Taught in Chapter 4.

Prefrontal Maturation¶

The slow development of the prefrontal cortex and its connections, which continues through adolescence into early adulthood and supports growing self-control and planning.

Example: Prefrontal maturation helps explain why older teens are generally better than younger teens at resisting impulses.

See also: Prefrontal Cortex, Adolescent Brain

Taught in Chapter 8.

Primary Motor Cortex¶

The strip of cerebral cortex at the back of the frontal lobe that sends commands for voluntary movement.

Example: A stroke in the left primary motor cortex can weaken the right arm.

See also: Corticospinal Tract, Homunculus

Taught in Chapter 7.

Primary Visual Cortex¶

The first area of the cerebral cortex to receive visual information, located in the occipital lobe, where basic features such as edges and their angles are detected.

Example: A stroke in the left primary visual cortex causes loss of the right half of vision in both eyes.

See also: Ventral and Dorsal Streams

Taught in Chapter 5.

Procedural Memory¶

Memory for skills and habits, shown by performing them rather than by describing them.

Example: Typing without looking at the keyboard relies on procedural memory.

See also: Declarative Memory, Motor Learning

Taught in Chapter 9.

Proprioception¶

The sense of the position and movement of your own body parts, based on receptors in muscles, tendons, and joints.

Example: Proprioception lets you touch your nose with your eyes closed.

See also: Vestibular System

Taught in Chapter 6.

Protein¶

A large molecule built from a chain of amino acids that folds into a specific shape, which lets it do a specific job such as forming structures, speeding up chemical reactions, or passing on signals.

Example: Ion channels, receptors, and the enzymes that make neurotransmitters are all proteins.

See also: Protein Misfolding

Taught in Chapter 1.

Protein Misfolding¶

An error in which a protein folds into the wrong shape, so it loses its normal function and may clump together in ways that harm cells.

Example: In Alzheimer's disease, misfolded amyloid-beta and tau proteins build up in the brain.

See also: Amyloid Plaque, Tau Tangle

Taught in Chapter 13.

Psychiatric Medication¶

A medicine used to treat mental illness by changing chemical signaling in the brain.

Example: Antidepressants, antipsychotics, mood stabilizers, and stimulants for ADHD are all psychiatric medications.

See also: Reuptake

Taught in Chapter 15.

R¶

Receptive Field¶

The specific part of the sensory world, such as a patch of skin or a region of what you see, in which a stimulus changes a particular sensory neuron's activity.

Example: Touch neurons in your fingertips have tiny receptive fields, which is why fingertips can feel fine details like Braille dots.

Taught in Chapter 5.

Receptor¶

A protein that recognizes and binds a specific signaling molecule, such as a neurotransmitter, hormone, or drug, and causes a change in the cell.

Example: Opioid drugs act by binding opioid receptors on neurons.

Taught in Chapter 3.

Reflex Arc¶

The neural circuit behind a reflex: a sensory receptor, a sensory neuron, a connection in the spinal cord or brainstem, a motor neuron, and a muscle or gland.

Example: Pulling your hand from a hot stove uses a reflex arc through the spinal cord before your brain even registers the pain.

See also: Stretch Reflex

Taught in Chapter 7.

Reflex Testing¶

Checking automatic responses, such as tapping a tendon with a reflex hammer, to assess sensory and motor nerves and the spinal cord and to spot signs of brain or nerve damage.

Example: In an adult, a big toe that points upward when the sole of the foot is stroked (the Babinski sign) suggests upper motor neuron damage.

Taught in Chapter 16.

Refractory Period¶

A short time after an action potential during which a neuron cannot fire again at all (absolute) or needs a stronger push to fire (relative).

Example: The refractory period keeps an action potential moving forward down the axon instead of bouncing backward.

See also: Action Potential

Taught in Chapter 2.

REM Sleep (Rapid Eye Movement Sleep)¶

A stage of sleep with quick eye movements, active brain waves, vivid dreams, and temporary paralysis of most skeletal muscles.

Example: Muscle paralysis during REM sleep keeps you from acting out your dreams.

Taught in Chapter 10.

Repolarization¶

The return of the membrane potential toward its negative resting level after depolarization, caused mainly by potassium ions flowing out of the cell.

Example: Right after the peak of an action potential, potassium channels open and repolarization begins.

See also: Depolarization

Taught in Chapter 2.

Research Ethics¶

The principles that guide responsible research, including informed consent, minimizing harm, protecting privacy, and the humane treatment of animals.

Example: Before a teen joins a brain imaging study, researchers explain the risks, and the teen agrees to take part along with a parent's permission.

Taught in Chapter 9.

Resting Potential¶

The steady membrane potential of a neuron that is not sending a signal, typically about −70 millivolts, with the inside negative compared with the outside.

Example: A very high potassium level in the blood makes the resting potential less negative, which can disturb how nerves and the heart work.

See also: Sodium-Potassium Pump

Taught in Chapter 2.

Return-to-Learn and Play¶

The gradual, step-by-step process of resuming school work and then sports after a concussion as symptoms allow.

Example: A student might start with shorter school days and less screen time, return to full classes, and only then begin light exercise before full practice.

See also: Concussion

Taught in Chapter 14.

Reuptake¶

The process in which a neuron pulls released neurotransmitter back into its axon terminal through transporter proteins, ending the signal.

Example: Selective serotonin reuptake inhibitors (SSRIs), a common type of antidepressant, block reuptake of serotonin.

See also: Serotonin

Taught in Chapter 3.

Reward Pathway¶

A brain circuit of dopamine neurons running from the midbrain (ventral tegmental area) to the nucleus accumbens and prefrontal cortex that drives motivation toward rewarding things.

Example: Eating good food, spending time with friends, and taking addictive drugs all activate the reward pathway.

See also: Dopamine, Addiction

Taught in Chapter 15.

Reward Prediction Error¶

The difference between the reward that was expected and the reward actually received, signaled by changes in the firing of dopamine neurons.

Example: Dopamine neurons fire more when a surprise reward arrives and fire less when an expected reward fails to show up.

See also: Dopamine, Decision-Making

Taught in Chapter 12.

Reward Sensitivity in Teens¶

The stronger response of the adolescent brain's reward circuits to excitement, new experiences, and social approval.

Example: In experiments, teens take more risks when friends are watching, partly because peers boost reward signals in the brain.

See also: Reward Pathway, Adolescent Brain

Taught in Chapter 8.

Rods and Cones¶

Rods are photoreceptors that work in dim light and see only shades of gray; cones work in bright light and provide color and sharp detail.

Example: At night you can spot a faint star better out of the corner of your eye, because rods are concentrated away from the center of the retina.

See also: Color Vision

Taught in Chapter 5.

S¶

Saltatory Conduction¶

The way an action potential travels along a myelinated axon by jumping from one node of Ranvier to the next.

Example: Saltatory conduction lets signals from your toes reach your spinal cord fast enough for quick reflexes.

See also: Myelin Sheath, Conduction Velocity

Taught in Chapter 2.

Schizophrenia¶

A serious mental illness that affects how a person thinks, perceives, and behaves, often including hallucinations, delusions, disorganized thinking, and reduced motivation.

Example: Schizophrenia usually first appears in late adolescence or early adulthood, and treatment helps many people manage their symptoms.

See also: Psychiatric Medication

Taught in Chapter 15.

Schwann Cell¶

A glial cell outside the brain and spinal cord that wraps myelin around one segment of a single axon and helps damaged nerves regrow.

Example: Because Schwann cells guide repair, a cut nerve in a finger can slowly regain feeling, while a damaged spinal cord usually cannot.

See also: Oligodendrocyte

Taught in Chapter 1.

Scientific Method in Neuroscience¶

The process of asking a question, forming a testable hypothesis, running controlled experiments, collecting data, and revising ideas based on the results, applied to the nervous system.

Example: To test whether sleep helps memory, researchers might compare word recall in a group that sleeps after studying with a group that stays awake.

See also: Correlation vs Causation, Animal Models

Taught in Chapter 9.

Seizure¶

A sudden burst of abnormal, excessive electrical activity in the brain that temporarily changes movement, sensation, awareness, or behavior.

Example: A seizure can look like whole-body jerking, or simply like a brief blank stare.

See also: Epilepsy, Seizure First Aid

Taught in Chapter 14.

Seizure First Aid¶

The steps a bystander takes to keep a person safe during a seizure, such as timing it, moving hard objects away, cushioning the head, turning the person on their side, and never putting anything in their mouth.

Example: If a seizure lasts longer than 5 minutes, call 911.

See also: Seizure

Taught in Chapter 14.

Sensory Neuron¶

A neuron that carries information from the body or the outside world toward the brain and spinal cord.

Example: When a mosquito lands on your arm, sensory neurons in your skin send that touch signal to your spinal cord and brain.

See also: Motor Neuron, Interneuron

Taught in Chapter 1.

Sensory Transduction¶

The conversion of a physical or chemical stimulus, such as light, sound, or pressure, into electrical signals in a sensory cell.

Example: In the eye, photoreceptors transduce light into changes in their membrane potential.

See also: Photoreceptor, Mechanoreceptor

Taught in Chapter 5.

Serotonin¶

A neurotransmitter that helps regulate mood, sleep, appetite, and digestion.

Example: Many antidepressants work by increasing the amount of serotonin available at synapses.

See also: Reuptake

Taught in Chapter 3.

Short-Term Memory¶

A brief, limited store that holds a small amount of information for seconds to under a minute unless it is rehearsed.

Example: Holding a verification code in mind just long enough to type it uses short-term memory.

See also: Working Memory

Taught in Chapter 9.

Skeletal Muscle Contraction¶

The shortening of a skeletal muscle fiber, triggered by a nerve signal that releases calcium inside the fiber and lets protein filaments (actin and myosin) slide past each other.

Example: Every sprint Maya makes at soccer practice depends on skeletal muscle contraction in her legs.

See also: Neuromuscular Junction, Motor Unit

Taught in Chapter 7.

Sleep and Memory¶

The relationship in which sleep helps stabilize, strengthen, and reorganize memories formed while awake.

Example: Students who sleep after learning new vocabulary usually remember more the next day than students who stay awake.

See also: Memory Consolidation

Taught in Chapter 10.

Sleep Deprivation¶

A condition of not getting enough sleep, either all at once or over many nights, which impairs attention, mood, memory, and reaction time.

Example: Drowsy driving is dangerous because sleep deprivation slows reaction time.

See also: Teen Sleep Needs

Taught in Chapter 10.

Sleep Stages¶

The repeating phases of sleep, made up of three stages of non-rapid eye movement sleep and one stage of rapid eye movement sleep, which cycle several times each night.

Example: One full cycle through the sleep stages takes roughly 90 minutes.

Taught in Chapter 10.

Sodium-Potassium Pump¶

A membrane protein that uses the cell's chemical energy (ATP) to move three sodium ions out of the cell and two potassium ions in, keeping the ion differences neurons need.

Example: A large share of the brain's energy use goes to running sodium-potassium pumps.

Taught in Chapter 2.

Somatosensory Cortex¶

The strip of cerebral cortex at the front of the parietal lobe, just behind the primary motor cortex, that receives and processes touch and body-position information.

Example: Gently stimulating the somatosensory cortex during brain surgery can make an awake patient feel a tingle in one specific body part.

See also: Homunculus

Taught in Chapter 6.

Somatosensory System¶

The sensory system that detects touch, pressure, vibration, temperature, pain, and body position from the skin, muscles, and joints.

Example: Recognizing a coin in your pocket without looking uses your somatosensory system.

See also: Mechanoreceptor, Somatosensory Cortex

Taught in Chapter 6.

Spinal Cord¶

A column of nerve tissue running inside the backbone that carries signals between the brain and body and controls some reflexes on its own.

Example: An injury to the spinal cord in the neck can affect movement and feeling in both the arms and the legs.

See also: Spinal Cord Injury, Reflex Arc

Taught in Chapter 4.

Spinal Cord Injury¶

Damage to the spinal cord that disrupts signals between the brain and the body below the injury, causing loss of movement, sensation, or automatic functions.

Example: An injury in the neck region can affect all four limbs, while one in the lower back may affect only the legs.

See also: Spinal Cord, Dermatome

Taught in Chapter 14.

Split-Brain Studies¶

Research on people whose corpus callosum was surgically cut to treat severe epilepsy, which revealed how each hemisphere works on its own.

Example: A split-brain patient could not name an object shown only in the left visual field, because that information reached only the right hemisphere.

Taught in Chapter 12.

Stigma and Treatment Access¶

Stigma is negative attitudes or stereotypes about people with a condition; together with cost and shortages of providers, it can keep people from getting care.

Example: Using person-first language, such as "a person with a substance use disorder," helps reduce stigma.

See also: Mental Illness

Taught in Chapter 15.

Stress Response¶

The body's set of reactions to a challenge or threat, including fast nerve signals that raise heart rate and slower hormone signals that release stored energy.

Example: Before a major exam, your stress response sharpens attention and makes your heart beat faster.

Taught in Chapter 11.

Stretch Reflex¶

An automatic muscle contraction that happens when a muscle is suddenly stretched, detected by stretch sensors called muscle spindles.

Example: The knee-jerk response to a tap below the kneecap is a stretch reflex.

See also: Reflex Testing

Taught in Chapter 7.

Stroke¶

A sudden loss of brain function caused by interrupted blood flow to part of the brain or by bleeding in or around the brain.

Example: Sudden weakness on one side of the face and body is a common sign of stroke.

Taught in Chapter 14.

Substantia Nigra¶

A dark-colored region in the midbrain whose neurons supply dopamine to the basal ganglia.

Example: In Parkinson's disease, the substantia nigra loses its dark color as its dopamine neurons die.

See also: Parkinson's Disease

Taught in Chapter 7.

Suprachiasmatic Nucleus¶

A tiny cluster of neurons in the hypothalamus, just above the optic chiasm, that acts as the body's master clock.

Example: Morning light signals from the eyes reach the suprachiasmatic nucleus and reset the body clock each day.

See also: Circadian Rhythm

Taught in Chapter 10.

Sympathetic vs Parasympathetic¶

The two main branches of the autonomic nervous system: the sympathetic branch prepares the body for action during danger or exercise, and the parasympathetic branch supports rest and digestion.

Example: Before a penalty kick, sympathetic activity speeds Maya's heart; after the game, parasympathetic activity slows it back down.

See also: Fight or Flight

Taught in Chapter 4.

Synapse¶

The junction where one neuron passes a signal to another neuron, a muscle cell, or a gland cell.

Example: Learning a new skill strengthens some synapses and weakens others.

See also: Chemical Synapse, Electrical Synapse

Taught in Chapter 3.

Synaptic Integration¶

The process by which a neuron combines all the excitatory and inhibitory inputs it receives at a given moment, which determines whether it fires.

Example: If EPSPs outweigh IPSPs enough to reach threshold at the start of the axon, the neuron fires.

Taught in Chapter 3.

Synaptic Pruning¶

The removal of weak or unused synapses during development, which makes brain circuits more efficient.

Example: During the teen years, pruning in the prefrontal cortex helps fine-tune circuits for planning and judgment.

See also: Synaptogenesis, Adolescent Brain

Taught in Chapter 8.

Synaptic Vesicle¶

A tiny membrane bubble inside an axon terminal that stores neurotransmitter molecules and releases them by fusing with the cell membrane.

Example: Calcium flowing into the axon terminal triggers synaptic vesicles to empty their contents into the synapse.

See also: Axon Terminal

Taught in Chapter 3.

Synaptogenesis¶

The formation of new synapses between neurons.

Example: Synaptogenesis is especially fast in infancy, when the brain forms enormous numbers of new connections.

See also: Synaptic Pruning

Taught in Chapter 8.

T¶

Tau Tangle¶

A twisted bundle of abnormal tau protein that builds up inside neurons and is a hallmark of Alzheimer's disease and some other dementias.

Example: The spread of tau tangles across the brain tracks closely with worsening memory and thinking.

Taught in Chapter 13.

Teen Sleep Needs¶

The amount and timing of sleep that adolescents need, shaped by a body clock that naturally shifts later during the teen years.

Example: Sleep experts at the American Academy of Sleep Medicine recommend 8 to 10 hours of sleep a day for teens aged 13 to 18, so a teen whose body clock makes it hard to fall asleep before 11 p.m. can easily come up short when school starts early.

See also: Circadian Rhythm, Sleep Deprivation

Taught in Chapter 10.

Temporal Lobe¶

The region on the lower side of each cerebral hemisphere, near the ears, involved in hearing, understanding language, recognizing objects and faces, and forming memories.

Example: Seizures that begin in the temporal lobe often start with an odd feeling such as déjà vu.

See also: Hippocampus

Taught in Chapter 4.

Teratogen¶

A substance, infection, or other factor that can cause birth defects or harm development when an embryo or fetus is exposed to it.

Example: Alcohol, certain medicines, and the Zika virus are teratogens that can affect brain development.

Taught in Chapter 8.

Thalamus¶

A pair of egg-shaped structures deep in the brain that relay and filter sensory and motor signals on their way to the cerebral cortex.

Example: Almost all sensory information except smell passes through the thalamus before reaching the cortex.

See also: Hypothalamus

Taught in Chapter 4.

Threshold¶

The membrane potential, often around −55 millivolts, at which enough voltage-gated sodium channels open to set off an action potential.

Example: A very light brush may depolarize a touch neuron a little without reaching threshold, so no signal is sent.

See also: All-or-None Principle

Taught in Chapter 2.

Tolerance and Withdrawal¶

Tolerance is needing more of a drug to get the same effect; withdrawal is the set of unpleasant symptoms that appear when a person stops or cuts back on a drug the body has adapted to.

Example: A daily coffee drinker may need more coffee to feel alert (tolerance) and get a headache on a day without it (withdrawal).

See also: Addiction

Taught in Chapter 15.

Traumatic Brain Injury (TBI)¶

Damage to the brain caused by an outside force, such as a blow, a jolt, or an object entering the skull, ranging from mild to severe.

Example: Falls and motor vehicle crashes are common causes of traumatic brain injury.

See also: Concussion

Taught in Chapter 14.

U¶

Upper vs Lower Motor Neuron¶

Upper motor neurons run from the brain to the brainstem or spinal cord; lower motor neurons run from the brainstem or spinal cord to the muscles, and damage to each gives a different pattern of signs.

Example: Upper motor neuron damage usually causes stiff muscles and brisk reflexes, while lower motor neuron damage causes floppy, shrinking muscles and weak reflexes.

Taught in Chapter 7.

V¶

Ventral and Dorsal Streams¶

Two main routes of visual processing beyond the primary visual cortex: the ventral ("what") stream through the temporal lobe identifies objects, and the dorsal ("where/how") stream through the parietal lobe tracks location and guides action.

Example: A person with ventral stream damage may be unable to name a cup yet still reach for it accurately.

See also: Primary Visual Cortex

Taught in Chapter 5.

Ventricles and CSF¶

The ventricles are connected, fluid-filled spaces inside the brain; cerebrospinal fluid (CSF) is the clear fluid made there that cushions and bathes the brain and spinal cord.

Example: If CSF cannot drain properly, the ventricles swell, a condition called hydrocephalus.

See also: Lumbar Puncture

Taught in Chapter 4.

Vestibular System¶

The sensory system in the inner ear that detects head movement, tilt, and gravity to support balance and keep vision steady.

Example: Spinning around and then stopping makes you dizzy because fluid in your vestibular system keeps moving for a moment.

See also: Proprioception

Taught in Chapter 6.

Visual Field¶

The whole area a person can see at one time while keeping the eyes fixed on a single point.

Example: A doctor tests the visual field by asking a patient to look at the doctor's nose and say when a wiggling finger appears from the side.

See also: Visual Field Defect

Taught in Chapter 5.

Visual Field Defect¶

A loss of vision in part of the visual field caused by damage somewhere along the visual pathway, from the eye to the brain.

Example: Loss of the outer halves of vision in both eyes (bitemporal hemianopia) points to damage at the optic chiasm.

See also: Visual Field, Lesion Localization

Taught in Chapter 5.

Visual Pathway¶

The route visual signals take from the retina through the optic nerve, optic chiasm, optic tract, and thalamus to the primary visual cortex.

Example: Doctors map which part of vision is lost to work out where along the visual pathway the damage lies.

See also: Visual Field Defect

Taught in Chapter 5.

Voltage-Gated Channel¶

An ion channel that opens or closes in response to changes in the membrane potential.

Example: Voltage-gated sodium channels snap open when a neuron reaches threshold, causing the rising phase of an action potential.

See also: Threshold

Taught in Chapter 2.

W¶

Wernicke's Area¶

A region in the upper back part of the temporal lobe, usually on the left, that is important for understanding language.

Example: Damage to Wernicke's area can cause fluent speech that makes little sense, along with trouble understanding others.

See also: Broca's Area, Aphasia

Taught in Chapter 12.

White Matter¶

Brain and spinal cord tissue made mostly of myelinated axons that connect regions of gray matter; the fatty myelin gives it a pale color.

Example: The corpus callosum is the largest bundle of white matter in the brain.

See also: Gray Matter

Taught in Chapter 4.

Working Memory¶

The ability to hold information in mind while actively using or changing it.

Example: Figuring out a 20 percent tip in your head uses working memory.

See also: Short-Term Memory, Executive Function

Taught in Chapter 9.