A critical neurotransmitter that controls functions such as memory, attention, sleep, heart rate, and muscular activity.

Action Potential

An electrical charge that travels along the axon to the neuron's terminal, where it triggers the release of a neurotransmitter. This occurs when a neuron is activated and temporarily reverses the electrical state of its interior membrane from negative to positive.


Loss of control over drug intake or compulsive seeking and taking of drugs, despite adverse consequences.


A neurochemical that inhibits wakefulness, serving the purpose of slowing down cellular activity and diminishing arousal. Adenosine levels decrease during sleep.

Adrenal Gland

An endocrine organ that secretes hormones. The outer layer (adrenal cortex) secretes the stress hormone cortisol. The inner portion (adrenal medulla) secretes epinephrine and norepinephrine in concert with the activation of the sympathetic nervous system in the “fight or flight” response.

Alzheimer's Disease (AD)

A major cause of dementia in the elderly, this neurodegenerative disorder is characterized by the death of neurons in the hippocampus, cerebral cortex, and other brain regions. The earliest symptoms of the disease include forgetfulness; disorientation as to time or place; and difficulty with concentration, calculation, language, and judgment. In the final stages, individuals are incapable of self-care and may be bedridden.


A memory impairment usually caused by brain damage or disease, or by drugs such as some anesthetics. People with amnesia may be unable to recall events from the past, form new memories, or both.


A structure in the forebrain that is an important component of the limbic system and plays a central role in emotional learning, particularly within the context of fear.

Amyotrophic Lateral Sclerosis (ALS)

Commonly known as Lou Gehrig's disease, ALS causes motor neurons in the brain and spinal cord to disintegrate, resulting in loss of control of voluntary muscle movements such as walking.


A drug that relieves pain without causing a loss of consciousness.


A state of heightened arousal characterized by intense worry.


Disturbance in language comprehension or production, often as a result of a stroke.


Programmed cell death induced by specialized biochemical pathways, often serving a specific purpose in the development of an animal.


A physiological state involving changes in the body and brain that motivate behavior and enable response to stimuli.


A star-shaped glial cell in the central nervous system that nourishes neurons; regulates the formation, maintenance, and pruning of synapses; and contributes to the blood-brain barrier.


A state of arousal in which the brain’s sensory processing is directed at a limited number of stimuli. Voluntary (endogenous) attention is a conscious decision to focus on a particular stimulus. Involuntary (exogenous) attention is an unplanned focus on a change in the environment, such as a loud noise or sudden movement. 

Attention-deficit/hyperactivity disorder (ADHD)

A condition characterized by excessively inattentive, hyperactive, or impulsive behaviors.

Auditory Nerve

A branch of the vestibulocochlear nerve that transmits auditory information from the cochlea of the ear to the brain.

Autism Spectrum Disorder (ASD)

A set of conditions characterized, in part, by impaired social communication and interaction, and narrow, obsessive interests or repetitive behaviors.

Autonomic Nervous System

A part of the peripheral nervous system responsible for regulating the activity of internal organs. It includes the sympathetic and parasympathetic nervous systems.


The fiber-like extension of a neuron by which it sends information to target cells.

Axon Terminal

The ends of axons where neurotransmitters are released to target cells.

Basal Ganglia

A group of interconnected structures located deep in the brain that play an important role in voluntary movement, motor skill learning, and habits. These structures include the caudate nucleus, putamen, nucleus accumbens, globus pallidus, and substantia nigra.


A class of drugs that enhance activity of the brain’s primary inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), to produce sedative and anti-anxiety effects. Benzodiazepines are often prescribed to treat anxiety disorders and insomnia.

Blood-Brain Barrier

A protective membrane composed of tightly packed endothelial cells lining the brain’s capillaries and highly specialized astrocytes, which controls the passage of certain molecules into and out of the brain.

Brain Waves

Oscillating patterns of brain activity that can be detected and recorded using electroencephalography (EEG).

Brain-Derived Neurotrophic Factor (BDNF)

A neurotrophic peptide that supports the growth and survival of neurons.


The major route by which the forebrain sends information to and receives information from the spinal cord and peripheral nerves. The brainstem includes the midbrain, pons, and medulla, and it controls, among other things, respiration and the regulation of heart rhythms.

Broca’s Area

A region of the frontal lobe — usually the left hemisphere — that governs speech production.

Cell Body

Also called the soma, the part of a neuron that contains the nucleus (with DNA) and the organelles, but not the projections such as the axon or dendrites.

Central Nervous System

The brain and spinal cord.


A large structure located at the roof of the hindbrain that helps to control the coordination of movement by making connections to the pons, medulla, spinal cord, and thalamus. It also may be involved in aspects of motor learning.

Cerebral Cortex

The wrinkled, outermost layer of the cerebrum consisting primarily of neuron cell bodies.


The largest part of the human brain associated with higher order functioning, such as thinking, perceiving, planning, and understanding language, as well as the control of voluntary behavior.

Circadian Rhythms

A cycle of behavior or physiological change lasting approximately 24 hours.


A snail-shaped, fluid-filled organ of the inner ear responsible for converting sound into electrical potentials to produce an auditory sensation.

Cognitive Behavioral Therapy

A form of counseling used to identify and change negative thought patterns that can contribute to anxiety and mood disorders.

Computational Neuroscience

A field of neuroscience research that uses computer programs and algorithms to analyze information about the brain, and develops mathematical models to explain brain function.


A primary receptor cell for vision located in the retina. It is sensitive to color and is used primarily for daytime vision.

Corpus Callosum

The large bundle of nerve fibers linking the left and right cerebral hemispheres.


A hormone manufactured by the adrenal cortex. In humans, cortisol is secreted in the greatest quantities before dawn, readying the body for the activities of the coming day.

Cranial Nerves

Twelve pairs of nerves that can be seen on the bottom surface of the brain. Some of these nerves transmit sensory information; some control the movement of face, head, and neck muscles; others transmit information to internal organs to regulate functions such as blood pressure and heart rate.

Critical Period

A period of heightened plasticity in brain development when certain experiences and sensory inputs are required for the formation of functional brain circuits.

Declarative Memory

Also called explicit memory, a type of memory that can be consciously retrieved. It includes memory of facts (semantic memory) and memory of personal experiences (episodic memory).

Default Mode Network

A collection of brain regions activated during quiet rest.


A decline in cognitive ability that interferes with day-to-day functioning.


A treelike extension of the neuron cell body. The dendrite is the primary site for receiving and integrating information from other neurons.


A change in a neuron’s membrane potential in which the cytoplasm becomes more positively charged. Neurons must depolarize beyond a certain threshold to generate an action potential.


A psychiatric disorder characterized by sadness, hopelessness, pessimism, loss of interest in life, reduced emotional well­being, and abnormalities in sleep, appetite, and energy level.


A catecholamine neurotransmitter present in three circuits of the brain: one that regulates movement; a second, thought to be important for cognition and emotion; and a third that regulates the endocrine system. Deficits of dopamine in the motor circuit are associated with Parkinson's disease. Abnormalities in the second circuit have been implicated in schizophrenia.

Down Syndrome

A condition that results from the presence of an extra copy of chromosome 21. This genetic anomaly is associated with physical and developmental characteristics, including mild to moderate intellectual disabilities; low muscle tone; and an increased risk of congenital heart defects, respiratory problems, and digestive tract obstruction.


A pronounced difficulty with reading despite normal intelligence, education, and motivation.

Electroencephalography (EEG)

A technology used to record electrical activity of the human brain in response to a variety of stimuli and activities.


Neurotransmitters produced in the brain that generate cellular and behavioral effects like those of morphine.


A disorder characterized by repeated seizures, which are caused by abnormal excitation of large groups of neurons in various brain regions. Epilepsy can be treated with many types of anticonvulsant medications.


A hormone released by the adrenal medulla and specialized sites in the brain. During times of stress, epinephrine, also known as adrenaline, is quickly released into the bloodstream. It then serves to put the body into a general state of arousal, which enables it to cope with the challenge.

Episodic Memory

A type of declarative memory consisting primarily of memory of personal experiences.


A female sex hormone produced primarily in the ovaries.


A change in the electrical state of a neuron that is associated with an enhanced probability of action potentials.


A type of neuron (or neurotransmitter) that excites target neurons and increases the likelihood of their firing an action potential.

Executive Function

Higher-level processing that takes place in the brain’s prefrontal cortex. Executive function comprises impulse control, working memory, and mental flexibility.


A region of the developing brain that goes on to become the cerebral hemispheres and major parts of the limbic system.


A small, pitted area in the center of the retina where visual acuity is highest, due to a high density of cones.

Fragile X Syndrome

A genetic condition resulting from a mutation in the FMR1 gene that causes intellectual disability.

Frontal Lobe

One of the four subdivisions of the cerebral cortex. The frontal lobe has a role in controlling movement and in the planning and coordinating of behavior.

Functional Magnetic Resonance Imaging (fMRI)

A technology that uses magnetic fields to detect activity in the brain by monitoring blood flow.

Gamma-Aminobutyric Acid (GABA)

An amino acid neurotransmitter in the brain whose primary function is to inhibit the firing of nerve cells.


Specialized cells that nourish and support neurons.

Glucocorticoid Hormones

Hormones that produce an array of effects in response to stress. Some of the actions of glucocorticoids help to mediate the stress response, while other, slower actions counteract the primary response to stress and help to re-establish homeostasis.


An amino acid neurotransmitter that acts to excite neurons. Glutamate stimulates N-methyl-d-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA). AMPA receptors have been implicated in activities ranging from learning and memory to development and specification of nerve contacts in developing animals. Stimulation of NMDA receptors may promote beneficial changes, whereas overstimulation may be a cause of nerve cell damage or death in neurological trauma and stroke.

Gray Matter

Portions of the brain that are gray in color because they are composed mainly of neural cell bodies, rather than myelinated nerve fibers, which are white. It includes the cerebral cortex as well as subcortical structures.

Growth Cone

A distinctive structure at the growing end of most axons. It is the site where new material is added to the axon.

Hair Cells

Sensory receptors in the cochlea that convert mechanical vibrations to electrical signals; they in turn excite the 30,000 fibers of the auditory nerve that carry the signals to the brainstem.


The most posterior part of the brain, comprising the pons, medulla, and cerebellum.


A seahorse-shaped structure located within the brain and considered an important part of the limbic system. One of the most studied areas of the brain, it is involved in learning, memory, and emotion.


A compound with multiple functions in the body. In the brain, histamine acts as a neurotransmitter to stimulate arousal. Local inflammatory responses in the body trigger the release of histamines from immune cells.


The normal equilibrium of body function.


Chemical messengers secreted by endocrine glands to regulate the activity of target cells. They play a role in sexual development, calcium and bone metabolism, growth, and many other activities.

Huntington's Disease

A genetic disorder characterized by involuntary jerking movements of the limbs, torso, and facial muscles, often accompanied by mood swings, depression, irritability, slurred speech, and clumsiness.


A change in a neuron’s membrane potential in which the cytoplasm becomes more negatively charged and therefore less likely to fire an action potential.


A complex brain structure composed of many nuclei with various functions, including regulating the activities of internal organs, monitoring information from the autonomic nervous system, controlling the pituitary gland, and regulating sleep and appetite.


A change in the electrical state of a neuron that is associated with a decreased probability of firing an action potential.


A type of neuron (or neurotransmitter) that prevents a target neuron from firing.


A sleep disorder in which people have trouble falling and/or staying asleep.


A neuron that exclusively signals another neuron.

Involuntary Movement

A movement that occurs without conscious control, such as a reflex.

Ion Channel

Proteins embedded in the cell membrane that allow ions or other small molecules to enter or leave the cell.

Limbic System

A group of structures deep within the brain involved in motivation and emotion. The hippocampus, amygdala, thalamus, and hypothalamus are all a part of the limbic system.

Long-Term Memory

The final phase of memory, in which information storage may last from hours to a lifetime.

Long-Term Potentiation (LTP)

A long-lasting increase in synaptic strength resulting from an increased number of neurotransmitter receptors on the post-synaptic neuron.

Magnetic Resonance Imaging (MRI)

A technique that uses magnetic fields to create a high-quality, three-dimensional image of organs and structures inside the body. This technology is noninvasive and does not expose the body to X-rays or other radiation.

Magnetoencephalography (MEG)

A technique that can quantitatively measure the strength of activity in various regions of the brain at millisecond resolution.


Also called the medulla oblongata, a structure of the brainstem that controls basic functions like swallowing, breathing, and heart rate.


A hormone produced in the pineal gland that regulates responses to light-dark cycles and induces sleep at night.

Membrane Potential

The voltage difference between the inside and outside of a neuron. The typical membrane potential of a neuron at rest is -70mV.


The ability to understand the mental states and thoughts of others and oneself.


Glial cells in the central nervous system that function as resident immune cells.


The most anterior segment of the brainstem. With the pons and medulla, the midbrain is involved in many functions, including regulation of heart rate, respiration, pain perception, and movement.


The process whereby new neurons find their proper position in the brain.


Small cylindrical organelles inside cells that provide energy for the cell by converting sugar and oxygen into special energy molecules, called adenosine triphosphate (ATP).


A general state of mind and emotional disposition.

Motor Cortex

A specialized region in the cortex involved in the planning and execution of movement.

Motor Neuron

A neuron that carries information from the central nervous system to muscles.

Motor Unit

A functional unit made up of an alpha motor neuron and all of the muscle fibers it contains and controls, ranging from a few to a hundred or more.


Compact fatty material that surrounds and insulates the axons of some neurons and accelerates the transmission of electrical signals.


A sleep disorder resulting from the loss of orexin neurons in the hypothalamus that causes pronounced sleepiness during the day.

Nerve Growth Factor (NGF)

A substance whose role is to guide neuronal growth during embryonic development, especially in the peripheral nervous system. Nerve growth factor also probably helps to sustain neurons in adults.


The progressive destruction and loss of neurons. Alzheimer’s, Parkinson’s, and amyotrophic lateral sclerosis (ALS) are examples of neurodegenerative diseases.


The production and growth of new nerve cells during development and, in select brain regions, throughout life.


A chemical messenger that alters the strength of a synapse by modifying the production and/or response to neurotransmitters. Neurotransmitters, hormones, and immune molecules can all function as neuromodulators.


A nerve cell specialized for the transmission of information and characterized by long, fibrous projections called axons and shorter, branchlike projections called dendrites.


Chemical messengers released by neurons at a synapse for the purpose of relaying information to other cells.

Neurotransmitter Receptors

Proteins embedded in the postsynaptic cell membrane that bind neurotransmitters to alter the cell’s excitability.


Nerve endings that signal the sensation of pain.

Nodes of Ranvier

Unmyelinated gaps in an axon’s myelin sheath along which electrical impulses travel.

Non-Declarative Memory

Also called implicit or procedural memory, a type of long-term memory that is stored and retrieved without conscious effort.


A catecholamine neurotransmitter produced both in the brain and in the peripheral nervous system. Norepinephrine is involved in arousal and sleep regulation, mood, and blood pressure.

Nucleus Accumbens

A region at the base of the forebrain that is a part of the basal ganglia and is important in motivation and reward.

Obsessive Compulsive Disorder

An anxiety disorder characterized by uncontrollable, recurring thoughts (obsessions) and repetitive behaviors (compulsions) that attempt to mitigate the obsessions.

Occipital Lobes

One of the four subdivisions of the cerebral cortex. The occipital lobe plays a role in processing visual information.

Olfactory Bulbs

Round, knoblike structures of the brain responsible for processing the sense of smell. Specialized olfactory receptor cells are located in a small patch of mucous membrane lining the roof of the nose. Axons of these sensory cells pass through perforations in the overlying bone and enter two elongated olfactory bulbs lying on top of the bone.


A type of glial cell in the central nervous system that forms myelin.


Substances that bind to opioid receptors in the brain to relieve pain. Endorphins are a type of endogenous opioid produced in the brain. Natural and synthetic opioids, such as morphine and codeine, can be prescribed to treat pain.

Optic Chiasm

The place in the brain where the optic nerves meet and some axons cross over to the opposite (contralateral) hemisphere in animals with binocular vision.

Optic Nerve

The bundle of neurons that transmit information from the retina to the brain.


A hormone produced in the hypothalamus that stimulates arousal.


A hormone produced in the hypothalamus and released by the pituitary gland that initiates the release of milk from mammary glands and stimulates uterine contractions. It is also involved in love and social bonding.


An unpleasant sensory and emotional experience often signaling tissue damage, or the potential for damage.


The loss of muscle function in all or part of the body, usually due to nerve damage.

Parasympathetic Branch

A branch of the autonomic nervous system concerned with the conservation of the body's energy and resources during relaxed states.

Parietal Lobes

One of the four subdivisions of the cerebral cortex. The parietal lobe plays a role in sensory processes, attention, and language.

Parkinson's Disease (PD)

A movement disorder caused by the death of dopamine neurons in the substantia nigra, located in the midbrain. Symptoms include slowness of movement, muscular rigidity, and walking and balance impairment.

Peripheral Nervous System

The nerves outside of the brain and spinal cord.


A nerve ending, cell, or group of cells specialized to sense or receive light.

Pineal Gland

A small endocrine gland in the brain that produces melatonin.

Pituitary Gland

An endocrine organ closely linked with the hypothalamus. In humans, the pituitary gland is composed of two lobes and secretes several different hormones that regulate the activity of other endocrine organs throughout the body.


The ability of the brain to modify its neural connections to adapt to challenges in the environment.


A part of the hindbrain that, with other brain structures, controls respiration and regulates heart rhythms. The pons is a major route by which the forebrain sends information to and receives information from the spinal cord and peripheral nervous system.

Positron Emission Tomography (PET)

A method of measuring brain function based on the detection of radioactivity emitted when positrons, positively charged particles, undergo radioactive decay in the brain. Computers then build three-dimensional images of changes in blood flow based on the amount of radiation emitted in different brain regions. The more brain activity, the more vivid the picture that is created.

Postsynaptic Neuron

In a synapse, the neuron receiving chemical messages.

Prefrontal Cortex (PFC)

A region at the front of the frontal lobe involved in the brain’s higher-level functions such as planning, decision-making, working memory, and inhibitory control.

Presynaptic Neuron

In a synapse, the neuron transmitting chemical messages to a target neuron.


Small lipid molecules that enhance nociceptor sensitivity to increase pain and prevent further tissue damage.

Rapid Eye Movement (REM) Sleep

The part of the sleep cycle when active dreaming takes place. It is characterized by neocortical EEG waves similar to those observed during waking. This state is accompanied by paralysis of the body's muscles; only the muscles that allow breathing and control eye movements remain active.


Considered the simplest and most fundamental movements, they are relatively fixed, automatic muscle responses to particular stimuli, such as the slight extension of the leg when a physician taps the knee with a small rubber hammer.


A multilayered sensory tissue that lines the back of the eye and contains the receptor cells to detect light.


A process by which released neurotransmitters are absorbed for later reuse.


A sensory neuron located in the periphery of the retina. The rod is sensitive to light of low intensity and is specialized for nighttime vision.

Saltatory Conduction

The process by which action potentials “jump” along the unmyelinated nodes of Ranvier, speeding electrical transmission.


A chronic disorder characterized by psychosis (e.g., hallucinations and delusions), flattened emotions, and impaired cognitive function.

Schwann Cell

A type of glial cell in the peripheral nervous system that forms myelin.

Selective Serotonin Reuptake Inhibitors (SSRIs)

Drugs that block the reuptake of serotonin, increasing its availability in the synapse. SSRIs are used to treat depression and other disorders.

Semantic Memory

A type of declarative memory that involves memory of facts.


A monoamine neurotransmitter believed to play many roles, including but not limited to temperature regulation, sensory perception, and the onset of sleep. Neurons using serotonin as a transmitter are found in the brain and gut. Several antidepressant drugs are targeted to brain serotonin systems.

Short-Term Memory

A phase of memory in which a limited amount of information may be held for several seconds or minutes.

Somatosensory Cortex

A region of the parietal lobe responsible for processing touch and pain signals from the body.

Spinal Cord

A bundle of nerve fibers running through the vertebral column that primarily functions to facilitate communication between the brain and the rest of the body.

Stem Cells

Unspecialized cells that renew themselves for long periods through cell division.


Any external stimulus that threatens homeostasis. Many kinds of stress have a negative effect on the body, but some kinds can be helpful.


A cluster of neurons deep within the brain divided into ventral and dorsal regions. The ventral striatum consists of the nucleus accumbens and the olfactory tubercle, while the dorsal striatum consists of the caudate and putamen. The striatum is a part of the basal ganglia and is involved in reward processing.


A block in the brain's blood supply. A stroke can be caused by the rupture of a blood vessel, a clot, or pressure on a blood vessel (as may be caused by a tumor). Without oxygen, neurons in the affected area die, and the part of the body controlled by those cells cannot function. A stroke can result in loss of consciousness and death.

Substantia Nigra

A region of the midbrain involved in movement and reward. Parkinson’s disease destroys the dopamine-producing neurons in this region.

Suprachiasmatic Nucleus (SCN)

A small group of nerve cells in the hypothalamus that express clock proteins, which go through a biochemical cycle of about 24 hours. This sets the pace for daily cycles of activity, sleep, hormone release, and other bodily functions.

Sympathetic Branch

A branch of the autonomic nervous system responsible for mobilizing the body's energy and resources during times of stress and arousal.


A physical gap between two neurons that functions as the site of information transfer from one neuron to another.

Synaptic Plasticity

The ability of synapses to alter their strength by changing their size, shape, number of receptors, and amount of neurotransmitter released.

Synaptic Pruning

The elimination of weak or non-functioning synapses to fine-tune neural circuitry.

Taste Buds

A sensory organ found on the tongue.

Temporal Lobes

One of the four major subdivisions of each hemisphere of the cerebral cortex. The temporal lobe functions in auditory perception, speech, and complex visual perceptions.


A sex hormone produced primarily in the testes but also in lower amounts in the adrenal cortex and ovaries.


A structure consisting of two egg-shaped masses of nerve tissue, each about the size of a walnut, deep within the brain. The key relay station for sensory information flowing into the brain, the thalamus filters out information of particular importance from the mass of signals entering the brain.

Trophic Factors

Small proteins in the nervous system that are necessary for the development, function, and survival of specific groups of neurons.

Vagus Nerve

The tenth cranial nerve, it transmits signals from the brain to the heart, lungs, and digestive tract.

Voluntary Movement

A motor action that is consciously planned and executed.

Wernicke's Area

A region in the temporal lobe responsible for comprehension of language.

White Matter

The part of the brain that contains myelinated nerve fibers. The white matter gets its color from myelin, the insulation covering nerve fibers.

Working Memory

A temporary type of declarative memory, the ability to keep a piece of information “in mind.” It is limited to a small amount of data and, unless transferred to long-term memory, decays within a few seconds.

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