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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.


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

Adrenal Cortex

An endocrine organ that secretes steroid hormones for metabolic functions; for example, in response to stress.

Adrenal Medulla

An endocrine organ that secretes epinephrine and norepinephrine in concert with the activation of the sympathetic nervous system; for example, in response to stress.


1.) A neurotransmitter, drug, or other molecule that stimulates receptors to produce a desired reaction. 2.) A muscle that moves a joint in an intended direction.

Alzheimer's Disease

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.

Amino Acid Transmitters

The most prevalent neurotransmitters in the brain, these include glutamate and aspartate, which have excitatory actions on nerve cells, and glycine and gamma-amino butyric acid (GABA), which have inhibitory actions on nerve cells.


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.


Sex steroid hormones, including testosterone, found in higher levels in males than females. They are responsible for male sexual maturation.


1.) A drug or other molecule that blocks receptors. Antagonists inhibit the effects of agonists. 2.) A muscle that moves a joint in opposition to an intended direction.


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.

Auditory Nerve

A bundle of nerve fibers extending from the cochlea of the ear to the brain that contains two branches: the cochlear nerve, which transmits sound information, and the vestibular nerve, which relays information related to balance.

Attention Deficit Hyperactivity Disorder (ADHD)

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

Autism Spectrum Disorders (ASD)

A condition characterized by impaired social skills; verbal and nonverbal communication difficulties; 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 fiberlike extension of a neuron by which it sends information to target cells.

Basal Ganglia

Structures located deep in the brain that play an important role in the initiation of movements. These clusters of neurons include the caudate nucleus, putamen, globus pallidus, and substantia nigra. Cell death in the substantia nigra contributes to Parkinson's disease.

Bipolar Disorder

Previously known as manic-depressive illness, this disorder is characterized by episodes of deep depression and manic highs. The depressive episodes are similar to those experienced by people with depression. Symptoms of mania include increased energy, decreased need for sleep, a marked interest in goal-directed activities, and poor judgment.


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

Broca's Area

The brain region located in the frontal lobe of the left hemisphere that is important for the production of speech.


The neurotransmitters dopamine, epinephrine, and norepinephrine, which are active in both the brain and the peripheral sympathetic nervous system. These three molecules have certain structural similarities and are part of a larger class of neurotransmitters known as monoamines.

Cell Body

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


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.


A large structure located at the roof of in the hindbrain that helps 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

A sheet of tissue covering the outermost layer of the cerebrum.

Cerebrospinal Fluid

A liquid found within the ventricles of the brain and the central canal of the spinal cord.

Circadian Rhythm

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.


The process or processes by which an organism gains knowledge or becomes aware of events or objects in its environment and uses that knowledge for comprehension and problem-solving.


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

12 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.

Declarative Memory

The ability to learn and consciously remember everyday facts and events.


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


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 typically occurs when, at the time of conception, an extra copy of chromosome 21 is present in the egg. 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.

Drug Addiction

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


Lipid-derived messengers sometimes referred to as the brain's marijuana. These messengers control the release of neurotransmitters, usually by inhibiting them, and can affect the immune system and other cellular parameters.  Endocannabinoids also play an important role in the control of behaviors.

Electroencephalography (EEG)

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

Endocrine Organ

An organ that secretes a hormone directly into the bloodstream to regulate cellular activity of certain other organs.


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.


A group of sex hormones found more abundantly in females than males. They are responsible for female sexual maturation and other functions.


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

Follicle-Stimulating Hormone

A hormone released by the pituitary gland that stimulates the production of sperm in the male and growth of the follicle (which produces the egg) in the female.


The largest part of the brain, which includes the cerebral cortex and basal ganglia. The forebrain is credited with the highest intellectual functions.


The centermost part of the eye located in the center of the retina and contains only cone photoreceptors.

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-Amino Butyric Acid (GABA)

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


Specialized cells that nourish and support neurons.


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


Primary sex gland: testis in the male and ovary in the female.

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.

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 comprises the pons, medulla oblongata, 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.


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 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 neuron that exclusively signals another neuron.


A synaptic message that prevents a recipient neuron from firing.


Electrically charged atoms or molecules.

Ion Channels

Selectively permeable water-filled channels that pass through the cell membrane and allow ions or other small molecules to enter or leave the cell.

Long-Term Memory

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

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.

Magnetic Resonance Spectroscopy (MRS)

Using the same machinery as MRI, MRS measures the concentration of certain chemicals, such as neurotransmitters, instead of blood flow.

Magnetoencephalography (MEG)

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


The sum of all physical and chemical changes that take place within an organism and all energy transformations that occur within living 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).

Motor Neuron

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

Motor Unit

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


Changes in DNA, such as “misspellings” in the gene sequence or incorrect amounts of DNA, that can prevent a gene from functioning properly.

Multiple Sclerosis (MS)

An autoimmune disease in which the body's natural defenses attack the myelin sheath covering the axons of neurons in the central nervous system. Symptoms include numbness, clumsiness, and blurred vision.

Myasthenia Gravis

A disease in which acetylcholine receptors on muscle cells are destroyed so that muscles can no longer respond to the acetylcholine signal to contract. Symptoms include muscular weakness and progressively more common bouts of fatigue. The disease's cause is unknown but is more common in females than in males. It usually strikes between the ages of 20 and 50.

Myelin Sheath

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

NMDA Receptors

N-methyl-d-aspartate (NMDA) receptors, one of three major classes of glutamate receptors, which have been implicated in activities ranging from learning and memory to development and specification of nerve contacts in a developing animal.

Nerve Growth Factor

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

Nerve Terminal

The tip of the axon where neurotransmitters are released.

Neural Induction

The process during embryonic development whereby molecules trigger ectoderm tissue to become nerve tissue.


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


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


Scientists who specialize in the study of the brain and the nervous system.


A chemical released by neurons at a synapse for the purpose of relaying information to other neurons via receptors.


Nerve endings that signal the sensation of pain.


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.

Occipital Lobe

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

Olfactory Bulb

A round, knoblike structure 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.

Orexin Neurons

Specialized neurons that provide an excitatory signal to the arousal system, particularly to the norepinephrine neurons. Orexin activation plays a critical role in preventing abnormal transitions into REM sleep during the day, as occurs in narcolepsy.

Parasympathetic Nervous System

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

Parietal Lobe

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

Parkinson's Disease

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


Chains of amino acids that can function as neurotransmitters or hormones.

Peripheral Nervous System

A division of the nervous system consisting of all nerves that are not part of the brain or spinal cord.


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

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.


A severe symptom of psychiatric disorders characterized by an inability to perceive reality. Psychosis can occur in many conditions, including schizophrenia, bipolar disorder, depression, and drug-induced states.

Rapid Eye Movement (REM)

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.


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

Second Messengers

Substances that trigger communication after the actions of neurotransmitters at their receptors have been completed. Second messengers convey the chemical message of a neurotransmitter (the first messenger) from the cell membrane to the cell's internal biochemical machinery. Second-messenger effects may endure for a few milliseconds to as long as many minutes. They also may be responsible for long-term changes in the nervous system.


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.

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 Cell

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


An environmental event capable of being detected by sensory receptors.


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


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 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.

Suprachiasmatic Nucleus

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 Nervous System

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.

Taste Bud

A sensory organ found on the tongue.

Temporal Lobe

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 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 brain that are necessary for the development, function, and survival of specific groups of neurons.


Comparatively large spaces filled with cerebrospinal fluid. Of the four ventricles, three are located in the forebrain and one in the brainstem. The lateral ventricles, the two largest, are symmetrically placed above the brainstem, one in each hemisphere.

Vertebral Column

The column of bones, or vertebrae, that extends down the back and functions as a structural element for the body while also surrounding and protecting the spinal cord.

Wernicke's Area

A brain region responsible for comprehension of language and production of meaningful speech.

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.

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