acetylcholine Neurotransmitter released at the neuromuscular junction; also released from certain synapses in the brain, where it can have either neurotransmitter or neuromodulatory effects, and from parasympathetic nervous system neurons.
action potentials The transient, all-or-nothing electrical signals that travel down axons carrying the output information of neurons.
adaptation The decline in responsiveness of sensory receptors to a sustained stimulus as a function of time.
adenosine triphosphate (ATP) An energy-rich molecule that powers biochemical reactions in cells.
adenylate cyclase The enzyme that converts ATP to cAMP.
agnosia (visual) The inability to recognize objects
amacrine cell One class of inner retinal neuron. Many amacrine cells are movement sensitive.
amblyopia The loss of visual acuity because of visual (form) deprivation of an eye or a crossed eye in a young animal or human.
amino acids The molecules that when strung together form proteins.
amygdala A region in the forebrain involved in integrating and coordinating emotional behaviors.
Aplysia californica An invertebrate used especially for studies of elementary forms of learning and memory.
area V1 The primary visual area of the brain, where visual information is first processed in the occipital cortex.
areas V2–V8 Areas in the occipital cortex that, along with area V1, are concerned with processing visual information.
association areas Regions of the cerebral cortex concerned with higher levels of processing.
association neurons Cells that mediate interactions between neurons.
autonomic nervous system The part of the nervous system that regulates our internal organs. Much of the regulation is involuntary and mediated by two opposing subdivisions, the sympathetic and parasympathetic systems.
autoreceptors Receptors found on synaptic terminals that are activated by the substances released by the terminals.
axon Thin cellular branch that extends from a neuron to contact another neuron or a muscle cell. Axons carry the output message from a neuron via action potentials.
axon terminals Enlargements of an axon near its site of termination where synapses are typically made.
axonal transport The special mechanism by which substances are moved rapidly down axons.
basal ganglia Five brain nuclei found in the forebrain that are concerned with the initiation and execution of movements.
basilar membrane The membrane in the organ of Corti in which the hair cells are imbedded.
bipolar cell A class of retinal neuron that carries the visual signal from the outer to inner retina.
birdsong The songs birds sing.
brain imaging (PET and fMRI) The imaging of increases of blood flow that occur when neurons in a brain area are active.
bride of sevenless (boss) A mutation in the fruit fly that prevents the R7 photoreceptor from developing.
Broca’s area An area (usually found in the left frontal lobe of the cerebral cortex) critical for the production of language.
calcium (Ca2+) A positively charged ion important in synaptic transmission and that also serves as a second messenger.
calmodulin A protein that binds calcium ions. Activated calmodulin activates a specific kinase termed CaM kinase (and other kinases as well) (CaMK, Ca2+/calmodulin-dependent protein kinase).
catecholamine A class of monoamine derived from the amino acid tyrosine. Dopamine, norepinephrine, and epinephrine are examples.
central nervous system The part of the nervous system that comprises the brain and spinal cord.
cerebellum A prominent hindbrain structure important for coordinating and integrating motor activity.
cerebral cortex A 2-millimeter-thick layer of cells that covers the forebrain. Highly infolded in man, the cortex is divided into two hemispheres , which are further subdivided into four lobes, frontal, parietal, occipital, and temporal.
cerebrum A collective term for the cerebral cortex, basal ganglia, and associated structures.
channels Membrane proteins that allow ions to cross the cell membrane. Channels are usually closed until activated by a specific stimulus.
chloride (Cl–) A negatively charged ion primarily involved in the inhibition of neurons.
circadian rhythm Endogenous rhythm that regulates various bodily functions depending on time of day.
cochlea A coiled structure containing three fluid-filled chambers as well as the sensory receptors (hair cells) and accessory structures that underlie audition (hearing).
complex cells Neurons recorded in the primary visual area of the cortex that respond best to oriented bars of light or dark moving at right angles to the bars’ orientation .
cone photoreceptors The photoreceptors responsible for color vision. Three types of cones exist in the human retina, sensitive to red, green, or blue light.
corpus callosum A thick band of axons found in the middle of the brain that carries information from one side of the brain to the other.
cortical columns Columns of neurons that run through and across the cortex that share similar properties, such as orientation or ocular dominance columns in area V1 of the visual cortex.
cortisol A steroid hormone released from the adrenal glands especially during stress.
cranial nerves Twelve nerves that enter the brain directly. Ten carry sensory and/or motor information related to the head, and two innervate the internal organs.
critical period The period of time during development when an animal is particularly sensitive to environmental conditions.
curare A drug that paralyzes muscles by blocking the acetylcholine receptors found on muscle cells.
current A measure of the flow of electrons through a wire, or ions across a membrane per unit of time.
cyclic adenosine monophosphate (cAMP) A second-messenger molecule formed by the enzyme adenylate cyclase from adenosine triphosphate (ATP).
cytoplasm The substance inside cells exclusive of the nucleus.
dark adaptation The time required for the eye to regain full sensitivity after light exposure.
declarative memory The memories of facts or events.
dendrites Bushy, branch-like structures that extend from the cell body of a neuron and receive the synaptic input to the cell.
direction-selective cell A cell in the visual system that responds selectively to a spot or bar of light or dark moving in a particular direction across the retina.
disinhibition The inhibition of an inhibiting neuron, resulting in a partial relief of inhibition.
disparity-tuned cells Neurons that respond only to stimuli precisely positioned within their receptive fields, thought to be critical for depth perception.
dopamine A neuromodulator released from brain synapses that has been associated with two diseases, Parkinson’s disease and schizophrenia.
eccentric cell Second-order neuron in the horseshoe crab eye.
ectoderm Cells on the outer surface of the embryo that become skin.
electrical synapse A junction where ions flow directly from one cell to the next.
electrons The small negatively charged particles that surround the protons in an atom.
endoderm Cells lining the inside of an embryo that form the gut and other internal organs.
enkephalins Small peptides released from synapses in the brain that have opiate-like effects.
epilepsy Seizures caused by diseased or damaged cells in the brain.
excitatory synapse A synapse that excites a neuron or muscle cell.
forebrain The most distal part of the brain, consisting principally of the thalamus, hypothalamus, basal ganglia, and cerebral cortex.
fovea The central region of the eye that mediates high-acuity vision.
frontal lobe The most anterior portion of the cerebral cortex, concerned primarily with movement, planning, programming, and smell.
functional magnetic resonance imaging (fMRI) A technique that measures increases in blood flow in an active region of the brain.
γ-aminobutyric acid (GABA) An inhibitory neurotransmitter in the brain.
ganglia Groups of nerve cells that usually serve a particular function; typically used to describe groups of neurons outside the central nervous system or in invertebrate nervous systems, but there are exceptions, such as the basal ganglia in the forebrain.
ganglion cells The third-order cells in the retina whose axons form the optic nerve; also, cells in a ganglion.
generator potential A collective term for excitatory synaptic and receptor potentials that lead to the generation of action potentials.
germinal zone Where neurons and glial cells are generated in the embryo; initially on the inner surface of the neural tube.
glia Supporting cells in the brain that help maintain neurons, prune processes, regulate the environment, and form the myelin around axons.
glomeruli Structures in the olfactory bulb that receive input from odorant receptors of a specific type.
glutamate An amino acid that serves as the major excitatory neurotransmitter in the brain.
glycine An amino acid that serves as an inhibitory neurotransmitter in the brain.
Golgi method A silver-staining method discovered by Golgi, an Italian histologist, but used prominently by Santiago Ramón y Cajal, a Spaniard, to elucidate neuronal structures.
G-protein A protein activated by postsynaptic membrane receptors, usually linked to an enzyme that makes a second-messenger molecule.
gray matter Those regions of the brain and spinal cord where neuronal cell bodies and dendrites are abundant.
grid cells Neurons found in the cortex near the hippocampus that are active when an animal is in several locations (place fields), which are arranged in a triangular pattern. Grid cells and place cells in the hippocampus are thought to interact, but how is not clear.
growth cone The specialized end of a growing axon.
growth factors Small proteins important for cell growth, differentiation, and survival.
guidepost neurons Specialized cells found in the developing brain that guide axonal growth.
habituation The decrease in the strength of a response following repeated elicitations of the response.
hair cells The sensory receptors for audition (hearing). They respond to bending of the fine hairs that project outward from their apical surface.
hemisphere (cortical) Half of the cerebral cortex. The two cortical hemispheres are each subdivided into four lobes, frontal, parietal, occipital, and temporal.
hindbrain The lowermost part of the brain, emerging from the spinal cord; consists mainly of the medulla, pons, and cerebellum.
hippocampus A region of the brain found under the temporal lobes important for establishing long-term memories.
histology The microscopic study of tissues.
horizontal cell An outer retinal cell class that mediates lateral inhibition between photoreceptors and bipolar cells.
horseshoe crab An invertebrate, Limulus polyphemus, useful for studies of visual mechanisms.
Huntington’s disease An inherited disease of the basal ganglia that causes movement dysfunction.
hypercolumn (module) The basic module of the visual cortex. A piece of cortex 1 × 1 × 2 millimeters that contains all the machinery needed to analyze a bit of visual space.
hypothalamus A forebrain region that contains nuclei concerned with basic acts and drives such as eating, drinking, and sexual activity. The hypothalamus also regulates the release of pituitary gland hormones and the autonomic nervous system and plays an important role in emotional behavior.
indoleamine A class of monoamine derived from the amino acid tryptophan. Serotonin is an example.
inhibitory synapse A synapse that inhibits neurons.
interneurons A general term for all neurons that are found between sensory neurons and motor neurons.
invertebrates Animals such as insects, crabs, flies, and mollusks that do not have a backbone.
ions Atoms that are charged; that is, have gained electrons and are thus negatively charged or have lost electrons and are thus positively charged.
kinases Enzymes that add phosphate groups to proteins, thereby altering their function.
lateral geniculate nucleus (LGN) The nucleus in the thalamus that receives input from the eye and transmits the visual signal to the cerebral cortex.
lateral inhibition The reciprocal inhibition of one neuron by another, best characterized in the horseshoe crab eye.
L-dopa A precursor of dopamine useful for treating Parkinson’s disease.
Limulus polyphemus The horseshoe crab.
long-term depression (LTD) A persistent decrease in synaptic potential amplitude induced in a neuron by a strong priming stimulus delivered to the neuron.
long-term memory Memories that last for long periods—weeks, months, or longer.
long-term potentiation (LTP) A persistent increase in synaptic potential amplitude induced in a neuron by a strong priming stimulus delivered to the neuron.
Mach bands Light and dark bands seen adjacent to dark and light borders, respectively, that enhance edge detection.
mechanoreceptor Sensory receptors that respond directly to deformation of membrane channels themselves or of the surrounding membrane.
medulla A hindbrain region that contains nuclei involved with vital body functions, including heart rate and respiration.
membrane (cell) The thin barrier surrounding a cell that keeps various substances in and other substances out; consists of a lipid bilayer in which are embedded various kinds of proteins, including channels, pumps, enzymes, and receptors.
mesoderm Cells between the ectoderm and endoderm in the embryo that develop into muscle, bone, and heart cells. In the early embryo, mesodermal cells induce overlying ectodermal cells to become neural plate cells.
messenger RNA The RNA that carries the code for a protein from the DNA in the nucleus to a ribosome in the cytoplasm where the protein is made.
midbrain That part of the brain between the hindbrain and forebrain.
mirror neurons Cells in the frontal lobes that become active when an animal observes another animal carrying out a specific task.
mitochondria Structures found in cells that provide the energy-rich molecules (i.e., adenosine triphosphate) that power cells.
mitral cells The major output neuron of a glomerulus in the olfactory bulb.
monoamine A type of substance released at synapses that functions mainly as a neuromodulator.
monosynaptic reflex A simple reflex circuit consisting of a sensory neuron impinging directly on a motor neuron.
motor output The activity of motor neurons that results in muscle movements.
multiple sclerosis (MS) A disease of the myelin that surrounds axons.
myasthenia gravis A disease of the neuromuscular junction.
myelin An insulating layer of membrane formed around axons by glial cells.
nerve growth factor (NGF) The first-identified and best-studied growth factor found in the brain.
neural crest Cells derived from the neural plate that form much of the peripheral nervous system.
neural plate Cells found on the dorsal side of an embryo that form the nervous system.
neural tube An early stage in the development of the brain formed by the infolding of neural plate cells.
neuromodulator Substance released at a synapse that causes biochemical changes in a neuron.
neurons Cells in the brain involved in the reception, integration, and transmission of signals.
neuropeptides Small proteins (peptides) released at synapses that act mainly as neuromodulators.
neurotransmitter Substance released at a synapse that causes fast electrical excitation or inhibition of a neuron.
night blindness Loss of visual sensitivity at night; can be caused by a deficiency of vitamin A or certain inherited eye diseases.
NMDA channels Channels that respond to glutamate and allow Na+ and Ca2+ to enter a neuron.
nodes Those regions along an axon where the myelin is interrupted and action potentials are generated.
norepinephrine A catecholamine released at certain synapses in the brain and also from sympathetic nervous system neurons.
nucleic acid (DNA) The genetic material found in the nucleus of a cell that codes for the proteins made by the cell.
nucleus A cluster of neurons in the brain that generally serves a particular function; also, the structure within cells that contains the genetic material (DNA).
nucleus accumbens A nucleus found in the basal region of the forebrain involved in reinforcing behaviors such as drinking when thirsty or eating when hungry. This nucleus has also been implicated in addictive behaviors.
occipital lobe The most posterior portion of the cerebral cortex, concerned with visual processing.
ocular dominance The phenomenon that results in most binocular cells in the cortex being driven more strongly by one eye than the other.
OFF-center cell A neuron whose activity decreases in response to stimulation of its receptive field center. Usually at the cessation of the stimulus such cells become active for a short period.
olfaction Smell.
oligodendrocytes Glial cells in the brain and spinal cord (central nervous system) that form myelin.
ommatidium The photoreceptive unit found in the eyes of invertebrates.
ON-center cell A neuron whose activity increases in response to stimulation of its receptive field center.
ON-OFF cell A neuron whose activity increases at the onset of a stimulus and again at the cessation of stimulation.
orbitofrontal cortex An area found in the lower part of the frontal lobes, important for the expression of emotional behaviors.
organ of Corti The structure in the inner ear that results in hearing, consisting of hair cells and the basilar and tectorial membranes.
Pacinian corpuscle A sensory mechanoreceptor receptor that responds to touch, pressure and vibration.
parallel processing The simultaneous processing of information along separate neural pathways.
parasympathetic nervous system The division of the autonomic nervous system that promotes “rest and digest” behaviors.
parietal lobe That region of the cerebral cortex between the frontal and occipital lobes concerned primarily with somatosensory information processing.
Parkinson’s disease A disease of the motor system caused by a deficiency of dopamine in the basal ganglia. Patients with the disease typically develop a tremor and have difficulty initiating movements.
peptide A small protein.
peripheral nervous system Parts of the nervous system outside of the brain and spinal cord.
phosphodiesterase (PDE) An enzyme that breaks phosphate bonds.
phosphorylation The addition of a phosphate group to a protein; serves to modify the properties of the protein.
pioneer axons Axons that form early in development and provide a path for other axons to follow.
pituitary gland A gland found at the base of the brain that releases a variety of hormones into the bloodstream.
place cells Neurons present in the hippocampus that signal when an animal is in a specific location
placebo An inert substance that can cause physiological effects under certain circumstances.
pleasure centers Regions of the brain that when stimulated appear to give pleasure to an animal. These regions appear to be related to the reinforcing and reward systems of the brain.
pons A hindbrain structure that relays information from the cortex to the cerebellum.
positron emission tomography (PET) A method for detecting increases in activity of a part of the brain.
postsynaptic Pertaining to structures or processes downstream of a synapse, for example, postsynaptic neuron, postsynaptic potential.
postsynaptic membrane That region of a cell membrane specialized to receive synaptic input.
potassium (K+) A positively charged ion primarily involved in establishing the resting potential.
potential Another term for voltage.
prefrontal lobotomy A surgical procedure that severs the connections between the orbitofrontal cortex and the rest of the brain.
premotor areas Regions in the frontal lobes thought to be important for the planning and programming of motor movements.
presynaptic Pertaining to structures upstream of a synapse, for example, presynaptic neuron, presynaptic terminal.
presynaptic synapse A synapse made onto a synaptic terminal.
primary motor area The region of the cerebral cortex where fine movements are initiated, found in the frontal lobes adjacent to the central sulcus.
primary sensory areas Regions where sensory information is first processed in the cerebral cortex.
procedural memory The memory of a motor skill such as riding a bicycle.
proprioceptive information Sensory information from muscles, joints, and tendons of which we are not aware—it does not reach our consciousness.
prosopagnosia The inability to recognize faces.
protein A chain of amino acids folded in complex ways that enable the molecule to carry out its prescribed function.
protons The positively charged particles found in the center of an atom.
pump A membrane protein that moves ions across the membrane of a cell. Pumps require energy to function.
Purkinje cell A large neuron found in the cerebellum.
pyramidal cell A prominent neuron found in all areas of the cerebral cortex.
radial glial cell A specialized glial cell found in the developing brain along whose processes precursor neurons travel to find their appropriate position.
rapid eye movement (REM) sleep An active phase of sleep during which most dreaming occurs.
receptive field That area of the retina that when stimulated causes a retinal cell to alter its activity.
receptor Membrane protein found in postsynaptic membranes that is usually linked to intracellular enzyme systems; also, a cell that responds to specific sensory stimuli, for example, a photoreceptor.
receptor potential The voltage change elicited in a sensory cell or neuron following the presentation of a specific sensory stimulus to the cell or neuron.
reflex An involuntary motor response in response to a specific stimulus.
resting potential The voltage across a cell membrane in the absence of any stimulus to the cell.
reticular formation Neurons found throughout the medulla that extend widely in the brain and are important for regulating states of arousal and levels of consciousness.
retinal The aldehyde form of vitamin A that, when combined with a specific protein, forms a visual pigment molecule.
retinitis pigmentosa An inherited disease that causes a slow degeneration of the photoreceptors.
rhodopsin The visual pigment of rods.
ribosomes Particles found in cells that are responsible for making proteins.
rod photoreceptors The photoreceptors responsible for dim-light vision.
schizophrenia A severe mental disease characterized by thought and mood disorders, hallucinations, and so forth.
Schwann cells Glial cells of the peripheral nervous system that form the myelin around axons.
second messenger A small molecule synthesized in a cell in response to a neuromodulator (the first messenger).
sensitization The increase in the strength of a response following the presentation of an adverse stimulus to an animal.
serotonin A substance released at synapses that most often acts as a neuromodulator. Decreased levels of serotonin in the brain have been linked to depression.
sevenless A genetic mutation in the fruit fly that prevents the R7 photoreceptor from developing.
short-term memory The initial storage of memories that lasts for a few minutes or so. Short-term memories are labile and easily disrupted.
simple cells Neurons recorded in the primary visual area of the cortex that respond best to oriented bars of light or edges projected onto the retina.
sodium (Na+) A positively charged ion involved in the generation of action potentials and in the excitation of neurons and sensory cells.
somatosensory Pertaining to sensory information coming from the skin and deeper tissues of the limbs and trunk, such as touch, pressure, temperature, and pain.
squid An invertebrate (a mollusk) that has giant axons.
stem cells Undifferentiated cells that can proliferate and become any type of cell.
sulcus A prominent and deep infolding of the cerebral cortex.
supplementary motor area A premotor area involved in the planning and programming of motor movements.
sympathetic nervous system The division of the autonomic nervous system that mediates “fight or flight” reactions.
synapse The site of functional contact between two neurons or a neuron and muscle cell.
synaptic potential The voltage change produced in a neuron following the activation of a synapse impinging on the cell.
synaptic terminal A site where a synapse is usually made.
synaptic vesicles Small vesicles found at synapses that contain the chemicals released at the synapse.
tectorial membrane The membrane in the organ of Corti that causes the bending of the hairs on the hair cells, resulting in cell excitation.
tectum A midbrain structure, especially prominent in nonmammalian species, which integrates sensory inputs and initiates motor outputs.
temporal lobe The lateral-most part of the cerebral cortex, concerned with hearing and memory.
tetrodotoxin An antagonist that blocks voltage-gated Na+ channels.
thalamus A forebrain region that relays sensory information to the cerebral cortex.
topographic Pertaining to the orderly projection of axons from one region of the brain to another.
transcription factor A protein that interacts with DNA to turn on or off the expression of a gene.
transducin A G-protein activated by a visual pigment and that activates phosphodiesterase.
transmagnetic stimulation Stimulation or inhibition of neural activity by a magnetic beam.
transporter A membrane pump that transports substances released at a synapse back into the synaptic terminal, thereby terminating the activity of the substances.
tricyclics Drugs that raise the levels of monoamines in the brain by inhibiting their reuptake into synaptic terminals.
tryptophan The amino acid from which indoleamines such as serotonin are derived.
tyrosine The amino acid from which catecholamines such as dopamine and norepinephrine are derived.
visual area Area in the occipital cortex involved with processing of an image.
visual pigment Molecule in photoreceptors that absorb light and leads to the excitation of the cell.
voltage A measure of the electrical charge difference between two points; in neurons, the charge difference across the cell membrane (i.e., membrane voltage or potential).
voluntary motor system The part of the nervous system that controls the muscles of the limbs, body, and head. The control is mainly voluntary.
Wernicke’s area An area in the left temporal lobe concerned with the comprehension of language, reading and writing.
what pathway The ventral visual pathway that is involved in object recognition.
where pathway The dorsal visual pathway that is involved in spatial visual tasks.
white matter Regions of the brain and spinal cord where there are abundant myelinated axons. The myelin gives the tissue its whitish appearance.
working memory A memory maintained for a short time to enable a specific task to be accomplished. An example is remembering a phone number until it is dialed.
X-chromosome The so-called sex chromosome. Males have just one; but females have two.