Glossary: The Nervous System

absolute refractory period: time during an action period when another action potential cannot be generated because the voltage-gated Na+ channel is inactivated

action potential: change in voltage of a cell membrane in response to a stimulus that results in transmission of an electrical signal; unique to neurons and muscle fibers

activation gate: part of the voltage-gated Na+ channel that opens when the membrane voltage reaches threshold

astrocyte: glial cell type of the CNS that provides support for neurons and maintains the blood-brain barrier

autonomic nervous system (ANS): functional division of the nervous system that is responsible for homeostatic reflexes that coordinate control of cardiac and smooth muscle, as well as glandular tissue

axon hillock: tapering of the neuron cell body that gives rise to the axon

axon segment: single stretch of the axon insulated by myelin and bounded by nodes of Ranvier at either end (except for the first, which is after the initial segment, and the last, which is followed by the axon terminal)

axon terminal: end of the axon, where there are usually several branches extending toward the target cell

axon: single process of the neuron that carries an electrical signal (action potential) away from the cell body toward a target cell

axoplasm: cytoplasm of an axon, which is different in composition than the cytoplasm of the neuronal cell body

biogenic amine: class of neurotransmitters that are enzymatically derived from amino acids but no longer contain a carboxyl group

bipolar: shape of a neuron with two processes extending from the neuron cell body—the axon and one dendrite

blood-brain barrier (BBB): physiological barrier between the circulatory system and the central nervous system that establishes a privileged blood supply, restricting the flow of substances into the CNS

brain: the large organ of the central nervous system composed of white and gray matter, contained within the cranium and continuous with the spinal cord

central nervous system (CNS): anatomical division of the nervous system located within the cranial and vertebral cavities, namely the brain and spinal cord

cerebral cortex: outermost layer of gray matter in the brain, where conscious perception takes place

cerebrospinal fluid (CSF): circulatory medium within the CNS that is produced by ependymal cells in the choroid plexus filtering the blood

chemical synapse: connection between two neurons, or between a neuron and its target, where a neurotransmitter diffuses across a very short distance

cholinergic system: neurotransmitter system of acetylcholine, which includes its receptors and the enzyme acetylcholinesterase

choroid plexus: specialized structure containing ependymal cells that line blood capillaries and filter blood to produce CSF in the four ventricles of the brain

continuous conduction: slow propagation of an action potential along an unmyelinated axon owing to voltage-gated Na+ channels located along the entire length of the cell membrane

dendrite: one of many branchlike processes that extends from the neuron cell body and functions as a contact for incoming signals (synapses) from other neurons or sensory cells

depolarization: change in a cell membrane potential from rest toward zero

effector protein: enzyme that catalyzes the generation of a new molecule, which acts as the intracellular mediator of the signal that binds to the receptor

electrical synapse: connection between two neurons, or any two electrically active cells, where ions flow directly through channels spanning their adjacent cell membranes

electrochemical exclusion: principle of selectively allowing ions through a channel on the basis of their charge

enteric nervous system (ENS): neural tissue associated with the digestive system that is responsible for nervous control through autonomic connections

ependymal cell: glial cell type in the CNS responsible for producing cerebrospinal fluid

excitable membrane: cell membrane that regulates the movement of ions so that an electrical signal can be generated

excitatory postsynaptic potential (EPSP): graded potential in the postsynaptic membrane that is the result of depolarization and makes an action potential more likely to occur

G protein: guanosine triphosphate (GTP) hydrolase that physically moves from the receptor protein to the effector protein to activate the latter

ganglion: localized collection of neuron cell bodies in the peripheral nervous system

gated: property of a channel that determines how it opens under specific conditions, such as voltage change or physical deformation

generator potential: graded potential from dendrites of a unipolar cell which generates the action potential in the initial segment of that cell’s axon

glial cell: one of the various types of neural tissue cells responsible for maintenance of the tissue, and largely responsible for supporting neurons

graded potential: change in the membrane potential that varies in size, depending on the size of the stimulus that elicits it

gray matter: regions of the nervous system containing cell bodies of neurons with few or no myelinated axons; actually may be more pink or tan in color, but called gray in contrast to white matter

inactivation gate: part of a voltage-gated Na+ channel that closes when the membrane potential reaches +30 mV

inhibitory postsynaptic potential (IPSP): graded potential in the postsynaptic membrane that is the result of hyperpolarization and makes an action potential less likely to occur

initial segment: first part of the axon as it emerges from the axon hillock, where the electrical signals known as action potentials are generated

integration: nervous system function that combines sensory perceptions and higher cognitive functions (memories, learning, emotion, etc.) to produce a response

ionotropic receptor: neurotransmitter receptor that acts as an ion channel gate, and opens by the binding of the neurotransmitter

leakage channel: ion channel that opens randomly and is not gated to a specific event, also known as a non-gated channel

ligand-gated channels: another name for an ionotropic receptor for which a neurotransmitter is the ligand

lower motor neuron: second neuron in the motor command pathway that is directly connected to the skeletal muscle

mechanically gated channel: ion channel that opens when a physical event directly affects the structure of the protein

membrane potential: distribution of charge across the cell membrane, based on the charges of ions

metabotropic receptor: neurotransmitter receptor that involves a complex of proteins that cause metabolic changes in a cell

microglia: glial cell type in the CNS that serves as the resident component of the immune system

multipolar: shape of a neuron that has multiple processes—the axon and two or more dendrites

muscarinic receptor: type of acetylcholine receptor protein that is characterized by also binding to muscarine and is a metabotropic receptor

myelin sheath: lipid-rich layer of insulation that surrounds an axon, formed by oligodendrocytes in the CNS and Schwann cells in the PNS; facilitates the transmission of electrical signals

myelin: lipid-rich insulating substance surrounding the axons of many neurons, allowing for faster transmission of electrical signals

nerve: cord-like bundle of axons located in the peripheral nervous system that transmits sensory input and response output to and from the central nervous system

neuron: neural tissue cell that is primarily responsible for generating and propagating electrical signals into, within, and out of the nervous system

neuropeptide: neurotransmitter type that includes protein molecules and shorter chains of amino acids

neurotransmitter: chemical signal that is released from the synaptic end bulb of a neuron to cause a change in the target cell

nicotinic receptor: type of acetylcholine receptor protein that is characterized by also binding to nicotine and is an ionotropic receptor

node of Ranvier: gap between two myelinated regions of an axon, allowing for strengthening of the electrical signal as it propagates down the axon

nonspecific channel: channel that is not specific to one ion over another, such as a nonspecific cation channel that allows any positively charged ion across the membrane

nucleus: in the nervous system, a localized collection of neuron cell bodies that are functionally related; a “center” of neural function

oligodendrocyte: glial cell type in the CNS that provides the myelin insulation for axons in tracts

peripheral nervous system (PNS): anatomical division of the nervous system that is largely outside the cranial and vertebral cavities, namely all parts except the brain and spinal cord

postsynaptic potential (PSP): graded potential in the postsynaptic membrane caused by the binding of neurotransmitter to protein receptors

precentral gyrus of the frontal cortex: region of the cerebral cortex responsible for generating motor commands, where the upper motor neuron cell body is located

process: in cells, an extension of a cell body; in the case of neurons, this includes the axon and dendrites

propagation: movement of an action potential along the length of an axon

receptor potential: graded potential in a specialized sensory cell that directly causes the release of neurotransmitter without an intervening action potential

refractory period: time after the initiation of an action potential when another action potential cannot be generated

relative refractory period: time during the refractory period when a new action potential can only be initiated by a stronger stimulus than the current action potential because voltage-gated K+ channels are not closed

repolarization: return of the membrane potential to its normally negative voltage at the end of the action potential

resistance: property of an axon that relates to the ability of particles to diffuse through the cytoplasm; this is inversely proportional to the fiber diameter

response: nervous system function that causes a target tissue (muscle or gland) to produce an event as a consequence to stimuli

resting membrane potential: the difference in voltage measured across a cell membrane under steady-state conditions, typically -70 mV

Schwann cell: glial cell type in the PNS that provides the myelin insulation for axons in nerves

saltatory conduction: quick propagation of the action potential along a myelinated axon owing to voltage-gated Na+ channels being present only at the nodes of Ranvier

satellite cell: glial cell type in the PNS that provides support for neurons in the ganglia

sensation: nervous system function that receives information from the environment and translates it into the electrical signals of nervous tissue

size exclusion: principle of selectively allowing ions through a channel on the basis of their relative size

soma: in neurons, that portion of the cell that contains the nucleus; the cell body, as opposed to the cell processes (axons and dendrites)

somatic nervous system (SNS): functional division of the nervous system that is concerned with conscious perception, voluntary movement, and skeletal muscle reflexes

spatial summation: combination of graded potentials across the neuronal cell membrane caused by signals from separate presynaptic elements that add up to initiate an action potential

spinal cord: organ of the central nervous system found within the vertebral cavity and connected with the periphery through spinal nerves; mediates reflex behaviors

stimulus: an event in the external or internal environment that registers as activity in a sensory neuron

summate: to add together, as in the cumulative change in postsynaptic potentials toward reaching threshold in the membrane, either across a span of the membrane or over a certain amount of time

synapse: narrow junction across which a chemical signal passes from neuron to the next, initiating a new electrical signal in the target cell

synaptic cleft: small gap between cells in a chemical synapse where neurotransmitter diffuses from the presynaptic element to the postsynaptic element

synaptic end bulb: swelling at the end of an axon where neurotransmitter molecules are released onto a target cell across a synapse

temporal summation: combination of graded potentials at the same location on a neuron resulting in a strong signal from one input

thalamus: region of the central nervous system that acts as a relay for sensory pathways

thermoreceptor: type of sensory receptor capable of transducing temperature stimuli into neural action potentials

threshold: membrane voltage at which an action potential is initiated

tract: bundle of axons in the central nervous system having the same function and point of origin

unipolar: shape of a neuron which has only one process that includes both the axon and dendrite

upper motor neuron: first neuron in the motor command pathway with its cell body in the cerebral cortex that synapses on the lower motor neuron in the spinal cord

ventricle: central cavity within the brain where CSF is produced and circulates

voltage-gated channel: ion channel that opens because of a change in the charge distributed across the membrane where it is located

white matter: regions of the nervous system containing mostly myelinated axons, making the tissue appear white because of the high lipid content of myelin

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