•motor neurons One of the three types of neurons, these efferent neurons direct
muscles to contract or relax, thereby producing movement.
•interneurons One of the three types of neurons, these neurons communicate only
with other neurons, typically within a speciﬁc brain region.
•action potential The neural impulse that passes along the axon and subsequently
causes the release of chemicals from the terminal buttons.
•resting membrane potential The electrical charge of a neuron when it is not active.
•all-or-none principle The principle whereby a neuron ﬁres with the same potency
each time, although frequency can vary; it either ﬁres or not, it cannot partially ﬁre.
•myelin sheath A fatty material, made up of glial cells, that insulates the axon and
allows for the rapid movement of electrical impulses along the axon.
•nodes of Ranvier Small gaps of exposed axon, between the segments of myelin
sheath, where action potentials are transmitted.
•synaptic cleft The small space between neurons that contains extracellular ﬂuid.
•neurotransmitter A chemical substance that carries signals from one neuron to
•receptors In neurons, specialized protein molecules on the postsynaptic membrane
that neurotransmitters bind to after passing across the synaptic cleft.
•reuptake The process whereby the neurotransmitter is taken back into the
presynaptic terminal buttons, thereby stopping its activity.
•enzyme deactivation The process whereby the neurotransmitter is destroyed by an
enzyme, thereby terminating its activity.
•autoreceptors A neuron!s own neurotransmitter receptors, which regulate the
release of the neurotransmitters.
•agonist Any drug that enhances the actions of a speciﬁc neurotransmitter.
•antagonist Any drug that inhibits the action of a speciﬁc neurotransmitter.
•acetylcholine (ACh) The neurotransmitter responsible for motor control at the
junction between nerves and muscles; also involved in mental processes such as
learning, memory, sleeping, and dreaming.
PSY100 CH3 Genetic and Biological Foundations