1.10: Glossary

Key Terms and Definitions 

• 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.
• 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.
• 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.
• 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.
• 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
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• Myelin sheath: lipid-rich layer of insulation that surrounds an axon, formed by oligodendrocytes in the CNS and Schwann cells in the PNS; 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.
• Neurotransmitter: chemical signal that is released from the synaptic end bulb of a neuron to cause a change in the target cell.
• Node of Ranvier: gap between two myelinated regions of an axon, allowing for strengthening of the electrical signal as it propagates down the axon.
• 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.
• 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.
• Repolarization: return of the membrane potential to its normally negative voltage at the end of the action potential.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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