BIO 201 Lecture 18: Neurons and Stimulus-Response Pathway

Comprehensive vocabulary flashcards covering neuron anatomy, structural and functional neuron classifications, CNS and PNS glial cells, action potential dynamics, and the stimulus-response pathway.

Neuron

The fundamental structural and functional unit of the nervous system (also known as a nerve cell) specialized for transmitting electrical impulses.

Soma (Cell Body)

The central body of a neuron that contains the nucleus and the majority of the major organelles.

Processes

Extensions that emerge from the neuronal cell membrane, including dendrites and axons.

Dendrites

Cellular extensions that receive incoming synaptic signals from other neurons and conduct graded potentials across the soma toward the axon hillock.

Axon Hillock

The tapered junction where the axon originates from the cell body; the site where graded potentials accumulate to threshold voltage to initiate an action potential.

Axon

A long nerve fiber that extends from the soma to transmit action potentials away from the cell body toward target cells.

Myelin

A multi-layered lipid and protein insulating substance produced by glial cell membranes that wraps axons to accelerate electrical impulse propagation.

Nodes of Ranvier

Unmyelinated gaps along the axon between adjacent myelin-forming glial cells where voltage-gated ion channels are localized.

Saltatory Conduction

The mechanism of rapid nerve impulse propagation in myelinated axons whereby the action potential leaps from one node of Ranvier to the next.

Continuous Conduction

The relatively slow wave of action potential propagation that travels along the entire uninsulated membrane of an unmyelinated axon.

Axon Terminal

The specialized, distal branched terminus of an axon that forms synapses and releases neurotransmitters into the synaptic cleft.

Multipolar Neuron

A structural class of neuron characterized by three or more processes (one axon and multiple dendrites); constitutes $99\%$ of all neurons and is the primary type in the mammalian brain and spinal cord.

Bipolar Neuron

A structural class of neuron with two processes extending from opposite poles of the soma (one axon and one dendrite); found in the retina, cochlea of the inner ear, and olfactory epithelium.

Unipolar Neuron

A structural class of neuron with a single short process emerging from the cell body that bifurcates T-like into proximal and distal branches; typical of most sensory neurons in the peripheral nervous system.

Sensory Neurons (Afferent Neurons)

Functional class of neurons that transmit electrical impulses from peripheral sensory receptors in the skin or internal organs toward or into the central nervous system.

Interneurons (Association Neurons)

Functional class of neurons located exclusively within the central nervous system that integrate sensory inputs and coordinate motor pathways; comprises $99\%$ of all neurons.

Motor Neurons (Efferent Neurons)

Functional class of neurons (typically multipolar) that carry electrical impulses away from the central nervous system to peripheral effector organs, muscles, or glands.

Astrocytes

Star-shaped glial cells of the central nervous system that regulate extracellular chemical composition, clear excess signaling molecules, assist in tissue repair, and support the blood-brain barrier (BBB).

Oligodendrocytes

Glial cells of the central nervous system whose membrane extensions wrap around segments of one or multiple axons to form myelin sheaths.

Microglia

Resident macrophage-like glial cells of the central nervous system that perform immune surveillance and phagocytose cellular debris, damaged cells, and pathogens.

Ependymal Cells

Ciliated epithelial-like glial cells lining the ventricles of the brain that filter blood components at the choroid plexus to produce and circulate cerebrospinal fluid (CSF).

Schwann Cells

Glial cells of the peripheral nervous system that wrap around a single axon segment to provide its insulating myelin sheath.

Satellite Cells

Supporting glial cells in the peripheral nervous system that encircle neuronal cell bodies within sensory and autonomic ganglia.

Action Potential

A rapid, transient electrical signal generated by the regulated movement of $Na^+$ and $K^+$ ions across the neuronal plasma membrane via voltage-gated ion channels.

Graded Potential

A local change in membrane voltage triggered at dendrites or sensory endings whose amplitude is directly proportional to the magnitude of the stimulus.

Depolarization

The rapid phase of an action potential during which voltage-gated channels open, permitting $Na^+$ influx and reversing the axon's membrane polarity so the interior becomes positive.

Repolarization

The restoration of the negative resting membrane potential following depolarization, primarily driven by the opening of voltage-gated $K^+$ channels and subsequent $K^+$ efflux.

Hyperpolarization

A brief overshoot at the termination of an action potential where membrane voltage drops below the resting threshold before equilibrium is restored.

Primary Afferent Axons

Peripheral sensory nerve fibers classified by diameter and conduction velocity: $A\alpha$ ($13\text{--}20\,\mu\text{m}$, $80\text{--}120\,\text{m/s}$), $A\beta$ ($6\text{--}12\,\mu\text{m}$, $35\text{--}75\,\text{m/s}$), $A\delta$ ($1\text{--}5\,\mu\text{m}$, $5\text{--}35\,\text{m/s}$), and $C$ ($0.2\text{--}1.5\,\mu\text{m}$, $0.5\text{--}2.0\,\text{m/s}$).

Thalamus

A deep bilateral sensory relay structure in the central nervous system that directs incoming sensory impulses to specific functional areas of the cerebral cortex.

Cerebral Cortex

The outer mantle of gray matter covering the cerebrum responsible for conscious sensory perception, processing, memory, and cognitive integration.

Precentral Gyrus

A prominent gyrus on the frontal cortex that serves as the primary motor area from which voluntary motor commands are executed.

Upper Motor Neuron

A cortical motor neuron whose cell body resides in the brain and whose axon descends the spinal cord to synapse onto lower motor neurons.

Lower Motor Neuron

An efferent neuron whose cell body resides in the ventral horn of spinal gray matter and whose axon travels via a peripheral nerve to directly innervate skeletal muscle.

Neuromuscular Junction

The specialized chemical synapse formed between the axon terminal of a lower motor neuron and the motor end plate of a skeletal muscle fiber.

Spinal Cord Gray Matter

The butterfly-shaped internal region of the spinal cord composed of neuronal cell bodies, dendrites, and synapses divided functionally into dorsal, lateral, and ventral horns.