Nervous System: Organization, Neurophysiology, and Synaptic Transmission

Vocabulary practice flashcards covering structural and functional organization, glial cells, resting membrane potential, action potential mechanics, synaptic transmission, and neuronal circuits from Chapter 12 of McKinley's Anatomy & Physiology.

Central Nervous System (CNS)

The structural division of the nervous system consisting of the brain and spinal cord.

Peripheral Nervous System (PNS)

The structural division of the nervous system consisting of nerves and ganglia.

Somatic Sensory System

The functional sensory division that detects stimuli consciously perceived by receptors of the five senses and proprioceptors.

Visceral Sensory System

The functional sensory division that detects stimuli typically not consciously perceived, originating from internal organs (such as the heart or kidneys) and blood vessels.

Somatic Motor System

The motor division of the nervous system that transmits voluntary motor commands from the CNS to skeletal muscles.

Autonomic Motor System

The visceral motor division that transmits involuntary commands to cardiac muscle, smooth muscle, and glands; divided into sympathetic and parasympathetic divisions.

Nerve

A bundle of parallel axons located in the peripheral nervous system (PNS).

Epineurium

A thick outer layer of dense irregular connective tissue that encloses an entire nerve and provides support and protection.

Perineurium

A layer of dense irregular connective tissue that wraps each fascicle, which is a bundle of axons within a nerve.

Endoneurium

A delicate layer of areolar connective tissue that wraps an individual axon, separating and electrically insulating it.

Ganglion

A cluster of neuron cell bodies located within the peripheral nervous system (PNS).

Excitability

The neuronal characteristic of responsiveness to a stimulus that produces a change in the cell's membrane potential.

Conductivity

The ability of a neuron to propagate electrical signals sequentially along voltage-gated channels of the plasma membrane.

Chromatophilic Substance (Nissl Bodies)

Aggregations of free and bound ribosomes located within the cell body (soma) of a neuron.

Axon Hillock

The triangular region of the neuron cell body (soma) from which the axon emanates.

Axoplasm

The cytoplasm contained within the axon of a neuron.

Axolemma

The specialized plasma membrane surrounding an axon.

Telodendria

Fine terminal branches (axon terminals) at the distal end of an axon that terminate in synaptic knobs.

Synaptic Knobs

Expanded tips at the distal ends of telodendria that house synaptic vesicles filled with neurotransmitter molecules.

Neurofibrils

Tensile-strength-providing bundles formed by the aggregation of intermediate filaments (neurofilaments) throughout a neuron.

Anterograde Transport

Axonal transport that moves newly synthesized substances from the cell body toward the synaptic knobs.

Retrograde Transport

Axonal transport that moves used materials and breakdown products from the axon toward the cell body for recycling.

Fast Axonal Transport

Microtubule-dependent movement powered by ATP-splitting motor proteins occurring at approximately $400\,\text{mm/day}$ in either anterograde or retrograde directions.

Slow Axonal Transport

Movement resulting from axoplasmic flow at a rate of $0.1\,\text{to}\,3\,\text{mm/day}$, exclusively transporting substances anterograde from the soma toward the knob.

Multipolar Neuron

The most common structural classification of neuron, characterized by possessing many dendrites and a single axon.

Bipolar Neuron

A neuron with two processes extending from the soma (one dendrite and one axon), found in limited locations such as the retina of the eye.

Unipolar Neuron

A pseudounipolar neuron possessing a single short process from the soma that branches into a peripheral process (receptive dendrites) and a central process (synaptic knobs in CNS).

Anaxonic Neuron

A neuron that possesses dendrites but lacks an axon.

Interneuron

Association neurons located entirely within the CNS that receive, process, and integrate information between sensory and motor neurons, making up $99\%$ of all neurons.

Electrical Synapse

A fast junction where presynaptic and postsynaptic neurons are mechanically connected by gap junctions, allowing direct flow of ions without synaptic delay.

Chemical Synapse

A junction where a presynaptic axon terminal releases neurotransmitters across a fluid-filled synaptic cleft to bind receptors on the postsynaptic membrane.

Synaptic Delay

The duration of time required for neurotransmitter release, diffusion across the synaptic cleft, and subsequent binding to postsynaptic receptors.

Astrocyte

A star-shaped CNS glial cell whose perivascular feet help form the blood-brain barrier, regulate interstitial fluid composition, provide structural support, guide neuronal development, and replace dying neurons.

Ependymal Cell

A ciliated glial cell lining the brain ventricles and central canal of the spinal cord that contributes to the choroid plexus to produce and circulate cerebrospinal fluid (CSF).

Microglial Cell

A small, phagocytic immune cell wandering the CNS that replicates during infection to engulf pathogens and cellular debris.

Oligodendrocyte

A large CNS glial cell with slender cytoplasmic extensions that wrap around portions of multiple axons to form myelin sheaths.

Satellite Cell

A PNS glial cell arranged around neuronal cell bodies within ganglia that electrically insulates them and regulates nutrient and waste exchange.

Neurolemmocyte (Schwann Cell)

An elongated, flattened PNS glial cell that wraps around a $1\,\text{mm}$ section of an axon to form a myelin sheath and neurilemma.

Neurilemma

The delicate outermost nucleated cytoplasmic layer formed at the periphery of a neurolemmocyte when its contents are pushed outward during myelination.

Neurofibril Node (Node of Ranvier)

Periodic unmyelinated gaps along an axon between adjacent neurolemmocytes or oligodendrocyte wraps where voltage-gated channels are concentrated.

Multiple Sclerosis

An autoimmune disorder characterized by progressive demyelination of CNS axons due to immune destruction of oligodendrocytes, leading to scarring and impaired conduction.

Guillain-Barré Syndrome

An inflammatory disease causing loss of myelin from peripheral nerves, characterized by distal muscle weakness that ascends proximally.

Wallerian Degeneration

The breakdown and degeneration of the distal segment of a severed axon and its myelin sheath following trauma, while the neurilemma remains intact.

Regeneration Tube

A conduit formed by the neurilemma and endoneurium in the PNS that directs and guides a severed axon back to its original effector or sensory receptor.

Leak Channel

A passive, continuously open membrane protein pore that allows specific ions (such as $K^+$ or $Na^+$) to diffuse down their concentration gradients.

Chemically Gated Channel

A normally closed membrane channel that opens only upon the binding of a specific neurotransmitter.

Voltage-Gated Channel

A membrane channel that remains closed until opened in response to changes in membrane electrical potential.

Modality Gated Channel

A membrane channel in sensory receptors that opens in response to a specific stimulus type, such as mechanical pressure, light, or temperature.

Receptive Segment

The functional region of a neuron (dendrites and cell body) equipped with chemically gated channels that receives neurotransmitter stimuli and produces graded potentials.

Initial Segment

The region of the axon hillock containing voltage-gated $Na^+$ and $K^+$ channels where graded potentials summate to initiate an action potential.

Conductive Segment

The length of the axon and its collaterals containing voltage-gated $Na^+$ and $K^+$ channels that propagates action potentials.

Transmissive Segment

The synaptic knobs of a neuron containing voltage-gated $Ca^{2+}$ channels and $Ca^{2+}$ pumps that release neurotransmitters.

Ohm's Law

The physical relationship stating that current is directly proportional to voltage and inversely proportional to resistance ($\text{Current} = \frac{\text{Voltage}}{\text{Resistance}}$).

Resting Membrane Potential (RMP)

The electrical charge difference across the plasma membrane of an unstimulated neuron, typically measured at $-70\,\text{mV}$.

Graded Potential

A small, local, short-lived change in the resting membrane potential initiated by the opening of chemically gated channels on the receptive segment.

Excitatory Postsynaptic Potential (EPSP)

A temporary local depolarization of the postsynaptic membrane produced primarily by $Na^+$ entry through chemically gated cation channels.

Inhibitory Postsynaptic Potential (IPSP)

A temporary local hyperpolarization of the postsynaptic membrane caused by $K^+$ exit or $Cl^-$ entry through chemically gated channels.

Threshold Membrane Potential

The critical voltage level (typically about $-55\,\text{mV}$) that must be reached at the initial segment to trigger opening of voltage-gated channels and initiate an action potential.

Spatial Summation

The cumulative effect of postsynaptic potentials generated simultaneously at multiple distinct locations across the receptive segment of a neuron.

Temporal Summation

The cumulative addition of postsynaptic potentials generated in rapid succession at a single location by a single presynaptic neuron.

All-or-None Law

The physiological principle that an action potential is either generated and propagated at full strength if threshold is reached, or not fired at all if subthreshold.

Depolarization

The phase of an action potential where membrane potential shifts from negative to positive ($-55\,\text{mV} \rightarrow +30\,\text{mV}$) due to rapid $Na^+$ influx through voltage-gated $Na^+$ channels.

Repolarization

The restoration of negative membrane polarity ($+30\,\text{mV} \rightarrow -70\,\text{mV}$) driven by $K^+$ outflow through open voltage-gated $K^+$ channels.

Hyperpolarization

A temporary drop in membrane potential below the normal resting potential (to approximately $-80\,\text{mV}$) due to delayed closure of voltage-gated $K^+$ channels.

Absolute Refractory Period

The period of approximately $1\,\text{ms}$ after action potential onset when voltage-gated $Na^+$ channels are open or inactivated, making initiation of a new action potential impossible.

Relative Refractory Period

The time immediately following the absolute refractory period where $Na^+$ channels have reset to resting state, but an exceptionally strong stimulus is required to trigger a potential due to hyperpolarization.

Continuous Conduction

Sequential propagation of an action potential along each successive unmyelinated segment of an axon's axolemma.

Saltatory Conduction

The rapid propagation of action potentials along myelinated axons where electrical signals appear to jump between successive neurofibril nodes.

Group A Nerve Fibers

Large-diameter, myelinated axons exhibiting conduction speeds up to $150\,\text{m/sec}$, characteristic of somatic motor and somatic sensory fibers.

Acetylcholinesterase

The enzyme residing within the synaptic cleft that rapidly hydrolyzes acetylcholine into choline and acetate to terminate neurotransmitter signaling.

Nicotinic Receptor

An ionotropic cholinergic receptor that acts as a chemically gated ion channel, directly causing $Na^+$ influx and producing an EPSP.

Muscarinic Receptor

A metabotropic cholinergic receptor that acts indirectly through G proteins and second messenger cascades to elicit either an EPSP or an IPSP.

Converging Circuit

A neuronal circuit configuration where inputs from multiple presynaptic neurons synapse onto a single postsynaptic neuron (e.g., sensory cues initiating salivation).

Diverging Circuit

A neuronal circuit where a single presynaptic neuron branches to transmit signals to multiple postsynaptic neurons (e.g., motor pathways coordinating skeletal muscles for walking).

Reverberating Circuit

A neuronal circuit utilizing positive feedback loops to sustain repetitive, cyclical output until disrupted by inhibition or synaptic fatigue (e.g., breathing rhythm during sleep).

Parallel-After-Discharge Circuit

A neuronal circuit in which an input transmits simultaneously along multiple parallel pathways containing variable numbers of synapses to reach a single postsynaptic neuron at different times.

Central Nervous System (CNS)

The structural division of the nervous system composed exclusively of the brain and the spinal cord.

Peripheral Nervous System (PNS)

The structural division of the nervous system consisting of nerves (bundles of axons) and ganglia (clusters of cell bodies) located outside the brain and spinal cord.

Somatic Sensory System

The functional sensory division that detects stimuli we consciously perceive, receiving input from receptors of the five senses and proprioceptors.

Visceral Sensory System

The functional sensory division that detects stimuli we typically do not consciously perceive, receiving input from internal organs (such as the heart and kidneys) and blood vessels.

Somatic Motor System

The functional motor division that initiates and transmits voluntary motor output from the CNS to skeletal muscles.

Autonomic Motor System

The visceral motor division that transmits involuntary commands from the CNS to cardiac muscle, smooth muscle, and glands; subdivided into sympathetic and parasympathetic divisions.

Epineurium

A thick outer layer of dense irregular connective tissue that encloses and protects an entire peripheral nerve.

Perineurium

A layer of dense irregular connective tissue that wraps a fascicle, which is a bundle of axons within a nerve.

Endoneurium

A delicate layer of areolar connective tissue that surrounds an individual axon, serving to separate and electrically insulate each axon.

Ganglion

A localized cluster of neuron cell bodies situated within the peripheral nervous system (PNS).

Excitability

A fundamental characteristic of neurons referring to their responsiveness to a stimulus, which induces a change in the cell's membrane potential.

Conductivity

The ability of a neuron to propagate an electrical signal along its plasma membrane via the sequential opening of voltage-gated ion channels.

Perikaryon

The cytoplasm contained within the cell body (soma) of a neuron, surrounding the nucleus.

Chromatophilic Substance (Nissl Bodies)

Aggregations of free and membrane-bound ribosomes found in the soma of a neuron that stain darkly with basic dyes.

Axon Hillock

The triangular, cone-shaped region of the neuron cell body that gives rise to the axon and serves as the initial segment for graded potential summation.

Neurofibrils

Parallel bundles of neurofilaments (intermediate filaments) that extend throughout the cytoplasm to provide structural tensile strength to the neuron.

Fast Axonal Transport

Microtubule-dependent movement powered by ATP-splitting motor proteins traveling at approximately $400\,mm/\text{day}$; can operate in both anterograde and retrograde directions.

Slow Axonal Transport

The passive anterograde movement of substances resulting from the flow of axoplasm from the cell body toward synaptic knobs at a rate of $0.1\text{ to }3\,mm/\text{day}$.

Multipolar Neuron

A neuron with many dendrites and a single axon extending from the soma; the most common structural type in the body, encompassing all motor neurons and most interneurons.

Bipolar Neuron

A neuron having two processes extending from the cell body: one dendrite and one axon; found in specialized sensory locations such as the retina of the eye.

Unipolar Neuron (Pseudounipolar)

A neuron with a single short process emerging from the cell body that bifurcates into a peripheral process and a central process; characteristic of most sensory neurons.

Anaxonic Neuron

A structural class of neuron containing multiple dendrites but lacking an axon, capable only of producing graded potentials rather than action potentials.

Interneuron (Association Neuron)

A multipolar neuron located entirely within the CNS that receives, processes, and integrates information between sensory and motor neurons, making up approximately $99\%$ of all neurons.

Electrical Synapse

A junction where presynaptic and postsynaptic membranes are physically coupled by gap junctions, allowing ionic current to flow directly and rapidly without synaptic delay.