Chapter 12: Nervous System: Nervous Tissue

Vocabulary practice flashcards covering Chapter 12: Nervous System: Nervous Tissue, including structural and functional organization, glial cells, nerve structure, membrane potentials, electrophysiology, and neurotransmitters.

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 composed of nerves and ganglia.

Somatic Sensory System

The portion of the sensory nervous system that detects stimuli we consciously perceive, including input from receptors of the five senses and proprioceptors.

Visceral Sensory System

The portion of the sensory nervous system that detects stimuli we typically do not consciously perceive, such as signals from internal organs (e.g., heart, kidneys) and blood vessels.

Somatic Motor Component

The subdivision of the motor nervous system that conducts voluntary motor output from the CNS to skeletal muscles.

Autonomic Motor System

The involuntary division of the motor nervous system (visceral motor) that transmits commands from the CNS to cardiac muscle, smooth muscle, and glands; divided into sympathetic and parasympathetic divisions.

Epineurium

A dense irregular connective tissue outer wrapping that encloses an entire nerve, providing support and protection.

Perineurium

A layer of dense irregular connective tissue that encloses each fascicle (bundle of axons) within a nerve.

Endoneurium

A delicate layer of areolar connective tissue that wraps and electrically insulates each individual axon within a nerve.

Ganglion

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

Mixed Nerves

Nerves containing both sensory and motor neurons, which represent the most common functional nerve classification.

Excitability

A general neuron characteristic describing the ability to respond to a stimulus, resulting in a change in the cell's membrane potential.

Conductivity

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

Secretion

The release of neurotransmitters stored in vesicles from synaptic knobs in response to conductive activity, influencing a target cell.

Chromatophilic Substance (Nissl Bodies)

Darkly staining aggregates composed of free and bound ribosomes located within the cell body of a neuron.

Axon Hillock

The triangular, cone-shaped region of a neuron's cell body from which the axon originates.

Axon Collaterals

Side branches that split off from the main axon to communicate with other cells.

Synaptic Knobs

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

Neurofibrils

Bundles of intermediate neurofilaments extending through the cytoplasm of a neuron to provide tensile strength and structural support.

Multipolar Neuron

A structural class of neuron possessing many dendrites and a single axon extending from the cell body; the most common neuron type.

Bipolar Neuron

A structural class of neuron with two processes extending from the cell body: one dendrite and one axon (e.g., in the retina of the eye).

Unipolar Neuron

A pseudounipolar neuron with a single short process emerging from the cell body that branches into a peripheral process and a central process.

Anaxonic Neuron

A structural category of neuron that possesses only dendrites and lacks an axon.

Interneurons

Association neurons located entirely within the CNS that receive, process, and integrate information from many neurons; make up approximately $99\%$ of all neurons.

Electrical Synapse

A type of junction where presynaptic and postsynaptic membranes are connected directly by gap junctions, allowing rapid signal transmission without synaptic delay.

Chemical Synapse

A junction where a presynaptic axon knob releases neurotransmitter molecules across a fluid-filled synaptic cleft to bind receptors on a postsynaptic cell.

Synaptic Delay

The interval of time required for neurotransmitter release from the presynaptic neuron, diffusion across the synaptic cleft, and binding to postsynaptic receptors.

Astrocyte

The most abundant star-shaped glial cell in the CNS; wraps perivascular feet around brain capillaries to form the blood-brain barrier, regulates interstitial fluid composition, provides structural support, and assists neuronal development.

Ependymal Cell

A type of ciliated CNS glial cell lining the brain ventricles and central canal of the spinal cord; forms part of the choroid plexus to assist in cerebrospinal fluid production and circulation.

Microglia

Small phagocytic immune cells that wander the CNS, replicating during infections to engulf harmful substances, microorganisms, and unneeded synapses.

Oligodendrocyte

A large CNS glial cell with slender cytoplasmic extensions that wrap around segments of multiple axons to form a myelin sheath.

Satellite Cell

A flattened PNS glial cell arranged around neuronal cell bodies in ganglia that electrically insulates soma and regulates nutrient and waste exchange.

Neurolemmocyte (Schwann Cell)

An elongated PNS glial cell that wraps around and insulates an axon segment with myelin to enable faster action potential propagation.

Neurilemma

The outer layer of a neurolemmocyte containing its pushed cytoplasm and nucleus that encloses the myelin sheath in PNS axons.

Neurofibril Node

Gaps (nodes of Ranvier) between successive myelin sheath wraps along an axon where voltage-gated ion channels are concentrated.

Multiple Sclerosis

An autoimmune disorder marked by progressive demyelination of CNS axons by immune attack against oligodendrocytes, causing scarring (sclerosis) and permanent functional impairment.

Guillain-Barré Syndrome

A peripheral nervous system inflammatory disorder where myelin is stripped from peripheral nerves, beginning with distal muscle weakness that ascends proximally.

Wallerian Degeneration

The breakdown and disintegration of the distal portion of a severed axon and its myelin sheath following traumatic injury, while the surrounding neurilemma survives.

Regeneration Tube

A pathway formed by the surviving neurilemma and endoneurium that guides a severed PNS axon back to its target effector or receptor.

Leak Channels

Continuously open membrane protein channels that permit passive ion diffusion down concentration gradients without requiring external gating.

Chemically Gated Channels

Normally closed ion channels that open specifically in response to the binding of a particular chemical messenger (neurotransmitter).

Voltage-Gated Channels

Ion channels that open or close in response to changes in the electrical charge (voltage) across the plasma membrane.

Modality-Gated Channels

Specific ion channels found in sensory receptor membranes that open in response to a stimulus like mechanical pressure, temperature changes, or light.

Voltage-Gated Sodium Channel States

The three functional states of a voltage-gated $Na^+$ channel: resting state (activation gate closed, inactivation gate open), activation state (both gates open), and inactivation state (inactivation gate closed, activation gate open).

Receptive Segment

The functional segment of a neuron (dendrites and cell body) equipped with chemically gated channels to bind neurotransmitters and produce graded potentials.

Initial Segment

The axon hillock of a neuron containing voltage-gated $Na^+$ and $K^+$ channels, responsible for summating graded potentials to initiate an action potential.

Conductive Segment

The axon and its branches containing abundant voltage-gated $Na^+$ and $K^+$ channels dedicated to propagating action potentials.

Transmissive Segment

The synaptic knobs of a neuron housing voltage-gated $Ca^{2+}$ channels and calcium pumps that regulate neurotransmitter exocytosis.

Ohm's Law

The physical law relating electrical properties: $\text{Current} = \frac{\text{Voltage}}{\text{Resistance}}$, where current increases with greater voltage or reduced resistance.

Resting Membrane Potential (RMP)

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

Graded Potential

A local, short-lived change in membrane potential established in the receptive segment whose amplitude varies with stimulus intensity.

Excitatory Postsynaptic Potential (EPSP)

A local graded depolarization of a postsynaptic membrane caused by the inflow of $Na^+$ through chemically gated cation channels.

Inhibitory Postsynaptic Potential (IPSP)

A local graded hyperpolarization of a postsynaptic membrane caused by either the outflow of $K^+$ or the inflow of $Cl^-$.

Spatial Summation

The addition of postsynaptic potentials generated simultaneously at multiple different locations across the receptive membrane.

Temporal Summation

The summation of postsynaptic potentials produced when a single presynaptic neuron fires in rapid succession at the same location.

Threshold Membrane Potential

The minimum membrane depolarization (typically $-55\,\text{mV}$) required to trigger opening of voltage-gated channels and initiate an action potential.

All-or-None Law

The principle stating that if threshold is reached, an action potential is generated and propagated at full amplitude; if threshold is not reached, no action potential is generated.

Depolarization

The phase of an action potential in which the intracellular potential shifts in a positive direction from $-55\,\text{mV}$ to $+30\,\text{mV}$ due to rapid influx of $Na^+$ ions.

Repolarization

The return of membrane potential from $+30\,\text{mV}$ back toward negative values ($-70\,\text{mV}$) driven by the outflow of $K^+$ through voltage-gated $K^+$ channels.

Hyperpolarization

A state during an action potential where the membrane potential becomes more negative than resting level (shifting to $-80\,\text{mV}$) because voltage-gated $K^+$ channels remain open.

Action Potential Events

The complete cycle of voltage changes including reaching threshold ($-55\,\text{mV}$), rapid depolarization to $+30\,\text{mV}$, repolarization to $-70\,\text{mV}$, hyperpolarization to $-80\,\text{mV}$, and reestablishing RMP via $Na^+/K^+$ pumps.

Absolute Refractory Period

The initial period of an action potential (about $1\,\text{ms}$) when voltage-gated $Na^+$ channels are open or inactivated, making it impossible to initiate a subsequent action potential.

Relative Refractory Period

The timeframe following the absolute refractory period when $Na^+$ channels have reset, but initiating another impulse requires a stronger-than-normal stimulus because the cell is hyperpolarized.

Continuous Conduction

The step-by-step propagation of an action potential along every adjacent segment of an unmyelinated axon.

Saltatory Conduction

The rapid mode of nerve impulse propagation along myelinated axons in which action potentials occur exclusively at neurofibril nodes.

Biogenic Amines

A chemical class of neurotransmitters derived from enzymatically modified amino acids, including catecholamines (synthesized from tyrosine) and indolamines (synthesized from histidine or tryptophan).

Neuropeptides

Neurotransmitter molecules consisting of chains of $2$ to $40$ amino acids, such as beta-endorphins and substance P.

Nicotinic Receptor

An ionotropic acetylcholine receptor that directly opens an ion channel to allow $Na^+$ entry, generating an EPSP.

Muscarinic Receptor

A metabotropic acetylcholine receptor that functions through a G protein and second-messenger system, resulting in either an EPSP or an IPSP.

Facilitation

A form of neuromodulation that increases postsynaptic responsiveness by elevating neurotransmitter concentration in the cleft or increasing the number of postsynaptic receptors.

Nitric Oxide

A short-lived, nonpolar gas synthesized by postsynaptic neurons that modulates synaptic strength in brain memory circuits and induces blood vessel dilation in the PNS.