Nervous System: Nerve Repair, Impulse Conduction, and Synaptic Transmission

Vocabulary flashcards covering peripheral nerve repair, the generation and conduction of action potentials, saltatory conduction, multiple sclerosis, and synaptic transmission mechanisms.

Neurilemma

The outer layer of Schwann cells essential for peripheral nerve regeneration; its absence in the central nervous system prevents injured brain and spinal cord fibers from regenerating.

Regeneration Tunnel

A tunnel formed by the neurilemma near the site of nerve injury where new Schwann cells grow, guiding sprouting axons to grow at a rate of $3\text{ to }5\,\text{mm/day}$.

Atrophy

The shrinking of muscle fibers that occurs when they are deprived of nervous input following damage to an innervating nerve fiber.

Synthetic Guidance Channels

Artificial conduits used in nerve repair research to direct newly growing axons across severed gaps, which may be implanted empty or filled with growth factors or neural cells.

Membrane Potential

The potential of oppositely charged ions separated by a cell membrane to move toward each other, characterized by an excess of positive charges on one side and negative charges on the other.

Resting Potential

The inactive, polarized state of a neuron when it is not conducting a signal, characterized by an exterior rich in $Na^+$ and an interior rich in $K^+$ with large negatively charged proteins and nucleic acids.

Sodium-Potassium Pump

A membrane transport mechanism that constantly works to restore $Na^+$ to the outside of the cell and $K^+$ to the inside, counteracting passive ion leakage and resetting the membrane after excitation.

Depolarization

A shift in the electrical charge of a neuron's interior from negative to positive that occurs when a stimulus causes $Na^+$ channels to open and $Na^+$ rushes into the cell.

Threshold Level

The minimum level of depolarization required to trigger the opening of adjacent ion channels, allowing an influx of $Na^+$ sufficient to produce an action potential.

Action Potential

Also called a nerve impulse; the rapid electrical conduction event triggered when membrane depolarization exceeds threshold, continuing along the length of an axon as each segment stimulates the next.

Repolarization

The process of restoring the neuron interior to a negative charge and the exterior to a positive charge, occurring when $K^+$ flows out of the cell and $Na^+$ channels shut.

Refractory Period

The period following repolarization during which a neuron cannot respond to a new stimulus because $Na^+$ and $K^+$ reside on the incorrect sides of the cell membrane until restored by the sodium-potassium pump.

"All or Nothing" Event

The principle stating that if a stimulus reaches threshold, the neuron fires at its maximum voltage, and if it fails to reach threshold, it does not fire at all; a stronger stimulus does not produce a stronger response.

Nodes of Ranvier

The evenly spaced gaps in the myelin sheath along an axon where ion exchange occurs and action potentials are generated.

Saltatory Conduction

The rapid transmission of a nerve impulse in myelinated fibers (up to $120\,\text{meters per second}$ or $268\,\text{miles per hour}$) where the impulse appears to leap from one node of Ranvier to the next.

Multiple Sclerosis (MS)

A progressive disease typically striking women between $20$ and $40\,\text{years}$ of age, in which the myelin sheaths of the central nervous system deteriorate and are replaced by scar tissue, disrupting impulse conduction.

Plaques

Hard scar tissue that replaces deteriorated myelin sheaths in the central nervous system during the progression of multiple sclerosis.

Gut Dysbiosis

An imbalance in the gut microbiome characterized by an excess of pro-inflammatory bacteria and a deficiency of anti-inflammatory species, implicated in the development of multiple sclerosis.

Synapse

The specialized junction where a nerve impulse transfers from one neuron to the next target cell, which can occur as an electrical or chemical interaction.

Synaptic Cleft

The narrow extracellular gap separating the presynaptic neuron from the postsynaptic neuron across which neurotransmitters diffuse in a chemical synapse.

Electrical Synapse

A synapse type found between cardiac muscle cells and certain smooth tissue cells where adjacent cell membranes physically touch, allowing action potentials to pass smoothly between them.

Calcium Ions ($Ca^{2+}$)

Charged particles that enter the synaptic knob when an action potential causes ion channels to open, triggering synaptic vesicles to fuse with the membrane and release neurotransmitters.

Excitation

The continuation of a nerve impulse caused when an excitatory neurotransmitter binds to postsynaptic receptors, opening $Na^+$ channels and depolarizing the postsynaptic membrane.

Inhibition

The arrest of a nerve impulse caused when an inhibitory neurotransmitter binds to postsynaptic receptors, opening $K^+$ channels and preventing further signal transmission.

Small-Molecule Neurotransmitters

A classification of neurotransmitters (such as acetylcholine, dopamine, and serotonin) responsible for triggering rapid synaptic actions.

Neuropeptides

A classification of neurotransmitters that modulate slower, ongoing synaptic functions rather than producing rapid synaptic actions.