The Nervous System, Action Potentials, and Muscle Physiology

Vocabulary practice flashcards covering neurophysiology, resting membrane potential, action potential mechanics, nerve conduction, skeletal muscle histology, neuromuscular junctions, the cross-bridge cycle, and muscular contraction in cardiac and smooth muscles.

Sensory (afferent) neurons

Neurons that conduct nerve impulses from sensory receptors into the Central Nervous System (CNS).

Motor (efferent) neurons

Neurons that conduct nerve impulses out of the Central Nervous System (CNS) to effector organs.

Association neurons (interneurons)

Neurons located entirely within the Central Nervous System (CNS) that serve an integrative function.

Pseudounipolar neurons

Neurons characterized by a single short process extending from the cell body that branches like a T; typical of sensory neurons.

Bipolar neurons

Neurons with two distinct processes extending from the cell body; found in specialized sensory structures such as the retina of the eye.

Multipolar neurons

Neurons possessing multiple dendrites and a single axon extending from the cell body; typical of motor neurons.

Resting Membrane Potential (RMP)

The potential voltage difference across a resting cell membrane, largely caused by trapped intracellular negatively charged organic molecules and limited diffusion of inorganic cations.

Excitability

The ability of a cell (such as a neuron or muscle cell) to produce and conduct electrical impulses in response to stimulation.

Leak channels

Membrane protein channels that allow $Na^+$ or $K^+$ to passively diffuse down their concentration gradients without requiring ATP. At rest, membrane permeability to $K^+$ is $50\text{--}70$ times higher than to $Na^+$ due to an abundance of $K^+$ leak channels.

$Na^+/K^+$ ATPase pump

An electrogenic pump driven by ATP hydrolysis that pumps $3\,Na^+$ to the outside and $2\,K^+$ to the inside of the cell, building and stabilizing the resting membrane potential.

Depolarization

A reduction in the membrane potential difference where the inside of the cell becomes more positive, such as shifting from $-70\,mV$ toward $+30\,mV$.

Repolarization

The return of the membrane potential toward the resting level, becoming more negative following depolarization.

Hyperpolarization

An increase in membrane potential magnitude making the inside of the cell more negative than its normal resting membrane potential.

Action potential (AP)

An all-or-none electrical signal conducted with constant size and shape along the entire length of an excitable cell membrane in response to a threshold stimulus.

All-or-none law

The principle stating that if a stimulus reaches threshold, an action potential of maximal amplitude is generated; stronger stimuli do not increase amplitude beyond approximately $+30\,mV$.

Stimulus intensity coding

The physiological mechanism where a greater stimulus strength is signaled by an increased frequency of action potentials rather than larger individual spikes.

Recruitment

The process by which stronger stimuli activate additional nerve axons that possess higher excitation thresholds.

Saltatory conduction

The rapid transmission of action potentials along myelinated axons achieved by impulses jumping node to node from one Node of Ranvier to the next.

Nodes of Ranvier

Gaps or interruptions in the myelin sheath along an axon containing concentrated voltage-gated $Na^+$ and $K^+$ channels where action potentials are generated.

Epimysium

The dense fibrous connective tissue sheath continuous with tendons that encloses the entire skeletal muscle.

Perimysium

The connective tissue sheath that surrounds individual bundles of skeletal muscle fibers known as fascicles.

Endomysium

The delicate connective tissue layer that surrounds each individual skeletal muscle fiber (muscle cell).

Sarcomere

The functional repeating contractile unit of a muscle myofibril extending from one Z disc to the adjacent Z disc.

A band

The dark, anisotropic band in a sarcomere containing thick filaments, which are primarily composed of the protein myosin.

I band

The light, isotropic band in a sarcomere containing thin filaments primarily composed of actin, bisected centrally by a Z disc.

Titin

A large elastic protein that anchors thick filaments, running through myosin from the M line to the Z disc and contributing to muscle elastic recoil.

M line

Protein filaments located in the middle of a sarcomere that anchor thick filaments (myosin) during contraction.

Neuromuscular Junction (NMJ)

The specialized synapse comprising the single axonal terminal ending of a motor neuron and the underlying motor end plate of the sarcolemma.

Acetylcholine (ACh)

The neurotransmitter stored in synaptic vesicles of motor neuron terminals that binds to motor end plate receptors to open chemically regulated $Na^+$ channels.

Transverse tubule (T tubule)

Invaginations of the muscle fiber sarcolemma that conduct action potentials into the cell interior to trigger calcium release from the sarcoplasmic reticulum.

Troponin

A regulatory protein complex associated with actin filaments that binds $Ca^{2+}$, causing a conformational shift in tropomyosin to uncover actin-binding sites.

Tropomyosin

A filamentous protein that covers the myosin-binding sites on actin thin filaments during muscle relaxation, preventing cross-bridge formation.

Power stroke

The mechanical action during contraction wherein the myosin head pivots and bends, pulling the thin actin filament toward the center of the A band while releasing inorganic phosphate ($P_i$) and ADP.

Gap junctions

Intercellular protein channels connecting adjacent cells (such as myocardial cells in cardiac muscle and smooth muscle cells) that allow electrical impulses to spread directly from cell to cell.

Calmodulin

An intracellular calcium-binding protein in smooth muscle that forms a $Ca^{2+}$-calmodulin complex to activate myosin light chain kinase.

Myosin light chain kinase (MLCK)

An enzyme in smooth muscle activated by the $Ca^{2+}$-calmodulin complex that phosphorylates myosin heads, enabling them to bind actin.

Dense bodies

Cytoplasmic structures in smooth muscle cells to which thin actin filaments and intermediate filaments attach, functionally analogous to Z discs.