Muscle Physiology: Somatic Motor Control, Neuromuscular Junction, and Excitation-Contraction Coupling

Vocabulary practice flashcards covering motor units, the neuromuscular junction, acetylcholine transmission and recycling, related pathologies (ALS and myasthenia gravis), and the molecular mechanisms of excitation-contraction coupling in skeletal muscle.

Corticospinal Tract

A descending motor pathway running through the ventral and interior lateral white matter that carries signals from the brain to the brainstem or spinal cord.

Upper Motor Neuron

A neuron that conducts motor impulses from the brain to the brainstem or spinal cord.

Alpha (Lower) Motor Neuron

A motor neuron with a large, myelinated axon conducting impulses at $15\text{--}120\text{ m/s}$ from the spinal cord (or brainstem) to innervate skeletal muscle fibers.

Motor Unit

A single alpha motor neuron and all the skeletal muscle fibers it innervates, all of which contract simultaneously upon stimulation.

Motor Unit Innervation Ratio

The variable number of muscle fibers per motor unit, ranging from $<10$ fibers for delicate, precise work to $>100\text{s}$ of fibers for powerful, less precise contractions.

Neuromuscular Junction (NMJ)

The specialized area where a lower motor neuron makes synaptic contact with a skeletal muscle fiber.

Three Components of the Neuromuscular Junction

The structural components consisting of: 1) the presynaptic motor neuron terminal filled with synaptic vesicles, 2) the synaptic cleft, and 3) the postsynaptic membrane of the skeletal muscle fiber.

Motor End Plate

The specialized, folded region of the sarcolemma directly beneath the nerve terminal at the neuromuscular junction containing nicotinic acetylcholine receptors.

Junctional Folds

Deep invaginations of the muscle sarcolemma at the motor end plate that serve to increase surface area for neurotransmitter reception.

Amyotrophic Lateral Sclerosis (ALS)

A neurodegenerative motor neuron disease in which upper and/or lower motor neurons degenerate, leading to muscle atrophy and weakness from disuse, and eventual respiratory failure.

Superoxide Dismutase (SOD)

An antioxidant defense enzyme that catalyzes the dismutation of superoxide ($O_2^-$) into oxygen and hydrogen peroxide; mutated in $25\%$ of familial (dominantly inherited) ALS cases.

Nicotinic Acetylcholine Receptor (nAChR)

A ligand-gated monovalent cation channel belonging to the cys-loop receptor family that is permeable to $Na^+$ and $K^+$ and requires the binding of two acetylcholine molecules to open.

End-Plate Potential (EPP)

A localized, excitatory depolarization of the motor end plate produced by net $Na^+$ entry through open nicotinic receptors that spreads to trigger adjacent voltage-gated $Na^+$ channels and initiate a muscle action potential.

Acetylcholinesterase (AChE)

An enzyme located in the synaptic cleft that terminates neural transmission by rapidly breaking down acetylcholine into acetate and choline.

Choline Acetyltransferase

The presynaptic enzyme that synthesizes acetylcholine by combining recycled choline with mitochondrial-derived Acetyl CoA.

Myasthenia Gravis

An autoimmune disorder of neuromuscular transmission in which autoantibodies bind to acetylcholine receptors, causing postjunctional fold degeneration, receptor loss, and severe muscle weakness (extraocular or generalized).

Transverse Tubules (T-tubules)

Deep tubular invaginations of the sarcolemma that encircle myofibrils at the A-band and I-band junctions, rapidly propagating action potentials into the interior of the muscle fiber.

Sarcoplasmic Reticulum (SR)

A specialized intracellular organelle arranged around myofibrils and T-tubules that acts as the primary storage and release reservoir for $Ca^{2+}$ in muscle fibers.

Terminal Cisternae

Specialized, enlarged end regions of the sarcoplasmic reticulum that store $Ca^{2+}$ and closely flank transverse tubules along their length.

Triad

The anatomical complex formed in skeletal muscle by one central transverse tubule associated with two adjacent terminal cisternae of the sarcoplasmic reticulum.

Excitation-Contraction (E-C) Coupling

The physiological process by which electrical excitation of the surface membrane triggers an increase in intracellular calcium ($[Ca^{2+}]_i$), leading to mechanical contraction.

Dihydropyridine (DHP) Receptor

A voltage-sensitive L-type $Ca^{2+}$ channel in the T-tubule membrane that undergoes a conformational shift in response to action potentials to mechanically open adjacent ryanodine receptors.

Ryanodine Receptor (RyR)

A $Ca^{2+}$ release channel embedded in the sarcoplasmic reticulum membrane that is mechanically activated by the DHP receptor to release stored $Ca^{2+}$ into the cytoplasm.

Calcium-Induced Calcium Release (CICR) in Skeletal Muscle

A secondary mechanism where RyR channels can be activated directly by $Ca^{2+}$, though neither CICR nor extracellular $Ca^{2+}$ entry is vital for skeletal muscle contraction.