Muscle Contraction - Microscopic Level

Comprehensive vocabulary flashcards covering the microscopic mechanisms of skeletal muscle contraction, neuromuscular junctions, the sliding filament theory, and sarcomere anatomy.

Motor Unit

A functional unit consisting of an $\alpha$-motor neuron and all the skeletal muscle fibers it innervates.

Neuromuscular Junction

The specialized intercellular site where a single motor neuron axon branch innervates an individual skeletal muscle fiber.

Acetylcholine (ACh)

A neurotransmitter released by neurons to alter the membrane properties of other cells, stored in vesicles at the synaptic terminal.

All-or-None Principle

The principle stating that a single nerve fiber will always give a maximum response and produce spikes of the same amplitude when stimulated, regardless of increased stimulus intensity.

Motor Unit Recruitment

The process by which multiple motor units work cooperatively to activate the number of muscle fibers required for a larger contractile response.

Excitation-Contraction Coupling

The link between the generation of an action potential in the sarcolemma and the initiation of muscle contraction, where an action potential reaching a triad triggers the release of $Ca^{2+}$ from terminal cisternae.

Contraction Cycle

The repeating microscopic sequence consisting of active-site exposure, cross-bridge attachment, myosin head pivoting (power stroke), cross-bridge detachment, and myosin reactivation.

Active-Site Exposure

The phase of the contraction cycle where $Ca^{2+}$ binds to troponin, weakening the bond between troponin and actin to uncover active binding sites.

Power Stroke

The pivoting of the stored-energy-loaded myosin head toward the M line upon the release of inorganic phosphate ($P_i$) and ADP, pulling the thin filament.

Cross-Bridge Detachment

The step during the contraction cycle in which a newly arrived ATP molecule binds to the myosin head, causing it to release from actin.

Myosin Reactivation

The process where the myosin head hydrolyzes (splits) ATP, re-cocking the cross-bridge back into its original resting orientation.

Sliding Filament Theory

The structural theory of contraction stating that thin filaments slide toward the center of the sarcomere alongside thick filaments, narrowing H zones and I bands while maintaining constant A band width.

Sarcomere

The fundamental repeating contractile unit of a myofibril, defined as the segment bounded between adjacent Z lines (Z discs).

A Band

The dark, dense band of a sarcomere containing thick filaments (encompassing the M line, H zone, and zone of overlap) that remains constant in width during contraction.

I Band

The light region of the sarcomere that contains only thin filaments and no thick filaments, which decreases in width as muscle contracts.

H Zone

The lighter area within the center of the A band containing only thick filaments and no overlapping thin filaments; it shrinks during contraction.

M Line

The structural line situated at the central portion of the thick filaments at the center of the sarcomere.

Z Lines (Z Discs)

The boundary boundaries that separate adjacent sarcomeres and draw closer together when thin filaments slide during contraction.

Zone of Overlap

The area in the A band where thin actin filaments interdigitate between thick myosin filaments; this region expands during muscle contraction.