Muscle Physiology: Motor Units, Contraction Dynamics, and Tissue Types

Comprehensive vocabulary flashcards covering skeletal, cardiac, and smooth muscle physiology, contraction mechanics, fiber classifications, and related pathologies.

Motor unit

A functional unit consisting of one somatic motor neuron and all the skeletal muscle fibers (myofibers) it innervates.

All-or-none principle

The physiological rule stating that if an action potential is transmitted down a motor neuron, every muscle fiber in that motor unit will contract completely; if the stimulus is insufficient, none will contract.

Twitch contraction

A brief contraction of all muscle fibers in a motor unit in response to a single action potential, lasting approximately 50 milliseconds.

Latent period

The brief delay of about five milliseconds between acetylcholine release and the onset of muscle shortening, during which electrical charges reverse and calcium is released from the sarcoplasmic reticulum.

Contraction period

The phase of a muscle twitch during which myosin heads bind actin, swivel, consume ATP, and shorten the sarcomere.

Relaxation period

The phase of a muscle twitch where acetylcholine and calcium are removed, calcium is actively transported back into the sarcoplasmic reticulum, and filaments slide apart.

Wave summation

The addition of contractile forces that occurs when a second action potential stimulates a muscle fiber before the preceding contraction has fully relaxed.

Unfused (incomplete) tetanus

A sustained, wavering muscle contraction caused by repeated stimulation at roughly 25 times per second, allowing partial relaxation between stimuli.

Fused (complete) tetanus

A smooth, sustained contraction with no relaxation between stimuli, achieved at high stimulation frequencies of roughly 90 times per second.

Motor unit recruitment

The activation of additional motor units within a muscle organ to increase the total force and strength of contraction.

Tonic contractions

Involuntary, low-level twitches that do not produce obvious movement but maintain baseline muscle tone and stimulate ongoing protein synthesis.

Tetanic contractions

Voluntary muscle contractions that produce noticeable movement, categorized into isotonic and isometric forms.

Isotonic contraction

A contraction in which the muscle changes length while moving a constant tension or workload throughout the entire movement.

Isometric contraction

A contraction in which muscle tension increases to stabilize a joint or posture, but the overall length from tendon to tendon remains unchanged and no movement occurs.

Slow oxidative (SO) fibers

Red skeletal muscle fibers containing high amounts of myoglobin, capillaries, and mitochondria, which produce ATP via aerobic respiration and resist fatigue.

Fast glycolytic (FG) fibers

White skeletal muscle fibers with low myoglobin and mitochondrial content but rich glycogen stores, adapted to split ATP quickly and perform short, powerful bursts via glycolysis before fatiguing.

Fast oxidative-glycolytic (FOG) fibers

Intermediate skeletal muscle fibers that exhibit intermediate structural and functional characteristics between slow oxidative and fast glycolytic fibers.

Myoglobin

A respiratory pigment protein in muscle cells that binds and stores oxygen for cellular respiration, giving endurance fibers a dark red color.

Intercalated discs

Thickenings of the sarcolemma where cardiac muscle cells join end to end, housing both desmosomes and gap junctions.

Desmosomes

Mechanical spot-weld junctions in intercalated discs that keep cardiac muscle cells firmly attached during contraction and expansion.

Gap junctions

Protein tunnels in intercalated discs that allow ionic current and action potentials to spread directly and rapidly from one cardiac cell to the next.

Autorhythmic cells

Specialized cells that generate their own rhythmic impulses intrinsically without requiring nervous system stimulation.

Sinoatrial (SA) node

A cluster of autorhythmic cells on the posterior wall of the right atrium that sets the pace of the heart, firing intrinsically at about 100 beats per minute but slowed to about 75 beats per minute at rest by parasympathetic input.

Single-unit (visceral) smooth muscle

Smooth muscle arrangement where autonomic nerves innervate select cells and the contraction spreads slowly from cell to cell, typical of the gastrointestinal and urinary tracts.

Multiunit smooth muscle

Smooth muscle arrangement where individual muscle cells form their own neuromuscular junctions, allowing simultaneous, rapid contraction as seen in the pupil, bronchioles, and arrector pili.

Pericytes

Stem cells located in the walls of small blood vessels (venules) that can divide by mitosis to regenerate new smooth muscle cells.

Satellite cells

Stem cells associated with skeletal muscle that can divide to assist in repair, but cannot fully regenerate dead myofibers into functional new muscle cells.

Fibrosis

The replacement of damaged or dead muscle tissue with non-contractile collagen connective tissue fibers.

Hypertrophy

An increase in muscle mass and strength resulting from the enlargement of existing cells through the synthesis of additional myofibrils and proteins.

Hyperplasia

An increase in tissue size resulting from the production of new cells through mitotic cell division.

Sarcopenia

The age-related loss of skeletal muscle mass and its progressive replacement with fibrous connective tissue and adipose.

Myasthenia gravis

An autoimmune disorder in which antibodies destroy acetylcholine receptors on the motor end plate, leading to progressive muscle weakness.