Editing Anatomy and Physiology (section)

== Muscular System ==

Three muscle types:
* '''Skeletal''': striated, voluntary, attached to bone by tendons.
* '''Cardiac''': striated, involuntary, connected by intercalated discs with gap junctions that allow the heart to contract as a functional unit.
* '''Smooth''': nonstriated, involuntary, found in vessel walls, the gastrointestinal tract, the bladder, the uterus, and the airways.

Skeletal muscle contracts by the '''sliding filament mechanism'''. A nerve impulse releases acetylcholine at the neuromuscular junction, the muscle fiber depolarizes, calcium is released from the sarcoplasmic reticulum, calcium binds troponin and exposes binding sites on actin, and myosin heads pull the actin filaments inward using ATP. Relaxation requires ATP as well, which is why rigor mortis occurs when ATP production stops.

'''Nursing application:'''
* Calcium is required for contraction and potassium is required for repolarization, so electrolyte disturbances present as muscular and cardiac symptoms. Hypocalcemia causes tetany, Chvostek and Trousseau signs, and laryngospasm. Hyperkalemia causes weakness and life-threatening dysrhythmias.
* Smooth muscle relaxation explains many drug effects: bronchodilators relax airway smooth muscle, and vasodilators relax vessel walls and can cause orthostatic hypotension.
* Isometric exercises maintain muscle mass in patients who cannot bear weight.
* Disuse atrophy and contractures begin within days of immobility, which is the rationale for range-of-motion exercises and correct positioning.