Physiology of a Muscle Contraction Human Anatomy and Physiology Dr. Steve W. Altstiel Naples Middle High School

Physiology of a Muscle Contraction Neuromuscular Junction (NJ) Motor Unit – one neuron + all skeletal muscle cells it stimulates.  Axon branches into axonal terminals, each forming a NJ with the sarcolemma of a different muscle cell. o Gap between the membrane of the axonal terminal and the sarcolemma is called a synaptic cleft (filled with tissue fluid).  Nerve impulses cause the release of a neurotransmitter to stimulate a muscle contraction. o Acetylcholine – stimulates muscle contractions by diffusing across the synaptic cleft and attaching to receptors on the sarcolemma.

Physiology of a Muscle Contraction The nervous system 'communicates' with muscle via neuromuscular (also called myoneural) junctions. These junctions work very much like a synapse between neurons. In Brief: 1.the impulse arrives at the end bulb, 2.chemical transmitter is released from vesicles (each of which contains 5, ,000 molecules of acetylcholine) and diffuses across the neuromuscular cleft, 3.the transmitter molecules fill receptor sites in the membrane of the muscle & increase membrane permeability to sodium, 4.sodium then diffuses in & the membrane potential becomes less negative, 5.and, if the threshold potential is reached, an action potential occurs, an impulse travels along the muscle cell membrane, and the muscle contracts. Some muscles (skeletal muscles) will not contract unless stimulated by neurons; other muscles (smooth & cardiac) will contract without nervous stimulation but their contraction can be influenced by the nervous system. Thus, the nervous and muscle systems are closely interconnected.

Physiology of a Muscle Contraction Returning Muscle Back to Resting State 1)Diffusion of K + out of the cell. 2)Activation of the Sodium-Potassium Pump

Physiology of a Muscle Contraction Characteristics of muscle: excitability - responds to stimuli (e.g., nervous impulses) contractility - able to shorten in length extensibility - stretches when pulled elasticity - tends to return to original shape & length after contraction or extension Functions of muscle: motion maintenance of posture heat production