University:
Glendale Community College
Course:
BIO160 | Introduction to Human Anatomy and Physiology
Academic year:

2019
Views:

412

Pages:

13
Author:

aroutteanittev7

Muscle Contraction and Relaxation • Four actions are involved in Muscle Contraction and Muscle Relaxation: – Excitation: action potentials in the nerve lead to formation of action potentials in a muscle fiber – Excitation-Contraction Coupling: action potentials on the sarcolemma activate myofilaments – Contraction: shortening of a muscle fiber or at least the formation of tension – Relaxation: return of a muscle fiber to its resting length Excitation • Nerve signal stimulates voltage-gated calcium channels at the synaptic knob resulting in exocytosis of synaptic vesicles containing Ach. • ACh is released into the synaptic cleft. Excitation (continued) • ACh released from the motor neuron binds to ACh receptors in the sarcolemma. • Binding of ACh to the receptor causes it to open a channel for Na+ and K+. A lot of Na+ rushes in and a little K+ rushes out resulting in a membrane voltage change called the EndPlate Potential. The voltage changes from negative to positive. Excitation (continued) • The End-Plate Potential (voltage change in the motor end-plate membrane) opens nearby voltage-gated Na+ and K+ channels in sarcolemma producing a depolarization that spreads across the sarcolemma. The spreading depolarization is the Muscle Action Potential. Excitation-Contraction Coupling • Action potential (membrane depolarization) spreads over the sarcolemma and down into the T tubules. • T tubule membrane depolarization triggers the opening of voltage-gated calcium channels in the sarcoplasmic reticulum allowing release of Ca++ from the sarcoplasmic reticulum into the sarcoplasm. Excitation-Contraction Coupling (cont.) • Calcium ions (Ca++) released by the SR binds to troponin. • Troponin-tropomyosin complex changes shape and exposes active sites on actin. Contraction • Myosin ATPase in myosin head hydrolyzes an ATP molecule. • Energy from the ATP “cocks” the myosin head in a high energy extended position. • Myosin head binds to an exposed active site on actin forming a cross-bridge. Contraction (continued) 12. Power Stroke; sliding of thin filament over thick filament • Power Stroke: myosin head releases the ADP and Pi as it flexes pulling the thin filament along. • If ATP is available, ATP binds to the myosin head and it releases the thin filament. • It will attach to a new active site further down the thin filament if Ca++ is still available and bound to troponin. – to prevent slippage, at any given moment, half of the heads are bound to a thin filament, while the other half of the heads are re-setting. Relaxation • Nerve stimulation ceases and acetylcholinesterase breaks down ACh. • ACh no longer bound to receptors so sarcolemma repolarizes. Acetylcholinesterase degrades ACh into acetate and choline which are reabsorbed and recycled by the presynaptic cell. Enzyme breaks down the ACh within 20 miliseconds. Relaxation (continued) • Active transport (using ATP) by integral membrane protein pumps in the sarcoplasmic reticulum (SR) removes Ca++ from the sarcoplasm and stores it in the SR where it is bound to the protein calsequestrin. • ATP is needed for muscle relaxation as well as muscle contraction. Relaxation (continued) • Loss of Ca++ from the sarcoplasm results in troponintropomyosin complex moving over the actin active sites which stops formation of cross bridges and prevents muscle tension. • Muscle fiber returns to its resting length due to elastic rebound of titin and the series-elastic components or contraction of antagonistic muscles. Rigor Mortis • Stiffening of the body beginning 3 to 4 hours after death peaks at 12 hours after death then diminishes over next 48 to 60 hours. • Deteriorating sarcoplasmic reticulum releases calcium. • Calcium activates actin-myosin cross bridging. • ATP is no longer produced after death and without any new ATP, the myosin head remains bound to the actin (does not release). • Actin and Myosin fibers remain bound until myofilaments decay.