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Unformatted text preview: ter of A band during contraction. • • • • As thin filaments slide inward, they pull Z lines closer together. Sarcomere shortens All sarcomeres throughout muscle fiber’s length shorten simultaneously Contraction is accomplished by thin filaments from opposite sides of each sarcomere sliding closer together b/t thick filaments. • • Transverse tubules (T-tubules) Run perpendicularly form the surface of muscle cell membrane into central portions of the muscle fibers. • Since membrane is continuous w/ surface membrane- AP on surface membrane also spreads down to T-tubule • Spread of AP down a T-tubule triggers released of Ca from sarcoplasmic reticulum into cytosol. • • • Sacroplasmic reticulum Modified ER Consists of fine network of interconnected compartments that surround each myofibril. • • • • • • • • • • • • • • Not continuous but encircles myofibril throughout its length. Segments are wrapped around each A band and each I band. Ends of segments expand to form saclike regions- lateral sacs (terminal cisternae). Relaxation Reuptake of Ca into sarcoplasmic reticulum (SR) Active transport with Ca pump Acetylcholinesterase breaks down Ach at neuromuscular junction. Muscle fiber AP stops. When local AP is no longer present, Ca moves back into SP. Less than optimal length Bunching thick filaments Thin filaments overlap Ca influx Too stretched • • • • • • • • • Less than optimal overlap. Muscle contractions Contractions of whole muscle can be of varying strength. Twitch Brief, weak contraction Produced from single AP Too short and too weak to be useful Normally does not take place in body Two primary factors which can be adjusted to accomplish gradation of wholemuscle tension • • • • • Number of muscle fibers contracting within a muscle Tension developed by each contracting fiber. Motor unit- one motor neuron and the muscle fiber it innervates. Number of muscle fibers varies among different motor units. Number of muscle fibers per motor unit and number of motor units per muscle vary widely. • Muscles that produce precise, delicate movements contain fewer fibers per motor unit. • Muscles performing powerful, coarsely controlled movement have larger number of fibers per motor unit. • • A synchronous recruitment of motor units helps delay or prevent fatigue. Factors influencing extent to which tension can be developed. • • • • • • Frequency of stimulation Length of fiber onset of contraction. Extent of fatigue Thickness of fiber. Twitch summation- results from sustained elevation of cystosolic Ca Tetanus- occurs if muscle fiber is stimulated so rapidly that it does no have a chance to relax b/t stimuli. • • Contraction is usually three to four times stronger than a single twitch....
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This note was uploaded on 09/23/2010 for the course NPB 8746546 taught by Professor Goldberg during the Spring '10 term at UC Davis.
- Spring '10