Anatomy & Physiology

Anatomy & Physiology Study Guide necessary energy available as ATP before the contraction begins. Instead, only enough ATP is available to initiate and sustain a contraction until more can be generated. Throughout the rest of the contraction, the muscle fiber will generate ATP at roughly the same rate as it is used. Relaxation A single stimulus has only a brief effect on a muscle fiber because the ACh released after a single action potential arrives at the synaptic terminal and does not remain intact for long. Whether it is bound to the sarcolemma or free in the synaptic cleft, the released ACh is rapidly broken down and inactivated by AChE. Inside the muscle fiber, the permeability changes in the SR are also very brief. Thus, a contraction will continue if additional action potentials arrive at the synaptic terminal continuously. When they do, the continual release of ACh into the synaptic cleft produces a series of action potentials in the sarcolemma that keeps Ca 2+ levels elevated in the sarcoplasm. Under these conditions, the contraction cycle will be repeated. If a single action potential arrives at the neuromuscular junction, Ca2+ concentrations in the sarcoplasmwill quickly return to normal resting levels. The contraction has then ended. Twitches A twitch is a single stimulus–contraction–relaxation sequence in a muscle fiber. Twitches vary in duration, depending on the type of muscle, its location, internal and external environmental conditions, and other factors. A single twitch can be divided into a latent period, a contraction phase, and a relaxation phase. The latent period begins at stimulation and typically lasts about two msec. During this period, the action potential sweeps across the sarcolemma, and the SR releases calcium ions. The contraction cycle has not begun; thus, the muscle fiber does not produce tension during this period. In the contraction phase , tension rises to a peak. As the tension rises, calcium ions bind to troponin, and active sites on thin filaments are exposed. The relaxation phase lasts about 25 msec. During this period, calcium levels are falling, and active sites are being covered by tropomyosin. As a result, tension falls to resting levels. Repeated stimulation before the relaxation phase ends may produce a summation of twitches (wave summation), in which one twitch is added to another. Incomplete tetanus occurs when tension peaks and falls at intermediate stimulus rates; whereas complete tetanus is demonstrated by the elimination of the relaxation phase by very rapid stimuli. Treppe If a skeletal muscle receives a second stimulus immediately after the relaxation phase has ended, the resulting second contraction will have a slightly higher maximum tension than the contraction of the first stimulus. This increase in peak tension will persist in the first 30–50 stimulations. After these initial stimulations, the amount of tension produced will remain constant. Because the tension rises in stages, like the steps in a staircase, this phenomenon is called treppe (German for staircase). Most skeletal muscles do not demonstrate treppe.

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