Skeletal muscle 3

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Biological Sciences
Stephen Reid

Lecture 10 Notes: Skeletal Muscle Continued Fatigue If a muscle is stimulated repetitively enough or at a fast enough frequency, it will ultimately fatigue and no longer be able to generate tension and contract. Muscle fatigue can occur as a result of processes in the muscle or processes at the neuromuscular junction (synapse). Within the muscle, fatigue can occur because of: 1) Glycogen depletion (glycogen being the primary source for ATP production). 2) A reduction in intracellular pH (accompanied by lactic acid buildup). The reduction in pH reduces the efficiency of enzymes and other proteins which have a pH optima at which they function best. 3) A calcium imbalance. At the neuromuscular junction, fatigue can occur because of the depletion of the neurotransmitter, acetylcholine. Measuring Synaptic Fatigue in the Laboratory The objective of the synaptic fatigue laboratory exercises is to observe and distinguish muscle fatigue due to fatigue within the muscle or fatigue within the neuromuscular junction (NMJ). You first stimulate the right sciatic nerve and watch the right gastrocnemius muscle contract. You continue to stimulate until the left muscle contracts. The contraction results from stimulus- induced nerve impulses traveling up the right sciatic nerve, through the spinal cord and into the left sciatic nerve. You continue stimulating the right nerve until the left muscle stops contracting. You then move the stimulating electrode to the left nerve. If stimulation of the left nerve leads to muscle contraction then you would conclude that the previous cessation of left muscle contraction (due to right nerve stimulation) was due to neuromuscular fatigue in the neurons connecting the right nerve, through the spinal cord, to the left nerve. If stimulation of the left nerve does not lead to left muscle contraction then there is fatigue either in the left muscle or in the left neuromuscular junction. To distinguish between the two you move the stimulating electrode onto the left muscle. If it contracts then any previous failure to contract (with left nerve stimulation) would be due to NMJ fatigue. If it doesnt contract then any previous failure to contract would have been due to fatigue within the muscle itself. Synaptic Delay Once an action potential (AP) arrives at the NMJ, there is a delay between the arrival of the AP and muscle contraction. This delay is due to the time taken for neurotransmitters to be released, post-synaptic nicotinic receptors on the muscle to be activated, the muscle membrane to
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