Lecture 10 Notes: Skeletal Muscle Continued
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
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 doesn’t contract then any previous
failure to contract would have been due to fatigue within the muscle itself.
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