Question 173

Question:

The nervous system increases the force of muscle contraction in a graded fashion primarily by recruitment of active motor units and by increasing which of the following?

Correct Answer: C.  Frequency of firing of individual motor neurons

There is a one-one relation between the motor neuron and muscle cells (ie one motor neuron for each muscle cell). There is only one active end-plate from each motor neuron per muscle cell; therefore, option D is incorrect.
In addition, the action potential in the neuron will be transferred to create a distinct action potential in each muscle innervated by the neuron. The action potential is constant in its duration and in its amplitude; therefore options A and B are incorrect because these parameters cannot be changed.
Although the rate of synthesis of acetylcholine would increase the amount released by the axon terminal, the normal amount of acetylcholine released into the neuromuscular junction is enough to excite the muscle cell beyond the action threshold potential. Therefore, increasing the rate of synthesis (option E) would not effect the force of contraction.
Increasing the frequency of firing of individual motor neurons (option C) will increase the force of contraction because with each action potential firing in the muscle cell, more calcium enters the cell. Calcium drives the force of contraction and therefore with more calcium in the cell, the stronger the force of contraction will be.