The neuromuscular junction. The point at which the motor nerve fiber invaginates the muscle fiber is called the endlplate. Transmission of the neuronal impulse across the synaptic cleft is made possible through the secretion of a acetylcholine. As the stimulus reaches the muscle fiber, cholinesterase also is secreted, which deactivates the acetylcholine by chemically breaking it down. This prevents further excitation of the muscle fiber following stimulation for that immediate time period.
The manner in which a stimulus is transmitted from the nerve to the muscle fiber is very similar to the way in which an impulse is transmitted from nerve to nerve through the neuronal synapse. Apparently, the major difference is that there is no inhibition mechanism at the neuromuscular junction. A muscle fiber recieves only one nerve fiber. However, the large fibers of an efferent (motor) nerve divide into numerous smaller fibers servicing as many as 200 muscle fibers. An individual nerve fiber plus all the muscle fibers it innervates is called a motor unit. As an impulse arrives at the neuromuscular junction, acetylcholine is released; the impulse is then able to cross the synapse, creating a potential in the muscle fiber. Such a potential, is called an excitatory postsynaptic potential (EPSP). The motorneuron activating the muscle fiber may receive impulses from several nerve fibers. If the postsynaptic potential is too small the muscle fiber will not contract; but when the EPSP rises to a certain level, discharge takes place and the muscle fiber contracts. All or nothing!
This progressive increase in the size of the EPSP as a result of a number of impulses is called spatial summation. In addition, successive discharges from the same postsynaptic terminal will summate eliciting an increased EPSP, provided that hte discharges occur in rapid succesion (within 15 milliseconds of each other). As before, this mechanism is called temporal summation.
To illustrate, suppose impulse A is initiated in the brain and impulse B comes from a pain receptor in the skin. For example, you decide to get a tan from a sun lamp and in a few minutes impulses from A+B inform you that the heat is too great; you turn off the lamp or perhaps move farther from it. However, you might fall asleep while under the lamp, in which case impulse A, which originated in the brain, could not be activated. Impulse B is not sufficiently strong by itself to arouse muscular activity (i.e. to create an adequate EPSP to fire the neuron). Consequently, you remain asleep while the sun lamp continues slowly to bake your tissue. When you do awaken, in all probability you will have received serious burns.
By inhibiting activation of brain cells, sleep, anesthetics, and too much alcohol prevent adequate stimulation of recipent neurons.
Before leaving this area let us consider the impulse firing rates of single motor units within various muscles of the human body. These rates are highly variable with an average of 30 +/- 10 impulses per second (HZ known as Hertz) for the biceps brachii and an average of 10+/-3 HZ for the soleus muscle. Remember that the biceps muscle of the arm commonly contains 50 percent or more of FT fibers and that the soleus is primarily comprised of ST fibers. The impulse frequencies also match up well with the general type of muscle being stimulated (i.e. a high firing rate form the fast muscle and a low rate for the slow muscle). The discharge frequencies for individual motor units of the tibialis anterior muscle of the lower leg are approximately 10Hz.
Other studies of the thumb adductor pollicis muscle indicate that maximal electrical stimulation through the ulnar nerve at high rates of 50 to 80 Hz produced rapid fatigue, wheras a stimulation rate of 20Hz produced fatigue curves similar to those for maximal voluntary contractions. The maximum discharge frequency of this latter muscle also has been shown to relate closely to the percentage of maximum voluntary contraction that is being performed. The average discharge frequency ranges from 35 to 55 Hz.
In summary, motor units receive impulse volleys at a rate that relates to the makeup of their constituent fiber types with an overall range between approximately 10 and 60 Hz. It is not surprising that these reports would indicate a higher rate of firing when more force is being applied. We have discussed that this is one important way to gradate the strength of muscle contraction.
Daryl Conant, M.Ed