Temporal Summation of Contractions Within a Single Motor Unit

When motor neurons are activated during naturally occurring movement, they do not typically fire just a single action potential, as has been the case in all our examples so far. Rather, action potentials tend to occur in bursts of several or as steady discharges at a relatively constant frequency. It is not uncommon for action potentials within a burst to be separated by only 10 msec or less. Under normal conditions, each of these many action potentials in a motor neuron will produce a corresponding action potential in each of the motor unit's muscle fibers. Because the tension resulting from a single action potential typically lasts for many tens or hundreds of milliseconds, there is considerable opportunity for summation of the effects of succeeding muscle action potentials in a series, as illustrated in Figure 11-6. Such temporal summation of individual

Figure 11-6 Isometric tension in response to a series of action potentials in a muscle. The dashed lines show the expected response if only the first action potential of a series occurred.

twitches is a major way in which the nervous system controls tension in skeletal muscles.

The amount of summation within a burst of muscle action potentials depends on the frequency of action potentials: the higher the frequency, the greater the resulting summed tension. However, as shown in Figure 11-6, when the frequency is sufficiently high the individual tension responses of the muscle fuse together into a plateau of tension. Further increase in frequency beyond this point does not increase tension: the muscle has reached its maximum response and cannot develop further tension. This plateau state is called tetanus. As expected from the examples shown in Figure 11-5, the frequency of stimulation required to produce tetanus varies considerably depending on whether slow or fast fibers are involved. For fast fibers, a frequency of more than 100 action potentials per second may be required, while for slow fibers a frequency of 20 per second may suffice.

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