Summation of Contractions Tetanus

When a muscle is stimulated twice in such rapid succession that the second stimulus falls during the response to the first, the tension developed is greater than in the single twitch. In some way, as yet not clearly understood, the tension developed as a result of the second stimulus adds to the tension remaining from the first stimulus. If a continuous series of rapidly repeated stimuli is sent into a muscle there is not sufficient time for relaxation between successive contractions and the result is a steady, prolonged contraction known as tetanus. The tension developed during tetanus may be three or four times that of a simple twitch in the muscle. If the rate of stimulation is not rapid enough to produce complete tetanus, there may be partial relaxation between contractions The result is a jerky type of contraction known as incomplete tetanus.

The All-or-none Law and the Motor Unit

If a muscle is stimulated with gradually increasing strengths of current, the tension developed increases progressively up to a certain point; beyond this point there is no increase in tension with further increase in stimulus strength. With increasing strength of stimulation the tension developed by each muscle fiber remains constant, but more fibers are activated. When a muscle is stimulated each fiber contracts maximally or not at all (the "all-or-none law"). It must be remembered, however, that the amount of tension developed by a maximal contraction will vary with such factors as fatigue and training.

It was mentioned earlier that in the body muscles contract only in response to nerve impulses from motor nerve cells in the spinal cord, and further that each motor nerve cell, through terminal branching of its nerve fiber (axon), supplies a large number (100 to 150) of muscle fibers; the motor nerve cell with its nerve fiber and the group of muscle cells supplied by its branches comprise the motor unit. The group of nerve cells in the spinal cord which together give rise to the motor nerve fibers to an entire skeletal muscle is called the motor pool. If the motor pool of a given muscle consists, for example, of 300 nerve cells, the tension developed by the muscle during contraction may theoretically be increased by 300 steps, each representing the activation of an additional motor unit.

Since the tension developed by each motor unit also varies according to the frequency of stimulation, which determines whether the response will be either a series of simple twitches, an incomplete tetanus or a complete tetanus, we have an additional means of varying the tension developed by the muscle. In brief, then, the strength of contraction of a muscle in the body can be varied by two means: (1) variation in the number of motor units activated and (2) variation in the frequency of stimulation of each active motor unit.

One additional point requires consideration. If an isolated muscle is stimulated maximally (that is, with a current sufficiently strong to stimulate every fiber in the muscle) but at a low frequency, the resulting contraction will be jerky because of the partial relaxation between contractions. Since voluntary and reflex contractions in the body are normally smooth, it was thought at one time that they must be invariably tetanic in nature. It is now known, however, that complete tetanus occurs in very powerful contractions and that most contractions involve incomplete tetanus in a variable proportion of the motor units. The smoothness of the contraction is due to the fact that the different motor units are not activated simultaneously; at any instant, the muscle fibers of some units are contracting while those of other units are relaxing. This asynchronous activity of the motor units results in smooth contractions even when the force of contraction is weak. If, as happens in certain nervous disorders, the contractions of the different motor units become synchronized, a jerky type of contraction known as a tremor results.

We are now in a position to understand the mechanism of adjustment of the strength of contraction of muscles to the task required. For example, it is obvious that a more powerful contraction is required to lift a 50 pound weight than to lift a 1 pound weight. A weak voluntary (or reflex) contraction is the result of partial tetanus in a fraction of the motor units responding asynchronously. A very powerful contraction is the result of more nearly complete tetanus in a larger fraction of the motor units of a muscle.