Motor Units, Recruitment and Summation
In normal skeletal muscle, fibres never contract as isolated individuals.
Several contract at almost the same time, as they are all supplied by the same α-motor neurone. The single motor neurone and all the fibres it innervates is called the motor unit (Fig. 14a). This is the smallest part of a muscle that can be made to contract independently of other parts of the muscle. The number of muscle fibres innervated by one motor unit can be as low as 5 or as high as 2000. The number is correlated with the precision with which the tension developed by the muscle is graded.
Within the muscle, the muscle fibres of each motor unit are widely distributed amongst the fibres of many other units. This, in effect, distributes the demands made on the muscle’s circulatory support. The ratio between the number of α-motor neurones and the total number of skeletal muscle fibres is small in muscles such as the extraocular muscles that provide fine smooth movements (1:5), but large in muscles such as the gluteus maximus that need to generate powerful but coarse movements (1:>1000).
Fibres have been classified into three types, on the basis of various structural and functional properties of motor units and their integral muscles. Table 14 outlines the relationship between the properties of the motor units (with their defining characteristics of conduction speed, resistance to fatigue and also size of activity pattern) and the properties of the muscle fibres they contain (the type of myosin which determines the speed of contraction, and the type of metabolism, which is highly correlated with the resistance to fatigue), and also the names given to these types in human muscles. Most muscles contain all three types (I, IIA, IIB), but differ in the proportions of each according to the function of the muscle as a whole. Posture muscles, such as the soleus, have mostly slow, fatigue-resistant, oxidative-type units, whereas movement muscles, such as the gastrocnemius, have a high proportion of the other two types. Training and exercising can alter these proportions.
The cell bodies of α-motor neurones also vary in size according to the type of motor unit: motor neurones innervating type I fibres have the smallest cell bodies, and those innervating type IIB fibres have the largest.
During graded contraction, there is a recruitment order of the units, such that the smallest cells discharge first and the largest last (the socalled size principle). Force is controlled not only by varying the unit recruitment, but also by varying the firing rate of the units. A single action potential in a single motor unit produces a delayed rise in tension in all the muscle fibres that make up that motor unit. A second and third action potential that occur soon after the first produce a summed con- traction, or a series of twitches. The tension developed by the first action potential has not completely decayed when the second contraction is grafted on to the first, and so on for the third action potential and contraction. This is called summation (Fig. 14b). If the muscle fibres are stimulated repeatedly at a faster frequency, a sustained contraction results in which individual twitches cannot be detected. This is called tetanus. The tension of tetanus is much greater than the maximum tension of a single, double or triple twitch (Fig. 14b). For most units, the firing rate for a steady contraction is between 5 and 8 Hz. It can rise to 40 Hz or more, but only for very brief periods. During a gradual increase in contraction of a muscle, the first units start to discharge and increase their firing rate and, as the force needs to increase, new units are recruited and, in turn, also increase their firing rate. When there is a need to gradually decrease the force output, the pattern is reversed, so that those units that were recruited last will be the first to decrease their firing and then stop, and the last units to fire will be the smallest units. Because the unitary firing rates for each motor unit are different and not synchronized, the overall effect is a smooth force profile from the muscle. The greater the desynchronized firing, the smoother the movements observed. When synchronized firing does occur, such as in fatigued states and Parkinson’s disease, marked muscle tremors are seen.
The summed excitatory impulses (action potentials) of the motor units can be recorded in an electromyogram (EMG). The EMG is an extracellular recording made from either the skin surface overlying a muscle or from electrodes inserted extracellularly within the body of the muscle. The increase in recruitment of individual motor units (motor unit recruitment), as well as the increased rate of firing of the units (rate or frequency recruitment), can sometimes be seen in the EMG during increased force of contraction (Fig. 14c).