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).
