Motor units
A motor unit consists of a motor neuron and the muscle fibers it innervates. In mammals each of the muscle fiber is supplied by only one motor neuron. Though, each motor neuron synapses with anything from six to a few thousand muscle fibers in a single muscle. The size of a motor unit is associated to the precision of motor control needed of a given muscle. Finely regulated muscles (example, extraocular eye muscles) consist of small motor units; less finely regulated muscles have bigger ones. The fibers of a single unit are scattered broadly throughout a muscle therefore no section of a muscle is controlled by merely one motor unit.
A single action potential in the motor neuron cause a twitch, single contraction, in all of the muscle fibers to which it is joined as shown in figure (a). The contraction and relaxation of muscle fibers is very much longer than the muscle action potential of around 3 ms. When a volley of action potentials is fired and there is inadequate time for the muscle to relax among successive impulses the twitches summate to increase the force that oscillates about a plateau value. This is known as unfused tetanus as shown in figure (b). As the firing frequency increases the oscillations smooth out and the plateau reaches maximum force. This is known as fused tetanus which is shown in figure (c).
Figure: Muscle fiber contraction: (a) single twitch; (b) un-fused tetanus (firing frequency 12 Hz); (c) fused tetanus (30 Hz). Note that the increase in force of contraction in going from (a) to (c).
Three kinds of motor unit can be differentiated by the firing behavior of their motor neurons and the properties of their muscle fibers.
The most several are the slow twitch (S) motor units that take about 50 ms to develop peak force and show little decline in force even after an hour of repetitive stimulation. The motor neurons of S units are little, containing low conduction velocity and quite long refractory periods as they consists a high density of Ca2+ -activated K+ channels that cause a long afterhyperpolarization. This limit maximum firing frequencies to quite low rates, though fused tetanus is achieved at low frequencies (15–20 Hz). The type 1 muscle fibers of S motor units are rich in mitochondria, contain high activities of Krebs cycle enzymes that fit them for high rates of aerobic metabolism and form red muscle since of their high myoglobin content. The slow twitch motor units are capable of exerting low forces for long periods. They form the bulk of the antigravity or postural muscles of the legs and trunk.
By contrast, fast twitch units contract maximally in 5–10 ms though cannot sustain the contraction for very long. With repetitive stimuli, the fatigue resistant (FR) units can sustain moderate force for 5 minutes or hence before a steady decline sets in that takes lots of minutes. Fast fatigue (FF) motor units can achieve the greatest force of the three types, although with repetitive stimuli the force falls precipitously after about 30 seconds. The motor neurons of both FR and FF units are large, with high conduction velocities. For short periods they fire at high rates although action potential volleys are of small duration mainly for FF units. Fast twitch units have type 2 muscle fibers which form white muscle since of their low myoglobin content and need firing frequencies of 40–60 Hz to form fused tetanus. Type 2b found in FF motor units are anaerobic, that explains why these fatigue so rapidly. Type 2a, found in FR units, are intermediate among types 1 and 2b in terms of metabolism. Both FR and FF are adapted for generating rapid, large forces and therefore are found mainly in muscles included in executing rapid movements.
In motor units the properties of the motor neurons and muscle fibers are matched for optimal performance. This occurs because muscle fiber properties are determined by the motor neurons that innervate them. When type 1 muscle fibers are denervated and the axon of an FF unit sprouts to begin new connections with the denervated fibers, they obtain the characteristics of type 2b muscle fibers.