NERVE

The fundamental property of nerve that distinguishes it from other cells in the body is its ability to produce and conduct the regenerative electrical signals known as action potentials. The generation of an action potential takes place at the neuronal surface membrane; once initiated, the signal may be conducted over relatively long distances.¹ This extraordinary feat is accomplished because neurons have excitable membranes that are capable of holding a charge.

The term motor unit was originally introduced by Sherrington² as a description to include an individual nerve fiber with the bunch of muscle fibers it activates. A contemporary view would, of course, include the entire motor neuron, dendrites, and cell body, as well as its axon (Fig. 81-1). There are three types of motor neurons and two basic categories of striated muscle fibers. α motor neurons are large cells with fast-conducting axons, which innervate the large muscle fibers that make up the bulk of a muscle. These muscle fibers are called extrafusal, to differentiate them from the much smaller specialized intrafusal muscle fibers, which are present only within muscle spindle stretch receptors. Intrafusal muscle fibers are innervated by their own specialized motor neurons and axons, which are referred to as fusimotor or γ motor neurons. There is a third group of motor neurons, referred to as skeletofusimotor or β motor neurons, which innervate both extrafusal and intrafusal motor fibers. α motor neurons are among the largest neurons in the mammalian central nervous system and have remarkably extensive dendritic trees. These cells exhibit a twofold range in average soma diameters and up to a fivefold range in cell body volume and total surface area. The size of α motor neurons is correlated with the diameter of their axons and with their physiological properties. The largest of the α motor neurons are classified as type II, which in turn innervate fast-twitch glycolytic muscle fibers. The smaller of the α neurons, type I, innervate slow-twitch oxidative muscle fibers (Table 81-1). The motor neurons that innervate different muscles are grouped into longitudinal columns that lie with the ventrolateral gray matter of the spinal cord. The position of the nuclear column for a particular muscle is predictable, and the number of motor neurons within these columns is approximately the same from one individual to another. α and γ motor neurons innervating a given muscle are mixed more or less randomly within its motor nucleus. On average, between 25% and 40% of motor neurons in a given motor nucleus are γ. Large muscles generally tend to possess more motor units than do small muscles, but the range of variation is less than might be expected on the basis of relative muscle size. For example, the nerve to the bulky medial gastrocnemius muscle of a human contains about 580 motor axons, whereas the much smaller first dorsal interosseous nerve has 119 axons. The nerve to the tiny lateral rectus muscle contains more than 1700 motor axons. Thus, function as well as muscle volume determine the number of motor neurons present in motor nuclei.

Figure 81-1 A diagram of a motor unit and neuromuscular junction.

TABLE 81-1 Motor Neurons

In 1874, Ranvier³ recognized a correlation between muscle color and contraction speed. He found that red muscle had slow-twitch properties and pale muscle had fast-twitch properties. These designations derived from the overall characteristics of whole muscles, but within individual muscles, motor unit properties are actually quite heterogeneous. Burke⁴ characterized three types of motor unit twitch properties: fast-fatiguing (“FF”), fast resistance (“FR”), and slow-twitch fatigue-resistant (“S”) motor units. As noted previously, slow “S”-type muscle units tend to be innervated by relatively slow-conducting motor axons, and motor neurons tend to present greater electrical resistance to currents passed into them through a micropipet electrode. They also have relatively long action potentials after hyperpolarization, which tends to limit their firing rates to slower frequencies. Fast-twitch motor neurons have lower input resistance and shorter-duration hyperpolarization positive action potentials after activation, which are associated with higher firing rates. In both instances, the firing frequencies of the motor unit appear to be physiologically matched to the twitch properties of the innervated muscle fibers. Fast-twitch muscle units have fast-twitch times and high tetanic fusion frequencies and generate high forces, but they fatigue in ways that do not enable these forces to be maintained for long periods. Slow-twitch motor units, in comparison, are activated by motor neurons that fire at relatively lower frequencies to produce the required tetanic fusion.

MUSCLE

Skeletal muscle is commonly referred to as striated muscle because of its appearance on both light and electron microscopy. Striated muscle is the major tissue component in the body, accounting for 40% to 50% of body weight. Striated muscle is under direct voluntary control and consists of two main categories of fibers: extrafusal and intrafusal.⁵

Intrafusal muscle fibers are further subdivided into nuclear chain and nuclear bag fibers. These are collectively referred to as the muscle spindle. Spindles are found in all skeletal muscles except facial muscles.⁶ Spindles are sensory receptors that signal information concerning the degree of stretch applied to a muscle and the velocity of the applied stretch. Extrafusal muscle fibers are the major component of skeletal muscle and are the fibers responsible for the generation of force in movement. The extrafusal fibers are the fibers that are attached to tendons and bone. This section deals primarily with extrafusal muscle fibers.