A motor unit consists of one somatic efferent (motor) neuron and all of the muscle fibers (cells) that it innervates. In the figure to the right, two motor units are illustrated diagrammatically. Notice that after an efferent axon enters the muscle, it branches and forms synapses with a number of muscle fibers. However, there is no overlap in the innervation of the muscle fibers by different efferent neurons. Each individual muscle fiber is connected to only one efferent neuron.
The figure, however, is quite diagrammatic. There are many more efferent neurons innervating a muscle and many more muscle fibers in one motor unit. For example, a large muscle used for powerful movements, such as the gastrocnemius in the calf, is controlled by hundreds of efferent neurons. Moreover, each efferent neuron innervates hundreds, or even thousands, of individual muscle fibers.
The synapse between the efferent neuron and one muscle fiber is very unusual. It is huge, and always releases enough acetylcholine to depolarize the muscle fiber above threshold. This is totally unlike synapses in the central nervous system.
Because the synapse is so large, here is the overall rule that governs the functioning of a single motor unit: one action potential in the efferent neuron leads to one action potential in all of the muscle fibers in the motor unit. This in turn triggers one brief, all-or-nothing contraction of all of the muscle fibers in the motor unit. Such a contraction is called a twitch.
Thus, each motor unit is either relaxed or each presynaptic action potential causes an action potential in all of the muscle fibers of the motor unit, causing them all to contract at once.
In the whole muscle, contractions of different strengths necessarily are created by activating different numbers of motor units.
Motor units are not all identical, but fall into three groups. In each group, both the efferent neuron and its muscle fibers have distinctive properties. There are three types: slow oxidative, fast oxidative and fast glycolytic. In the chart below, concentrate primarily on the slow oxidative and the fast glycolytic. Think of the fast oxidative as intermediate between the other two.
In the chart, refer to the first line in purple, which indicates the size of the efferent neurons, which can be small, intermediate or large. Developmentally, the type of the efferent neuron determines the characteristics of the muscle fibers that it innervates. For example, if an efferent neuron degenerates in a disorder, a neighboring efferent axon can sprout branches and innervate the muscle fibers that lost their original efferent neuron. When this happens, the muscle fibers adopt the characteristics associated with the new efferent neuron.
The muscle fibers innervated by small efferent neurons are also small.
The small motor neurons are the easiest to excite above threshold. Thus, with the smallest type of contraction of the muscle, the smallest motor neurons and the smallest muscle fibers begin contracting. With progressively stronger contractions, the intermediate and then the large efferent neurons successively add their contributions to the contraction.
Thus, contractions of increasing strength occur through the progressive recruitment of larger and larger motor units.
The next line, in blue, indicates the primary specialization of the corresponding muscle fibers. This is the rate at which the myosin uses ATP. One type of myosin uses ATP relatively slowly. Thus, these slow muscle fibers contract relatively slowly. The second type of myosin uses ATP rapidly. Thus, these fast muscle fibers contract rapidly.
The remaining characteristics support two modes of contraction of the slow and fast muscle fibers.
The next three lines in red show specializations that support the slow muscle fibers. Since these use ATP relatively slowly, the circulatory system usually can deliver enough oxygen to allow oxidative phosphorylation to generate the required ATP. Thus, these muscle fibers are surrounded by many capillaries to provide oxygen, have many mitochondria to use the oxygen, and have a high myoglobin content. The latter binds oxygen and speeds it diffusion into the muscle fibers. Finally, note that the muscle fibers are small, which also allows for rapid diffusion of oxygen to the interior of the muscle fibers.
The fast glycolytic muscle fibers do not have many mitochondria and rely on glycolysis to generate most of the ATP. Since glycolysis does not depend on oxygen, the fast muscle fibers are not limited by the rate at which the circulatory system delivers oxygen to the muscle. But they rapidly run through their glycogen and release lactic acid into the blood. The lactic acid is either converted to glucose in the liver or used by the slow muscle fibers.
|innervating neuron size||small||large|
|muscle fiber diameter||small||large|
|myosin ATPase activity||low||high|
Thus, while the slow muscle fibers do not produce as much force as the fast, they are equipped to generate ATP for a long time and fatigue slowly. By contrast, the fast glycolytic muscle fibers generate much more force, but fatigue comparatively rapidly.
Most muscles are a mixture of fast and slow motor units. The quadriceps, for example, has approximately an equal proportion on average. But this can vary greatly, depending on a person's genetics. Postural muscles along the back, by contrast, are largely slow motor units. At the other extreme, are the extraocular muscles that aim the eye. These are completely fast motor units.