A motor pool consists of all individual motor neurons that innervate a single muscle. Each individual muscle fiber is innervated by only one motor neuron, but one motor neuron may innervate several muscle fibers. This distinction is physiologically significant because the size of a given motor pool determines the activity of the muscle it innervates: for example, muscles responsible for finer movements are innervated by motor pools consisting of higher numbers of individual motor neurons. Motor pools are also distinguished by the different classes of motor neurons that they contain. The size, composition, and anatomical location of each motor pool is tightly controlled by complex developmental pathways.
Anatomy
Distinct skeletal muscles are controlled by groups of individual motor units. Such motor units are made up of a single motor neuron and the muscle fibers that it innervates. The cell bodies of motor neurons are located in the ventral horn of the spinal cord and the brainstem. These neurons innervate skeletal muscle fibers through the propagation of action potentials down their axons (through ventral roots and cranial nerves), and they stimulate skeletal muscle fibers at neuromuscular junctions where they synapse with the motor end plates of muscle fibers. In humans, these axons can be as long as one meter. Motor neurons themselves fall into three main classes: alpha-motor neurons control extrafusal muscle fibers, meaning that they innervate skeletal muscles leading to movement; gamma-motor neurons innervate intrafusal muscle fibers, controlling the sensitivity of muscle spindles to stretch; beta-motor neurons are capable of synapsing on either type of muscle fiber. Alpha-motor neurons can further be divided into three separate subclasses, distinguished according to the contractile properties of the motor units that they form: fast-twitch fatigable (FF), fast-twitch fatigue-resistant (FR), and slow-twitch fatigue-resistant (S). The composition of a motor pool may consist of multiple classes and subclasses of motor neurons. Motor pools in the spinal cord are clustered in distinct columns of motor neurons extending over multiple spinal cord segments; although, there is significant overlap. Motor pools that control proximal muscles are generally located medial to the ventral horn, while those that control distal muscles are located laterally. Motor pools that control flexor muscles are located dorsally to the ventral horn while those that control extensor muscles are located ventrally. The number of motor neurons in an individual motor pool is highly variable and can generally be predicted by the level of nuanced control that a specific muscle requires. For example, some muscles have relatively low numbers of motor units in their respective motor pools while others, with highly nuanced control (such as the muscles in the human hand) have higher densities of motor units.
Function Motor pools function primarily to integrate synaptic input from higher CNS centers into precise and consistent contraction patterns. Individual motor neurons within a given motor pool fire in accordance with what is known as the 'size principle'. The size principle was proposed by Elwood Henneman and his group in the 1960s as an explanation of the characteristic pattern with which individual motor neurons in a motor pool fire. The size principle stipulates that when the motor neurons of a motor pool fire, leading to the contraction of a terminal muscle fiber, the motor units containing the smallest motor neurons fire first. As excitatory signalling increases, larger motor neurons are subsequently recruited and contraction strength increases. Further, this differential recruitment of motor neurons occurs in instances of both increasing and decreasing contraction strength. As contraction strength is increased, the smallest motor units fire first and are also the last to stop firing as the contraction strength decreases. The size principle has important functional benefits. Primarily, this system frees higher centers of the CNS from having to signal specific contraction patterns for distinct levels of muscle contraction. The level of synaptic input that higher centers provide to a given motor pool must determine the contraction strength, and this simplifies the process of contraction strength modulation. This system allows for very precise and consistent modulation of contraction strength from just increased or decreased levels of synaptic input: with additional motor units of increasing size, there will be a consistent and precise effect on the force of contraction. Another key benefit derived from the size principle is that smaller neurons will be fired more regularly and for a longer duration of time compared to larger neurons. These smaller motor units are more resistant to fatigue, and as such, are better suited to this role.
Specialization and development There are several layers of differentiation and specialization to consider the complicated development of motor pools.
Alpha- and gamma-motor neuron differentiation Alpha motor neurons and gamma motor neurons do not merely differ in their postsynaptic targets. The physiological differences between these two classes are significant. The axonal diameter of gamma-motor neurons is half of that of alpha-motor neurons, resulting in a higher cytoplasmic resistance and therefore a slower signal propagation velocity. Additionally, gamma-motor neurons display far simpler branching patterns than that of their alpha- counterparts. The differentiation into these two classes is regulated by complex interactions between several neurotrophic factors, and all of these interactions are not yet well understood. Glial cell line-derived neurotrophic factor (GDNF) has been discovered to play an especially important role in all layers of motor pool development. In the case of alpha- and gamma- differentiation, it has been shown that gamma motor neurons express significantly higher levels of certain GDNF receptor subunits.
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