Spinal Effector Mechanisms

Motor Neurons. Except for muscles innervated by the cranial nerves, each somatic muscle receives its motor supply from a column of motor neurons arranged longitudinally in the anterior horn of the spinal cord. Motor neurons fall into three classes. The large alpha motor neurons supply the extrafusal fibers of the muscle, and each motor neuron may innervate several to more than one thousand fibers distributed throughout the muscle. A single motor neuron and all of the muscle fibers that it innervates are called a motor unit. The small gamma motor neurons (fusimotor neurons) innervate the intrafusal muscles of the spindles, thus regulating proprioceptive feedback of information about muscle length. The intermediate-sized beta motor neurons project to both extrafusal and intrafusal muscle fibers; their activity causes contraction and also adjusts length feedback to compensate for that contraction. The beta motor neurons are divided into dynamic and static, depending on the type of intrafusal muscle fibers that they innervate and their physiologic effects.

Motor nuclei may extend longitudinally over several segments of the spinal cord. Despite this, the nuclei supplying different muscles tend to be arranged in an orderly, somatotopic pattern (see Plate 2-13). In the upper cervical and the thoracic segments, which innervate only axial muscles, the anteromedial group is the only group of somatomotor neurons present; in the cervical and lumbar enlargements of the spinal cord (lower part of illustration), additional motor columns supplying limb muscles appear more laterally in the anterior horn. Moving from the rostral to the caudal end of the two enlargements, the motor nuclei supplying the proximal limb muscles appear first, lying adjacent to the anteromedial column. They are followed by the nuclei supplying the more distal muscles, which tend to lie more posteriorly and laterally. Nuclei supplying the extensor muscles also tend to lie anteriorly and laterally to those supplying the flexor muscles. Even the small movement of an extremity involves activity in the medial and lateral cell columns extending over several spinal cord segments.

Proprioceptive and Exteroceptive Fibers. Motor neurons can be influenced by both the proprioceptive and the exteroceptive fibers that enter the spinal cord. Upon entering the spinal cord (upper part of illustration), these fibers may give off ascending and descending branches that send terminal branches into the posterior or anterior horn over a distance of several segments, which approximately matches the extent of the corresponding motor nucleus. Further afferent fiber branches may continue rostrally to various sensory relay nuclei (see Plate 2-7).

The most numerous proprioceptive fibers are those carrying information from muscle spindles (groups Ia and II fibers), and from Golgi tendon organs (group Ib fibers). The Ia fibers are unique in that they enter the anterior horn motor nuclei and establish direct connections with motor neurons. These connections form the basis of the muscle stretch reflex (see Plate 2-15). One Ia afferent fiber from a spindle in a given muscle produces direct excitation in virtually every motor neuron supplying that muscle and in a smaller proportion of motor neurons supplying closely related synergistic muscles. This selectivity may be explained by the fact that the terminal field of the la fiber is approximately coextensive with the motor nucleus of its muscle and overlaps slightly with synergist motor nuclei located nearby.

The principal reflex elicited by exteroceptive fibers is the flexor withdrawal reflex (see Plate 2-15). The distribution of motor effects in this reflex is much broader than that of the stretch reflex, comprising most of the flexor muscles of the limb, as well as crossed activation of the contralateral extensor muscles. This distribution does not derive from the projection pattern of the afferent fibers, however, but rather from the divergent projection of chains of interneurons in the posterior and anterior horns that subserve the withdrawal reflex.

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