The Spinal Cord Is the Most Caudal and Simplest Level of the Movement Control Hierarchy
The spinal cord contains the lower motor neurons that represent the final common path to the muscles of the trunk and limbs (Figure 10-1). As noted in Chapter 6, an α lower motor neuron innervates several extrafusal muscle fibers of a single muscle, forming a motor unit (see Figure 6-6).
The neuronal cell bodies of the motor units of a given muscle are clustered into a motor neuron pool located in the ventral horn of spinal cord gray matter. The motor neuron pool of a muscle has a cigar-shaped, longitudinal organization in the cord, often extending rostrocaudally over a few spinal cord segments (e.g., Ll-L3; see Figure 10-2). These motor neuron pools have a Somatotopic organization in the ventral horn; that is, their relative position in the CNS corresponds to the relative body position of the muscles that their neurons innervate. In other words, motor neuron pools whose neurons innervate distal muscles of the limbs tend to be located in more lateral parts of the ventral horn, whereas motor neuron pools associated with axial and proximal musculature tend to be located more medially within the ventral horn.Lower motor neurons projecting out to muscles are often Synaptically activated by premotor neurons whose cell bodies are usually located in the intermediate zone of spinal cord gray matter (Figure 10-2). Activating a premotor neuron in the lateral part of the intermediate zone on one side of the body will generally activate a modest number of α motor neurons, in the lateral part of the ventral horn, on the same side of the body. This in turn will result in the activation of a modest number of distal limb muscles that would generally be used for skilled, voluntary movement. Premotor neuron activation in the medial part of the intermediate zone on one side of the body will generally activate a larger number of α motor neurons, in the medial part of the ventral horn, often on both sides of the body and often over more than one spinal cord segment. This in turn will result in the extensive activation of axial or proximal antigravity muscles on both sides of the body. Such a complement of muscles would be required for the involuntary stabilization or adjustment of posture. It can therefore be seen that more lateral parts of the spinal cord gray matter are involved in control of the distal limb musculature of skilled voluntary movement, whereas more medial parts are associated with the axial and proximal musculature of postural control.
The simplest type of motor behavior, the spinal segmental reflex (e.g., the knee jerk reflex; see Chapter 7), can be organized at the level of the spinal cord, without significant control from more rostral divisions of the CNS (e.g., the brain). Under different circumstances, however, the same spinal premotor and α motor neurons that participate in a simple spinal reflex could be activated by the brain to participate in an elegant and skilled sequence of movement.