Cell columns

Each of the columns of motor neurons in the anterior gray horn supplies a group of muscles having similar functions. The individual muscles are supplied from cell groups (nuclei) within the columns. Axial (trunk) muscles are supplied from medially placed columns, proximal limb segment muscles from the midregion, and distal limb segment muscles from lateral columns (Figure 16.1). Columns supplying extensor muscles lie anterior to columns supplying flexors; hence the presence of ventromedial and dorsomedial columns for the trunk, and ventrolateral and dorsolateral columns for the limbs. A retrodorsolateral nucleus is devoted to the intrinsic muscles of the hand and foot. An isolated, central nucleus supplies the diaphragm.

Figure 16.1 Cell columns in the anterior gray horn of the spinal cord: somatotopic organization.

The segmental levels of the six somatomotor cell columns are listed in Table 16.1. The autonomic nervous system is represented by the intermediolateral cell column.

Table 16.1 The somatomotor cell columns

Cell types

Large, α (alpha) motor neurons supply the extrafusal fibers of the skeletal muscles. Interspersed among them are small, γ (gamma) motor neurons supplying the intrafusal fibers of neuromuscular spindles.

Corticospinal tract

The corticospinal tract is the great voluntary motor pathway. About 40% of its fibers take their origin from the primary motor cortex in the precentral gyrus. Other sources include the supplementary motor area on the medial side of the hemisphere, the premotor cortex on the lateral side, the somatic sensory cortex, the parietal lobe, and the cingulate gyrus (Figure 16.2). The contributions from the two sensory areas mentioned terminate in sensory nuclei of the brainstem and spinal cord, where they modulate sensory transmission.

Figure 16.2 Pyramidal tract visualized from the left side. The supplementary motor area is on the medial surface of the hemisphere. Arrow indicates level of pyramidal decussation. Non-motor neurons are shown in blue.

The corticospinal tract descends through the corona radiata and posterior limb of the internal capsule to reach the brainstem. It continues through the crus of the midbrain and the basilar pons to reach the medulla oblongata (Figure 16.3). Here it forms the pyramid (hence the synonym, pyramidal tract).

Figure 16.3 Coronal section of embalmed brain, following treatment with copper sulfate (Mulligan’s stain) showing unstained corticospinal fibers displacing nuclei pontis en route to the pyramid.

(Illustration kindly provided by Professor David Yew, University of Hong Kong.)

During its descent through the brainstem, the corticospinal tract gives off fibers which activate motor cranial nerve nuclei, notably those serving the muscles of the face, jaw, and tongue. These fibers are called corticonuclear (Figure 16.4). (The term ‘corticobulbar’ is sometimes used, but ‘bulb’ is open to different interpretations.)

Figure 16.4 The pyramidal tract. ACST, anterior corticospinal tract; LCST, lateral corticospinal tract. Note: Only the motor components are shown; the parietal lobe components are omitted.

Just above the spinomedullary junction (Figure 16.5):

Figure 16.5 Ventral view of medulla oblongata and upper spinal cord, showing the three spinal projections of the left pyramid.

The corticospinal tract contains about one million nerve fibers. The average conduction velocity is 60 m/s, indicating an average fiber diameter of 10 µm (‘rule of six’ in Ch. 6). About 3% of the fibers are extra large (up to 20 µm); they arise from giant neurons (cells of Betz), located mainly in the leg area of the motor cortex (Ch. 26). All corticospinal fibers are excitatory and appear to use glutamate as their transmitter substance.

Targets of the lateral corticospinal tract

Distal limb motor neurons

In the anterior gray horn, LCST axons synapse upon the dendrites of α and γ motor neurons supplying limb muscles, notably in the upper limb. A unique property of these corticomotoneuronal fibers of LCST is that of fractionation, whereby small groups of neurons can be selectively activated. This is most obvious in the case of the index finger, which can be flexed or extended quite independently, although three of its long tendons arise from muscle bellies devoted to all four fingers. Fractionation is essential for the execution of skilled movements such as buttoning a coat or tying shoe laces. Skilled movements are lost, and seldom recover completely, following damage to the corticomotoneuronal system anywhere from motor cortex to spinal cord.

Although fractionation is a manifestly important function, even simple voluntary movements, such as flexion of the elbow or abduction of the shoulder, are initiated by corticomotoneuronal fibers.

As mentioned already in Chapter 10, the α and γ motor neurons are coactivated by the LCST during a given movement, so that spindles in the prime movers are signaling active stretch while those in the antagonists are signaling passive stretch.

Renshaw cells

The number of possible functions served by LCST synapses on Renshaw cells is large because some of the cells synapse mainly upon Ia inhibitory internuncials, and others upon other Renshaw cells. Probably the most important function is to permit co-contraction of prime movers and their antagonists, in order to fix one or more joints, e.g. when a chopping or shoveling action is required of the hand. Co-contraction is achieved by inactivation of Ia inhibitory internucials by Renshaw cells.

Excitatory internuncials

In the intermediate gray matter and the base of the anterior horn, motor neurons supplying axial (vertebral) and proximal limb muscles are recruited mainly indirectly by the LCST, by way of excitatory internuncials.

Ia inhibitory internuncials

Also located in the intermediate gray matter are the Ia inhibitory internuncials, and these are the first neurons to be activated by the LCST during voluntary movements. Activity of the Ia internuncials causes the antagonist muscles to relax before the prime movers (agonists) contract. In addition, it renders the antagonists’ motor neurons refractory to stimulation by spindle afferents passively stretched by the movement. The sequence of events is shown in Figure 16.6 and its caption for voluntary flexion of the knee.

Figure 16.6

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