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Descending Motor Pathways

The reflex circuits demonstrate that sophisticated neural processing occurs at the lowest level of the motor hierarchy. These automatic reflexes can be modulated, however, by higher levels of the hierarchy. For example, when touching an iron to see if it is hot, your flexor reflex may be hypersensitive. As a result, you pull your hand away repeatedly before even touching the iron, anticipating that it may be hot. Conversely, if you remove a hot dish from the oven and the heat starts to go through the oven mitt, you will suppress the flexor response so that you do not drop your dinner as you rush to put it down on a table. These modulations (both facilitatory and inhibitory) of the spinal reflexes arise from the descending pathways from the brainstem and cortex. Voluntary movement and some sensory-driven reflex actions are also controlled by the descending pathways. The corticospinal system controls motor neurons and interneurons in the spinal cord. The corticobulbar system controls brainstem nuclei that innervate cranial muscles.

Parallel and Serial Processing

Although the motor system is organized hierarchically, the hierarchy is not a simple chain of processing from higher to lower areas. Many pathways enable the different levels of the hierarchy to influence each other. Thus, the flow of information through the motor system has both a serial organization (communication between levels) and a parallel organization (multiple pathways between each level). This parallel organization is critically important in understanding the various dysfunctions that can result from damage to the motor system. If the motor hierarchy had a strictly serial organization, like a series of links on a chain, then damage to any part of the system would produce severe deficits or paralysis in almost all types of movements. However, because of the parallel nature of processing, paralysis is actually a relatively rare outcome, produced by damage to the lowest level of the hierarchy. Damage to higher levels results in deficits in motor planning, initiation, coordination, and so forth, but movement is still possible. The parallel nature of organization is also important for the ability of undamaged parts of the motor system to compensate (at least partially) for injuries to other parts of the system.

Descending motor pathways arise from multiple regions of the brain and send axons down the spinal cord that innervate alpha motor neurons, gamma motor neurons, and interneurons. The motor neurons are topographically organized in the anterior horn of the spinal cord according to two rules: the flexor-extensor rule and the proximal-distal rule (Figure 2.8).

Figure 2.8 Flexor-extensor rule and proximal-distal rule.

Flexor-extensor rule: motor neurons that innervate flexor muscles are located posteriorly to motor neurons that innervate extensor muscles.

Proximal-distal rule: motor neurons that innervate distal muscles (e.g., hand muscles) are located lateral to motor neurons that innervate proximal muscles (e.g., trunk muscles).

Descending motor pathways are organized into two major groups:

1. Lateral pathways control both proximal and distal muscles and are responsible for most voluntary movements of arms and legs. They include the
   1. lateral corticospinal tract
      
   2. rubrospinal tract
      
2. Medial pathways control axial muscles and are responsible for posture, balance, and coarse control of axial and proximal muscles. They include the
   
   1. vestibulospinal tracts (both lateral and medial)
      
   2. reticulospinal tracts (both pontine and medullary)
      
   3. tectospinal tract
      
   4. anterior corticospinal tract
      

 

Figure 2.9 Corticospinal tracts (also called pyramidal tracts). Click on the labels to see the highlighted area.

Corticospinal tracts. The corticospinal tract originates in the motor cortex (Figure 2.9). The axons of motor projection neurons collect in the internal capsule, and then course through the crus cerebri (cerebral peduncle) in the midbrain. At the level of the medulla, these axons form the medullary pyramids on the ventral surface of the brainstem (hence, this tract is also called the pyramidal tract). At the level of the caudal medulla, the corticospinal tract splits into two tracts. Approximately 90% of the axons cross over to the contralateral side at the pyramidal decussation, forming the lateral corticospinal tract. These axons continue to course through the lateral funiculus of the spinal cord, before synapsing either directly onto alpha motor neurons or onto interneurons in the ventral horn. The remaining 10% of the axons that do not cross at the caudal medulla constitute the anterior corticospinal tract, as they continue down the spinal cord in the anterior funiculus. When they reach the spinal segment at which they terminate, they cross over to the contralateral side through the anterior white commissure and innervate alpha motor neurons or interneurons in the anterior horn. Thus, both the lateral and anterior corticospinal tracts are crossed pathways; they cross the midline at different locations, however.

Function. The corticospinal tract (along with the corticobulbar tract) is the primary pathway that carries the motor commands that underlie voluntary movement. The lateral corticospinal tract is responsible for the control of the distal musculature and the anterior corticospinal tract is responsible for the control of the proximal musculature. A particularly important function of the lateral corticospinal tract is the fine control of the digits of the hand. The corticospinal tract is the only descending pathway in which some axons make synaptic contacts directly onto alpha motor neurons. This direct cortical innervation presumably is necessary to allow the powerful processing networks of the cortex to control the activity of the spinal circuits that direct the exquisite movements of the fingers and hands. The percentage of axons in the corticospinal tract that innervate alpha motor neurons directly is greater in humans and nonhuman primates than in other mammals, presumably reflecting the increased manual dexterity of primates. Damage to the corticospinal tract results in a permanent loss of the fine control of the extremities. Although parallel descending pathways can often recover the function of more coarse movements, these pathways are not capable of generating fine, skilled movements. In addition to the fine control of distal muscles, the corticospinal tract also plays a role in the voluntary control of axial muscles.

Figure 2.10 Rubrospinal tract. Click on the labels to see the highlighted area.

Rubrospinal tract. The rubrospinal tract originates in the red nucleus of the midbrain (Figure 2.10). The axons immediately cross to the contralateral side of the brain, and they course through the brainstem and the lateral funiculus of the spinal cord. The axons innervate spinal neurons at all levels of the spinal cord.

Function. The rubrospinal tract is an alternative by which voluntary motor commands can be sent to the spinal cord. Although it is a major pathway in many animals, it is relatively minor in humans. Activation of this tract causes excitation of flexor muscles and inhibition of extensor muscles. The rubrospinal tract is thought to play a role in movement velocity, as rubrospinal lesions cause a temporary slowness in movement. In addition, because the red nucleus receives most of its input from the cerebellum, the rubrospinal tract probably plays a role in transmitting learned motor commands from the cerebellum to the musculature. The red nucleus also receives some input from the motor cortex, and it is therefore probably an important pathway for the recovery of some voluntary motor function after damage to the corticospinal tract.