Chapter 8A - Pyramidal System

Pyramidal system

Many corticofugal axons terminate in the brain stem (corticobulbar) and others continue through the brain stem and into the spinal cord where they are called the corticospinal or "pyramidal" tract.

Corticospinal

The corticospinal tract (figure 22) represents the highest order of motor function in humans and is most directly involved in control of fine, digital movements. This tract arises in pyramidal neurons of layer V of the precentral gyrus, the "primary motor cortex." Betz cells are the largest of these pyramidal neurons. There is a motor homonculus in this gyrus, with the feet represented near the superomedial part of the motor cortex and the leg, trunk, arm, hand and head represented progressively further inferior on the lateral side of the brain. Axons arising from neurons in the precentral gyrus exit through the white matter and pass through the internal capsule where they are topographically arranged in the posterior limb. The fibers controlling the lower extremity are posterior to those of the upper limb. Corticospinal fibers traverse the middle portion of the cerebral peduncle of the midbrain and then the basal pons. They enter the pyramids of the medulla (from whence they get their name). Over 90% of the axons in the pyramids decussate just before reaching the upper cervical spinal cord (the pyramidal decussation) and they enter the lateral funiculus of the spinal cord to become the (lateral) corticospinal tract (figure 22). Most of these axons terminate in the intermediate gray matter of the cord, although some enter the dorsal horn (where they can have an effect on sensory transmission) and a few terminate directly on alpha motor neurons, contributing to rapid voluntary movement. Most of these fibers terminate on interneurons of the spinal cord. These interneurons are responsible for reflexes and, therefore, most motor activity actually occurs by the regulation of reflex excitability in the spinal cord.

A few corticospinal axons descend the anterior funiculus of the spinal cord as the anterior (ventral) corticospinal tract. This is more involved in axial (trunk and neck) movements and terminates bilaterally.

Corticobulbar

Many projections from the cerebral cortex terminate in the brain stem (generically called corticobulbar projections). These projections have several functions including voluntary control over cranial nerves, relay to the cerebellum, activation of other descending pathways (i.e., "indirect corticospinal projections") and modulation of sensory processing.

Many cranial nerve nuclei receive direct and indirect (through the reticular formation) cortical input via nerve fibers arising from the motor cortex and traversing the genu of the internal capsule. Most corticobulbar connections are bilateral, meaning that unless both sides of the nervous system are affected, there is no loss of motor control. However, the facial nucleus to the lower face receives only input from the contralateral motor cortex and, therefore, there will be weakness of voluntary movement of the lower face on the side opposite damage to corticobulbar neurons (with sparing of movements of the upper face).

The majority of corticobulbar projections terminate in the ipsilateral basal pontine nuclei. These nuclei relay to the cerebellar cortex via projections that decussate in the pons and enter the cerebellum through the middle cerebellar peduncle (see below). These represent, by far, the largest input to the cerebellum.

Bulbospinal projections

There are several brain stem nuclei that project to the spinal cord. The cerebral cortex projects to most of these and, therefore, may affect them as "indirect corticospinal projections." These areas include the red nucleus, which gives rise to the rubrospinal tract that decussates in the midbrain and descends the lateral funiculus near to the location of the lateral corticospinal tract (figure 14). The reticular formation gives rise to several descending pathways, one from the rostral pons that helps pattern locomotion, one from the caudal pons that can affect head movement to coincide with eye movement and one from the medulla that mostly inhibits reflex activity in the spinal cord. This latter tract is excited by cortical input and, therefore, cerebral motor cortical output is mostly inhibitory to spinal cord reflexes via this indirect pathway. For this reason, interruption of corticobulbar projections typically increases reflexes.

Cerebral cortical projections also go to the superior colliculus, a region that gives rise to a tectospinal tract as well as projections to eye movement centers. The superior colliculus is mostly responsible for reflex head and eye movement toward novel stimuli. The cerebral cortical projections to the superior colliculus may effect movement via these projections.

 

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