Corticospinal tracts

These are the primary motor pathway involved with voluntary, discrete and skilled movements. The tracts start in the cerebral cortex and terminate in the spinal cord. Approximately one-third of corticospinal fibres arise from the pre-central gyrus (area 4), referred to as the primary motor cortex. Another one-third of fibres arise from the secondary motor cortex (area 6), which is located rostral to the pre-central gyrus. The remaining fibres arise from the post-central gyrus, influencing sensory input to the nervous system.

Pre-central gyrus: site of origin of the fibres controlling motor activity (see Figure 54.1). The pre-central gyrus is organised somatotopically, where a body region corresponds to a particular point of the motor cortex (homunculus; Figure 54.2). The homunculus is a distorted image of the human body. Parts of the body that are responsible for fine movement (e.g. digits) occupy a much larger portion of the cortex, compared with body regions that are more coarsely controlled (e.g. trunk).

Corticospinal tracts: axons from pyramidal cells of the fifth layer of the cerebral cortex descend and converge in the corona radiata before passing to the posterior limb of the internal capsule. Fibres are grouped into those associated with the upper and lower extremities. The internal capsule continues into the basis pedunculi of the midbrain. On reaching the pons the tracts get broken up into fasciculi (bundles), which become grouped together to form the pyramids in the medulla oblongata (Figure 54.3). The pyramids are swellings that run down the ventral midline of the medulla oblongata. At the point where the medulla oblongata becomes continuous with the spinal cord, most of the fibres (∼85%) have crossed the midline (pyramidal decussation). Decussated fibres enter the spinal cord running in the lateral white columns (lateral corticospinal tracts). These fibres descend in the spinal cord before terminating in the ventral horn at their target spinal cord level. Fibres that did not cross the midline (anterior [ventral] corticospinal tract) continue to descend in the ipsilateral spinal cord within the anterior white columns (Figure 54.3). Most of these fibres cross the midline within the cervical and upper thoracic spinal cord regions, to terminate in the contralateral ventral horn. Only the largest corticospinal fibres synapse directly with motor neurones. Most will synapse with interneurones, which in turn will synapse with alpha motor neurones and some gamma motor neurones. Early in the corticospinal tracts descend some fibres which return to the cortex to inhibit the activity of adjacent cortical regions. Other returning fibres pass to additional brain regions (e.g. reticular formation and olivary nuclei) to inform subcortical areas of the cortical motor activity, in case they are required to react to this via other descending pathways.

Corticobulbar tracts: fibres accompany the corticospinal tracts to the level of the brainstem where they synapse within the reticular formation (near cranial nerve nuclei) or on lower motor neurones associated with the cranial nerves.