Motor Systems

Refer to lectures 23, 24, 38, 40, 41 and 52 of your BOM (2001) notes
as well as your Laboratory notes.

A. The Corticospinal (pyramidal) and Sub-corticospinal Tracts:

          The cerebral cortex projects to the spinal cord in the corticospinal tract. This tract originates in the precentral gyrus, the premotor and supplementary motor areas (anterior to the precentral gyrus), and the postcentral gyrus of each cerebral hemisphere. From the ventral surface of the midbrain within the cerebral peduncles, the tract continues through the pons to the ventral surface of the medulla, where it is visible as a pyramidal shaped bundle of fibers on each side known as the pyramids (thus the other name, pyramidal tract). The right and left corticospinal tracts decussate at the level of the caudal medulla forming the decussation of the pyramids.

          The fibers that cross over to the other side will course down the lateral funiculi forming the lateral corticospinal tract, which is involved in control of limb muscles. The lateral corticospinal tract thus carries fibers from the contralateral cerebral cortex. The fibers of the tract terminate mostly upon interneurons to indirectly influence motor neurons although some also terminate directly on motor neurons.

          A contingent of fibers does not decussate at the caudal medulla but continues in a ventral ipsilateral position to form the medial corticospinal tract. The latter carries fibers from the ipsilateral cerebral cortex involved in control of axial and trunk muscles, predominantly by way of interneurons.

          Both medial and lateral cortico-spinal tracts control girdle muscles.

The corticobulbar tract innervates motor neurons of the brainstem, i.e. cranial nerve nuclei 5, 7, and 12, and the nucleus ambiguus. The corticobulbar tract innervation is primarily bilateral. One important exception to this bilateral innervation is the cranial nerve nuclei VII.           As it can be seen in the diagram, corticobulbar innervation of the cranial nerve nuclei VII is bilateral for the upper face motor neurons but exclusively contralateral for the motor neurons of the lower face.

          The superior colliculus (tectum) receives axons from the optic tract and projects to the contralateral spinal cord via the tectospinal tract. By this tract (and input to abducens neurons), the superior colliculus contributes to control of head (and eye) movements in response to visual input.

          The vestibular complex is involved in the control of balance and influences spinal cord motor neurons. The medial vestibulospinal tract projects bilaterally to the cervical cord and is involved in reflex adjustments of the head in response to vestibular stimulation. The lateral vestibulospinal tract is uncrossed and projects the full length of the ipsilateral spinal cord to activate muscles involved in balance and gravity control.

          The medial reticular formation (MRF) integrates a variety of sensory inputs and commands from the cortex and projects the integrated commands (e.g. postural adjustment) to the spinal cord via the reticulospinal tract, which is mostly uncrossed, and courses in the ventral funiculi.

          The red nucleus, which receives input from motor cortex, projects to the spinal cord via the rubrospinal tract. The latter tract is mostly crossed, runs in the lateral funiculus and terminates near lateral motor neurons.

          The tectospinal tract, the medial and lateral vestibulospinal and reticulospinal tracts, and the anterior corticospinal tract form the medial tracts. These tracts descend in the medial, ventral white matter of the spinal cord and terminate in the medial regions of the ventral horns where motor neurons that innervate axial and proximal limb musculature are located. The medial tracts are therefore involved in reflex postural adjustments and maintenance of balance. The lateral corticospinal and rubrospinal tracts are involved in limb movements.

B. The Extra-Pyramidal Tracts (and Basal Ganglia):

          Deep to the cortex of each cerebral hemisphere are located several massive nuclei forming the basal ganglia. This structure, also known as the corpus striatum, is composed of the caudate nucleus and the putamen. In turn, the putamen and globus pallidus form together the lentiform nucleus.

Click here to view Dr. Chalk's functional
representation of the basal ganglia.

          The basal ganglia establish feedback loop-like connections with the different motor areas of the cortex. The cortex projects fibers to the basal ganglia, which then project to the thalamus (ventro-lateral and ventro-anterior nuclei), which project back to the cortex. The basal ganglia establish also several connections with other nuclei such as the substantia nigra and the subthalamus. This network of interconnections forms the extra-pyramidal system.

          A complete understanding of the role of these structures has not yet been fully established. It is known that damage of any of these different structures can result in:

• the generation of a variety of involuntary movements, or
• a paucity of movement or inability to initiate movements.

Clinical Problems:

1) Discuss the consequences of a lesion an upper motor neuron (UMN) compared to one of a lower motor neuron (LMN).
2) Describe the conditions known as chorea and Parkinson's Disease.
3) What would happen if you were to sustain a lesion to your left facial motor cortex?

Questions or comments.

Saturday, September 7, 2002 2:59 PM