Overview
Efferent (motor) pathways run from the cerebral cortex to the brainstem and spinal cord. The pyramidal system (corticospinal and corticobulbar tracts) carries voluntary, skilled movements, whereas the extrapyramidal system (rubrospinal, reticulospinal, vestibulospinal and tectospinal tracts) regulates tone, posture and balance, modulated by the basal ganglia and cerebellum.
The motor system is a complex network of structures and pathways within the nervous system that coordinates and controls voluntary and involuntary movements.
Pyramidal system: corticobulbar and corticospinal tracts
Pyramidal System:
- This system begins from the pyramidal cells of the primary motor cortex.
- Axons of these upper neurons descend into the brain stem and spinal cord to synapse with on lower motor neurons that control skeletal muscles.
- It includes 3 pairs of tracts:
- Cortico-bulbar tracts: (to motor cranial nerves nuclei in the brain stem).
- Lateral cortico-spinal tracts: to the spinal cord, running in the lateral white column.
- Anterior cortico-spinal tracts: to the spinal cord, running in the anterior white column
Extrapyramidal tract:
- This system is concerned with subconscious regulation of muscle tone and movement, in addition to subconscious control of the balance needed for each movement.
- It includes 2 pathways:
- Medial pathway: controlling movements of trunk and proximal limb muscles (vestibulo-spinal, tecto-spinal, and reticulo-spinal tracts).
- Lateral pathway: controlling distal limb muscles with its more discrete movements (rubro-spinal tracts).
Corticoneuclear (Corticobulber) system:
- Fibers of each tract descend in the corona radiata, genu of internal capsule, then brain stem.
- They terminate on motor nuclei of 3rd, 4th, 5th, 6th, 7th, 9th, 11th, and 12th cranial nerves supplying head muscles (muscles of the eye, jaw, face, and some neck muscles).
- Fibers mainly terminate on contralateral motor nuclei of cranial nerves and also on ipsilateral nuclei.
- 2 nuclei lack this double control (they receive contralateral fibers only):
- The part of motor nucleus of facial nerve controlling muscles of the lower half of face.
- The part of hypoglossal nucleus controlling genioglossus muscle.
Pyramid cells ↓ brainstem. (motor cranial nerves)
Motor nucleus of facial nerve:
The part controlling muscles of the upper half of the face receive bilateral corticobulbar fibers, while the part controlling muscles of lower half of face receives only contralateral fibers.
Corticospinal:
- Axons of neurons in the cerebral cortex descend and converge in corona radiata. Then pass throgh the anterior 2/3 of posterior limb of internal capsule (upper limb fibers are anterior, while lower limb fibers are posterior).