MOTOR SYSTEM
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 :
- 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 fascial 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).
- Then fibers descend in brain stem (midbrain: middle 3/5 of cerebral peduncles, pons: in the basis pontis separated by transverse pontine fibers, then medulla oblongata: forming the pyramid of medulla).
- Along the length of the pyramid, 85% of fibers cross the midline (the motor decussation) then descend in the spinal cord as (lateral corticospinal tract).