Somatic Motor Pathways
All excitatory and inhibitory signals that control movement converge on the motor neurons that extend out of the brain stem and spinal cord to innervate skeletal muscles in the body. These neurons are called Lower Motor Neurons (LMNs) and have their cell bodies in the brain stem and spinal cord. From there, LMNs extend through cranial nerves to innervate skeletal muscles of the face and head and through spinal nerves to innervate skeletal muscles of the limbs and trunk. Upper Motor Neurons (UMNs) originate in the brain and brain stem and synapse with local circuit neurons (interneurons). The axons of UMNs extend from the brain to LMNs through 2 types of somatic motor pathways: Direct and Indirect. Direct Motor Pathways : provide input to LMNs via axons that extend directly from the cerebral cortex.
Indirect Motor Pathways : provide input to LMNs from motor centers in the brain stem.
Direct Motor Pathways Aka Pyramidal Pathways
Nerve impulses for voluntary fine, isolated, precise & specific movements propagate from the cerebral cortex to LMNs via direct motor pathways. These pathways consist of axons that descend from pyramidal cells. Pyramidal cells are UMNs with pyramid-shaped cell bodies located in the primary motor area and the premotor area of the cerebral cortex.
Horses don’t have significant pyramidal tracts in the spinal cord.
The direct motor pathways consist of the Corticospinal Pathways and the Corticobulbar Pathway.
1. Corticospinal Pathways
This pathway is responsible for conducting impulses that control skeletal muscles of the limbs, neck and trunk. Axons of UMNs in the cerebral cortex form the Corticospinal Tracts which descend through the internal capsule of the cerebrum and the cerebral peduncle of the midbrain. In the medulla, these axon bundles of corticospinal tracts form the ventral bulges known as the pyramids. ~90% of corticospinal axons decussate to the contralateral side in the medulla and then descend into the spinal cord where they synapse with a local circuit neuron (interneuron) or LMN. The remaining 10% remain on the ipsilateral side and eventually decussate at the spinal cord levels where they synapse with a local circuit neuron or LMN.
There are 2 types of corticospinal tracts :
Lateral Corticospinal Tract : these are corticospinal axons that have decussated in the medulla. These axons synapse with local circuit neurons or LMNs in the ventral gray horn of the spinal cord. Axons of the these LMNs exit the cord in the ventral roots of spinal nerves to terminate at skeletal muscles that control movements of distal parts of limbs which are responsible for precise, agile, and highly skilled movements distal limbs.
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