VPB 121: Veterinary Physiology - II (2+1)

CORTICO SPINAL TRACT

• It originates from the pyramidal (Betz) cells of the motor cortex‚ (60% from primary motor cortex and 40% from somasthetic motor cortex). 
• Enter the internal capsule, pass through the cerebral peduncles, form the pyramids at he base of the medulla and then enter the spinal cord. 
• In the medullary region, it divides into two unequal portions.It partially decussates before entering the spinal cord so the crossed ventral, uncrossed lateral, uncrossed ventral  tracts are also formed.
• 80% of this tract decussate to the opposite side (contralateral) and descends as lateral corticospinal tract, in the lateral funiculus of the spinal cord.
• It extends throughout the length of the spinal cord in primates and carnivores and regulates fine motor control both in the fore and hind limbs by its direct synapse with the spinal alpha motor neurons. While in other animals it synapses with the internuncial neurons, which in turn control, the motor activities of the alpha and the gamma  neurons.
• The remaining 20% of  the tract descend from the medulla  as uncrossed (ipsilateral), forming the ventral corticospinal tract in the ventral funiculus of  the spinal cord and descend  up to caudal cervical and cranial thoracic spinal segments. 
• At this point, it decussates to the opposite side of the spinal cord and synapse with the alpha  motor neurons, ventral corticospinal tract is totally absent in dogs.
• Four in number in domestic animals :
  • Crossed lateral
  • Uncrossed lateral 
  • Crossed ventral  
  • Uncrossed ventral  

• All four tracts degenerate in the cervical and thoracic portions of the spinal cord
• The differential termination make them important in various function that can be grouped as :
  • Involving components from the sensory cortex : function primarily to alter activities of ascending sensory systems such as spinocerebellar, spinothalamic, spinotectal and spinoreticular tracts
  • Involving components from the motor cortex: function primarily to alter somatic and sensory motor activities
• There is always one Internuncial neuron between the corticospinal tract fibers and spinal alpha motor neurons
• Thus a high frequency discharge is necessary to overcome synaptic resistance and produce a motor activity
• In ungulates (horse, cow, pig, sheep and goat) the Corticospinal tract terminates upon the Internuncial neurons of the spinal cord. From here it is carried to the thoracic and lumbo sacral levels of through propriospinal systems.
• Primary function is the control of spinal cord reflexes for the elicitation of motor mechanisms in animals
• Produces excitation of alpha motor neurons supplying flexor muscles and inhibition of alpha motor neurons supplying extensor muscles of the limbs
• Also exerts control over the ANS.
• The pyramidal tract is involved in the voluntary motor control of the skeletal muscles. 
• The pyramidal system is important in maintaining muscle tone.