Descending tract of Spinal cord
Cortico spinal tract
Cortico spinal tract 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 (All four tracts degenerate in the cervical and thoracic portions of the spinal cord)
- Crossed lateral
- Uncrossed lateral
- Crossed ventral
- Uncrossed ventral
- 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.
Reticulo spinal tract
Originate from Pontine and medullary reticular formations. Pontine reticular fibres are ipsilateral. Medullary reticular fibres descend bilaterally.
They exert an influence upon the motor neurons (alpha and gamma) of the spinal cord without the involvement of interneurons. But the majority use interneurons for their influence upon somatic motor activity
Visceral functions are mediated by this tract include the control of respiration , micturition, defecaton,cardiovascular alterations and gastrointestinal activity.
This tract is tonically active and influences lower gamma motor neurons, the gamma loop mechanism mediated by this tract causes reflex contraction in the anti gravity muscle to maintain postural muscle tone.
Vestibulo spinal tract
- Consists of two descending fiber tracts
- Vestibulospinal tract and
- Medial longitudinal fasciculus
Vestibulo spinal tract oringinates from the lateral vestibular nucleus and descends ipsilaterally through th length of the spinal cord. Medial longitudinal fasciculus fibres terminate monosynaptically upon alpha motor neurons.
The tract ipsilaterally exerts an excitatory influence upon extensor muscle tone and an inhibitory influence upon flexor muscle tone and contralaterally exerts an inhibitory influence upon extensor muscle tone and an inhibitory influence on flexor muscle tone. Also alters the tonus of the muscles of neck and forelimb.
Exert considerable influence upon the ANS by the production of motion sickness involving the constant acceleration and deceleration of the head.
Rubro spinal tract
Rubro spinal tract is the more important motor tract in domestic animals.it arises from the red nucleus of the midbrain. Decussates near its origin and enters the spinal cord contralateral to its origin.
Descends in the lateral funiculus throughout the entire length of the spinal cord. There is always involvement of Internuncial neuron between the rubrospinal tract fibers and spinal alpha motor neurons.
They are rapidly conducting. This controls semi skilled movements. It functions to stimulate the flexor motor activity and inhibit the extensor and motor neuron activity.
Tecto spinal tract
Tecto spinal tract arises from the deep layers of rostral colliculus. It deccussates within the dorsal tegmentum. Descends caudally in the brain stem.
In cat Tecto spinal tract descends contralaterally.
Terminates in the rostral region of cervical spinal cord. Plays a major role in mediation of auditory and visual reflexes and modifies neck reflexes. There is always involvement of Internuncial neuron between the tectospinal tract fibers and spinal alpha motor neurons.