Spinal cord tracts and ascending tracts

What are SPINAL CORD Tracts?

Groups of nerve fibers passing through spinal cord are known as tracts of the spinal cord.

The spinal tracts are divided into two main groups:

      1. Short tracts

      2. Long tracts.


  1. Short tracts

     Fibers of the short tracts connect different parts of spinal cord itself. Short tracts are of two types:  

           i.  Association Tracts 

          ii.  Commissural Tracts

   i. Association or Intrinsic Tracts, which connect adjacent segments of spinal cord on the same side.          

   ii. Commissural Tracts, which connect opposite halves of same segment of spinal cord.

 

  2. Long Tracts

     Long Tracts of spinal cord, which are also called projection tracts, connect the spinal cord with other parts of central nervous system. Long tracts are of two types:

        i. Ascending Tracts 
       ii. Descending Tracts    
  
   i. Ascending Tracts, which carry sensory impulses from the spinal cord to brain 

  ii. Descending Tracts, which carry motor impulses from brain to the spinal cord. 


 Ascending tracts of spinal cord

Ascending tracts of spinal cord carry the impulses of various sensations to the brain.

Sensation is formed by two or three different types  of neuron are as follows-

1. First order neurons

2. Second order neurons

3. Third order neurons.


1. First Order Neurons

First order neurons receive sensory impulses from the receptors and send them to sensory neurons present in the posterior gray horn of spinal cord through their fibers.
Nerve cell bodies of these neurons are located in the posterior nerve root ganglion.                                             

 2. Second Order Neurons

All the ascending tracts are formed by fibers of second order neurons of the sensory pathways except the ascending tracts in the posterior white funiculus, which are formed by the fibers of first order neurons.

3. Third Order Neurons

Third order neurons are in the Subcortical areas. Fibers of these neurons carry the sensory impulses from  Subcortical  areas to Cerebral cortex.


List of ascending tracts


1. Anterior white column-   

  • Anterior spinothalamic tract.
  • Lateral spinothalamic tracts.
  • Ventral spinocerebellar tracts.
  • Dorsal spinocerebellar tracts.
  • Spino tectal tracts.


2. Posterior column-

  • Fasciculus cuneatus.
  • Fasciculus gracilis.

3Lateral white column-

  • Spinoreticular tracts.
  • Spinovestibular tracts.
  • Spino-olivary tract

 

Anterior spinothalamic tracts 

 

Fibers of anterior spinothalamic tract arise from the neurons of chief sensory nucleus of posterior gray horn, which form the second order neurons of the crude touch pathway.

  • First order neurons are situated in the posterior nerve root ganglia.
  • These neurons receive the impulses of crude touch sensations from the pressure receptor.
  • Axons of the first order neurons reach the chief sensory nucleus through the posterior nerve roots.
  • Anterior spinothalamic tract contains crossed fibers. After taking origin, these fibers cross obliquely in the anterior white commissure and enter the anterior white column of opposite side.
  • Here, the fibers ascend through other segments of spinal cord and brainstem (medulla, pons and midbrain) and reach thalamus.c order neurons of the pathway of crude touch sensation.
  • Fibers of anterior spinothalamic tract terminate in the ventral posterolateral nucleus of thalamus.
  • Neurons of this thalamic nucleus form third order neurons of the pathway. Fibers from thalamic nucleus carry the impulses to somesthetic area (sensory cortex) of cerebral cortex. 

                                                                        

Lateral spinothalamic tract

  • Lateral spinothalamic tract is formed by the fibers from second order neurons of the pathway for the sensations of pain and temperature.
  • Lateral spinothalamic tract is situated in the lateral column towards medial side, i.e. near the gray matter.
  • Few fibers may ascend one or two segments,  then cross to the opposite side and then ascend in the lateral column.
  • All the fibers pass through medulla, pons and midbrain and reach thalamus along with fibers of anterior spinothalamic tract.     
  • Some of the fibers of lateral spinothalamic tract form collaterals and reach the reticular formation of brain stem.                                                  
  • Fibers of lateral spinothalamic tract terminate in the ventral posterolateral nucleus of thalamus along with anterior spinothalamic tract fibers.
  • Third order neuron fibers run to somesthetic area (sensory cortex) of cerebral cortex.
  • Function Fibers of lateral spinothalamic tract carry impulses of pain and temperature sensations.
  •  Fibers arising from this marginal nucleus transmit impulses of fast pain sensation.
  • Fibers arising from substantia gelatinosa of Rolando transmit impulses of slow pain and temperature sensations.



Ventral spinocerebellar tract

  • Ventral spinocerebellar tract is also known as Gower tract, indirect spinocerebellar tract or anterior spinocerebellar tract.
  • It is constituted by the fibers of second order neurons of the pathway for subconscious kinesthetic sensation .
  • This tract is situated in lateral white column of the spinal cord along the lateral periphery.
  • Origin Fibers of this tract arise from the marginal nucleus in posterior gray horn.
  • Neurons of marginal nucleus form the second order neurons. Fibers from these neurons make their first appearance in lower lumbar segments of spinal cord.
  • First order neurons are in the posterior root ganglia and receive the impulses of proprioception from the proprioceptors in muscle, tendon and joints.
  • Fibers from neurons of posterior root ganglia reach the marginal cells through posterior nerve root.


Dorsal spinocerebellar tract

  • Dorsal spinocerebellar tract is otherwise called Flechsig tract, direct spinocerebellar tract or posterior spinocerebellar tract.
  • Like the ventral spinocerebellar tract, this tract is also constituted by the second order neuron fibers of the pathway for subconscious kinesthetic sensation.
  • The first order neurons are in the posterior nerve root ganglia. But, the fibers of this tract are uncrossed.

 

Spino-olivary tract

  • Spino-olivary tract is situated in anterolateral part of white column.
  • Origin of the fibers of this tract is not specific. However, the fibers terminate in olivary nucleus of medulla oblongata.
  •  From here, the neurons project into cerebellum. This tract is concerned with proprioception.

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