Question

Explain the ascending process of the spinothalamic tract from when the fibers leave the DRG. Make sure to include 1st, 2nd and 3rd order neurons and point of decussation.

Answer 1

Anterior and lateral spinothalamic tracts

Location : The anterior spinothalamic tract is located in the anterior white funiculus near the periphery, while lateral spinothalamic tract is located in the lateral funiculus towards medial side, i.e. near the grey matter .
Origin. The spinothalamic tracts are formed by the axons
of the chief sensory cells of posterior grey horn, which form the second-order sensory neurons. The first-order neurons of this pathway are located in the spinal ganglia. These neurons receive the impulses from the cutaneous receptors. Central processes of these neurons enter the spinal cord and terminate in relation to the chief sensory cells of spinal grey matter.
Course: After taking origin from the chief sensory cells, the
fibres of anterior spinothalamic tract ascend in posterior
grey horn for 2–3 segments in the same side. Then, they
cross obliquely to the opposite side of the spinal cord in the white commissure (but some fibres may remain uncrossed). The fibres of lateral spinothalamic tract cross within the same segment of spinal cord and reach the lateral column of the same segment.The two tracts also carry about 10% uncrossed fibres and run up in the spinal cord, medulla, pons and mid brain and reach the thalamus. In the brain stem, they form the so-called spinal lemniscus.
Termination : All the spinothalamic fibres running in the
spinal lemniscus terminate in the ventral posterolateral
nucleus of thalamus. The neurons of this thalamic nucleus
form the third-order neurons of this sensory pathway and
relay the impulses to the somaesthetic area of the cerebral
cortex.
Functions : Traditionally, it has been said that the anterior
spinothalamic tracts carry sensations for crude touch and
pressure, while the lateral tracts carry sensations of pain
and temperature.

Dorsolateral spinothalamic tract

The dorsolateral spinothalamic tract also called fasciculus
dorsolateralis or tract of Lissauer.
Origin : This is formed by the fibres arising from the neurons of the posterior root ganglia, which form the first-order neurons.
Location : It is located in the lateral white column between
the periphery of spinal cord and tip of posterior grey horn
Course : These fibres enter the spinal cord through the lat-
eral division of the posterior nerve root and pass upwards
or downwards for few segments on the same side and
synapse with cells of substantia gelatinosa of Ronaldo
situated in the posterior grey column. The processes
of these cells (second-order neurons) cross to the opposite side and ascend in the dorsolateral fasciculus to reach the ventral posterolateral nucleus of thalamus where they synapse.
Functions: This tract carries impulses arising in skin (mainly pain and temperature).