Question

Explain the two main somatosensory pathways that carry signals from the periphery to the cortex.

a) Write down the routes of these pathways on their way to cortex.

b)Describe and compare the properties of these pathways.

Answer 1

The sensory information processed by the somatosensory systems travels along different anatomical pathways depending on the information carried. The two main pathway are the posterior column medial lemniscus pathway and the anterolateral pathways.

The dorsal column-medial lemniscal pathway (DCML) or  the posterior column medial lemniscus pathway has two major structures In the spinal cord, information travels via the dorsal (posterior) columns. In the brainstem, it is transmitted through the medial lemniscus.There are three neurons involved in this pathway,

The first order neuron takes two pathways, Signals from the upper limb (travel in the fasciculus cuneatus (the lateral part of the dorsal column). They then synapse in the nucleus cuneatus of the medulla oblongata.Signals from the lower limb travel in the fasciculus gracilis (the medial part of the dorsal column). They then synapse in the nucleus gracilis of the medulla oblongata.The second order neurones which begin in the cuneate nucleus or gracilis receives impulses from first order neuron. Axons of the second-order neurons decussate as the internal arcuate fibers and then form the medial lemniscus on the other side of the medulla. The next major synapse occurs when the medial lemniscal axons terminate in the ventral posterolateral nucleus (VPL) of the thalamus. The neurons of the VPL then project through the posterior limb of the internal capsule in the thalamic somatosensory radiations to reach the primary sensory cortex.

The anterolateral system, also enters the spinal cord via the dorsal root; however, these axons make their first synapse immediately in the gray matter of the spinal cord, mainly in the dorsal horn. The first order neurones arise from the sensory receptors in the periphery. They enter the spinal cord, ascend and synapse at the tip of the dorsal horn ( substantia gelatinosa.) From here the second order neurones carry the sensory information from the substantia gelatinosa to the thalamus. After synapsing with the first order neurones, these fibres cross over within the spinal cord, and then form two distinct tracts, the anterior spinothalamic tract. and the lateral spinothalamic tract. Both these tracts travel superiorly within the spinal cord, synapsing in the thalamus.The third order neurones carry the sensory signals from the thalamus to the ipsilateral (same side) primary sensory cortex of the brain. They ascend from the ventral posterolateral nucleus of the thalamus, travel through the internal capsule and terminate at the sensory cortex.

b) Posterior column medial leminscus pathway, carries and processes discriminative touch and proprioceptive information from the body.Level of decussation for this pathway is Internal arcuate fibers (lower medula). Anteriolateral pathway are responsible for conveying, pain, temperature sense and crude touch, level of decussation for this pathway is Anterior commisure (spinal cord)