Spinothalamic tracts

In this note, we will continue discussing the anatomy and function of the tracts of the brain and spinal cord.

Let’s look at the spinothalamic tracts: lateral and anterior.

They represent the ascending pathways and carry sensory (afferent) information from peripheral receptors to the structures of the central nervous system for further processing.

The spinothalamic tracts carry superficial sensations.

The lateral spinothalamic tract is responsible for pain and temperature sensations, while the anterior spinothalamic tract carries “crude” (or non-discriminative) sensations of touch and pressure.

The lateral spinothalamic tract is responsible for pain and temperature sensations, while the anterior spinothalamic tract carries “crude” (or non-discriminative) sensations of touch and pressure.

Fibers exiting the receptors head towards the spinal ganglion, where they synapse with the first-order neuron.

Then, they pass directly into the spinal cord via the dorsal root, into the posterior horn, where they synapse with the second-order neuron.

The axons of the second-order neurons decussate to the contralateral side of the spinal cord via the anterior white commissure. And they do so not in their own spinal segment, but 2 to 3 segments higher, that is obliquely, diagonally.

Only a very small portion of fibers does not decussate.

Then, the lateral spinothalamic tract travels in the lateral funiculus,

and the anterior, correspondingly, in the anterior funiculus of the spinal cord.

Upon reaching the level of the pons,

some fibers terminate on neurons of the reticular formation, and are referred to as the “spinoreticular tract”.

Other fibers terminate a bit higher, within the superior colliculus of the quadrigeminal plate,

forming the spinotectal tract.

Another portion terminates within the so-called parabrachial nuclei, which are located at the boundary between the pons and the midbrain, as well as within the neurons of the periaqueductal gray matter. These fibers are referred to as the spinomesencephalic tract.

All other fibers continue even higher as part of the so-called spinal lemniscus and terminate in the thalamus, where the third-order neurons are located in the ventral posterolateral (VPL), ventral posteroinferior (VPI), and intralaminar nuclei.

Then, the fibers pass through the posterior limb of the internal capsule, reaching the primary somatosensory cortex of the postcentral gyrus in the parietal lobe.

The postcentral gyrus has a particular pattern of somatotopic arrangement.

Its upper and medial parts are responsible for the sensory input from the lower limbs, and the lower and lateral parts, respectively, from the head and neck.

This concept is called Penfield’s sensory homunculus.

Moreover, the proportions of the homunculus differ from those of an actual human. These differences are due to the number of receptors in a specific area of the body. For example, there are significantly more tactile receptors on the lips than on the back, which is why the cortical area responsible for the lips is significantly wider.

Golosary