John Thomas
The somatosensory system, one of the five main sensory systems in our body enables perception through touch, to help decipher the shape and texture of the objects, and know about the relative position of our body parts. It also helps avoid noxious stimuli by analysing the information about temperature and pain. The system comprises of three major ascending sensory pathways: the anterolateral or spinothalamic system (ALS), the dorsal column-medial lemniscal pathway (DCML), and the somatosensory pathways to cerebellum. These sensory pathways differ in the functional component of the mechanosensory information from the cutaneous and deep mechanoreceptors that is conveyed. The anterolateral system is further subdivided into two components, the lateral pathway that conveys pain and temperature information, and anterior tracts that convey crude touch and pressure. The DCML system carries inputs that enable sensations of fine touch, vibration, two point discrimination and proprioceptive information about joint position and movement. The spinocerebellar pathways carry proprioceptive inputs, pain and pressure information. These ascending pathways convey the tactile and other somatosensory information to multiple nuclei in the brainstem and thalamus, which further ascends to the primary sensory areas in the post central gyrus, and further to the higher order association cortices in the parietal lobe and upper bank of the lateral sulci. Sensory transduction is the conversion of mechanical stimulus energy into an electrical signal and is the first step in sensory processing. This is mediated by receptors whose location and properties decide the quality and strength of the stimulus conveyed. Based on the function, these sensory receptors can be grouped as: mechanoreceptors (touch and movement), nociceptors (pain) and thermoceptors (temperature). The functional importance of a body part in discriminative touch for a species correlates with the density of the mechanoreceptors on the skin. In primates, the digit tips have the maximum receptor density, closely followed by sensitive areas of the lips and face. In rodents, the primary sensory organs are the whiskers on the face. The information from the mechanoreceptors flows through the peripheral nerve afferents whose cell bodies lie in the dorsal root ganglia or cranial nerve ganglia to reach the spinal cord and brain stem. The afferents bifurcate into long ascending and short descending fibers upon entering the posterior funiculus of the spinal cord.
