By Janani Rajan
Have you ever pet a dog and felt it’s soft, sleek fur? Or have you accidentally burned yourself with a stove while cooking? Have you felt your phone vibrate against your leg? All of these perceptions are the result of your somatosensory system at work. Generally, the somatosensory system is simply referred to as the sense of “touch.” However, it is actually responsible for a diverse array of perceptions, also known as somatosensations. These somatosensations include touch, pain, temperature, vibration, and pressure.
The somatosensory system is quite different from other sensory systems as it’s parts are not localized anywhere in the body. Rather, it is distributed throughout the entire body. The somatosensory system starts with sensory receptors in the periphery, with each somatosensation containing a type of receptor. These receptors connect with their corresponding spinal pathways, which are series of neurons that transmit information from the somatosensory receptors to the brain. The information transmitted through spinal pathways travels along the spinal cord, which is the primary method of communication between sensory and motor information between the brain and the body. Spinal pathways can be ascending or descending. Ascending pathways are responsible for relaying sensory information from the periphery, up the spinal cord, and into the brain. On the other hand, while descending pathways are in charge of relaying motor information from the brain, down the spinal cord, and into the peripheral nervous system. Therefore, the pathways that make up the somatosensory system are ascending.
The two major pathways of the somatosensory system are the spinothalamic pathway and the posterior column-medial lemniscus (PCML) pathway. The spinothalamic pathway is integral for a person’s survival, as it senses noxious (harmful) stimuli and allows a person to turn away from them. The lateral spinothalamic pathway relays the majority of information about temperature and pain perception. Meanwhile, the anterior spinothalamic pathway mediates information about pressure and crude touch. The spinothalamic pathway starts when an afferent neuron (a neuron that receives input from a sensory organ) reaches the dorsal horn of the spinal cord. The dorsal horn is a group of cell bodies of excitatory and inhibitory interneurons which process somatosensory information. It also contains projection neurons that transmit the processed somatosensory information from the spinal cord to the brain. The afferent neuron then reaches the Lissauer Tract, a tract of white matter on the back of the spinal cord. Axons from this tract then extend past the gray matter to the opposite side of the spinal cord. They travel up the spinal cord through the brainstem, and finally terminate in the ventral posterolateral nucleus, a group of neurons which establishes pathways to different cortical areas in the brain.
The PCML pathway is responsible for relaying somatosensory information about fine touch, proprioception (more commonly known as balance), vibration, and pressure. This pathway mediates information from mechanoreceptors (neuron synapses that respond to touch) to the somatosensory cortex from the skin and joints. The PCML pathway consists of a bundle of myelinated axons that travel up the posterior column of the spinal cord, decussate (form an X) at the medulla, and reaches the brain from there. It is made up of three types of neurons: first-order, second-order, and third-order neurons. First-order neurons make up the bundle of myelinated axons, and propagate signals from the skin and joints to the medulla, where they connect with second-order neurons. The second-order neurons propagate the signal to the thalamus, where the third-order neurons take over. The third order neurons ascend the signal to the postcentral gyrus, where the somatosensory cortex is located. The somatosensory cortex then processes the sensory input, resulting in the perception of these somatic sensations.
Together, these two pathways allow us to sense various characteristics of our environment and be aware of our body parts, their positioning, as well as the actions they are performing. From sensory receptors to the somatosensory cortex, electrical signals propagate from neuron to neuron in order for us to perceive our environment in the way we perceive it, which is a truly special thing.
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