Somatosensory System: Introducing the Parts and Paths

Neuroscience Foundation
Aug 3
4 min read

Updated: Aug 6

By Sucharita Desu The human body is made up of a vast number of systems, coming together to help us perceive and interact with the world around us. Oftentimes, multiple systems are grouped together to fit under the umbrella of one larger, more general system. An example is the sensory system, which includes the gustatory system, the somatosensory system, the olfactory system, the auditory system, and the visual system. You might know these as our five senses - taste, touch, smell, hearing, and sight, respectively.

While every system has its own important role in our body, let’s focus on the somatosensory system. This system is mainly involved in processes related to touch. It allows you to feel different temperatures, feel objects on your skin and their textures, identify where your body is in space, feel pain, or sense that you have to go to the bathroom. Essentially, almost every part of your body is somehow involved in this system. So, how does it work?

Well, to be able to sense anything, we need receptors. Receptors can be either inside a cell or outside, and its function is to cause a specific response in the cell according to what binds to it. Unlike the other sensory systems, receptors of the somatosensory system are spread throughout our entire body. For the purpose of this article, let’s focus on our largest organ, our skin!

Most of the receptors in our skin cells are known to be mechanoreceptors that are sensitive to physical changes. This means that these receptors react to the bending and stretching of our skin. Within this class of receptors, there is variety in the shape, size, and location of the receptors as well! For starters, one of the receptors are known as Merkel’s discs and look disc-shaped, as the name suggests. These receptors are found near the border of the epidermis (outermost and thinnest layer) and the dermis (second layer) of our skin. Second, we have our Meissner’s corpuscles. These are found just below the epidermis and look like flattened cells. Third, our Ruffini’s endings. They look like cylindrical capsules with branched fibres inside and are found in the dermis. Lastly, there’s the Pacinian corpuscles. These are the largest of the four, have onion-like layers, and surround the nerve fibers deep in our dermis.

Mechanoreceptors have different adaptations rates and different receptive field sizes. Typically, sensory receptors are known to react quickly to any type of stimulus. But, eventually, they get used to the stimulus. The speed of this is known as the adaptation rate. Both the Pacinian corpuscles and the Meissner’s corpuscles are fast to adapt; they get used to a stimulus very quickly. On the other hand, Merkel’s discs and Ruffini’s endings are slow to adapt; they take longer to get used to a stimulus. Receptive field size refers to the area “covered” by a single receptor. It can be thought of as how much skin each receptor is responsible for reacting to. Receptors like Meissner’s corpuscles and Merkel discs are closer to the surface of your skin, and therefore have smaller, more specific, receptive fields. Receptors deeper inside your skin, like the Pacinian corpuscles and Ruffini’s endings, have larger receptive fields. The size of the receptor may also influence its field size, as larger receptors cover larger areas.

Finally, let’s cover how we really sense stimuli. What path do touch sensations follow, from our fingertips to our brain?

Starting us off, afferent neurons bring information through their axons from the touch receptors to the spinal cord (or directly to the brain stem). The cell bodies of these axons lie in a structure known as the dorsal root ganglion found within our spinal cord.

From there, there are two possible pathways the sensation can take.

First, the medial lemniscal pathway. This pathway is responsible for touch, vibration, and proprioception. The axons travel up the spine, through the dorsal columns, and end in the medulla (in the brain stem). Specifically, they end at the dorsal column nuclei. Then, the axons cross to the other side of the medulla and travel up a tract known as the medial lemniscus. This tract ends in the thalamus, which then projects the sensation to our somatosensory cortex for processing.

The second possible pathway uses the spinothalamic pathway. This pathway is responsible for feelings of pain and temperature. When information about a sensation comes into the spinal cord, it is sent to the substantia gelatinosa. This is a region in the grey matter of our spinal cords. From there, axons take the signal to the other side of the spinal cord before they climb up the spinothalamic tract. This pathway also leads the signal to the thalamus, from which the sensation is projected to the somatosensory cortex.

Then, your brain processes the sensory information in the cortex. This includes integrating the sensation, generating an output, and then transmitting the output to the necessary muscles and/or neurons.

However, it is extremely important to note that multiple neural pathways and systems come together in the brain to be able to generate any outputs. For example, while feeling a bug crawling up your arm alerts you of the bug, you would also need to see the bug to be able to react to it appropriately. Seeing it is a butterfly might mean you merely watch the butterfly on your arm. However, seeing a spider on your arm might trigger you to react in a much more panicked manner (i.e., quickly brushing it off).

A similar thing happens during pain processing. Touching a hot stove, would trigger you to quickly retract your arm. However, considering this scenario is time sensitive, the output would be generated through the neurons in your spinal cord even before the signal reaches the brain. Now, when the sensation does reach your brain, it is encoded for future reference rather than reacted on. Thus, it teaches you to not touch the stove, helping you anticipate the consequences of your movements/actions.

While this may have been an informative article, it is important to note that it is merely an introduction to this topic. There are many more types of receptors, somatosensory pathways, and related details that we recommend you to check out! To help you get started, feel free to look through some of the resources below:

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