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The somatosensory system is extremely complex, accounting for a diverse array of perceptions. It allows a person to feel the texture of a material, the frequency in which something is vibrating, the amount of pressure applied on something, and even balance and body positioning. However, this myriad of somatosensations wouldn’t be possible without somatosensory receptors, the dendrites of sensory neurons that detect the somatosensory stimuli. There are a plethora of somatosensory receptors in the human body, but they are classified into three main types: mechanoreceptors, nociceptors, and thermoreceptors. Mechanoreceptors are responsible for picking up touch, pressure, and vibration, as well as proprioception (balance and body positioning). More information about thermoreceptors and nociceptors can be found in Somatosensory Receptors Part II: Thermoreceptors and Nociceptors.
Mechanoreceptors can be categorized into two types: encapsulated and unencapsulated mechanoreceptors. Encapsulated mechanoreceptors contain a structure (or a “capsule”) which enhances their ability to perceive stimuli. When a mechanoreceptor perceives a stimulus, its ending becomes deformed, which transmits an electrical signal. The function of the capsule is to enhance the deformation done to the receptor, allowing it to perceive weaker stimuli, such as fine touch. Unencapsulated mechanoreceptors do not have this capsule, as they are able to detect weaker stimuli without it. The four main types of mechanoreceptors are Pacinian corpuscles, Ruffini endings, Meissner’s corpuscles, and Merkel Cells. These mechanoreceptors account for the vast majority of the mechanical sensations of touch, vibration, and pressure.
Somatosensory Science Facts, Page 68, Question 52
Firstly, Pacinian corpuscles are encapsulated mechanoreceptors located in the dermis, and play a major role in sensing vibration and pressure. They are made up of an outer and inner lamella (thin tissue layer or membrane), and lamellar fluid in between, and are in the shape of an oval. The outer lamella and lamellar fluid serve as a filter that allows nerve endings to be stimulated with high frequency vibrations and hard pressure and blocks out any low frequency vibrations and sensations of light pressure. They are phasic receptors, meaning they quickly adapt to stimuli, and even if the stimulus is present for a long time, they will stop generating action potentials shortly after it is first perceived. For instance, when you feel the temporary vibration of a phone, or if you press your hand against a wall, the Pacinian Corpuscles in your hand will temporarily detect the vibration and pressure.
Ruffini endings are unencapsulated receptors that are also located in the dermis. They sense stretching, sustained pressure, and heat, and their location in the dermis helps them perceive these strong stimuli. They are also more elongated, compared to other mechanoreceptors, and like Pacinian Corpuscles, they have an oval shape. These receptors are tonic receptors, meaning they adapt very slowly to stimuli. As long as the stimulus is present, they will consistently generate action potentials, allowing for the continuous perception of the stimulus.
Meissner’s corpuscles are encapsulated receptors located in the upper layer of the dermis. They allow for the perception of subtle tactile details, including light pressure, fine touch, and low-frequency vibration. They are phasic receptors that simply consist of a coiled nerve ending within a thin capsule. The thin capsule allows for higher sensitivity because it makes the nerve ending easily sensitive to minimal deformation, allowing the corpuscle to perceive fine touch. They have the highest concentration in the fingertips, the primary organs used to feel various substances.
Merkel Cells are unencapsulated somatosensory receptors that are in charge of perceiving pressure, fine touch, and low-frequency vibrations. They are tonic receptors that are located in the deepest layer of the epidermis. They consist of large myelinated nerve endings which release neuropeptides (small proteins produced by neurons) when strong stimuli (such as heavy pressure) are detected. There is a high density of these receptors in the palms of the hands, soles of the feet, face, lips, and mouth, which are the most sensitive body regions. Merkel Cells are very unique from other sensory receptors because while other sensory receptors contain their endings and myelinated axons in the same cell, Merkel Cells contain a separate axon that detects changes within the cell to receive tactile input.
Though the four sensory receptors listed above are considered as the key mechanoreceptors in the somatosensory system, mechanoreceptors also include proprioceptors, the sensory receptors responsible for the sensations of balance and body positioning. Proprioceptors include muscle spindles in the muscles, golgi tendon organs in tendons, and joint receptors located in ligaments. Proprioceptive signals are conveyed from these receptors into the brain. When moving, inputs from these receptors are transmitted to the somatosensory homunculus, which determines the precise location of a limb.
All these types of mechanoreceptors, as well as various others, allow us to detect different characteristics about our surroundings through the sense of touch. However, somatosensory receptors are only the beginning of the complex and extensive somatosensory pathways. Somatosensory receptors fire action potentials which propagate from neuron to neuron until they reach the somatosensory cortex in the postcentral gyrus, which detects where the somatosensory receptor was activated. Then, the insula (a brain region which plays a major role in homeostatic functions) interprets the stimulus as pleasant and painful. Before a perception is available, all other sensory systems, including vision, hearing, smell, and taste, process other stimuli. Then, all the sensory information is integrated into Conscious Perception. It is important to appreciate these receptors, because without them, there would be no touch or feeling in the world.
This information and more on Somatosensory can be found in our book, “Somatosensory Science Facts” (in press) and the lead Authors are Sehej Bindra and Charles Pidgeon, Ph.D. Several other students from the Neuroscience Sensory Unit are co-authors.
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