The somatosensory system is distributed throughout all major parts of our body. It is responsible for sensing touch, temperature, posture, limb position, and more. It includes both sensory receptor neurons in the periphery (eg., skin, muscle, and organs) and deeper neurons within the central nervous system.

Sensory receptors are classified into five categories: mechanoreceptors, thermoreceptors, proprioceptors, nociceptor, and chemoreceptors. These categories are based on the nature of the stimuli that each receptor class transduces.

Mechanoreceptors

There are four primary tactile mechanoreceptors in human skin: Merkel’s disks, Meissner’s corpuscles, Ruffini endings, and Pacinian corpuscle; two are located toward the surface of the skin, and two are located deeper.

Merkel’s disks are found in the upper layers of skin. Merkel’s disks are densely distributed in the fingertips and lips. They are slow-adapting, unencapsulated nerve endings, which respond to light touch. Meissner’s corpuscles are found primarily in the glabrous skin on the fingertips and eyelids. They respond to fine touch and pressure, but they also respond to low-frequency vibration or flutter. Deeper in the dermis, near the base, are Ruffini endings, which are also known as bulbous corpuscles. These are slow-adapting, encapsulated mechanoreceptors that detect skin stretch and deformations within joints. Pacinian corpuscles, located deep in the dermis are rapidly-adapting mechanoreceptors that sense deep, transient (not prolonged) pressure, and high-frequency vibration. Pacinian receptors detect pressure and vibration by being compressed, which stimulates their internal dendrites. There are fewer Pacinian corpuscles and Ruffini endings in the skin than there are Merkel’s disks and Meissner’s corpuscles.

Proprioceptors

Proprioception is the sense of the position of parts of our body and force being generated during movement. Proprioception relies on two, primary stretch receptors: Golgi tendon organs and muscle spindles. Muscle spindles that detect changes in the length of this muscle. The Golgi organ is a proprioceptive sensory receptor organ that is located at the insertion of skeletal muscle fibers into the tendons of skeletal muscle.

Thermoreceptors

Thermoception is the sense by which an organism perceives temperatures. The details of how temperature receptors work are still being investigated. Mammals have at least two types of sensors: those that detect heat (i.e., temperatures above body temperature) and those that detect cold (i.e., temperatures below body temperature).

Pain receptors

A nociceptor is a sensory receptor that responds to potentially damaging stimuli by sending nerve signals to the spinal cord and brain. This process, called nociception, usually causes the perception of pain. Nociceptors receive and send pain signals through myelinated fast fibers and nonmyelinated slow fibers that are only activated with intense or prolonged input.

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Key Points

• The four major types of tactile mechanoreceptors include Merkel’s disks, Meissner’s corpuscles, Ruffini endings, and Pacinian corpuscles.

• Merkel’s disk is slow-adapting, unencapsulated nerve endings that respond to a light touch; they are present in the upper layers of skin that have hair or is glabrous.

• Meissner’s corpuscles are rapidly-adapting, encapsulated neurons that respond to low-frequency vibrations and fine touch; they are located in the glabrous skin on fingertips and eyelids.

• Ruffini endings are slow adapting, encapsulated receptors that respond to skin stretch and are present in both the glabrous and hairy skin.

• Pacinian corpuscles are rapidly-adapting, deep receptors that respond to deep pressure and high-frequency vibration.

• The various types of receptors, nociceptors, mechanoreceptors (both small and large), thermoreceptors, chemoreceptors, and proprioceptors, work together to ensure that complex stimuli are transmitted properly to the brain for processing.

• The distribution of mechanoreceptors within the body can affect how stimuli are perceived; this is dependent on the size of the receptive field and whether single or multiple sensory receptors are activated.

• Thermoception is the sense by which an organism perceives temperatures

• Proprioception is the sense of the position of parts of our body and force being generated during movement.

• A nociceptor is a sensory receptor that responds to pain.

Key Terms

dendrite: branched projections of a neuron that conduct the impulses received from other neural cells to the cell body

glabrous: smooth, hairless, bald

mechanoreceptor: any receptor that provides an organism with information about mechanical changes in its environment, such as movement, tension and pressure

thermoreceptor: a nerve cell that is sensitive to changes in temperature

somatosensory: of or pertaining to the perception of sensory stimuli produced by the skin or internal organs

nociceptor: a sensory receptor that sends signals that cause the perception of pain in response to a potentially damaging stimulus

myelinated: a nerve wrapped in a myelin sheath to speed up transmission

proprioception: the sense of the position of parts of our body and force being generated during movement