What is mechanoreception?

Introduction

The human body is supplied with an abundance of sensory receptors that detect touch and pressure. These receptors are members of a larger group called mechanoreceptors; they are able to detect energy in mechanical form and convert it to the energy of nerve impulses. Mechanoreceptors occur both on body surfaces and in the interior, and they detect mechanical stimuli throughout the body. Touch receptors are located over the entire body surface; pressure receptors are located only under the skin and in the body interior. The two sensations are closely related. A very light pressure on the body surface is sensed by receptors in the skin and is felt as touch. As the pressure increases, mechanoreceptors in and immediately below the skin and at deeper levels are stimulated, and the sensation is felt as pressure.

Types of Mechanoreceptors

Several different types of mechanoreceptors are located in the skin and primarily detect touch. One type, known as free nerve endings, consists simply of branched nerve endings without associated structures. Although located primarily in the skin, some mechanoreceptors of this type are also found to a limited extent in deeper tissues, where they detect pressure.

A second mechanoreceptor type, termed Meissner’s corpuscles, consists of a ball of nerve endings enclosed within a capsulelike layer of cells. These mechanoreceptors, which are exquisitely sensitive to the lightest pressure, occur in nonhairy regions of the skin, such as the lips and fingertips.

A third mechanoreceptor type, the expanded-tip tactile receptor, occurs in the same nonhairy regions as Meissner’s corpuscles and, in smaller numbers, in parts of the skin that are covered with hair. These mechanoreceptors often occur in clusters that are served by branches of the same sensory nerve cell. Meissner’s corpuscles and the expanded-tip tactile receptors, working together in regions such as the fingertips, are primarily responsible for a person’s ability to determine the size, surface texture, and other tactile features of objects touched.

A fourth type of mechanoreceptor consists of a network of nerve endings surrounding the root of a hair. The combined nerve-hair root structure, called a hair end organ, is stimulated when body hairs are displaced. These mechanoreceptors, because hairs extend from the body surface, give an early warning that the skin of a haired region of the body is about to make contact with an object. The remaining mechanoreceptors of this group are located in deeper regions of the body; because of their location, they detect pressure rather than touch.

Pacinian corpuscles, which occur just under the skin and in deeper regions of the body, consist of a single sensory nerve ending buried inside a fluid-filled capsule. The capsule is formed by many layers of connective tissue cells, which surround the nerve ending in concentric layers, much like the successive layers of an onion. Pressure displaces the capsule and deforms its shape; the deforming pressure is transmitted through the capsule fluid to the surface of the sensory nerve ending. In response, the sensory nerve generates nerve impulses.

The remaining type of pressure receptor, Ruffini’s end organ, consists of a highly branched group of nerve endings enclosed in a capsule. These mechanoreceptors occur below the skin, in deeper tissues, and in the connective tissue capsules surrounding the joints. They detect heavy pressures on the body that are transmitted to deeper layers, and, through their locations in the joints, contribute to proprioception
—the sense of the position of the body’s limbs.

The various types of mechanoreceptors are believed to convert mechanical energy into the electrical energy of nerve impulses by essentially the same mechanism. In some manner, as yet incompletely understood, the mechanical forces deforming the cell membranes of sensory nerve endings open channels in the membranes to the flow of ions. The ions, which are electrically charged particles, produce the electrical effects responsible for generating nerve impulses.

Mechanoreceptor Adaptation

The different mechanoreceptor types exhibit the phenomenon of adaptation to varying degrees. In adaptation, the number of nerve impulses generated by a sensory receptor drops off with time if the stimulus remains constant. In the Pacinian corpuscle, for example, which is highly adaptive, adaptation results from flow of the capsule fluid. If pressure against the corpuscle is held at steady levels, deforming the capsule in one direction, the fluids inside the capsule flow in response to relieve the pressure. The new fluid distribution compensates for the applied pressure, and the nerve impulses generated by the Pacinian corpuscle drop in frequency. Any change in the pressure, however, is transmitted through the fluid to the sensory nerve ending before the fluid has a chance to shift in response. As a result, a new volley of nerve impulses is fired by the sensory neuron on a change of pressure until the fluid in the corpuscle shifts again to compensate for the new pressure. In Pacinian corpuscles, compensating movements of the fluid take place within hundredths or even thousandths of a second. Meissner’s corpuscles and the hair end organs also adapt quickly.

The expanded-tip tactile receptors and Ruffini’s end organs adapt significantly more slowly than the other mechanoreceptors. Expanded-tip tactile receptors adapt initially to a steady touch or pressure but reach a base level at which they continue to generate nerve impulses under steady pressure. The Ruffini’s end organs adapt only to a limited extent. The continuing nerve impulses arriving from these mechanoreceptors provide continuous monitoring of a constant stimulus. Thus, some of the mechanoreceptors are specialized to detect changes in touch or pressure and some to keep track of constant stimuli.

Mechanosensory Abilities

The combined effects of touch and pressure receptors, along with the varying degrees of adaptation of different receptor types, allow the detection of a range of stimuli, varying from the lightest, most delicate, glancing touch, through moderate pressures, to heavy pressures that stimulate both the body surfaces and interior. People can explore the surface, texture, and shape of objects and can interpret the various levels of touch and pressure so well that they can reconstruct a mental image of objects touched by the fingers with their eyes closed.

Much of this mechanosensory ability depends on the degree to which the different receptor types adapt. The rapid adaptation of Meissner’s corpuscles and the hair end organs explains why, if a steady, light to moderate pressure (not heavy enough to cause pain) is maintained on the body surface, the sensation of pressure quickly diminishes. If the pressure is heavy enough to cause pain, a person continues to be aware of the painful sensation, because pain receptors are very slow to adapt. If the degree or location of the pressure is altered, a person again becomes acutely aware of the pressure.

Awareness of continued touch depends primarily on the expanded-tip tactile receptors, which initially adapt but then continue to send nerve impulses when a light surface pressure is held constant. This allows a person to continue to be aware, for example, that some part of the body surface is touching an object. The limited adaptation of Ruffini’s corpuscles keeps a person aware of stronger pressures that are felt deeply in the body. Through their locations in joints, these slow-adapting mechanoreceptors also help keep a person continually aware of the positions of the limbs.

The sensory effects of the fast- and slow-adapting mechanoreceptors can be demonstrated by a simple exercise such as pinching the skin on the back of the hand with a steady pressure strong enough to cause only slight pain. The feeling of pressure dissipates rapidly; however, one remains aware of the touch and pain. The rapid dissipation of the sensation of pressure is caused by the fast adaptation of Meissner’s corpuscles and any Pacinian corpuscles that may have been stimulated. Some degree of touch sensation is maintained, however, by residual levels of nerve impulses sent by the expanded-tip tactile receptors. The sensation of pain continues at almost steady levels because, in contrast to most of the mechanoreceptors, pain receptors adapt very little. If the pressure is released, the pain stops, and another intense sensation of pressure is felt as all the receptor types fire off a burst of nerve impulses in response to the change.

Mechanoreceptors located at deeper levels keep a person constantly aware of the positions of body parts and the degree of extension of the limbs with respect to the trunk. Ruffini’s and Pacinian corpuscles located within the connective tissue layers covering the bones, and within the capsules surrounding the joints, keep track of the angles made by the bones as they are pulled to different positions by the muscles. Ruffini’s and Pacinian corpuscles are among the most important mechanoreceptors keeping track of these movements.

Muscle Spindles and Golgi Tendon Organs

Touch and pressure receptors represent only a part of the body’s array of mechanoreceptors. Other mechanoreceptors located more deeply in the body help monitor the position of body parts and detect the degree of stretch of body cavities.

In addition to the Ruffini’s and Pacinian corpuscles detecting the positions of the bones and joints, two further types of mechanoreceptors constantly track the tension developed by the muscles moving the limbs. One is buried within the muscle itself, and one is in the tendons connecting the muscles to the bones. The mechanoreceptors buried within muscles, called muscle spindles, consist of a specialized bundle of five to twelve small muscle cells enclosed within a capsule of connective tissue. Sensory nerve endings surround the muscle cells in a spiral at the midpoint of the capsule and also form branched endings among the muscle cells of the capsule. Because of their position within the muscle spindle, the nerve endings are stretched, and generate nerve impulses, when the surrounding muscle tissue contracts.

The mechanoreceptors of tendons, called Golgi tendon organs, are formed by nerve endings that branch within the fibrous connective tissue cells forming a tendon. The nerve endings of Golgi tendon organs detect both stretch and compression of the tendon as the muscles connected to them move and place tension on the limbs. The combined activities of the deeply located mechanoreceptors keep a person aware of posture, stance, and positions of the limbs. They also allow a person to perform feats such as bringing the thumbs or fingers together behind the back or touching the tip of the nose with the forefinger with the eyes closed.

Somatic Senses

Mechanoreceptors are one of five different types of sensory receptors that also include thermoreceptors, which detect changes in the flow of heat to or from the body; nociceptors, which detect tissue damage and whose nerve impulses are integrated and perceived in the brain as pain; chemoreceptors, which detect chemicals in locations such as the tongue, where they are responsible for the sense of taste, and in the nasal cavity, where they contribute to the sense of smell; and photoreceptors, which detect light. The mechanoreceptors, thermoreceptors, and nociceptors together form what are known as the somatic or body senses.

Sensory nerve tracts originating from mechanoreceptors, particularly those arising from the body surfaces, and their connecting neurons within the spinal cord and the brain are held in highly organized register with one another. Sensory fibers and their connecting nerves originating from the hand, for example, are located in a position near those originating from the wrist. In the cerebral cortex, the organization is retained, so that there is a projection of the body parts over a part of the cerebrum called the somatic sensory cortex. In this region, which occupies a band running from the top to the lower sides of the brain along anterior segments of the parietal lobes, segments corresponding to major body parts trace out a distorted image of the body from the top of the brain to the sides, with the genitalia, feet, and legs at the top, the arms and hands at the middle region, and the head, lips, tongue, and teeth at the bottom. Sensory information from the right side of the body is received and integrated in the somatic sensory cortex on the left side of the brain, and information from the left side of the body is received and integrated on the right side of the brain. The area of the somatic sensory cortex integrating signals from various body regions depends on the numbers of touch and other sensory receptors in the body regions. The lips and fingers, for example, which are generously supplied with sensory receptors, are represented by much larger areas in the somatic sensory cortex than the arms and legs. Reception and integration of signals in the somatic sensory cortex are partly under conscious control; a person can direct attention to one body part or another and concentrate on the signals arriving from the selected region. The activities of touch, pressure, and other sensory receptors, integrated and interpreted in the somatic sensory cortex, supply people’s link to the world around them and supply the information people require to survive and interact with the environment.

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