Somatic Sensations : I. General Organization, Tactile and Position Senses

Other Classifications of Somatic Sensations

Somatic sensations are also often grouped together in other classes, as follows:

Exteroreceptive sensations are those from the surface of the body. Proprioceptive sensations are those relating to the physical state of the body, including position sensations, tendon and muscle sensations, pressure sensations from the bottom of the feet, and even the sensation of equilibrium, which is often considered a “special” sensation rather than a somatic sensation.

Visceral sensations are those from the viscera of the body. When using this term, one usually refers specifically to sensations from the internal organs.

Deep sensations are those that come from deep tissues, such as from fasciae, muscles, and bone. They include mainly “deep” pressure, pain, and vibration.

Interrelations Among the Tactile Sensations of Touch, Pressure, and Vibration

Although touch, pressure, and vibration are frequently classified as separate sensations, they are all detected by the same types of receptors. There are three principal differences among them: (1) touch sensation generally results from stimulation of tactile receptors in the skin or in tissues immediately beneath the skin; (2) pressure sensation generally results from deformation of deeper tissues; and (3) vibration sensation results from rapidly repetitive sensory signals; however, some of the same types of receptors as those for touch and pressure are used.

Tactile Receptors

There are at least six entirely different types of tactile receptors, but many more similar to these also exist. Some were shown in Figure 47-1 (previous chapter); their special characteristics are as follows.

First, some free nerve endings , which are found everywhere in the skin and in many other tissues, can detect touch and pressure. For example, even light contact with the cornea of the eye, which contains no other type of nerve ending besides free nerve endings, can nevertheless elicit touch and pressure sensations.

Second, a touch receptor with great sensitivity is the Meissner’s corpuscle (illustrated in Figure 47-1 and Figure 48-1 ), an elongated encapsulated nerve ending of a large (type Aβ) myelinated sensory nerve fiber. Inside the capsulation are many branching terminal nerve filaments. These corpuscles are present in the nonhairy parts of the skin and are particularly abundant in the fingertips, lips, and other areas of the skin where a person’s ability to discern spatial locations of touch sensations is highly developed. Meissner corpuscles adapt in a fraction of a second after they are stimulated, which means that they are particularly sensitive to movement of objects over the surface of the skin, as well as to low-frequency vibration.

Third, the fingertips and other areas that contain large numbers of Meissner’s corpuscles usually also contain large numbers of expanded tip tactile receptors , one type of which is Merkel’s discs , shown in Figure 48-1 . The hairy parts of the skin also contain moderate numbers of expanded tip receptors, even though they have almost no Meissner’s corpuscles. These receptors differ from Meissner’s corpuscles in that they transmit an initially strong but partially adapting signal and then a continuing weaker signal that adapts only slowly. Therefore, they are responsible for giving out steady-state signals that allow one to determine continuous touch of objects against the skin.

Merkel discs are often grouped together in a receptor organ called touch domes , which project upward against the underside of the epithelium of the skin. This upward projection causes the epithelium at this point to protrude outward, thus creating a dome and constituting an extremely sensitive receptor. Also note in Figure 48-1 that the entire group of Merkel’s discs is innervated by a single large myelinated nerve fiber (type Aβ). These receptors, along with the Meissner’s corpuscles discussed earlier, play extremely important roles in localizing touch sensations to specific surface areas of the body and in determining the texture of what is felt.

Fourth, slight movement of any hair on the body stimulates a nerve fiber entwining its base. Thus, each hair and its basal nerve fiber, called the hair end-organ ( Figure 48-1 ), are also touch receptors. A receptor adapts readily and, like Meissner’s corpuscles, detects mainly the following: (1) movement of objects on the surface of the body; or (2) initial contact with the body.

Fifth, located in the deeper layers of the skin and also in still deeper internal tissues are many Ruffini’s endings , which are multibranched encapsulated endings, as shown in Figure 47-1 and Figure 48-1 . These endings adapt very slowly and, therefore, are important for signaling continuous states of deformation of the tissues, such as heavy prolonged touch and pressure signals. They are also found in joint capsules and help to signal the degree of joint rotation.

Sixth, Pacinian corpuscles , which were discussed in detail in Chapter 47 , lie both immediately beneath the skin and deep in the fascial tissues of the body. They are stimulated only by rapid local compression of the tissues because they adapt in a few hundredths of a second. Therefore, they are particularly important for detecting tissue vibration or other rapid changes in the mechanical state of the tissues.

Transmission of Tactile Signals in Peripheral Nerve Fibers

Almost all specialized sensory receptors, such as Meissner’s corpuscles, Iggo dome receptors, hair receptors, Pacinian corpuscles, and Ruffini’s endings, transmit their signals in type Aβ nerve fibers that have transmission velocities ranging from 30 to 70 m/sec. Conversely, free nerve ending tactile receptors transmit signals mainly via the small type Aδ myelinated fibers that conduct at velocities of only 5 to 30 m/sec.

Some tactile free nerve endings transmit via type C unmyelinated fibers at velocities from a fraction of a meter up to 2 m/sec. These nerve endings send signals into the spinal cord and lower brain stem, probably subserving mainly the sensation of tickle.

Thus, the more critical types of sensory signals—those that help to determine precise localization on the skin, minute gradations of intensity, or rapid changes in sensory signal intensity—are all transmitted in more rapidly conducting types of sensory nerve fibers. Conversely, the cruder types of signals, such as pressure, poorly localized touch, and especially tickle, are transmitted via much slower, very small nerve fibers that require much less space in the peripheral nerve bundle than the fast fibers.

Detection of Vibration

All tactile receptors are involved in detection of vibration, although different receptors detect different frequencies of vibration. Pacinian corpuscles can detect signal vibrations from 30 to 800 cycles/sec because they respond extremely rapidly to minute and rapid deformations of the tissues. They also transmit their signals over type Aβ nerve fibers, which can transmit as many as 1000 impulses/sec. Low-frequency vibrations from 2 up to 80 cycles/sec, in contrast, stimulate other tactile receptors, especially Meissner’s corpuscles, which adapt less rapidly than do Pacinian corpuscles.

Detection of Tickle and Itch by Mechanoreceptive Free Nerve Endings

Neurophysiological studies have demonstrated the existence of very sensitive, rapidly adapting mechanoreceptive free nerve endings that elicit only the tickle and itch sensations. Furthermore, these endings are found almost exclusively in superficial layers of the skin, which is also the only tissue from which the tickle and itch sensations usually can be elicited. These sensations are transmitted by very small type C, unmyelinated fibers similar to those that transmit the aching slow type of pain.

The purpose of the itch sensation is presumably to call attention to mild surface stimuli such as a flea crawling on the skin or a fly about to bite; the signals elicited then activate the scratch reflex or other maneuvers that rid the host of the irritant. Itch can be relieved by scratching if this action removes the irritant or if the scratch is strong enough to elicit pain. The pain signals are believed to suppress the itch signals in the cord by lateral inhibition, as described in Chapter 49 .

Dorsal Column–Medial Lemniscal Pathway

Note in Figure 48-3 that nerve fibers entering the dorsal columns pass uninterrupted up to the dorsal medulla, where they synapse in the dorsal column nuclei (the cuneate and gracile nuclei ). From there, second-order neurons decussate immediately to the opposite side of the brain stem and continue upward through the medial lemnisci to the thalamus . In this pathway through the brain stem, each medial lemniscus is joined by additional fibers from the sensory nuclei of the trigeminal nerve; these fibers subserve the same sensory functions for the head that the dorsal column fibers subserve for the body.

In the thalamus, the medial lemniscal fibers terminate in the thalamic sensory relay area, called the ventrobasal complex . From the ventrobasal complex, third-order nerve fibers project, as shown in Figure 48-4 , mainly to the postcentral gyrus of the cerebral cortex, called somatic sensory area I (as shown in Figure 48-6 , these fibers also project to a smaller area in the lateral parietal cortex called somatic sensory area II ).