Structure and Functions An essential attribute of the survival of a species is the ability to detect both the internal and external environment. This is necessary so that appropriate and life-sustaining actions can be taken at all times. When changes or modifications of these environments take place, many responses can occur within an individual, ranging from rolling over during sleep to restore blood flow to an arm to avoiding contact with a prickly pear cactus. This kind of monitoring occurs within the general and special sense organs or structures found in humans and many other species.
One way in which the body monitors its internal and external environments is through the general senses. General sensations include temperature, pain, touch, pressure, vibration, tickle, and proprioception (internal sensations relating to how one’s body is situated in space). Sensations of touch usually originate at or very near the skin surface; some originate from receptors found in deeper, subcutaneous (below the skin) layers. Many of the general senses are collectively called the "tactile senses"—notably, those of touch, pressure, vibration, and tickle. In addition, the term “somatic senses” refers to the sensory receptors associated with skin, muscles, joints, and visceral organs. Receptors in the muscles or joints are essential for the awareness of body movement. Visceral receptors play important roles in monitoring changes in body pain, as in stomach pain, hunger, and thirst. The somatic senses provide a means by which the internal and external environments are monitored with regard to touch, pressure, stretch, pain, and temperature. While these terms are not precisely interchangeable, there is overlap in the sensations of touch, pressure, vibration, and tickle with all three categories: general, tactile, and somatic sensations.
Another means whereby a human interacts with the environment is controlled by the special senses. Special senses include taste, vision, smell, hearing, and body equilibrium in space (or balance). Special senses involve relatively large and specialized structures of the tongue, eyes, nose, and ear and inner ear, in contrast to the general senses, in which the structures are relatively simple but are more widely dispersed throughout the entire body. The neurological pathways are also simplified relative to the neurological events that occur in the special senses’ organs and pathways. Combined, the general and special senses form an intricate and elegant system that allows for an individual’s survival.
Sensations occur when a receptor receives a stimulus from the external or internal world. The result is a neural impulse that can be utilized in the brain to provide some awareness of the body and its immediate environment. Perception results from the interpretation of the sensory information at a conscious level. Conscious awareness of sensory stimulation generally occurs only when a sufficiently large, or sometimes abrupt, change in the status quo happens in either the internal or external environment. At that point, the perception of a sense will be registered and noted in the cerebral cortex.
An untold number of sensory receptors are stimulated at any given second, including receptors that note where each body part is placed—from the tiniest portion of each finger and toe, to the position of the body in a chair. Other receptors respond to inhalation and exhalation pressure changes or feel air brushing past. All receptors work simultaneously and in harmony in a healthy person, but most of this activity is on a subconscious level. Only if a sufficient change in either the internal or external world occurs will a sensation no longer be simply monitored but instead cause a conscious response. An example is feeling the hard coolness of a bench when being seated: at first, the perception of the hard and cold surface is pronounced, but this perception will decrease over time until one seemingly “forgets” about being seated on an uncomfortable bench. This kind of decreased sensitivity to a stimulus is called "adaptation."
It appears that adaptation prevents the conscious mind from being overloaded with “meaningless” data or data that require no particular response. Adaptation may also allow for new, perhaps more important, stimuli to be noted at the receptor sites. Thus, the adaptation mechanism acts as a “reset” button. Consider the bench example again: in spite of one’s “forgetfulness” about the bench being beneath the body, the stimulus is not, in fact, gone. Rather, the mind conveniently elects to ignore the stimulus unless a change occurs that reminds the brain of the bench’s presence. This new perception of the bench might recur if a shift in body position causes new receptors to receive stimuli from new contact places between the body and the bench. Eventually, these new sensations will undergo the adaptation process and the bench will once again be “forgotten.” (Although thermal equilibrium will be reached between the bench and the person, this process is slow and does not account for the quick rate of adaptation.)
There are four necessary components of sensation: a stimulus, which is generally caused by a change in the environment; a receptor, which can experience a stimulus and produce a generator potential to initiate a nerve impulse; an impulse, which carries the signal from the point of stimulation of the receptor to the brain; and a translation of the impulse within the brain, so that a meaningful interpretation of the kind of sensations experienced, such as a tickle or a floral fragrance, can be made.
All sense receptors are very excitable, but only if stimulated by the specific sensation that they are designed to monitor. This means that sensory receptors are highly specialized in their function. Sense receptors have a low threshold of response to the type of stimuli to which they are designed to respond, while having a high threshold of response to other kinds of stimuli. (Pain receptors are an exception to this rule, perhaps because of the variety of stimuli that can include a sense of pain.) An example of specialization of the receptors is seen in the fact that certain regions of the body are more susceptible to sensing a tickle than others. Specialization of receptors is attributed to the unique structures of the receptors, even though all sense receptors contain dendrites from sensory neurons.
At a receptor site, a stimulus may induce a generator potential, which sometimes is called the "receptor potential." The generator potential is a localized and graded response that may reach its threshold if enough depolarization of the dendrites occurs. In other words, if threshold potential is achieved at the receptor, then a nerve impulse will ensue. This kind of response is an all-or-none response, the landmark characteristic of nerve cells.
The cerebral cortex is essential in the perception (interpretation) of a sensation. All different impulses arriving at the cerebral cortex are chemically the same; the only difference between an impulse carrying a message of a soft touch or of a heavy pounding is where the impulse arrives within the cerebral cortex. Impulses arriving at different locations of the cortex allow the brain to identify, classify, and locate the origins of a stimulus. The ability to distinguish one sensation from another is called "modality"; the ability to locate precisely the point at which the stimulus is applied is called "projection". Modality and projection are functions of the cerebral cortex. In addition, the cortex can prompt many shifts in body position (such as stretching after reading) or body chemistry (such as the release of epinephrine to increase heart rate and blood flow in a crisis) if a response to the stimulus is deemed necessary.
Receptors can be classified according to the location or type of stimuli that cause the receptors to respond. Touch receptors can be classified as exteroceptors by virtue of the fact that these sensory receptors are externally located, mainly on the skin surface. Other classes of somatic receptors are located internally. Visceroreceptors monitor internal organs for data on hunger, thirst, pressure, and nausea. Proprioceptors monitor the muscles, tendons, joints, and inner ear for exacting knowledge of body position and body movement.
"Mechanoreceptors" are an alternative classification for touch receptors. This label is based on the type of stimulus—a mechanical displacement or disfiguring, for which touch receptors have a low threshold. Light touch is felt when the skin is very gently touched and no indentation or distortion of the skin results. Touch pressure is felt when a heavy touch causes a distortion of the skin surface, either laterally, as in a tugging sensation, or vertically, as in a depression of the skin surface.
Six types of touch receptors have been identified in the human anatomy: root hair plexuses, free nerve endings, tactile disks (or Merkel’s disks), corpuscles of touch (or Meissner’s corpuscles), type II cutaneous mechanoreceptors (or end organs of Ruffini), and Pacinian corpuscles (or lamellated corpuscles). At these structures, a mechanical stimulus can be transformed into a sensation, provided that the stimulus is sufficiently strong to bring about a threshold potential.
Root hair plexuses are located in networks at the hair
roots. On the scalp, these generate a sensation of touch when the hair is being pulled, brushed, or stroked. On the body surface, these receptors are sensitive to movement of the hair, as can occur if a small breeze passes over the skin or if a silk scarf is dragged lightly over the skin hairs. Root hair plexuses are not structurally supported, nor are they protected by any surrounding structure.
Free nerve endings are everywhere on the skin surface and seem to be responsive to many kinds of stimuli. These little dendritic processes of sensory neurons are not protected or supported by surrounding structures.
Made of disklike formations of dendrites, Merkel’s disks, or tactile disks, are found in the deeper layers of the skin. Merkel’s disks are particularly abundant on the fingertips, palms, soles of the feet, eyelids, lips, nipples, clitoris, tip of the penis, and tip of the tongue. Merkel’s disks are particularly suited to receiving stimuli of fine touch and pressure.
Meissner’s corpuscles, or corpuscles of touch, are egg-shaped dendritic masses that are sensitive to light touch and vibrations of a low frequency. Found in the hairless portions of the skin, these receptors are also used in making judgments about the textures of whatever the skin may contact. Two or more sensory nerve fibers enter each corpuscle of touch; the nerve fibers terminate as tiny knobs within the corpuscle. In addition, Meissner’s corpuscles contain dendritic extensions. The entire mass is enclosed in connective tissue, which offers some support and protection to the disks. Corpuscles of touch are found in the external genitalia, tip of the tongue, eyelids, lips, fingertips, palms and soles, and nipples of both sexes.
The end organs of Ruffini, or type II cutaneous mechanoreceptors, are found all over the body but especially in the deep dermis (skin) and deeper tissues below the dermis. These are also called "corpuscles of Ruffini" and are responsive to heavy and continuous touch and pressure.
Finally, the Pacinian corpuscles are relatively large ellipses that are found in the subcutaneous tissue (below the skin) and deeper subcutaneous tissues. Also known as "lamellated corpuscles," these touch receptors are found under tissues that contain mucous membranes, serous membranes, joints and tendons, muscles, mammary glands, and external genitalia. Pacinian receptors are sensitive to deep and heavy pressure and vibrations of low frequency. As such, these receptors detect pulsating, vibrating stimuli. There is an abundance of Pacinian receptors in the penis, vagina, feet and hands, clitoris, urethra, breasts, tendons, and ligaments. Inside each ellipsoid are dendrites from sensory nerves; the bundle itself is wrapped in connective tissue that can serve as a protective support.
Disorders and Diseases Loss of tactile senses is a symptom rather than a disease. In general, a lost ability to sense touch, pressure, vibration, or tickle is a result of physical damage to a group of nerves or of a disease of the nervous system. Sensory receptors are not themselves targets of disease, but they can be physically or chemically impaired, especially if the skin is severely damaged.
An example of severe damage to the skin that will cause a loss of tactile sensations is a third-degree
burn of the body. Third-degree burns are marked by the total destruction of the full thickness of the skin. This destruction includes the epidermis, the dermis, and any associated skin structures, such as secretion glands, hair, and the general sensory receptors. No pain is sensed when regions of the body that have received a third-degree burn are touched, because the nerve fibers that innervate the touch receptors and the free nerve endings, as well as nerves located in the subcutaneous layers, have been destroyed by the burn. In such cases, total destruction of the nerve fibers and the skin has occurred. Third-degree burns can have a charred, dry appearance or a mahogany or ash-white color. Regeneration of the dermis and the subcutaneous structures is slow and painful as the healing occurs. Although skin grafting
can facilitate the regeneration
process, it is not uncommon for scarring to result from the rapid contraction of the wounded area as it heals.
The sense of touch is largely lost in scar tissue, since new nerve fibers cannot be formed. (Nerve cells are formed only during gestation and early life and are designed to last a lifetime.) Some tactile senses can return to scarred regions through a process called "sprouting." This process involves the branching forth, or the sprouting, of dendritic processes originating in undamaged nerve cells near to, but removed from, the injured site. In this kind of recovery, the healthy nerves assist in restoring tactile senses, to a limited degree, in the damaged regions.
Aside from a total loss of sensation, a sense of numbness can indicate a loss of proper blood circulation to a body region. For example, if one sits or folds oneself into a position so that a leg is receiving pressure from other body parts, the sensation of touch and pressure on the leg will eventually cause a sense of numbness; a form of pain ensues that feels like a tingling sensation that is often described as “pins and needles” all over the leg. The loss of blood circulation to the numbed region actually triggers pain receptors, sending an impulse to the cerebral cortex that warns of an odd feeling. The cerebral cortex will perceive the problem and command responses of the skeletal and muscular systems to change body position. Proprioceptors will sense the new body position, and as blood circulation restores a normal environment, the tingling decreases until it disappears. Numbness can also be a symptom of nerve damage or can result from the use of certain drugs, such as Novocain, that are used in dental and medical applications.
Changing of body position is an important outcome of the touch, pressure, and pain senses. Without the ability to change the position of the body, as can happen in some elderly or quadriplegic persons, damage to the areas on which the whole of the body is resting can occur. Neglect of these persons in home care or in health care facilities will result in bedsores (pressure ulcers) developing at these pressure points. If left unchecked, these bedsores will grow and can lead to gangrene. As a part of their care, such individuals require physical therapy or physical aid in moving body parts and in changing sitting and sleeping positions several times each day.
Damage to the right lobe of the brain may cause abnormalities in the senses of touch and pressure, which in turn can cause a deficit in the ability to locate precisely where a tactile sensation originates on the body surface (the ability to project tactile sensations), making adjustment to and interaction with the external environment challenging. Right lobe damage, therefore, may give rise to a condition called "passive touch deficit." Passive touch deficits are revealed by an impaired ability to discriminate touch sensations and by altered thresholds of touch and pressure that cause a sensation to be perceived.
Damage to the right lobe may also lead to deficits in active touch, meaning that such descriptors as size, shape, and texture cannot be readily discerned in touch tests. Generally, right lobe damage also leads to the loss of fine motor control of the fingers, which is especially challenging for musicians, authors, computer operators, visual artists, surgeons, and others who require exacting motor control of the fingers.
Sometimes, lesions in the right lobe of the cerebral cortex will lead to a condition called "tactile agnosia." Tactile agnosia will occur only in the left hand, given the contralateral arrangement of the hands and neurological pathways to the cortex. The symptom of tactile agnosia is the diminished ability, or the inability, to identify common items (such as a key, pencil, or comb) when it is placed in the left hand. Fortunately, this condition is not common.
Another interesting form of loss of touch can occur in an odd behavior called "neglect." Again, the problem with this loss of sensation originates from damage to the cerebral cortex, not to the touch receptors of the body. In neglect, lesions of the right parietal lobe are generally present but the contralateral neural pathway results in lost sense on the left side of the body. The ability to perceive a left-side stimulus is lost; patients may not notice anything in the internal or external environments on the left side of the body. In left-side neglect, patients may step into the right leg of a pair of pants but not the left leg and will not know that anything is wrong. Left-side neglect can also result in only right turns being made in walking patterns, and stimuli from the visual, auditory, and tactile sensations are not at all perceived. The sensation is traveling from the point of stimulation, but the cerebral cortex cannot process the sensation.
Finally, the issue of the phantom limb has significance in the medically related aspects of the sense of touch. “Phantom limb” is the term used to describe a sensation that seems to arise from a limb that has been amputated. Patients who have lost body parts to amputation, either surgically or mechanically (as a result of trauma such as a car accident or an accident while operating a meat-processing machine), often describe sensations of itching, burning, or heat or cold, as well as other general sensations in the limb that is missing.
Although the limb—such as a finger, toe, or part of a leg or arm—may be absent, general sensations seem to arise from these absent body parts because of the neurological pathways that would normally connect the limbs to the cerebral cortex. In addition, a sensation is only as accurate as the cortex’s ability to locate and identify the stimulus. This recall is called "perception." Perception, however, is not an exact science; it results from experiences and associations that teach the cortex how to sort and analyze sensory data. Much information comes into the brain on common pathways, as if only a few streets led into a special city and all cars wanted to travel those streets to get there. An index finger may have had priority access to the paths during the training days of a young concert pianist. If, however, the pianist loses that index finger, neurological activity along the pathway to which the finger was once connected has not stopped. In fact, during the adjustment and recovery period immediately following the amputation, neurological activity may be heightened. As these sensory receptors and neurons send impulses along the avenues as before, these impulses will now be dominant in the absence of the index finger. Although the conscious mind is fully aware that the finger is absent, the subconscious mind has not yet acclimated itself to the change. Thus, a false association is made before the conscious mind can “correct” the perception and realize that the stimulus must be originating from a place other than the absent body part.
Perspective and Prospects Responsiveness to different kinds of touch at different locations on the body reveals a relationship between receptor structures and their subsequent function. Specifically, the hands of the human body are exquisitely sensitive to touch. The calculated innervation (number of nerve connections) for touch alone on the palms of the hands is seventeen thousand units.
On the palms, the most abundant type of touch receptors are Meissner’s corpuscles. Accounting for 43 percent of all the touch receptors of the palms, they are responsible for the sensations of texture, light touches, and low-frequency vibrations and clearly play an important role in the human experience. Meissner’s corpuscles are particularly abundant on the tips of all fingers and both thumbs. Although these receptors adapt at a moderately rapid pace, they are not so fast at adapting that the pleasure of stroking a cat or caressing a baby’s head is lost.
After Meissner’s corpuscles, Merkel’s disks are second most abundant on the palms. Constituting 25 percent of the touch innervation density, these cells are suited for fine touch and pressure. Mainly confined to the digits’ and thumbs’ full length, Merkel’s disks are important in tactile pursuits such as painting, drawing, sewing, writing, and dentistry. They are equally important in the expression of loving gestures to other people, animals, and plants via touch. Merkel’s disks are slow to adapt; thus, these sensations are somewhat sustained.
Constituting 19 percent of the palms’ innervation are the Ruffini endings. These are scattered throughout the palm surface area and are not localized. Ruffini endings are receptive to heavy and continuous touch and are slow adaptors. While a person is carrying a stack of books, for example, these endings are “firing” the nerves that connect to them.
Finally, making up only 13 percent of the palms’ innervation to touch are the Pacinian corpuscles. Having a slight clustering in the fingertips, these are very quick at adapting to stimuli. They are receptive to deep and heavy pressure, such as the sensations that can be felt while massaging the hands.
Hairless regions of the skin, such as the palms, soles, penis, and vagina, contain Merkel’s disks, Ruffini endings, Meissner’s corpuscles, and Pacinian corpuscles. The combinations of receptors within these regions make these body parts acutely aware of and sensitive to light or heavy touch, rough or velvety textures, and pulsating or vibratory stimuli. These areas are also associated with pleasure centers of the body, in part because of their heightened tactile sensitivity.
Hairy skin, such as on the legs, chest, and arms, contains tactile disks, Ruffini endings, root hair plexuses, and Pacinian corpuscles. These body parts are sensitive to vibratory stimuli, breezes and other forms of displacement of body hairs, and pressure and tugging or pulling of the skin.
People are often willing to go to extra lengths to take care of their special sensory organs—the eyes, ears, mouth, and nose—because of their unique and important functions in the human body, but rarely do the general senses receive such attention. In spite of being largely overlooked, the general senses provide humans and other species with something so fundamental to life that it is often forgotten: touch. Offering a means of experiencing the most intimate communication and connection between self and others or between self and the environment, touch is an integral aspect of life.
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