CHAPTER XVIII THE ORGANS OF SPECIAL SENSE The nerve system is the means by which the individual is brought into conscious relation with the external world. This consciousness is excited by numerous material impressions which develop nerve impulses in the end-organs of the skin, tongue, nose, eye, and ear, and are conveyed by afferent nerves to the centres in the cortex of the brain where they awake sensations. These sensations vary in character. Thus one may feel happy, fatigued, hungry, thirsty, etc., as a result of material changes going on within the body. These are usually spoken of as common or ordinary sensations. The important or special sensations arise as a result of the definite impressions made upon the highly sensitive end-organs, as touch, pain, temperature, pressure, taste, smell, light and its varying qualities, sound and its varying qualities. The physiologic mechanisms underlying these special sensations are spoken of as tactile, touch; temperature, pain, gustatory, taste; olfactory, smell; optic, sight; auditory, hearing, and are known as the special senses. The factors necessary to the production of the sensations are: (1) A special physical stimulus; (2) a specialized terminal organ (end-organ); (3) an afferent pathway which conveys the impulse to the centres in the cortex of the brain; (4) a specialized receptive sensor cell in the cortex of the brain. The special senses are five in number: sense of touch, sense of smell, sense of sight, sense of hearing, and sense of taste. THE SENSE OF TOUCH 391 THE SENSE OF TOUCH The structures essential to the appreciation of the sense of touch are the skin and mucous membrane of the mouth, etc., the end-organs therein and the afferent nerves which convey the nerve impulses by pathways through the cord and brain to the cells in the tactile area of the cerebrum which are located in the parietal lobes. The end-organs are the highly specialized organs found not only in the skin, but in other sense organs. They are the sensitive bodies intervening between the surface coming in contact with the skin, etc., and the terminal filaments of afferent nerves. They are more sensitive than the terminals of afferent nerves and receive specific stimuli which excite them to activity; and they in turn transmit the impulses to the aft'erent nerves. However, these end-organs are usually spoken of as the terminations of afferent nerves. Classification of End-organs. — (1) Free Endings.^ Club-shaped processes found in and among the cells of the epidermis (upper layer of skin) ; they are termina- tions of minute fibers of afferent nerves. 2. Tactile Cells. — ^They are oval, nucleated bodies found in the deep layer of the epidermis. They are embraced by a crescentic-shaped body (tactile disk) which is directly connected with the afferent nerve. 3. The Corpuscles of Meissner and Wagner. — These are found in the papillae of the derma (deep layer of skin), especially in the palm of the hand and finger tips. They are bodies which consist of numbers of tactile disks surrounded by connective tissue and are in connection with the terminals of afferent nerves. 4. Hair Wreaths. — ^They are minute nerve fibers, consisting of a whole axis-cylinder, which surround the hair follicle just beneath the opening of the seba- ceous glands. They are arranged in the form of a wreath. 5. Corpuscles of Vater or Pacini. — They are oval- shaped structures situated along the course of afferent nerves distributed to the skin on the palms of the hands and soles of the feet, external genital organs, joints, etc. They consist, when examined under the microscope, of bulbs composed of granular pro- toplasm surrounded by layers of connective tissue, and are joined by the axis-cylinder of the afferent nerve. The sense of touch is the sensation conveyed to the brain by an object we touch coming in contact with the end-organs in the skin and mucous mem- branes. By this sense we are enabled to touch and be touched by objects and determine their size and weight; quality, whether hard or soft, rough or smooth, sharp or dull, etc.; also the temperature of a body or surface coming in contact with the skin and mucous membranes, whether it is hot or cold. The Skin. — The skin possesses (1) touch sense and (2) temperature sense. The touch sense is subdivided into (a) pressure sense and (6) place sense. The touch sense is stimulated by mechanical pressure coming in contact with the end-organs in the skin and mucous membrane. Touch Spots. — The areas of the skin and membrane, which when stimulated by an object, as pin, knife, etc., give rise to the sensation of touch, are not general throughout the surface of the skin, but are localized spots in the skin (these areas are called touch spots) with intervals which are insensitive to stimuli. The greater number of touch spots in a given area the greater is the acuteness of the touch. These spots are more numerous in the hands and in the neighbor- hood of hair follicles. The skin of the index finger over the last phalanx is particularly well supplied with touch spots localized in the corpuscles of Meissner, thus rendering the tip of the finger more acute in the sense of touch. It must be remembered that when the pressure and temperature (extremes of heat or cold) of an object are too severe the sense of touch and temperature is lost in the sense of pain. The sense of touch is replaced by the sense of pain when the skin is bruised or burned, so that the epidermis is destroyed, leaving the nerves too exposed. The sense of pressure is the variety of the touch sense which is based on the fact that, when pressure is brought to bear along with touching an object, the pressure of the object coming in contact with the skin must reach a certain intensity before the sensation of weight will be appreciated, and permit the individual to determine the amount of the pressure, and gauge the comparative pressure of low weights. The Sense of Place. — Is, based on the fact that when a stimulus touches any portion of the skin the sensa- tion caused thereby is, under normal conditions, always referred to the place stimulated. This is always the case, whether the place of stimulation is at two points near or distant from each other on the same side of the body, or the corresponding place on the opposite side. These areas for the localization of sensations are usually arranged in circles throughout the surface of the skin. The Temperature Sense. — This is supposed to be due to the presence in the skin of special nerve-endings which give rise to sensations of heat and cold, and are different from each other, as well as from those end-organs which give rise to sensations of touch. It has been proved by investigation that throughout the surface of the skin there are spots, called heat and cold spots, which if stimulated give rise to sensations of heat and cold. Each spot responds to only one kind of stimulus, thus a warm object applied to the skin will only affect the heat spots, and give rise to the sensation of warmth, and the application of a cold one will only influence the cold spots and create a sensation of coldness. The specific physiologic stimuli to the end-organs controlling the temperature sense are termed thermic vibrations.
Key Takeaways
- The skin contains specialized end-organs like tactile disks and corpuscles for touch sensation.
- Touch spots in the skin, particularly on the fingertips, enhance our ability to perceive small objects.
- Pressure sense is a component of the overall sense of touch.
Practical Tips
- Regularly check your skin for injuries or changes that could affect your sense of touch and temperature.
- Wear gloves when handling hot or sharp objects to protect your skin from damage.
- Practice using your fingertips, which are rich in tactile receptors, to improve fine motor skills.
Warnings & Risks
- Severe pressure or extreme temperatures can cause loss of sensation, leading to accidents.
- Bruised or burned skin is more sensitive and can lead to pain instead of touch.
- Ignoring changes in your skin's sensitivity can delay the detection of serious conditions like nerve damage.
Modern Application
Understanding the anatomy and physiology of the sense of touch remains crucial for modern survival preparedness. While the specific end-organs have been identified, the principles of how our body perceives pressure and temperature are still relevant. Modern techniques in wound care and pain management can help prevent complications from injuries that might affect these senses.
Frequently Asked Questions
Q: What are tactile disks and where are they found?
Tactile disks are oval, nucleated bodies found in the deep layer of the epidermis. They are part of the end-organs responsible for touch sensation.
Q: How do the corpuscles of Meissner contribute to our sense of touch?
The corpuscles of Meissner are found in the papillae of the derma, especially on the fingertips and palms. They enhance the sensitivity of these areas, making them more acute in detecting touch.
Q: What happens when the skin is bruised or burned?
When the skin is bruised or burned, the epidermis may be destroyed, leaving the nerves exposed. This can cause a shift from the sense of touch to pain.