Intro | Free Nerve Endings | Meissner's Corpuscle | Merkel's Disks | Pacinian Corpuscle | Ruffini Corpuscles | Subcutaneus Fat | Sweat Gland
Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test
The sensations of the body are referred to as somatosensation. Three separate systems interact to provide us with information about the state of our body. The exteroceptive system senses stimuli applied to the skin of the body; including mechanical stimuli for the sensation of touch, thermal stimuli for the sensation of temperature, and nociceptive stimuli for the sensation of pain. The interoceptive system provides general information about internal body conditions; including temperature and blood pressure. These organic senses ("senses of the organs") originate in and around organs of the body, including the receptors in the lining of muscles, outer layers of the gastrointestional system, and the linings of the thoracic and abdominal cavities. The majority of these receptors are sensitive to stretch alone, but some are sensitive to changes in temperature and blood pressure. Finally, the proprioceptive system (or kinesthesia) senses the position of body parts via stretch receptors located in the muscles (see Tutorial 31), joints, tendons, and organs of balance within the inner ear.
This tutorial illustrates and describes receptors of the exteroceptive system found in the skin (the cutaneous receptors). These cutaneous receptors are found in both hairy skin and in hairless or glabrous skin (fingertips and palms), and respond to a number of different stimuli including pressure (tissue displacement), heat, cold, vibration, and pain-inducing events associated with tissue damage. Hairy skin contains primarily free nerve endings and Ruffini corpuscles. However, Pacinian corpuscles and Merkel's disks are also present. Glabrous skin is more complex, with free nerve endings and axons that end in specialized end organs known as Meissner's corpuscles. Pacinian corpuscles, Ruffini corpuscles, and Merkel's disks are also found in hairless skin. Meissner's corpuscles, however, are found exclusively in hairless skin.
The sensation of touch is accomplished through movement of the skin causing low-frequency pressure or high-frequency vibration. The specialized receptors of glabrous skin support fine touch discrimination, and differ in their rate of adaptation to stimuli, receptive field size, and depth in the cutaneous layer. Skin is a complex and vital organ of the body that is often underrated in its importance in sustaining life. It is composed of two primary layers; from the surface down they are the epidermis and the dermis. Below the cutaneous layers is a layer of subcutaneous fat, which contains primarily fat and blood vessels.
Tutorial 28 will discuss the somatosensory pathways for the mechanical and thermal divisions of the exteroceptive system and the proprioceptive system. Click here for a detailed description of the sensation of pain and brain pain pathways (the nociceptive division of the exteroceptive system).
| Advanced |
A more detailed study of the skin's histology provides a more refined understanding of the composition of cutaneous tissue (Ham, 1974). The epidermis of the skin is actually composed of multiple layers. There are several layers of living tissue that contain the Merkel's disks and the free nerve endings. The outermost epidermal layer is composed of nonliving keratin; a protective layer that varies in thickness depending on the body region. The keratin layer is usually thickest on the soles of the feet, where the greatest protection is needed. The dermis of the skin is composed to two layers of connective tissue, the pupillary layer and the reticular layer. The pupillary layer of the dermis is in direct contact with the epidermis and contains many capillaries that help to control body temperature via their control of blood flow close to the surface of the skin. The reticular layer (meaning "network") of the dermis contains collagen and elastic fibers, which help to maintain the structure of skin.
| Reference |
Ham, A.W. (1974). Histology (7th Ed.). Philadelphia: J.B. Lippincott Company.
| Suggestions for further study |
Bower, T.G. (1971, October). The object in the world of the infant. Scientific American, 255(4), 30-38.
Geldard, F.A., Sherrick, C.E. (1986, July). Space, time and touch. Scientific American, 255(1), 90-95.
Geschwind, N. (1979, September). Specializations of the human brain. Scientific American, 241(3), 180-199.
Rock, I, Harris, C.S. (1967, May). Vision and touch. Scientific American, 216(5), 96-104.
Stix, G. (1991, February). Reach out. Touch is added to virtual reality simulations. Scientific American, 264(2), 134.
http://faculty.washington.edu/chudler/receptor.html
(Skin Somatosensory Receptors)
http://nba5.med.uth.tmc.edu/academic/neuroscience/lectures/section_2/lecture22_06.htm
(Somatosensory System)
Chiyeko Tsuchitani, Department of Neurobiology and Anatomy The
University of Texas-Houston Health Science Center, Advanced Tutorials.
http://thalamus.wustl.edu/course/body.html
(Somatosensory Pathways From The Body)
The Washington University School of Medicine, Neuroscience Tutorial.
http://pegasus.cc.ucf.edu/~Brainmd1/brain2.html
(Brain Tutorial)
http://pegasus.cc.ucf.edu/~Brainmd1/lobes2.html
(Human Cortex)
Mark Darty, University of Central Florida, Brain Tutorial.
http://www.med.jhu.edu/somlab/
(The Somatosensory Laboratories)
K. Johnson, S. Hsiao, The Mind/Brain Institute, Johns Hopkins University.
http://www.ctiphysicaltherapy.com/somato.htm
(Somatosensory Evoked Potentials)