Sensory Receptors and the Transmission of Sensory Information

Introduction

When humans respond to various stimuli — cold, pain, heat, light, sound, or touch — it is because sensory receptors have transmitted the body's response to those stimuli to the central nervous system. Sensory receptors are specialized cells that detect environmental stimuli and first pass those signals to sensory neurons; those neurons, in turn, send the information to the brain, or central nervous system (CNS) (Darpan, 2006). To make it easier to picture how this works, sensory receptors can be thought of as "portals through which nervous systems experience the world" (Darpan).

Types of Sensory Receptors

Mechanoreceptors receive stimulation from touch and from pressure on the skin; they also respond to sound, vibrations, and balance. Found in the ears, they alert the individual to what is happening in the surrounding environment (Darpan).

Photoreceptors — found in the rods and cones of the retina — are stimulated by light and changes in light. In darker environments they adjust so that the person can still receive stimulation and maintain balance (Darpan). The photoreceptor cells of the retina are among the most studied sensory structures in human physiology.

Sensory Receptors in Human Skin

Human skin, the largest organ in the body, contains a number of important sensory receptors. Among these is the Pacinian corpuscle, found beneath the surface of the skin. When pressure is applied to the Pacinian corpuscle, that stimulation is converted into "a neural message that is relayed to the brain" (Hockenbury et al., 2008). If pressure is constant, "sensory adaptation" occurs and the Pacinian corpuscle "either reduces the number of signals sent, or quits responding altogether" (Hockenbury). Pacinian corpuscles are found in greatest numbers on the soles of the feet and on the palms of the hands.

Human skin also contains nociceptors — tiny sensory fibers that are present in the muscles as well. These are called free nerve endings, and there are millions of them in the body, particularly in the skin (Hockenbury).

The Biology of the Skin extends scientific understanding of free nerve endings further. Free nerve endings are associated with "individual Merkel cells of the epidermis," and single Merkel cells are found at the tips of the rete ridges in the glabrous skin of the fingertips, the lips, gingival tissue, and nail bed (Freinkel, 2001). Merkel cells are also found in hair follicles in "two belt-like clusters." These cells function as sensory receptors, but they also play a role in the paracrine and autocrine regulation of inflammatory diseases. While they transmit stimuli to the CNS, they also alert the body to the existence of developing inflammatory conditions (Freinkel, 158).

Conclusion

While the discussion of how sensory receptors respond to stimuli requires considerable explanation, these processes themselves are instantaneous. Prick a finger, and in a microsecond the sensory receptor transmits that information through compression and transmission to the central nervous system. It is a remarkable system, and one that few people fully appreciate — which is precisely what makes physiology so fascinating when examined in depth.

Works Cited

Darpan, P. (2006). Competition Science Vision. 8(96).

Freinkel, R.K., and Woodley, D.T. (2001). The Biology of the Skin. Boca Raton, FL: CRC Press.

Hockenbury, D.H., and Hockenbury, S.E. (2008). Psychology. New York, NY: Macmillan.

Rhoades, R.A., and Bell, D.R. (2012). Medical Physiology: Principles for Clinical Medicine. Philadelphia, PA: Lippincott Williams & Wilkins.