Sensory Receptors and Neural Transmission in Human Skin

Introduction

This paper reviews the sensory cells in the human body and examines the transmission of sensory information from various parts of the body to the central nervous system. The paper relies on peer-reviewed scholarly publications as sources for all materials presented.

Sensory Receptors

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 stimuli notices to sensory neurons, which in turn send the messages to the brain (central nervous system / CNS) (Darpan, 2006). To visualize how this works, sensory receptors can be understood as "portals through which nervous systems experience the world" (Darpan).

Types of Sensory Receptors

The "mechanoreceptors" receive stimulation from touch and pressure on the skin; they also respond to sound, vibrations, and balance. These receptors are found in the ears and alert the individual to what is happening in the surrounding environment (Darpan).

The "photoreceptors"—found in the rods and cones of the retina—are stimulated by light and changes in light intensity. In darker environments, photoreceptors adjust so the person can still receive stimulation and maintain balance (Darpan).

In human skin, which is the largest organ, there are several important sensory receptors. The Pacinian corpuscle, found beneath the surface of the skin, converts pressure stimulation "into a neural message that is relayed to the brain" (Hockenbury et al., 2008). When there is constant pressure, "sensory adaptation" activates and the Pacinian corpuscle "either reduces the number of signals sent, or quits responding altogether" (Hockenbury). Pacinian corpuscles are found in greater numbers on the soles of the feet and on the palms of the hands.

Human skin also contains "nociceptors"—tiny sensory fibers also present in the muscles, called free nerve endings. There are "millions of nociceptors in the body and particularly in the skin," according to Hockenbury.

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

Transmission of Sensory Information to the Central Nervous System

Once stimuli have been initially processed by the sensory receptor, some encoding takes place before the stimuli can reach the central nervous system. According to Rodney A. Rhoades, the first encoding step is what he calls "compression," in which "the range in the intensity of the stimulus" is substantial. There is a "hundred-fold" potential variation in the compressed stimulus (Rhoades, 2012). However, there is also an upper limit to the number of "action potentials" in the stimuli because a neuron's "per second" stimuli has a finite refractory period (Rhoades).

The second step is "transmission"—once the dynamics at the compression level have occurred, the sensory message must be sent to the CNS in order for the body to know that a stimulus has taken place.

Conclusion

In conclusion, while these stimuli and the discussion of how sensory receptors respond to them require considerable explanation of their mechanisms and functions, these processes are instantaneous. Pricking your finger triggers a response in which the sensory receptor transmits information through compression and transmission to the central nervous system in a microsecond. It is an remarkable system, and few people fully understand how it works, which is what makes physiology so fascinating when examined in depth.