Sensory Information and the Central Nervous System

¶ … sensory cells in the human body and delves into the transmission of sensory information from various parts of the body to the central nervous system. This paper uses bona fide scholarly publications as sources for the 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 are working for humans by detecting environmental stimuli and first passing those stimuli notices to "sensory neurons," and those neurons, in turn, send the messages (information) to the brain (Central nervous system / CNS) (Darpan, 2006). So, to make it easier to picture how this works, it could be said that the sensory receptors are basically "portals through which nervous systems experience the world" (Darpan).

Types of Sensory Receptors

The "mechanoreceptors" receive stimulation from touch, and from pressure that occurs on the skin; they also respond to sound, to vibrations and to balance, in essence they are found in the ears and alert the individual to what is happening around him or her (Darpan).

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

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

The human skin also has "nociceptors" -- tiny sensory fibers -- that are also in the muscles. These are called free nerve endings. There are "millions of nociceptors in the body and particularly in the skin, Hockenbury explains.

A book called The Biology of the Skin takes the free nerve endings further into scientific understanding. 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 fingertips, the lips, gingival and nail bed" (Freinkel, 2001). The Merkel cells are also found in hair follicles "in two belt-like clusters." Merkel cells function as sensory receptors, but they also serve a purpose along with the paracrine and autocrine regulation of inflammatory diseases. So while they provide the ability to send stimuli to the CNS, they also alert the body (brain) to the existence of inflammatory diseases that may be brewing (Freinkel, 158).

The transmission of sensory information to the Central Nervous System

Once stimuli has been partially / initially processed by the sensory receptor, according to Rodney A. Rhoades, there is some encoding that takes place before the stimuli can reach the central nervous system. The first encoding step is what Rhoades calls "compression," and the "range in the intensity of the stimulus" is substantial. In fact there is a "hundred-fold" potential variation in the compressed stimulus (Rhoades, 2012). That said, it is also true that there is an upper limit to the number of "action potentials" in the stimuli because the potential is that a neuron's "per second" stimuli has a finite period of refraction (Rhoades).