RECEPTION [ MESSENGERS ( THE NERVOUS SYSTEM ) ]

A healthy brain needs a constant stream of incoming information. Picture what happens without it: When volunteers enter a sensory deprivation tank a body temperature pool of water in which they are forced to go without sights, sounds, smells, tastes, and skin sensations they begin to hallucinate; their brain creates stimuli to stay occupied. Insanity awaits those whose brain starves for external stimulation. Conversely, a healthy body needs the brain to send it signals. Deprived of adequate motion because of nerve damage or a sedentary lifestyle, for example, once strong muscles of the body will quickly atrophy.

Sensory receptors come in five types. The mechanoreceptors create nerve impulses when their physical shape changes in response to external force, such as pressure or touch.

Touching a devil's club thorn stimulates pressure-sensitive mechanoreceptors and, possibly, pain-sensitive nociceptors in the fingertips.
Touching a devil’s club thorn stimulates pressure-sensitive mechanoreceptors and, possibly, pain-sensitive nociceptors in the fingertips.

Photoreceptors respond to light. Curiously, not all photoreceptors exist in the eyes; some are found in the skin. Scientists at Cornell University and at White Plains, New York, found they could combat jet lag and insomnia by shining lights on the back side of the sufferer’s knees. Thermo receptors register heat and cold. Chemoreceptors register the presence of chemicals, such as the sugars in an orange when you bite into it.

Photoreceptors in the eye begin the neural circuitry that registers sensations of visible light.
Photoreceptors in the eye begin the neural circuitry that registers sensations of visible light.

And last are the nociceptors, which respond to external stimuli that have the potential to create, or do create, pain. The body needs to process painful feelings in order to warn it of possible larger dangers that pose threats to life and limb.

SENSORY RECEPTORS

Nociceptors are able to act in concert with other sensory receptors. For example, the warmth of a fire on a wickedly cold day feels good on the feet because it stimulates thermo receptors in the skin. If the toes get too close to the flames, however, extreme heat activates the nociceptors and the sensation changes from pleasure to pain.

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