Wednesday, March 21, 2012

Sensory System

A sensory system is a part of the nervous system that consists of sensory recepor cells that receive stimuli from the external or internal environment, the neural pathways that conduct information from the receptors to the brain or spinal cord, and those parts of the brain that deal primarily with processing the information. Information processed by a sensory system may or may not lead to conscious awareness of the stimulus. Regardless of whether the information reaches consciousness, it is called sensory information. If the information does reach consciousness, it can also be called a sensation.Aperson’s understanding of the sensation’s meaning is called perception. For example, feeling pain is a sensation, but awareness that a tooth hurts is a perception. Sensations and perceptions occur after modification or processing of sensory information by the CNS. This processing can accentuate, dampen, or otherwise filter sensory afferent information. At present we have little understanding of the final stages in the processing by which patterns of action potentials become sensations or perceptions.

The initial step of sensory processing is the transformation of stimulus energy first into graded potentials (the receptor potentials) and then into action potentials in nerve fibers. The pattern of action potentials in particular nerve fibers is a code that provides information about the world even though, as is frequently the case with symbols, the action potentials differ vastly from what they represent. Intuitively, it might seem that sensory systems operate like familiar electrical equipment, but this is true only up to a point. As an example, compare telephone transmission with our auditory (hearing) sensory system. The telephone changes sound waves into electrical impulses, which are then transmitted along wires to the receiver. Thus far the analogy holds. (Of course, the mechanisms by which electrical currents and action potentials are transmitted are quite different, but this does not affect our argument.) The telephone receiver then changes the coded electrical impulses back into sound waves. Here is the crucial difference, for our brain does not physically translate the code into sound. Instead, the coded information itself or some correlate of it is what we perceive as sound.