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5. SOME GENERAL CONCEPTS ASSOCIATED WITH CODING AND NEURONAL PROCESSING IN THE NERVOUS SYSTEMNeural Signals: Label Line Codes and Pattern Codes (Fig. 3.13) The body responds to external and internal environmental stimuli through receptors that are associated with modalities of the sensory systems. The recognized receptors are mechanoreceptors, nociceptors, thermoreceptors, photoreceptors, and chemoreceptors. The modalities comprise general somatic senses, balance, audition, vision, taste, and smell. Most sensory receptors are sensitive to specific stimulus energies, called receptor specificity that results in generating a neural signal as a signalline code in a first-order neuron. The code of specific modalities associated with, for example, touch (Chap. 10) is conveyed via specific Chapter 3 Basic Neurophysiology 71 lines though a series of neurons and neural centers of the CNS where they are processed to evoke the perception of the sensation. Each sensory receptor responds when it is adequately stimulated, in a specific manner regardless of the stimulus. Whether activated by a natural or an artificial (e.g., electrical) stimulus, the same sensation is elicited. Chemoreceptors lack the specificity of responding to a single stimulus signal and, thus, do not project their information along signal lines. Rather, the modalities associated with taste and smell use a pattern code signal in which several receptors are activated. The resulting discharge pattern of signals formed by a group of neurons is the basis for the perception of a given flavor or odor.
Transformation of the Neural Signals at Processing Centers of Pathways
Neural processing occurs within the neural centers (laminae, nuclei, and cortices) of each pathway system. Neurons within these centers process the patterns of incoming signals and transform them so that the relay neurons project a different pattern of signals to other centers. Each center performs a transformation function. Each center consists of (1) relay neurons whose axons project to other centers, (2) interneurons with processes located wholly within the center, and (3) terminations of axons from other sources.
Processing Within the Nervous System
The ascending (and descending) pathways function both as processors and as transmitters of coded information. The processing within a center of a pathway is information linked, not energy linked. For example, stimulation of the optic system evokes sensations related to vision, regardless of whether the stimulus is light, an electrical shock, or a blow to the eye.
The sensory inputs to the relay neurons within a center are examples of convergence (input of axons from many neurons to one relay neuron) and divergence (input of an axon to more than one relay neuron). These inputs form a basis for some of the complexities of interactions among neurons. The numerous signals arriving at each processing center and interaction with the local interneurons act to bias, enhance or dampen signals and their transmission. In effect, each center acts as an editor. A major role in the processing is performed by the local inhibitory interneurons in (1) feedback inhibition, (2) feed-forward inhibition, and (3) reflected inhibition.
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