The nervous system is a rapid communication system that interacts continuously with the endocrine system to control coordination of body functions. In the vertebrates it plays three basic roles: (i) it acquaints the organism with its external environment, and stimulates the organism to orient itself favorably to that environment; (ii) it participates in regulation of the internal environment, and; (iii) it serves as storage for information These functions are accomplished by the nerves, spinal cord, and brain in association with receptors (sense organs) and effectors (muscles and glands). The basic unit of nervous integration in all animals is the neuron, a highly specialized cell designed to conduct self-propagating impulses, called action potentials, to other cells. Action potentials are transmitted from one neuron to another across synpses which may be either electrical or chemical. The thin gap between neurons at chemical synapses is bridged by a chemical neuro-transmitter molecule, which is released from the synaptic knob, and can be either stimulatory or inhibitory.

The simplest organization of neurons into a system is the nerve net of Cnidarians, basically a plexus of nerve cells that, with additions, is the basis of nervous systems of several invertebrate phyla. With the appearance of ganglia (nerve centers) in bilateral flatworms, nervous system is differentiated into central and peripheral divisions. In vertebrates, the central nervous system consists of a brain and spinal cord. Fishes and amphibians have a three-part linear brain, whereas in mammals, the cerebral cortex has become a vastly enlarged multicomponent structure that has assumed the most important integrative activities of the nervous system. It completely overshadows the ancient brain, which is consigned to the role of relay center and to serving numerous unconscious but nonetheless vital functions such as breathing, blood pressure, and heart rate.

In human the left cerebral hemisphere is usually specialized for language and mathematical skills while the right hemisphere is specialized for visual, spatial, and musical skills.

The peripheral nervous system connects the central nervous system to receptors and effector organs. It is divided broadly into an afferent system, which conducts sensory signals to the central nervous system, and an efferent system, which conveys motor impulses to effector organs. This efferent system is subdivided into the somatic nervous system, which innervates skeletal muscles, and the autonomic nervous system, which innervates smooth and cardiac muscle and glands. The autonomic nervous system is subdivided into anatomically distinct sympathetic and parasympathetic systems, each of which sends fibers to most body organs. Generally the sympathetic system governs excitatory activities and the parasympathetic system governs maintenance and restoration of body resources.




Sensory organs are receptors designed especially to respond to internal or environmental change. The most primitive and ubiquitous sense is chemoreception. Chemoreceptors may be contact receptors, such as the vertebrate sense of taste, or distance receptors such as smell, which detects airborne molecules. In both cases, a specific chemical interacts with a particular receptor and results in impulses that are transmitted to, and interpreted by the brain. In spite of the similarity between these two senses, the sense of smell is far more sensitive and complex.

Receptors for touch, pain, equilibrium, and hearing are all mechanical force receptors. Touch and pain receptors are characteristically simple structures, but hearing and equilibrium are highly specialized senses based on special hair cells that respond to mechanical deformation. Sound waves received by the ear are mechanically amplified and transmitted to the inner ear where different areas of cochlea respond to different sound frequencies. Equilibrium receptors, also located in the inner ear, consist of two saclike static balance organs and three semicircular canals that detect rotational acceleration.

Vision receptors (photoreceptors) are associated with special pigment molecules that photochemically decompose in the presence of light and, in doing so, trigger nerve impulses in optic fibers. The advanced compound eye of arthropods is especially well suited to detecting motion in the visual field. Vertebrates have a camera eye with focusing optics. The photoreceptor cells of the retina are of two kinds: rods, designed for high sensitivity with dim light, and cones, designed for color vision in daylight. Cones predominate in the fovea centralis of human eyes, the area of keenest vision. Rods are more abundant in peripheral areas of the retina.


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