One of the characteristics of higher animals is their possession of a more or less elaborate system for the rapid transfer of information through the body in the form of electrical signals, or nervous impulses. At the bottom of the evolutionary scale, the nervous system of some primitive invertebrates consists simply of an interconnected network of undifferentiated nerve cells. The next step in complexity is the division of the system into sensory nerves responsible for gathering incoming information, and motor nerves responsible for bringing about an appropriate response. The nerve cell bodies are grouped together to form ganglia. Specialized receptor organs are developed to detect every kind of change in the external and internal environment; and likewise there are various types of effector organ formed by muscles and glands, to which the outgoing instructions are channelled. In invertebrates, the ganglia which serve to link the inputs and outputs remain to some extent anatomically separate, but in vertebrates the bulk of the nerve cell bodies are collected together in the central nervous system. The peripheral nervous system thus consists of afferent sensory nerves conveying information to the central nervous system, and efferent motor nerves conveying instructions from it. Within the central nervous system, the different pathways are connected up by large numbers of interneurons which have an integrative function.
Certain ganglia involved in internal homeostasis remain outside the central nervous system. Together with the preganglionic nerve trunks leading to them, and the postganglionic fibres arising from them which innervate smooth muscle and gland cells in the animal's viscera and elsewhere, they constitute the
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Fig. 1.1. Schematic diagrams (not to scale) of the structure of: a, a spinal motoneuron; b, a spinal sensory neuron; c, a pyramidal cell from the motor cortex of the brain; d, a bipolar neuron in the vertebrate retina.
autonomic nervous system. The preganglionic autonomic fibres leave the central nervous system in two distinct outflows. Those in the cranial and sacral nerves form the parasympathetic division of the autonomic system, while those coming from the thoracic and lumbar segments of the spinal cord form the sympathetic division.
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