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PinkMonkey Online Study Guide-Biology

(ii) Spinal nerves : There are 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal pairs of spinal nerves (31 pairs in all), which leave the spinal cord between adjacent vertebrae. Each spinal nerve is formed by the joining of two roots, a dorsal root (somatic and visceral sensory) and a ventral root (somatic and visceral motor), to form a mixed nerve (Figure 23.6).

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Figure 23.6 Formation of spinal nerve and its divisions

The nerve soon splits into three branches, each containing some sensory and some motor fibers. Ramus dorsalis serves the skin and muscles of the back, another innervates skin and muscles of the front of the body, and the third serves internal organs (autonomic branch). The cervical spinal nerves innervate diaphragm, skin and muscles of the neck and arms, the thoracic to skin, muscles of thorax, the lumbar nerves innerrate to skin and muscles of thigh, hip and lower leg, while the sacral and coccygeal nerves innervate skin and muscles of leg, foot, genitalia and abdomen. After many spinal nerves emerge from the cord, they branch extensively to form networks of nerves called a plexus. Four of such plexuses are formed (i.e. cervical plexus in the neck region, brachial plexus in thorax, lumbar plexus in the abdomen and sacral plexus in the lumbo-sacral region).

Figure 23.5 Cranial nerves (Underside of brain and part of spinal cord)

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Table of Contents

23.0 Introduction
23.1 Central nervous system
23.2 The automatic nervous system
23.3 Receptors and effects
23.4 Reflex action - mechanism of nervous action

Chapter 24


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