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PinkMonkey Online Study Guide-Biology

(B) The Spinal Cord

The spinal cord extends from the medulla or brain stem down to the second lumbar vertebra. It is surrounded and protected by the meninges (same as covering the brain) and the vertebrae of vertebral column. It is a hollow, oval shaped cylinder, tapering slightly as it descends.

A transverse section of the spinal cord (Figure 23.4) has an H-shaped region of grey matter. The grey matter contains nerve cells and non-myelinated fibers.

A small central canal containing cerebrospinal fluid runs through the center of the chord. Outside the grey matter is white matter comprised of numerous medullated nerve fibers running up (ascending) and down (descending) the cord or passing out to the spinal nerves. These fibers usually cross over from one part of the spinal cord to the other along their pathways to and from the brain. Thus, the right side of brain receives impulses that originate on the left side of the body and vice versa.

Functions : (1) The spinal chord serves as a pathway for the condition of impulses between the receptors and effector organs and the brain. (2) It acts as a reflex center (spinal reflexes) for many local simple reflexes.

(i) Cranial nerves : There are 12 pairs of cranial nerves in mammals (10 pairs in fishes and amphibians) and most of them lead away from the medulla oblongata (Figure 23.5). Of these 12 pairs, some are purely sensory, some are purely motor and the others are mixed. The table shows the name, number, nature, function and distribution of the cranial nerves.

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Figure 23.4 Transverse section of a Spinal Cord

Cranial nerves of man

No. Name Nature Function Distribution
1) Olfactory Sensory Smell From nose
2) Optic Sensory Vision From eye
3) Oculomotor Motor Eye movement To muscles of eye ball
4) Trochlear Motor Eye movement To muscles of eye ball
5) Trigeminal Mixed Sensitive and jaw movement From and to face, teeth, lips, tongue,jaws
6) Abducens Motor Eye movement To muscles of eye ball
7) Facial Mixed Taste sensation, jaw movement From taste buds, to sali vary glands and face
8) Auditory Sensory Hearing and balance From ear
9) Glossopharyngeal Mixed Muscle movement and sensations From and to pharynx, from taste buds Salivary glands
10) Vagus Mixed Sensory to chest and abodomen From and to visceral organs
11) Spinal Motor Movement of shoulder muscles To shoulder muscles
12) Hypoglossal Motor Movement of tongue To tongue

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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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