In a typical neuron, the cytoplasm is drawn out into short processes called dendrites and a single long process called an axon. There are many variations on this basic pattern (Campbell, 3rd ed. page 985; 4th ed. page 996; 5th ed. page 962-963).
The simplest nervous system, as seen in the cnidarians (such as Hydra), consists of a diffuse network of nerve cells and little or no concentration of nervous tissues. Among the more complicated multicellular animals there is a marked tendency for the cell bodies of neurons to be grouped into masses known as ganglia. Ganglia tend to be lined up along one or more central nerve cords. The most anterior of these ganglia tends to form a "brain". Associated with the accumulation of nervous tissue in the anterior region is an accompanying proliferation of sense organs (eyes, antennae, chemoreceptors, ears, etc.) in this same area. The term cephalization is used to describe the overall evolutionary process and it seems to be definitely linked with bilateral symmetry (see Campbell, 3rd ed. Fig 44.16 page 1001; 4th ed. Fig 44.13 page 1010; 5th ed. page 977 Fig 48.13).
The spinal nerves are best studied in the frog because they are exposed on the dorsal wall of the body cavity, instead of being deep in the flesh as in the rat. The frog has 10 pairs of spinal nerves, one pair corresponding to each of its 10 vertebrae.
Examine the frog and observe the spinal nerves, whitish, thread-like structures that emerge from between the vertebrae and lie along the dorsal wall of the body cavity.
If you are having problems finding the nerves try this image
| Spinal Nerve I | The most anterior spinal nerve, leaves the vertical column, bends posteriorly then anteriorly. In the frog, it will go the chin region. |
| Spinal Nerve II | The brachial nerve, running to the arm |
| Spinal Nerve III | Forms a connection with I and II before it passes laterally to the body wall. Where the three join, they form the brachial plexus. |
| Spinal Nerves IV, V and VI | Small delicate nerves to the abdominal wall |
| Spinal Nerves VII, VIII and IX | Unite to form a complicated sciatic plexus which results in the sciatic nerve to the hind leg |
| Spinal Nerve X | very small nerve to the pelvic region. It may also contribute fibers to the sciatic plexus. |
Each spinal nerve is connected to the spinal cord by two connections, a dorsal and a ventral root. The dorsal root has an enlargement which contains numerous nerve cell bodies and is known as the dorsal root ganglion. Impulses from receptors, such as pain receptors in the skin, enter the CNS by way of the dorsal root. Motor impulses exit via the ventral root which contains no nerve cell bodies. Both roots soon join to form a spinal nerve which is termed mixed since it contains both sensory and motor fibers.
In the next section we will examine the Spinal cord in cross section
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First published Jan 99: Modified Jan 99
Copyright © Michael Shaw 1998 (Images and Text)
