Chapter-15
Autonomic Nervous System
1. The preganglionic fibres of sympathetic nervous system arise from the lateral horn cells of the spinal cord.
2. They pass through anterior nerve roots of spinal nerves.
3. Then they run for a short distance in the spinal nerve.
4. From the spinal nerve, they are communicated to ganglia of sympathetic chain through white rami communicantes. (The sympathetic chain consists of 21 pairs of ganglia present on either side of the vertebral column. These ganglia are connected with each other by means of fibers.
Postganglionic fibres:
They are formed by grey rami communicantes which arise from ganglia of sympathetic chain. They enter into spinal nerve of the same level and reach the organ which they supply. Some of the fibres may ascend to higher ganglia or descend to lower ganglia of the sympathetic chain. They come out from a ganglion of appropriate level through grey rami communicates and enter into spinal nerves.
Parasympathetic Nervous System
Preganglionic fibres:
The preganglionic fibres of parasympathetic nerves arise from cells present in
i) mid brain
ii) medulla
iii) sacral portion of spinal cord as follows :
1. From the mid brain, the fibres emerge through oculomotor nerve.
2. From the medulla, they emerge through facial, glassopharyngeal and vagus nerve.
3. At the sacral portion of spinal cord, they arise from the anterior column of 2nd, 3rd and 4th lumbar segments. They then pass through anterior' roots of the corresponding spinal nerves.
All these nerves (which have three different origins) end in a ganglion each. These parasympathetic ganglia are very near to the structure which these nerves supply.
Postganglionic fibres:
They arise from the ganglia and then reach the structures which these nerves supply.
CHEMICAL TRANSMITTERS OF AUTONOMIC NERVES
Parasympathetic system :
1. The transmitter in preganglionic parasympthetic nerve is acetylcholine (Ach) which is liberated at the ganglion.
2. Again, acetylcholine is the transmitter in postganglionic parasympathetic nerve. It is liberated at the postganglionic parasympathetic nerve ending.
Sympathetic system :
1. Similar to preganglionic parasympathetic nerves, acetylcholine is the transmitter in preganglionic sympathetic nerves also. 2. But in postganglionic sympathetic nerves, norad- renaline is the transmitter. It is liberated at the postganglionic sympathetic nerve ending.
Adrenergic receptors :
The receptors present at the postganglionic sympathetic nerve ending are called 'ad- renergic receptors'. The adrenergic receptors are classified as :
1. Alpha receptors, the action on which is excitatory (except intestine)
2. Beta receptors, the action on which is inhibitory (except heart)
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