Wednesday, March 15, 2017

Earthworm: Nervous System

It is well developed and consists of central nervous system, peripheral nervous system and sympathetic nervous system.
Central nervous system:-   It consists of nerve ring and ventral nerve cord.



Nerve ring consists of supra-pharyngeal ganglia, circum-pharyngeal connectives and sub-pharyngeal ganglia.
A pair of supra-pharyngeal ganglia are present at the middle of 3rd  segment on dorsal surface of depression between buccal chamber and pharynx. Each ganglion is small, round and white coloured  structure and they remain fused with each other at middle. They are also called as cerebral ganglia or brain.
From outer side of each supra-pharyngeal ganglia a thick and stout circum-pharyngeal connectives arises and pass posterior-ventrally encircling the pharynx and meet with sub-pharyngeal ganglia.
A pair of sub-pharyngeal ganglia are present ventrally below the pharynx in 4th segment. Each ganglion is small, round and white coloured structure and they remain fused with each other at middle.
Due to the presence of supra-pharyngeal ganglia, circum-pharyngeal connectives and sub-pharyngeal ganglia a complete ring like structure is formed around the pharynx and is called as nerve ring or nerve collar.
Ventral nerve cord arises from sub-pharyngeal ganglia and runs mid-ventrally upto posterior end of body. At the middle of each segment from 5th segment , it remains enlarged to form segmental or ventral nerve cord ganglia. Externally, it seems single but consists of two nerve cords each arising from each sub-pharyngeal ganglia.
The ventral nerve cord consists of nerve cells and nerve fibers. The fibers are present at center forming core of fibers. Two cores of fibers are present in ventral nerve cord which proves double nature of ventral nerve cord. These cores of fibers remain fused with each other in segmental ganglia. The nerve cells are present on sides and under surface of core of fibers. The number of cells is more in segmental ganglia. Four giant nerve fibers are present dorsal to core of fibers. All these are present in mass of connective tissue called as neuroglea and all these remain inside a capsular structure called as epineurium outer to which a layer of longitudinal muscles is present and externally it remains covered by peritoneum.
Peripheral nervous system
       It consists of nerves arising from central nervous system. About 8-10 pairs of nerves arise from supra-pharyngeal ganglia and pass into prostomium and buccal chamber. Two pairs of nerves arise from circum-pharyngeal ganglia and they pass into peristomium and buccal chamber. Three pairs of nerves arise from sub-pharyngeal ganglia and each pair pass into 2nd, 3rd and 4th segment respectively. Three pairs of nerves arise from each segmental ganglia and pass in to different parts of that segment.
The nerves are of both types. Some are sensory or afferent  carrying information from receptors up to central nervous system while some are motor or efferent carrying information from central nervous system up to effectors.
Sympathetic nervous system

It consists of network of nerves present in muscles of body wall and muscles of alimentary canal. These network of nerves i.e. nerve plexus remains connected with circum-pharyngeal  connectives.