Nervous System of Earthworm

Well developed and concentratednervous system.
Consists of 3 parts: thecentral, peripheral,andsympatheticnervous system.

Figure: Nervous System of Earthworm. Image Source:Meridian Technical Charter High School.

Table of Contents

1. Central nervous system

It comprises of the anteriornerve ringorbrain ringand the posteriorventral nerve cord.

Nerve ring

Comprises ofcerebral ganglia or supra-pharyngeal ganglia,circumpharyngeal connectives,andsubpharyngeal ganglia.
A pair of closely united white, pear-shaped,cerebral,orsupra-pharyngeal ganglia, forming the so-calledbrain.
The brain lies dorsally in 3^rdsegment in the groove between the buccal cavity and pharynx.
From the brain a pair of thick stoutcircumorperi-pharyngealconnectives arise, one on each side, embracing the pharynx and meeting ventrally in4th segment by a pair of fusedsub-pharyngeal ganglia.
In this way, anerve collaris formed around the pharynx called anerve ringorbrain ring.

Ventral nerve cord

Originates from the sub-pharyngeal ganglia and runs backward in the mid-ventral line to the posterior end of the body.
It appears to be single but really is double and made of 2 longitudinal cords fused together in a transverse section.
In each segment from 5^thto the last, it presents a slight enlargement or aganglion.
Ganglia fused together as swellings representing thesegmental ganglia.
Histologically, the nerve cord consists ofnerve fibersandnerve cells.
Externally, nerve cords ofPheretimaare solid an enclosed in a sheath called theperineurium.
Perineurium consists of 3 layers- the outerperitoneum, middle thin layer oflongitudinal muscle fibers,and inner thick fibrous layer of anepineurium.
Both nerve cords are separated internally by a vertical epineurium.
In the region of segmental ganglia,2 cores of nerve fibers are completely fused along the middle line.
The ventrolateral region of the nerve cord containsbipolarandtripolar nerve cellsall along the length.
Nerve cells occur more in ganglia.
Nerve cells and nerve fibers lie embedded in a mass of connective tissues, calledneuroglia.
Dorsally,4 giant nerve fibers(1 median, 1 submedian, and 2 lateral) runs through the mass of connective tissues along the entire nerve cord.
These nerve fibers surrounded by epineurium, all are tubular and filled with homogenous plasma-like matter which helps in their contraction.
These fibers are responsible for the rapid conduction of impulses throughout the nerve cord.
In the region of segmental ganglia, the partition between the 2 nerve cords are absent.

2. Peripheral nervous system

从大脑8到10双神经而产生h supply to the prostomium, buccal cavity, and pharynx.
2 pairs of nerves arise from the circumpharyngeal connectives supply 1^stsegment and buccal cavity.
From the sub-pharyngeal ganglia, 3 pairs of the nerve arise supply to 2^nd, 3^rd,and 4^thsegments.
Each segmental ganglion of the ventral nerve cord gives off 3 pairs of lateral nerves.
One pair in front and 2 pairs behind the row of setae, they supply the various parts of the segment in which the ganglia are situated.
因此,神经来自中央神经us system to supply the different parts of the body constitute the peripheral nervous system.
Nerve are of mixed type, consists of bothafferentorsensory fibersandefferentormotor fibers.

3. Sympathetic nervous system

It consists of an extensivenerve plexussituated in the wall of the alimentary canal and some other internal organs.
Such plexuses are connected with the per-pharyngeal connectives by fine nervules.
They help in coordinating the functions of the related organs.

Working mechanism of the Nervous System

All the activities of earthworm are under the control of the nervous system, though not necessarily of the brain.
Like higher animals, they have bothsensoryandmotor neurons.
The nerves of the nerve cord have both sensory and motor fibers, i.e., mixed.
Sensory fibers start from asensory cellorreceptor organsin the epidermis and terminate in the ventral nerve cord in fine branches.
Near the branches in the cord, and forminga synapsewith them, arise similar branches of motor fibers which run outward and terminate in muscles.
Stimuliorsensory impulsesare conducted from the receptor by sensory fibers to the ventral nerve cord, from where they are reflected motor impulses along efferent fibers to muscles which contract.
The circuit of stimuli or impulses forms a simplereflex arc.
The movement of circular and longitudinal muscles are co-ordinated, so the contraction of one bring about relaxation of the other.
Giant fibersof the nerve cord conduct impulses more rapidly than other fibers resulting in sudden contraction of the whole body of earthworm when one point is strongly stimulated.

Video: Nervous System of Earthworm & its types By Studio Biology

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References

Kotpal RL. 2017. Modern Text Book of Zoology- Invertebrates. 11th Edition. Rastogi Publications.
Jordan EL and Verma PS. 2018. Invertebrate Zoology. 14th Edition. S Chand Publishing.