Table of Contents
In this parts of earthworm nervous system post we have briefly explained about earthworm nervous system, central nervous system (nerve ring or brain ring, nerve cord), peripheral nervous system, sympathetic nervous system and working mechanism of earthworm nervous system.
Earthworm nervous system is well developed and concentrated. The earthworm nervous system is divided into three parts: the central, peripheral, and sympathetic nervous systems.
Earthworm Nervous System
Central Nervous System
The anterior nerve ring of earthworm nervous system sometimes known as the brain ring and the posterior ventral nerve cord make up the central nervous system.
Parts of Earthworm Nervous System
Nerve Ring or Brain Ring, It is ring like spherical structure which lies around the pharynx in 3rd and 4th segment.
Cerebral ganglia: There are two pear shaped cerebral ganglia fused together at 3rd segment called brain. They are bilobed in structure. A pair of whitish pear-shaped supra pharyngeal ganglia fused to form brain. Lies dorsally in the 3rd segment in the depression between the buccal cavity and the pharynx.
Circumpharyngeal connective: Two circumpharyngeal connectives arise from each cerebral ganglion laterally. They encircle pharynx and fuse at 4th segment. The fused portion is called sub pharyngeal ganglia. It is outer buldging part which extends from 3rd to 4th segment.
Sub-pharyngeal ganglia: Sub-pharyngeal ganglia found in lower region of nerve ring which lies in 4th segment. Ventrally Circum-pharyngeal ganglia meet with a pair of subpharyngeal ganglia. Thus, a complete nerve ring is formed around the pharynx.
Nerve cord of the earthworm nervous system is white rod like structure, which starts running from subpharyngeal ganglia towards posterior end. In each segment, ventral nerve cord swells which is called segmental ganglia. Actually, there are two cords fuse together to form single ventral nerve cord. Ventral nerve cord is composed of nerve cells and nerve fibers. There are 4 giant fibers on mid dorsal side of nerve cord which conducts impulses rapidly. The outer covering of ventral nerve cord is called peritoneum.
Peripheral Nervous System
PNS of the earthworm nervous system includes nerve fibers or nerve, which arises from central nervous system. From cerebral ganglia, 8-10 nerves arise and supply to prostomium, buccal chamber, and pharynx.
From circumpharyngeal connectives, two pairs of nerves arise and supply to 1st and 2nd segment. From subpharyngeal ganglia, three pairs of nerve arise and supply to 2nd, 3rd and 4th segment.
From each segmental ganglion, three pairs of nerves arise and supply to respective segment. Thus, the nerve originating from the central nervous system to supply the different parts of the body constitute the peripheral nervous system.
Nerve are of mixed type, consists of both afferent and sensory fibers and efferent or motor fibers.
Sympathetic nervous system
Sympathetic nervous system of the earthworm nervous system consists of nerve plexuses extensively branched and distributed beneath epidermis, alimentary canal that is connected to circumpharyngeal connectives.
The earthworm nervous system, although not necessarily the brain, is in charge of all of the earthworm’s functions. They have both sensory and motor neurons, just like higher animals.
Sensory and motor fibres coexist in the nerves of the nerve cord, resulting in mixed nerves. Sensory fibres originate from sensory cells or receptor organs in the epidermis and branch out into fine branches in the ventral nerve cord.
Similar branches of motor fibres develop near the cord’s branches, creating a synapse with them, and run outward and terminate in muscles.
Sensory fibres carry stimuli or sensory impulses from the receptor to the ventral nerve cord, where they are reflected as motor impulses by efferent fibres to muscles that contract.
Circular and longitudinal muscle movement is coordinated, so contraction of one causes relaxation of the other.
When one spot on the earthworm’s body is forcefully stimulated, giant fibres of the nerve cord conduct impulses more quickly than other fibres, resulting in an abrupt contraction of the entire body.