Nervous system of Earthworm - Online Biology Notes

Do earthworms have nervous system?

Earthworm has a well-developed nervous system.
The nervous system is metamerically segmented.
The nervous system is divisible into central, peripheral and autonomic nervous system (sympathetic nervous system).

Nerve ring is an oblique ring around the pharynx in 3^rd and 4^th segments.
Its mid-dorsal part comprises of a pair of small and fused supra-pharyngeal ganglia, also called cerebral ganglia or brain.
Likewise, its mid ventral part is formed of a pair of small and fused sub-pharyngeal ganglia.
The dorsal and ventral ganglionic parts are connected by a pair of loop-like circum-pharyngeal or peri-pharyngeal connective to complete the ring.

Nerve cord arises from the sub-pharyngeal ganglia.
It extends behind up to the posterior end of body in mid ventral axis beneath the ventral vessel.
Both cords appear as a single nerve cord as they are fused and enclosed in a common sheath.
Starting from the 5^th segment behind, the nerve cord has a ganglionic swelling in the posterior part of each segment. It is known as segmental ganglion.
Histologically, the double nerve cord is solid and formed of nerve cells and fibres.
Nerve cord is surrounded by epineurium (a common sheath of connective tissue).
Outside epineurium, there is a layer of longitudinal muscle fibers.
This is finally surrounded by a layer of visceral peritoneum.
Nerve cells are mainly present in ventral and lateral sides of the nerve cord.
Nerve fibers are mainly present in dorsal and middle portion of nerve cord.
Nerve fibers are of two types ordinary fibers and giant fibers/neurocords.
Four peculiar giant fibers (one median, one sub-median, and 2 laterals) are found mid-dorsal to the ventral nerve cord.
Giant fibers functions for the rapid conduction of impulses throughout the nerve cord.
In lateral giant fibers, impulse is conducted antero-posteriorly.
However, in median and sub-median fibers, the impulse is conducted postero-anteriorly.

All nerves arise from CNS and supply to various parts of body.
All the nerves present in earthworm are of mixed type as they contain both afferent and efferent nerve fibres and adjustors (association neurons).
From the cerebral ganglia/brain, 8-10 pairs of nerves arise.
From the peri-pharyngeal or circum-pharyngeal connectives, 2 or 3 pairs of nerves arise.
From the sub-pharyngeal ganglia, 3 pairs of nerves arise.
Each segmental ganglion of nerve cord gives out 3 pairs of nerves in each segment.
First pair arises just in front of setal ring while the other 2 pairs arise closely but behind setal ring.

ANS of Pheretima posthuma possess sympathetic nervous system only with extensive nerve plexus that are connected to the nerves from circum-pharyngeal connectives.

All earthworm operations are regulated by the nervous system, but not necessarily by the brain.
They have both sensory and motor neurons, like larger animals.
Sensory fibers start in the epidermis from sensory cells or receptor organs and end in fine branches in the ventral nerve cord.
Similar branches of motor fibers arise near the branches of the cord and form a synapse with them, running outward and ending in muscles.
Stimuli or sensory impulses are conducted to the ventral nerve cord from the receptor through sensory fibers, from where motor impulses are reflected along efferent fibers to contracting muscles.
A simple reflex arc forms the circuit of stimuli or impulses.
The motion of the circular and longitudinal muscles is coordinated, so the contraction of one causes the other to relax.
Giant fibres of nerve cord conduct impulses more rapidly than other fibres, resulting in the sudden contraction of the entire earthworm body when strongly stimulated at one point.