Anatomy of finfish and shellfish (2+1)

Unit 7 - Nervous system in fishes

7.2.1. Peripheral nervous system
There are two kind of nerves, spinal and cranial nerves. The former take their origin from the spinal cord and are metamerically arranged, that is to lay, their number is same as that of the vertebrae.
Spinal Cord:
The spinal cord, or nerve cord is similar in all fish. It is a thick sheath of nervous material that runs from the base of the brain back along the fish's body through, and protected by, the neural canal of the spinal column. Normally it extends the full length of the fish's body, but a notable exception to this is the giant Sunfish (Mola mola) wherein the spinal cord is actually shorter than the brain. It serves as the basis of many simple responses and as the major link to the brain for sensory input and brain-mediated responses.
Cranial nerves
The cranial nerves arise from the brain and Ten (10) pairs of them are typically present in a teleost (twelve in higher vertebrates).
They are as follows:
1) Olfactory 2) Optic 3) Oculomotor 4) Trochlear 5) Trigeminal 6) Abducens 7) Facial 8) Acoustic or Auditory 9) Glorrophoryngeal 10) Vagus.
Olfactory nerve: It is purely sensory one connecting nasal organ with olfactory lobe. It is a special sensory nerves conveying smell impulses to the brain.
Optic nerve: Likewise sensory and supplies to the eyes. In bony fishes, the two crosses each other below the brain immediately after leaving the optic lobes. The nerve from the left lobe joins to the right eye and vice versa. It supplies the visual impulses.
Oculomotor nerve: Arises from the lower surface of the brain and innervates four of the six striated muscles of the eye ball.
Trochlear nerve: Arises form the dorsolateral side of the brain between the optic lobes and the cerebellum and supplies the superior oblique muscle of the eye ball.
Trigeminal nerve: Arises from the lateral side of the medulla oblongata and supplies the snout and upper and lower jaws. It is a mixed nerve and is divided into three important branches – the ophthalmic, maxillary and mandibullar.
Abducens nerve: Arise from the ventral side of the medulla oblongata, a little behind the trigeminal nerve. It also enters the orbit and supplies the posterior rectus muscle that moves the eye ball.
The Facial nerve: has independent origin from the side of medulla oblongata behind the trigeminal, but soon joins the later to form the trigemino – facial complex which divides into three branches, the supra–orbital, infra orbital and hyomandibular.
The Auditory nerve or acoustic nerve: Arises form the side of the medulla oblongata behind the facial and supplies nerves to the “inner ear”.
The Glassopharyngeal nerve: Arises from the ventro – lateral aspect of the medulla oblongata behind the auditory and enters the first gill slit. It is a mixed nerve and supplies a part of the lateral line system, taste buds in the pharynx and the muscles of the first gill slit.
The Vagus: The vagus nerve arises behind the glasso-pharyngeal and has an extensive distribution. It divides into branchia – visceral trunk and a lateralis branch. The branchio visceral trunk divides into three branchiolis nerves and a visceralis branch. Each branchial branch gives off a slender branch to supply the muscles of the gills. The visceralis branch supplies various organs of the viscera. The lateralies is a stout nerve that runs up to the end of the tail along the lateral line canal and innervates it by a several branches.