Nervous System of Frog

Anatomy of Earthworm,Cockroach & Frog of Class 11

Central Nervous System

CNS consist of two parts : Brain and spinal cord. Brain has grey matter on the outer side and white matter on the inner side. In spinal-cord, grey matter is on the inner side and white matter on the outer side.

Fig. Brain of frog from ventral (A) and dorsal (B) views: CN : cranial nerves originating from brain

Brain is covered by two meninges: outer tough, fibrous duramater and inner vascular, pigmented piamater closely attached to the brain. Sub-dural space between duramater and piamatar is filled with cerebrospinal fluid which acts as shock absorber.

Forebrain or prosencephalon consist of olfactory lobes, cerebrum (cerebral hemispheres) and diencephalon. Midbrain or mesencephalon consist of optic lobes and optic chiasma. Hindbrain or rhombencephalon is composed of cerebellum and medulla oblongata.

Olfactory lobes are a pair of large, fused, bulbous lobes in front of cerebral hemispheres. Cerebrum consists of two cerebral hemispheres separated by a median longitudinal fissure. Each cerebral hemisphere encloses a cavity called as lateral ventricle. Both the lateral ventricles open in 3rd ventricle or diacoel through foramen of Monro. Diencephalon lies just next to cerebral hemispheres and encloses the 3rd ventricle or diacoel. From its ventral side arises infundibulum with pituitary body. Pineal body and stalk is present on dorsal side. Pineal body lies outside the skull on the skin as brown spot in adult.

Optic lobes are paired, large, hollow enclosing optocoel which open into fourth ventricle through iter or aqueduct of Sylvius. X-shaped optic chiasma lies on the ventral surface.

Cerebellum behind the optic lobes is not well developed in frog. Medulla oblongata is the posterior most part of the brain and encloses 4th ventricle or metacoel. Its roof has vascular posterior choroid plexus.

Olfactory lobes are centres of smell, intelligence, memory. Voluntary actions are controlled by cerebrum. Diencephalon regulates genital function and sleep. Optic lobes are with the sense of sight. Equilibrium is maintained by cerebellum. Medulla controls involuntary action like heart beat, respiration, digestion, excretion etc.

Spinal cord is the continuation of medulla. It has a dorsal fissure and ventral fissure. Spinal cord has a central canal as an extension of 4th ventricle of medulla oblongata. It is surrounded by H-shaped grey matter. Corners of grey matter are produced into a pair of dorsal and a pair of ventral horns. Dorsal horn is sensory while the ventral horn is motor in nature.

White chalky masses called calcareous bodies or glands of Swammerdam cover the dorsal root ganglia and are believed to form reserves of calcium.

Peripheral Nervous System

  • Ten pairs of cranial nerves and 10 pairs of spinal nerves are present. 1st, 2nd and 8th are sensory ; 3rd, 4th and 6th are motor while 5th, 7th, 9th and 10th are mixed cranial nerves.
  • Spinal nerves are formed by fusion of dorsal and ventral root of the spinal cord before coming out through intervertebral foramen. All spinal nerves are mixed.
  • 1st, 2nd and 3rd spinal nerves of each side form brachial plexus 4th, 5th and 6th of each side are separate and supply to muscles and ventral skin. 7th, 8th, 9th of each side form sciatic complex and it innervates hind limb.

Fig. T.S. of Frog’s Spinal Cord

  • Tenth pair is called coccygeal nerve and supply to cloaca and urinary bladder. It is generally absent in Rana tigrina or may be present only on one side.

Autonomic Nervous System

Sympathetic nervous system consists of two longitudinal nerve chains, each made up of ten sympathetic ganglia, on either side of dorsal aorta. Sympathetic nerve ending secrete sympathine to increase the activity of heart, blood vessels, stomach, intestine, kidney, urinary bladder, eyes etc.

Parasympathetic nervous system is in the form of ganglia in the wall of visceral organs. Parasympathetic nerve ending secrete acetylcholine which inhibits the activity of organ concerned.

Fig. Vertical section of Frog’s Eye (diagrammatic)

Sense Organs

Eyes are guarded by immovable upper eyelid, movable lower eyelid and transparent nictitating membrane.

Outer sclerotic ring is cartilagenous and cornea is the part exposed out. Middle highly vascular, pigmented layer is choroid. Iris is yellow pigmented and perforated by a central aperture, the pupil. Retina is the innermost coat of eyeball and consist of inner pigmented layer and an outer receptor layer. Rods and cones are light sensitive structures found in retina.

Rods have rhodopsin or visual purple meant for night vision while cones have iodopsin responsible for day light and colour vision.

The anterior chamber is present in front of lens filled with aqueous humour while the chamber behind the lens is vitreous chamber filled with vitreous humour. Eye ball is moved in the eye orbit by a set of six muscles – two are oblique and four are recti. Besides, there are retractor bulbi and levator bulbi muscles for intrusion or protrusion of eye ball into the eye orbit, respectively.

Frog is myopic (short-sighted) at land and hypermetropic (long-sighted) in water. It has monocular vision.

Ear of frog has only middle and internal ear. Tympanic membrane is present at the body surface. Middle ear has single bone called as columella auris. Pressure of middle ear is controlled by eustachian tube.

Membranous labyrinth or internal ear consist of utriculus, sacculus, and semicircular canals. Endolymph fills the membranous labyrinth.

Semicircular canals are three in number : two vertical and one horizontal. The three canals lie at right angles to each other. At their ends, these canals are dilated into ampullae. From the posterior side of sacculus arises two blind sacs – lagena or cochlea and para basilaris. Endolymph contains numerous calcareous crystals, ear stones or octoconia. Membranous labyrinth bears inwardly projections – acoustic ridges. The ridges in the ampulla are cristae and those with vestibule are maculae. These are involved with balance.

Jacobson’s organ are chemoreceptors and open into the nasal chamber.

Jacobson organs are also called vomero-nasal organs.

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