I. Central Nervous system of frog

1. Brain of Frog:

• Brain is white in color, elongated and somewhat flattened structure.
• It lies well protected inside the cranium of skull.
• It is surrounded by two meninges.
• Meninges are connective tissue membranes.
• The inner or piamater is delicate and pigmented.
• Piamater is vascular and closely applied to brain.
• The outer durameter is tough and fibrous which lines the cranial cavity.
• The narrow space between the membranes and the inner cavities of brain are filled with cerebrospinal fluid.
• Cerebrospinal fluid is a clear, watery and lymphatic fluid. It protects and nourishes the brain.
• The ventricles (inner cavities of brain) are continuous with one another.

Brain can be explained under 3 main parts:

i. Forebrain:

• It consists of two olfactory lobes, two cerebral hemispheres and a diencephalon.
• Olfactory lobes:
  • Olfactory lobes are two anterior most, rather small and spherical lobes.
  • Each lobe sends a small olfactory nerve to the nasal chamber of its side.
  • The two lobes are united but each contains a separate small cavity.
  • This cavity is termed as the olfactory ventricle or rhinocoel.
  • Olfactory lobes are concerned with sense of smell which is poorly developed in frog.
• Cerebral hemispheres:
  • Posteriorly, the olfactory lobes are demarcated by a slight constriction from two large cerebral hemispheres that forms cerebrum.
  • Cerebral hemispheres are long, oval, and smooth structures.
  • It is narrow in front but broad behind.
  • It is separated from one another by a deep mid-longitudinal groove or fissure.
  • The large cavities of hemispheres, called I, II, or lateral ventricles or paracoels, are continuous anteriorly with olfactory ventricles.
  • Posteriorly they unite with each other and with the III ventricle or diocoel of diencephalon through foramen of Monro.
  • Foramen of Monro is the common opening.
  • The roof of cerebrum is thin and is called as pallium.
  • The each ventro-lateral side is thick called as corpus striatum (singular).
  • Corpus striatum is made of white medullated nerve fibers and cells.
  • A transverse fibrous tract, the anterior commissure joins the two corpora striata (pleural) together.
  •  The hemispheres are the place of memory, intelligence, consciousness and will.
  • It also regulates voluntary actions.
• Diencephalon:
  • It is the short, rhomboid, depressed region.
  • It lies just behind the cerebrum.
  • Its small cavity is termed as third ventricle or diocoel.
  • Diocoel has thick lateral walls called as optic thalamic and a thick floor, called as hypothalamus.
  • Its dorsal roof contains a network of blood capillaries, called as anterior choroid plexus.
  • Behind anterior choroid plexus, arises a small hollow projection, the pineal stalk, that runs to the brow spot.
  • In case of tadpoles, a small spherical pineal body is attached to the stalk.
  • However, in adult frog, it is detached and comes to lie outside skull.
  • The two optic nerves when crossed, forms the X-shaped optic chiasma.
  • It lies on the ventral surface of diencephalon.
  • A large median bilobed projection, the infundibulum is present just posterior to optic chiasma.
  • The infundibulum bears a flattened oval pituitary body or hypophysis.
  • Diencephalon is concerned with vision and balance.

ii. Midbrain:

• It is the broadest part of brain.
• It consists of a narrow canal, called as iter or aqueduct of Sylvius, which is continuous with III ventricle in front and IV ventricle behind.
• Two large rounded optic lobes, called copro bigemina are present dorsolaterally in midbrain.
• Their canals called as optocoels open into iter.
• There are two thick longitudinal bands of nerve fibers, called as crura cerebri.
• They run longitudinally beneath optic lobes connecting diencephalon and medulla.
• The inhibition of spinal reflexes is done by optic lobes, and each lobe controls the opposite side of body.

iii. Hindbrain:

• It is the posterior part of brain.
• It includes cerebellum and medulla oblongata.
• Cerebellum:
  • It is a poorly developed narrow ridge or band.
  • It is placed dorsally just behind the optic lobes.
  • It controls equilibrium and muscular co-ordination which are not that important in case of frog.
• Medulla oblongata:
  • It is the last small part of brain.
  • It is continuous with spinal cord without distinction.
  • It has triangular cavity which is called as IV ventricle or metacoel.
  • Metacoel is continuous anteriorly with iter and posteriorly with the central cavity of spinal cord.
  • Metacoel has thin and highly vascular dorsal roof which forms the posterior choroid plexus.
  • Medulla regulates essential involuntary functions as such heartbeat, metabolism, respiration, etc. which keeps on going even if rest of the brain is removed.
  • However, the removal of medulla is soon followed by death.

2. Spinal cord of frog:

• Posteriorly the spinal cord extends from medulla oblongata through foramen magnum.
• It lies protected within the neural canal of vertebral column.
• It is short, thick, cylindrical, somewhat flattened and white in color.
• Similar to brain, spinal cord is also surrounded by the two meninges, piamater and duramater that contains protective and nourishing cerebrospinal fluid.
• Posteriorly, it ends into a fine, non-nervous filament, called as filum terminale in the urostyle.
• Spinal cord shows swelling in two places:
• Brachial enlargement: between forelimbs.
• Sciatic or lumbar swelling: anterior to filum terminale.
• Two longitudinal grooves run throughout the length of spinal cord.
• The mid-dorsal called as dorsal fissure and the mid-ventral is termed as ventral fissure.
• It encloses a narrow central canal which is a continuation of the ventricles of brain.
• Spinal cord comprises of:
  • Outer white matter: composed mainly of nerve fibres.
  • Inner gray matter: composed largely of nerve cells.
  • Spinal cord is chiefly concerned with the reflex actions.

II. Peripheral nervous system of frog

i. Cranial nerves:

• From the brain of frog, 10 pairs of cranial nerves are originated.
• Some claim to have O or terminal nerves making that count to 11 pairs.
• Their number, name, origin, distribution and nature are shortly enlisted in the following table:

ii. Spinal nerves:

• There are 10 pairs of spinal nerves in case of frog, which are often reduced to 9pairs.
• This unusual small number is expected in animal with short spinal cord.
• Every spinal nerve on either side arises from spinal cord by two roots which unite just as the nerve comes out of neural canal through an intervertebral foramen.
• Dorsal root has a ganglion containing nerve cells.
• It consists entirely of afferent and sensory nerve fibres.
• These nerve fibres carry impulses from various body parts towards the spinal cord.
• Ventral root consists of only efferent or motor nerve fibres.
• These nerve fibres carry impulses from spinal cord to the tissues of body.
• Hence, all spinal nerves are mixed in nature i.e. made up of both sensory as well as motor fibres.
• In frogs, white soft chalky masses, called calcareous bodies or glands of  Swammerdam, cover the dorsal root ganglia and are believed to form reserve supplies of calcium.
• Just after its origin, each spinal nerve gives off 3 branches:
• a short ramus dorsalis to dorsal skin and muscles
• a large ramus ventralis to ventral skin and muscles
• a very small ramus communicans to join the nearest sympathetic ganglion.
• The first spinal nerve is called as hypoglossal.
• It comes out of neural canal between first and second vertebrae.
• It turns anteriorly to supply the muscles of tongue.
• The second spinal nerve is large and stout.
• Joined by the third spinal nerve and a small branch from hypoglossal, it forms a network, the brachial plexus.
• Then, it forms the brachial nerve that supplies the forelimb.
• Fourth, fifth, and sixth spinal nerves are small and run obliquely to skin and muscles of abdomen.
• Seventh, eighth, and ninth nerves are large and run backwards to form the sciatic plexus.
• From this sciatic plexus, a large sciatic nerve and some small nerves supply the hind limb.
• Tenth spinal nerve is usually absent in case of Rana tigrina, but may be present only on one side, when it arises through a hole in urostyle near its anterior end.
• Besides a branch to sciatic plexus, it supplies the urinary bladder, cloaca and other parts.

III. Sympathetic nervous system of frog

• Sympathetic nervous system consists of two slender delicate thread-like nerve cords i.e. the sympathetic trunks.
• These sympathetic trunks run beneath the vertebral column, one on either side of the dorsal aorta.
• Each trunk has a series of 10 sympathetic ganglia.
• These ganglia are connected with spinal nerves by small nerves termed as rami communicantes.
• Corresponding ganglia of both the sympathetic cords are also connected together by small transverse commissures.
• On each side, the sympathetic cord continues anteriorly to enter the skull.
• It joins first the vagus ganglion of X nerve and then forward again to join the gasserian ganglion of V or trigeminal nerve where it finally ends.
• Sympathetic ganglia distribute nerves mainly to circulatory system, digestive tract and glandular organs.
• It controls activities not under the control of will such as rate of heartbeat, muscular tone of blood vessels, secretion of digestive juices.
• It also controls activities like muscular movements of stomach and intestine, etc.

Nervous system of frog

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