Nervous system of frog

• Similar to other vertebrates, the nervous system of frog is composed of 3 main sub-divisions:

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:

Number	Name of cranial nerve	Origin	Distribution	Nature
O	Terminal	Forebrain	Lining of nose	Sensory(probably)
I	Olfactory	Olfactory lobe	Lining of nose	Sensory(small)
II	Optic	Diencephalon	Retina of eye	Sensory(vision)
III	Oculomotor	Midbrain ventrally	4 muscles of eye	Motor
IV	Trochlear	Midbrain dorsally	Superior oblique muscle of eye	Motor
V	Trigeminal Ophthalmic Maxillary     Mandibular	Medulla laterally	Skin of snout Skin of upper jaw Muscles of lower jaw, tongue	Mixed Somatic sensory   Somatic sensory       Visceral motor
VI	Abducens	Medulla ventrally	External rectus of eye	Motor
VII	Facial Palatinus       Hyomandibularis	Medulla laterally	Roof of buccal cavity     Tympanum, skin of lower jaw, tongue	Mixed Visceral sensory       Visceral motor
VIII	Auditory	Medulla laterally	Internal ear	Sensory (hearing)
IX	Glossopharyngeal	Medulla laterally	Tongue, hyoid, pharynx	Mixed
X	Vagus (Pneumogastric) Laryngeal     Gastric     Pulmonary     Cardiac	Medulla laterally	Laryngotracheal chamber   Stomach     Lung     Heart	Mixed

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