Peripheral nervous system

It is formed of a number of long, thin, whitish threads called nerves extending between central nervous system and body tissues. Each nerve is formed of bundles of nerve fibres, fasciculi, held together by connective tissue and surrounded by a white fibrous connective tissue sheath called epineurium.
 The nerve fibres are classified into two categories on the basis of presence or absence of myelin (white fatty) sheath.
         (1) Medullated or Myelinated nerve fibres.
         (2) Non-medullated nerve fibres.
On the basis of function, the nerves are of three types
(1) Sensory nerve
(i) It contains only sensory nerve fibres.
(ii) It conducts nerve impulses from sense organs to CNS to produce sensation. e.g. Optic nerve, auditory nerve.
(2) Motor nerve
      (i) It contains only motor nerve fibres.
      (ii) It conducts nerve impulses from CNS to some muscles or glands to
            control their activities. e.g. Occulomotor nerve, hypoglossal nerve.
(3) Mixed nerve
     (i) It contains both sensory and motor nerve fibres.
     (ii) It conducts both sensory and motor impulses. e.g. All spinal nerves,
           Trigeminal nerve. On the basis of their origin, nerves are of two types
     (1) Cranial or cerebral nerves which either arise from or end into brain.
     (2) Spinal nerves which arise from spinal cord.
 (4) Cranial nerves
      (1)10 pairs of cranial nerves are present in an anamniote (fishesand amphibians).
      (2) Number of cranial nerves found in frog is ten pairs (20).
      (3) 12 pairs of cranial nervers are present in an amniote (reptiles, birds and mammals).
      (4) Number of cranial nerves found in rabbit and man is 12 pairs (24).
      (5) The first 10 pairs are common for frog and rabbit. The additional
            pairs found in rabbit are spinal accessory and hypoglossal,
      (6) The smallest cranial nerve is trochlear in humanben]ings, but all
             animals smallest cranial nerve is abducens
      (7) The largest cranieal nerve is trigeminal in human beings but vagus is largest
            Cranial nerve in all animals.
      (8)  Vagus supplies the regions other than head.
      (9)  The sensory cranial nerves are
             I Olfactory          -    Smell
             II Optic            -    Vision
             VIII Auditory        -    Hearing and equilibrium
      (10) The motor cranial nerves are III, IV, VI, XI and XII
      (11) Extraocular muscle nerves are III, IV and VI
      (12) The mixed cranial nerves are V, VII, IX and X (4 pairs)
      (13) Number of cranial nerves in snake (Amniota) 10 pairs
Spinal nerves : Spinal nerves arise from gray matter of spinal cord. There are 31 pairs of spinal nerves in man (37 pairs in rabbit). All spinal nerves are mixed. The spinal nerves in man are divided into 5 groups.
(1) Cervical (C)         —>         8 pairs         -    in Neck region
(2) Thoracic (T)        —>         12 pairs     -    in thoracic region
(3) Lumbar (L)             —>         05 pairs     -    upper part of abdomen
(4) Sacral (S)             —>         05 pairs    -     lower part of abdomen
(5) Coccygeal (CO)         —>         01 pairs     -    represent the tail nerves
                                      Total      =        31 pairs
 Number of spinal nerves in frog is 10 pairs. In some frog like Rana tigrina, 10th pair may reduced or absent. The first pair of spinal nerves in frog is hypoglossal. The last pair of cranial nerves of mammals has the same name. Brachial plexus is formed by 2nd and 3rd spinal nerves in frog. Sciatic plexus is formed by 7, 8 and 9 spinal nerves in frog. Glands of Swammerdam are calcareous glands found at the places emerging of spinal nerves in frog.
 Spinal nerve formula can be written as — C8, T12, L5, S5, CO1, Spinal nerves exit via intervertebral foramen.  Each spinal nerve arises from spinal cord by 2 roots
(1) Dorsal (= Afferent = Sensory = Posterior) root is a continuation of dorsal horn and is formed of gray matter. It presents a ganglionic swelling in middle, called dorsal root ganglion. These transmit sensory nerve impulses from the sense organs to spinal cord (touch, pain, temperature). They activate involuntary reflexes. (2) Ventral (= Efferent = Motor) root are continuation of ventral horn and is also formed of gray matter. No ganglion are present. It is formed of only efferent nerve fibers. They transmit motor nerve impulses to effector organs e.g., glands and muscles. Each spinal nerve has 4 branches —
(1) Ramus dorsalis : Supplies to skin and muscles of dorsal side.
(ii) Ramus ventralis: Supplies to skin and muscles of ventral and lateral sides and also to upper and lower limbs. Ventral root of certain spinal nerve form 5 nerve plexi on either side, i.e., cervical, thoracic, lumber, sacral, caudal.
(iii) Ramus communicans: It joins sympathetic ganglion of autonomic nervous system.
 (iv) Meningeal branch: Vertebra, vertebral blood vessel.