• The human neural system divided into two parts –
    • The central nervous system (CNS)
    • The peripheral nervous system (PNS)

  • The CNS includes the brain and spinal cord and is the site of information processing and control.
  • The PNS comprises all nerves of the body associated with CNS.
    • Cranial nerves: nerves arises from the brain (12 pairs)
    • Spinal nerves: nerves arises from the spinal cord (33 pairs)

  • The nerve fibres (Cranial and spinal nerves) are of two types –
    • Afferent fibres: transmits impulses from the tissues to the CNS
    • Efferent fibres: transmits impulses from the CNS to the tissues.

  • The PNS is divided into two divisions –
    • Somatic neural system.
    • Autonomic neural system.
      • Sympathetic neural system.
      • Parasympathetic neural system.

  • The somatic neural system relays impulses from the CNS to skeletal muscles.
  • The autonomic neural system transmits impulses from the CNS to the involuntary organs and smooth muscles of the body.


  • A neuron composed of three major parts –
    • Cell body
    • Dendrites
    • Axon

  • The cell body contains cytoplasm with typical cell organelles and specific granular body called Nissl’s granules.
  • Short fibres which profusely branched projects out of cell body called dendrites.
  • The axon is a long fibre, branched at the end.
  • Each branch terminates as a bulb-like structure called synaptic knob.
  • Based on the number of axon and dendrites the neurons are of following types –
    • Multipolar: one axon and several dendrites - found in cerebral cortex.
    • Bipolar: one axon and one dendrite - found in retina of eye.
    • Unipolar: cell body with one axon only – found in embryonic stage.

  • The axon may be myelinated or non-myelinated.
  • The myelinated nerve fibres are enveloped with Schwann cells, which form myelin sheath around the axon. The gaps between two adjacent myelin sheath are called Nodes of Ranvier.
  • Cranial and spinal nerves are myelinated.
  • Autonomic and somatic neural fibres are non-myelinated.


Polarized membrane/Resting Potential :

  • In resting phase when neuron is not conducting an impulse, the axonal membrane is called polarized. This is due to difference in concentration of ions across the axonal membrane.
  • At Rest : 
    • Axoplasm inside the axon contains high conc. of K+ and low conc. of Na+.
    • The fluid outside the axon contains low conc. of K+ and high conc. of Na+.

  • As a result the outer surface of axonal membrane is positively charged and inner surface is negatively charged. The electric potential difference across the resting plasma membrane is called resting potential.

Action Potential :

  • When a nerve fibre is stimulated, the permeability of membrane to Na+ is greatly increased at the point of stimulus (rapid influx of Na+) and hence polarity of membrane is reversed and now membrane is said to be depolarized.
  • The electric potential difference across the plasma membrane at that site is called action potential, which in fact termed as nerve impulse.
  • Depolarization is very rapid, so that conduction of nerve impulse along the entire length of axon occurs in fractions of second.
  • Depolarization is followed by the increase in permeability of K+ to the membrane leads to change in polarization i.e. +ve charge outside and –ve charge inside. It is called repolarization.
  • Regain of resting potential takes place due to action of Na+/K+ ATPase enzyme which transports three Na+ inside and two K+ inside with expense of one ATP. It continues till the resting potential becomes -70 mv.

Transmission of impulses through synapse :

  • The functional junction between two neurons is called synapse.
  • A synapse is formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron, which may or may not be separated by a gap called synaptic cleft.
  • There are two types of synapses:
    • Electrical synapse: pre and post synaptic membrane with close proximity without any synaptic cleft.
    • Chemical synapse: the pre and post synaptic membrane is separated by a fluid filled synaptic cleft.


CBSE Biology (Chapter Wise) Class XI ( By Mr. Hare Krushna Giri ) 
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