Class 12 Nervous System notes NEET/AIIMS

Nervous System

The nervous system is a system of neurons (nerve cells), nerves and nervous organs that coordinates and controls the activity of various organs by conducting and responding to various nerve impulses.

Nerve Cell (Neuron)

           The basic unit of the nervous system is neuron or nerve cell. It has two basic properties – 

       (a)  The longest cell in the body (up to a metre)       

        (b)       No or minimum power of regeneration.

    The neuron consists of the main cell body and the cytoplasmic process is arising from it.          

Cyton/cell body/soma

           It consists of cytoplasm, nucleus and cell membrane.

           The cytoplasm is called Neuroplasm and it contains neurofibril, microtubules and Nissl’s granules.

           Nissl’s granules are present in cytons, dendrites and made up of ribosomes and RNA.

          Nissl’s granules principle function is nutrition and it controls the physiology of nerves. And if nerve cell degenerates than first Nissil granules degenerate.

           In nerve cells, body centrosomes are absent.

           Cyton is concerned with metabolic maintenance and growth.  

Appendages of Neuron

           These are called neurites. These are of two types–

             (a)  Dendrons   

             (b) Axon      

       (a)  Dendrons

           Are called afferent process – as they conduct nerve impulses towards the cell body.

           These are branched and tapering structures and contains, Nissl’s granules, neurofibrils etc.

       (b) Axon

           It is a single long process arising from a part of cyton and from where it arises is called Axon Hillock. It is the most sensitive part of Neuron.

           Axon hillock is the most sensitive part of Neuron.

           Axon has neurofibril but does not possess Nissil granules.

           The membrane surrounding axon is called axolemma and cytoplasm is called Axoplasm,

           Axon ends up in branches called terminal arborisation, telodendria or axon terminal.

           On muscle fibres, terminal arborization ends as the motor endplate.

           Axon conduct nerve impulse away from the cell body and hence called different processes,

           The direction of nerve impulse is always from dendron cyton  Axon.

           Nissl’s granules are absent.       

Types of Neuron

           Neurons are classified on the basis of their structure and function.

       (A) On the basis of Structure –      

              (i) Non-Polar (ii) Unipolar

             (iii) Bipolar             (iv) Multipolar

(i)    Non-Polar

           Each neuron has several branched processes. Differentiation between dendrites and axon is absent, This neuron occurs in cinidarians (Hydra).

       (ii) Unipolar

           A unipolar neuron comes with an axon and no dendrites, Basically, it is sensory in nature, Ex. – Dorsal root ganglion of spinal cord

       (iii) Bipolar

           The neurons have only one process, axon on one end and dendrites on the other end. Ex. – Rods and cones on the retina, olfactory epithelium, etc.

       (iv)  Multipolar

           These neurons have several dendrites and axon,  The forms majority of neurons and are most common, Occur in Brain and Spinal Cord.

(B) On the Basis of Function :

       (i)   Sensory/Afferent Neuron

           Neurons connecting sense organ with the central nervous system. They bring impulse towards CNS.

2.   Motor/Efferentneuron

Neurons connect the central neurons system with effector organs. They carry impulses away from CNS.

3.   Connector/Interneuron

           They occur in between sensory and motor neuron for transmission of the impulse.     

Nerve Fibre

           Axon and dendrites of a nerve cell covered with two or three sheaths are called Nerve Fibre.

           Nerve fibre may be Myelinated or Non-myelinated.        

Myelinated Nerve Fibre

           The fibre in which myelin sheath is present.

           Schwann cell forms myelin sheath of Neuron.

Non-Myeunated Nerve Fibre

           Nerve fibre in which myelin sheath is absent.

           They conduct nerve impulses slower than myelinated nerve fibres.

Grey and White Matter

           Nervous tissue which forms the brain and spinal cord are of two types of grey and white matter.

           Grey matter consists of cell bodies of neurons, most fibres in the grey matter are non-myelinated. White Matter: In white matter nerve fibres are myelinated.