Class 12 Nervous System notes NEET/AIIMS
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.
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–
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.
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
Each neuron has several branched processes. Differentiation between dendrites and axon is absent, This neuron occurs in cinidarians (Hydra).
A unipolar neuron comes with an axon and no dendrites, Basically, it is sensory in nature, Ex. – Dorsal root ganglion of spinal cord
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.
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.
Neurons connect the central neurons system with effector organs. They carry impulses away from CNS.
They occur in between sensory and motor neuron for transmission of the impulse.
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.