Class 12 and NEET AIIMS Nervous System Notes
A nerve consists of several bundles of nerve fibre called Fasciculi.
Each fasciculi is surrounded by a layer of connective tissue called the perineurium.
A dense layer of connective tissue surrounding the entire nerve is called the epineurium.
Types of Nerves
Basically nerve is of three types depending upon fibre they are composed of –
(i) Sensory Nerve – composed of sensory nerve fibres. Eg. – optic nerve.
(ii) The Motor Nerve – composed of motor nerve fibres. Eg. – Abducens nerve.
(iii) Mixed Nerve – Having both sensory and motor nerve fibres. Eg. – Vagus nerve.
The giant neuron is present in Crayfish.
. Another cell is present in the nervous system.
1. Ependymal Cell – The ependymal cells form a columnar epithelium that lines the cavities in the brain and spinal cord.
They are ciliated in the embryo, Cilia may persist at some places in the embryo.
The ependymal cells can divide and develop long processes that penetrate the nervous tissue.
2. Neurosecretory cell
In the nervous system, these specialised cells work as an endocrine organ.
They release special chemicals from their axon in the bloodstream.
Neurosecretory cells of hypothalamus secrete neurohormones.
These neurohormones are then taken to the anterior lobe of the pituitary.
3. Neuroglia and Glia cells
Neuroglia consists of supporting and packing cells found in the brain.
They are through to separate and insulate adjacent neurons so that impulses pass from one neuron to the next only over the synapse.
They are of following types –
(a) Microglia: They perform Phagocytosis, they arise from the monocytes. (Mesoderm, all other neuroglia cells have the same origin as the neurons).
(b) Astrocytes: They provide nutrition
(c) Oligodendrocytes: They help to repair wear and tear in the brain, the oligodendrocytes from a myelinated sheath around the axons in the CNS.
Physiology of Nerve Impulse Transmission
Nerve impulse may be defined as a wave of physiological activity which travels in a neuron.
The transmission of nerve impulse is an “Electrochemical Process.”
Nerve fibre remains bathed in ECF with Na+ ion concentration greater on the outer side and K+ ion concentration greater on the inner side.
Transmission of a nerve impulse along nerve fibre involves various processes like depolarisation, repolarisation etc.
The electric potential across nerve fibre axon when no nerve impulse is passed is called
Resting Membrane Potential
Resting membrane potential is about – 80 mv, this is maintained by Na+ K+ pump.
The sodium-potassium pump with the utilisation of ATP throws. 3Na+ outside and 2K+ inside.
Due to this activity, Na+ is extracellular and K+ is intracellular.
The electric potential developed when nerve impulse is passed is called action potential (Depolarisation)
According to Hodgkin and Huxley, at the point of application of a stimulus, the permeability for K+ is decreased and for Na+ is increased. As a result, the outer surface of axolemma becomes negative and the inner surface becomes positive due to the inward movement of Na+. It is called depolarization.
The action potential is about + 60 mv.
Due to the development of action potential, there is an increase in permeability of sodium ion.
This action potential move like a spark along with nerve fibre and constitutes nerve impulse.
Repolarisation – As due to development of action potential Na+ ions inside the cell increase so now sodium, potassium pump gets activated and it expels sodium from outside and withdraws potassium inside. Thus restoring positive potential outside and negative potential inside and this is called Repolarisation.
During nerve transmission, the outside membrane is + – +
During nerve transmission, the inside of the membrane is – + –
Factors Affecting Conduction of Nerve Impulse
1. Stimulus Strength
The minimum strength of stimulus to which a neuron responds is called the threshold stimulus.
So, for a nerve to be stimulated, the stimulus must be greater than the threshold stimulus.
When a stimulus is weaker than the threshold value and is given in rapid succession, then the additive effect of stimulus occurs. This is called summation.
3. All or None Rule
The all or none rule means that whenever a neuron is provided with a stimulus then neuron either conducts or does not conduct stimulus.
4. Saltatory Conduction
In a myelinated neuron, the nerve impulse jumps from one node of Ranvier to the other node, known as saltatory conduction.
Polarity only changes at these points.
By these Ions do not pass from neuron to outside
This rate is faster than non-myelinated transmission.
The speed of nerve impulses is more in –
(i) Thick neuron
(ii) Myelinated neuron (ii) Long neuron
In squid (Mollusca) all nerve fibres are non-myelinated which results in slow nerve transmission hence they possess thick nerve fibre so to make nerve impulse transmission faster.
Axon is in contact with four dendrites, the speed of nerve impulse will be same as in axon.