Nervous tissue is a highly specialized tissue for reception, discharge and transmission of stimuli. It is ectodermal in origin. It is composed of nerve cells or neurons and neroglia.
Nerve Cells or Neurons:
Neuron is the structural and functional unit of nervous tissue. Each nuron has 3 parts- cyton, Dendron and axon.
Cyton is also called cell body or perikaryon. It is irregularly spherical in shape.its cytoplasm is called neuroplasm. It has a central nucleus and other cell organelles like mitochondria, ribosomes, lysosomes, endoplasmic recticulum, Nissl’ s bodies.Nissl’ s bodies are made of rough endopasmic reticulum with ribosomes. Several neurofibrils present in the cytoplasm. Neurofibrils help in transmission of nerve impulses to and from the cell body.Centrosome is absent in the cyton. Mature nerve cells are called post-mitotic cells as they never divide. Cell bodies are mostly found in brain, spinal chord and ganglia.
The Dendron is a process of nerve cell which carries impulses towards the cell body. Therefore, it is called afferent process. Each Dendron forms fine branches called dendrites. The dendrons contain both Nissl’s granules and neurofibrils.
An axon is the tubular process of the nerve cell that carries impulses away from the cell body. It arises from the cyton at axon hillock. It has only neurofibrils. Here Nissl’s granules are absent. The fine branches of terminal end of axon are called telodendria. The telodendria terminate with tiny knobs called synaptic knobs. The end of the knobs carries synaptic vesciles which carry neurotransmiiters. The lateral branches of axon called collateral fibres.its plasma membrane are called axolemma and cytoplasm is called axoplasm. The axons secrets a fatty protinaceous layer, the myelin sheath around axolemma.
Myelin sheath acts as a insulating material. The myelin sheath is further covered by a layer of Schwann’s cells called neurilemma. Axon with these two membranes forms a nerve fibre. The myelin sheath breaks at interval by constrictions called nodes of ranvier. At the nodes of Ranvier the cytoplasm of Schwann’s cell comes in contact with axon. This area is called cementing disc.
Classification of Neurons:
On the basis of the function, neurons are of 3 types :
(i) Sensory Neuron- It carries impulses from the sense organ to the central nervous system.
(ii) Motor neuron- It carries impulses from the central nervous system to effector organ.
(iii) Connector neuron- They lie between sensory and motor neurons.
Types on the basis of number of processe:
(i) Non-Polar Neuron:
The dendrons or axons are not functionally differentiated in the nerve cells of coelenterates. These are called Non-polar neurons.
(ii) Unipolar Neuron:
This types of neuron has one axon but there is no dendrons. They are found in early embryos.
(iii) Bipolar Neurons:
It has a single Dendron and a single axon. It is found in retina of eyes and nasal chambers.
(iv) Multipolar Neuron:
It has a single axon and many dendrons. It is found in brain, spinal chord and ganglia of autonomic nervous system.
When the axon is covered with a sheath, it is called nerve fibre. There are two types of nerve fibres:
Myelinated Nerve Fibre:
It is also called medullated nerve fibre. Here the axon is covered with myelin sheath and Schwann’s cells. Myelin is white in colour; hence myelinated nerve fibres appear white. The white matter of central nervous system consists of myelinated nerve fibres. In Myelinated nerve fibres, nurve impulses are transmitted by jumping from node to node. This is called salutatory conduction.
Non-Myelinated Nerve Fibre:
It is also called Non-medullated nerve fibre. It is not covered by myelin sheath. It is grey in colour and found in grey matter of the brain and spinal chord.
Structure of Nurves:
The nerve is externally covered by a connective tissue layer called epineurium. Each nerve is made up of several bundles of nerve fibres. These bundles are called fascicule. Each fasciculus is surrounded by perineurium and consists several nerve fibres. Each nerve fibre is covered by loose connective tissue covering called endoneurium.
Neuroglia are additional supporting cells of nervous tissue. They act as packing cells. They provide nutrition to neuron. They insulate adjacent neurons. They are like nerve cells but axon and Nissl’s granules are absent.
(1) Oligodendrocyte– These are small cells which bear many processes. They form myelin sheath.
(2) Astrocytes– These are stra-shaped cells. They are found in grey matter of brain and spinal chord.
(3) Ependyma– They form lining of ventricles of brain and spinal chord.
(4) Mycroglia– These are phagocytic cells found in the central nervous system.
(1) Excitability– Nerve fibre can be stimulated by a suitable stimulus which may be mechanical thermal, electrical and chemical in nature.
(2) Conductivity– The velocity of conduction depends on diameter of the nerve fibre. Thick fibres have higher velocity of conduction.
(3) All or non laws- A single nerve fibre always gives a maxium response to a suitable stimulus.
When a nerve fibre is once stimulated, it will not respond to second stimulus for a short period called refractory period.
In a nervefibre summation of two stimuli are possible.
Synapse and Synaptic Transmission:
The functional gap between neurons or neurons and muscle cells is called synapase. Transmission of impulse from one neuron to target cell through synapse is called synaptic transmission.
At the resting stage, the outside of the plasma membrane of the nerve cell is more positively charged than the inside. This potential difference is called resting potential.
Action potential- On stimulation the membrane potential is reserved i.e. the outside becomes negatively charged and the inside positively charged. This is called action potential.