Nervous Tissue, Neuron, Types of Neuron, Nerve Fibre, Animal Tissue

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Keywords: Nervous Tissue, Animal Tissue, Defining Neuron, Cyton, Types of Neuron, Structure of Nerve Fibre, Neuroglia, Sodium-Potassium Pump, Zoology Help, Biology Help, Nervous Tissue Assignment Help, Nervous Tissue Homework Help, Nervous Tissue Projects Writing Help, Live Experts, Online Tutors

Nervous Tissue

This constitutes about 2.4% of the body weight of man viz.

Brain-1400 gram

Spinal cord-30 gram

Spinal nerve-150 gram

Cranial nerve-10 gram

C.S.F. is altogether absent in nervous tissue. About half of the solid nervous tissue is lipid and 40% is constituted by proteins.

Nervous tissue is composed of two different types of cells-

(1) The excitable cells are nerve cells or neuron.

(2) The non-excitable cells are neuroglial cells, Schwann cells and ependymal cells.

Neuron: It arises from a single embryonic cell called neuroblast. It has-

Cyton: It is main cell body also called soma or perikaryon. Nucleus is centrally placed. Cyton of most neurons occurs in grey matter. If they are outside the grey matter, they are called ganglia. Cytoplasm is called neuroplasm. It contains nucleus, ribosome, E.R., golgi bodies, fat globules, mitochondria, Nissl's granules etc. Neurons with flask shaped cyton are called Purkinje cells?

Cell processes are called neurite.

They are of two types-

1. Many short afferent processes which are branched and bushy are called dendrite.

2. A single long efferent process with occasional collateral fibres, is called axon. Its terminal branches are called telodendria. Axons form tracts in C.N.S. and nerves in P.N.S. Telodendria ends in a bulb like structure which is called terminal buttons.

Types of neuron:

(1) According to number of process¬

(a) Unipolar: Only axon is present. e.g. mesencephalic nucleus.

(b) Pseudounipolar: Only axon is present but it gets divided to form dendron also." e.g. sensory ganglia.

(c) Non-polar: Only cyton is present. e.g. Hydra and other cnidarians.

(d) Bipolar: One axon and one dendron is present. e.g. retinal and cochlear ganglia.

(e) Multipolar: Single axon and several dendrons are present. e.g. most of the neuron of the body.

(2) According to length of axon

(a) Goigi type I: With long axon.

(b) Golgi type II: With short or no axon.

(3) According to their function

(a) Sensory or afferent: Bring impulse from receptor or organ to C.N.S.

(b) Motor or efferent: Carry impulse from C.N.S. to organ or effector.

(c) Adjustor neuron: They occur only in C.N.S. and join two different neurons.

Structure of nerve fibre: An axon may be enveloped by one, two or three sheath like investments viz.

(1) Axon of P.N.S. has an outermost endoneurial sheath of connective tissue, middle neurilemma and an inner sheath of myelin.

(2) Axon of C.N.S. has an outermost sheath of neuroglial fibres and middle oligodendroglial sheath and inner myelin sheath.

A nerve fibre is an axon with its investments. The tube formed by myelin sheath is filled with myelin. At the regular intervals the myelin sheath is interrupted. These interrruption or constrictions are called Node of Ranvier. Segment between two successive node is called internode. Myelin sheath bearing fibres are called myelinated while which are without myelin sheath are called non¬myelinated. Internodal segment of myelin sheath is formed by single Schwann cell.

The junctions between the neurons are called synapses. It is not the continuity but contiguity. At the synapses, various types of neurotransmitter are found viz. Acetylcholine, GABA, catecholamines etc. Most common type of synapses are axo-axonic and axo¬somatic. A synapse may be either inhibitory or excitatory.

Neuroglia: It is non-excitable supporting component of nervous tissue.

These are -

1. Neuroglial cell — In C.N.S.

2. Ependymal cells — In internal cavities.

3. Capsular cells — Surrounding autonomic ganglion.

4. Schwann cells — Forming sheath.

5. Supporting cells — Sheathing nerve terminals.

The glial cells are protective and supporting in function. Specially microglia cells are 'protective and do it by phagocytosis. Ependymal cells are concerned with the exchange of materials between brain and C.S.F. The glial cells also act to repair the damage in the nervous tissue by the process gliosis.

Impulse transmission: The conduction of impulse in a nerve fibre is. an electrochemical process. Maximum speed of transmission of nerve impulse can be 13 metre/second because axoplasm is much resistant to current flow. Impulse conduction depends upon-

1. Permeability of axon membrane.

2. Osmotic equilibrium between axoplasm and E.C.F.

3. Electrical equivalence between axoplasm and E.C.F.

Sodium-Potassium pump:

(1) The resting nerve membrane is about 50-100 times as permeable to K+ and Cl- as to Na+.

(2) Concentration of Na+ and Cl- in E.C.F. is about 10 and 14 times more than cytoplasm.

(3) Concentration of K+ in cytoplasm is about 30 times more than E.C.F.

(4) Due to this difference in concentrations, there is continuous influx of Na+ and Cl- and out flux of K+ into the E.C.F. Three Na+ are exchanged by two K+ ions.

(5) This passive exchange can cause disturbance in osmotic equilibrium of cell which is maintained by expelling of Na+ into E.C.F. and taking in of K+ from it by active transport. This is called N+-K+ pump. It is facilitated by Na+-K+- ATPase enzyme.

Resting membrane potential: This maintenance of osmotic equivalence causes net difference of about 42 m Eq between cytoplasm and E.C.F. Thus the resting cell is in a polarized state with an electrical potential across its membrane of about -90 mV. This is called resting membrane potential.

Action potential: When any factor that disturb the resting state of membrane, elicit the membrane and increases its permeability to Na+, causes origination of action potential. At the onset of action potential, the permeability of membrane increases by 5000 fold (about).

Once the action potential is established the impulse will transmit all through the length of fibre. This is called all or none principle. The value below which action potential is not established is called threshold value. Impulse conduction is such a triggered phenomenon which occurs like an explosion but only when it reaches ignition point.

The sudden loss of normal negative potential inside the fibre during action potential is called membrane depolari¬zation and positive potential that develops momentarily is called reversal potential.

When the excessive +ve charge inside the fibre is transferred back out of fibre and the normal negative potential is established, then this state is called repolarization.

Period between which the point on the fibre is in depolarized state, no other stimulus can be taken or transmitted, is called the refractory period. It means it is the time taken in repolarization. In the myelinated fibres the action potential jumps from one Node of Ranvier to another successively. This type of conduction is called saltatory conduction.

Transmission at the synapse in most of the fibre is chemical and it is facilitated by neurohumors e.g. acetylcholine. These are also called as chemical mediators.

Some important applicative words

(1) Fracture: Break in the continuity of bone.

(2) Paralysis: Loss of motor power.

This is due to inability of muscle to contract.

(3) Muscular spasm: It is caused by muscle pull and is quite painful.

(4) Disused atrophy: When the muscle is not used for very long time, it becomes thin and weak. It is called disused atrophy.

(5) Hypertrophy: Excessive development of a muscle due to over¬use of it.

(6) Wasting: Reduction in size of muscle.

(7) Muscle fatigue: Inability of muscle to contract and to carry on metabolic processes of fibre. It is because of accumulation of lactic acid due to prolonged and strong contraction of muscle.

(8) Rigor mortis: Contraction of muscle several hours after the death.

(9) Rouleaux formation: Adhering of R.B.Cs. in the form of a stake or piles of coins. It is due to surface tension.

(10) R.B.C. ghost: R.B.C. loses its content by simple diffusion but it is intact and called R.B.C. ghost.

(11) Anaemia: Destruction or loss of R.B.C. or a sharp fall in erythropoesis causes anaemia.

(12) Polycythemia: Abnormal rise in the number of red blood corpuscles.

(13) Twitch: Sudden partial contraction of a muscle.

(14) Summation: Superposition of the 2nd contraction over first causes further contraction of fibre. This is called summation.

(15) Muscle tone: Partial contraction of a muscle in resting condition to maintain posture of body.