nerve physiology 2..pptx

Zoology

1. Types of Neurons Dr. Chitra D. Morey Dept. of Zoology, Shri Shivaji College, Motala , Buldhana

2. Types of Neurons: Neurons are cells within the nervous system/ basic working unit of the brain that transmit information to other nerve cells, muscle, gland cells. Neuron is the structural and functional unit of nervous system. Histologically, the neurons are categarised into two categaries : A) myelinated Or medullated neurons. B) Non- myelinated Or non-Medullated neurons.

3. A) Myelinated Or medullated neurons: The axons, both in vertebrates and invertebrates are often covered with a sheath called the myelinsheath . Such fibres ( axons) are called myelinated fibres. Usually fibres of larger diameter are myelinated. In mammals, all fibres larger than 1u in diameter are covered with myelin sheath. The structure of myelinated fibres is complicated. The axon is surrounded by myelin sheath. The central core of axon is called axoplasm, which is pasty ( semifluid) in nature.

4. The axoplasm is surrounded by membrane called axolemma. Outside the axis of axon lies myelin sheath. The myelin sheath is surrounded by the Cytoplasm of special type of cell called schawnn cells. Infact , the schawnn cells repeatedly surrounds the axis with many concentric rings of its cell wall to form myelin sheath of particular region. The myelin sheath provides both mechanical and electrical insulation to the axon.

5. At regular intervals there is an apparently Constricted area called ‘node of Ranvier’ At the node of Ranvier there is no myelin sheath. The neurilemma is in direct contact with the axis of axon. The portion between two successive nodes Called internode. The presence of nodes and internodes gives the appearance of bamboo stick. The myelin sheath is made up of lipid material and is particularly rich in sphingomyelin ( a phospholipid). Myelin has whitish appearance.

6. The white matter of brain look white because they are myelinated fibres. Each node of myelinated axon fibre contain one and only one nucleus. Deosition of myelin sheath:- myelination begins at 4 th month of intrauterine life and the process is completed a few years after birth.

7. Non-myelinated or non –medulated neurons: There is no diposotion of myelin sheath. The diameter of these nerve fibres are very small. There is no nodes of Ranvier. Schawan cells are also associated with non- myelunated fibres. In some cases a number of non-myelinated fibres pass through a single schawnn cells which acts as a supporting structure.

8. Importance of myelin sheath: Propagation of action potential is very fast in myelinated fibres but slow in non- myelinated fibres. This is because the saltatory conduction is possible only in a myelinated fibres. Myelination, thus result in quick mobility in the higher animals.

9. Neurons are divided into four major types: unipolar, bipolar, multipolar, and pseudounipolar. Unipolar neurons have only one structure extending from the soma; bipolar neurons have one axon and one dendrite extending from the soma. Multipolar neurons contain one axon and many dendrites; pseudounipolar neurons have a single structure that extends from the soma, which later branches into two distinct structures.