Neuron or the nerve cells are the structural and functional unit of nervous system. There are innumerable neurons in human nervous system. The neurons are linked together and can sense stimulus from both the external environment as well as the inside parts of the body. It’s main function is signal transmission. There are different types of neuron present in our body and are differentiate according to their structure and function.
TYPES OF NEURON: Neurons are classified on the basis of their structure and function.
One the basis of structure, neurons are three types- 1. Unipolar neurons, 2. Bipolar neurons, and 3. Multipolar neurons. There are also two other types of neurons- Pseudo-unipolar neuron and non-polar neuron.
STRUCTURE OF A TYPICAL NEURON: A typical neuron primarily consist of the- Cell body and processes, also called neurites. Neurites are two types – A. Dendrites and B. Axon.
CELL BODY: The cell body of neuron is also called the soma or perikaryon. It is round, pyramidal or fusiform in shape. Cell body contain cytoplasm and some cytoplasmic organelle.
- The mass of cytoplasm present in the nerve cell is called neuro-plasm.
- Some cytoplasmic organelles like Mitochondria, Golgi-bodies, Endoplasmic Reticulum, Ribosome, Lysosome etc.
- It contains a prominent spherical nucleus with one or two nuclei but there is no centrosome.
- Neuron also consist Nissl’s granules. These are basophilic granules and are comparatively large and irregular masses of Ribosomes and Rough Endoplasmic Reticulum. They synthesize proteins in the cell or the main site of protein synthesis in the nerve cells.
- Cell body also contain neurofibrils and fat globules. Some pigment granules are also seen in the cell body of neurons.
PROCESSES OF NEURON: The processes of neuron are called neurites. These are two types- Dendrite and Axon.
- DENDRITES: The dendrites (dendron) are multiple small branched processes which contain Nissl bodies and neurofibrils. These are usually shorter and tapering process. Dendrites are the receptive processes of neuron. They receive signals from other neurons via the synapse with axon terminal.
- AXON: Axon is the single longer process of the nerve cell. It arises from the conical extension of the cell body, called axon hillock. This part is devoid from Nissl bodies. Axon hillock is the most sensitive part of a neuron. In axon the cell membrane continues as axolemma and the cytoplasm is called axoplasm. Neuro-plasm of the axon or axoplasm contains abundant neurofibrils and mitochondria. Nissl’s granules, Ribosomes, Golgi bodies and fat globules are absent in the axoplasm. Myelin sheath is present around the middle part of the axon, just after the axon hillock. Myelin sheath which consist of protein-lipid complex is produced by the glial cells called Schwann cells, which incircle the axon. The nerve cell containing the myelin sheath is called myelinated nerve fiver, and which does not contain the myelin sheath is known as non-myelinated nerve fibre. This axon terminates by dividing into a number of synaptic knobs also known as terminal buttons or axon telodendrion.
FUNCTION OF NEURON: Neuron is the main functional unit of the nervous system. It helps to transmit the signals, generated from both inside the body part and outside environment to the brain. It this transmission is done by the sensory neurons. The motor neurons help to transmit the signal generated in the brain to the place of reaction.
NOTABLE: The absence of the centrosome in the nucleus of the nerve cells indicates that the neuron has lost ability for division. So that if neurons once destroyed are replaced only by the neuroglia cells.