Structure of Neuron:
The neuron consists of-
- Cell body or cyton or soma.
- Fine cytoplasmic processes.
- Nerve fibre (axon).
Cell Body- It may be a fusiform, rounded or oval enlarged structure that contains the vesicular nucleus. The cytoplasm that surrounds the nucleus often is called the perikaryon (neuroplasm). The nucleus is a pale-coloured, spherical or ovoid organelle and is centrally located within the cell body. The perikaryon is densely packed with mitochondria, endoplasmic reticulum, Golgi complex, neurofilaments and the basophilic- Nissle’s granules. The Nissle’s granules mainly synthesize protein for the cell. The mitochondria in the cell body provides energy to neuron for impulse conduction.
Cytoplasmic Processes- The cytoplasm of the cell body extends into a number of short branching processes called dendrites and one long process called an axon. The dendrites are characterized by their profuse branching which increases the functional receptive surface of the nerve cell and ultimately, conduct impulses towards the cell body. The axon frequently originates from the cell body in a conical elevation of perikaryon-the axon hillock. The axon is a longer and thinner structure bodies that attains a considerable length and sends out the number of branches called axon collaterals and at its terminal end, it arborizes into fine branches called terminal arborization (terminal buttons) on muscle fibres, glands or other neurons. These terminations contain synaptic knobs bearing synaptic vesicles and mitochondria in which neurotransmitters are stored for nerve conduction.
The Medullated Nerve Fibre (Axon)- It is surrounded by two layers- neurolemma and myelin sheath. The myelin sheath is a whitish structure consisting of a protein-lipid complex and it envelops the axon continuously in the brain and spinal cord. This sheath is insulative, nutritive and plays an important functional role in the transmission of nerve impulse. The myelin sheath of an axon in the peripheral nerves is completely interrupted at regions called the nodes of Ranvier.
The neurolemma is a thin, transparent, cylindrical and cellular investment that completely invests the axon. It is made up of a single layer of flat expanded Schwann cells. The Schwann cells are neuroglial cells that have migrated from CNS and form complex neurilemmal sheaths about the nerve fibres. The myelin actually is a part of the Schwann cell and consists of layers of the membrane of the Schwann cell that is wrapped in a spiral fashion about the axon.
Myelinated nerve fibres are found in spinal and cranial nerves. Unmyelinated nerve fibre is enclosed by a Schwan cell that does not form a myelin sheath around the axon and is commonly found in autonomous and somatic neural systems.