N EURAL S YSTEM A neuron is a microscopic structure composed of three major parts, namely, cell body , dendrites and axon (Figure . ). The cell body contains cytoplasm with typical cell organelles and certain granular bodies called Nissl’s granules . Short fibres which branch repeatedly and project out of the cell body also contain Nissl’s granules and are called dendrites.
These fibres transmit impulses towards the cell body. The axon is a long fibre, the distal end of which is branched. Each branch terminates as a bulb-like structure called synaptic knob which possess synaptic vesicles containing chemicals called neurotransmitters . The axons transmit nerve impulses away from the cell body to a synapse or to a neuro-muscular junction.
Based on the number of axon and dendrites, the neurons are divided into three types, i.e., multipolar (with one axon and two or more dendrites; found in the cerebral cortex), bipolar (with one axon and one dendrite, found in the retina of eye) and unipolar (cell body with one axon only; found usually in the embryonic stage). There are two types of axons, namely, myelinated and non- myelinated . The myelinated nerve fibres are enveloped with Schwann cells, which form a myelin sheath around the axon. The gaps between two adjacent myelin sheaths are called nodes of Ranvier .
Myelinated nerve fibres are found in spinal and cranial nerves. Unmyelinated nerve fibre is enclosed by a Schwann cell that does not form a myelin sheath around the axon, and is commonly found in autonomous and the somatic neural systems. . .
Generation and Conduction of Nerve Impulse Neurons are excitable cells because their membranes are in a polarised state. Do you know why the membrane of a neuron is polarised? Different types of ion channels are present on the neural membrane. These ion channels are selectively permeable to different ions.
When a neuron is not conducting any impulse, i.e., resting, the axonal membrane is comparatively more permeable to potassium ions (K + ) and nearly impermeable to sodium ions (Na + ). Similarly, the membrane is impermeable to negatively charged proteins