NEURITES -

The processes of neurons are called neurites. These are of two types: dendrites or dendrons and an axon or axis cylinder or neuraxon.

(a)      Dendrites (Dendrons). These are usually shorter, tapering and much branched processes. They may be one to several. The dendrites contain neurofibrils, neurotubules and Nissl's granules. They conduct nerve impulse to- wards the cell body and are called afferent processes (= receiving processes). It's fine branches are telodendria.

(b)      Axon. Axon is a single, usually very long process of uniform thickness. The part of cyton from where the axon arises is called axon hillock.

• Most sensitive part of neuron is axon hillock.
• The axon contains neurofibrils and neurotubules but does not have Nissl's granules, Golgi complex, ribosomes, pigment granules, fat globules, etc.
• In the absence of Nissl's granules, the axon depends on the cell body for the supply of proteins.
• The cell membrane of the axon is called axolemma and its cytoplasm is known as axoplasm.
• The axon ends in a group of branches, the terminal arborizations (= axon terminals or telodendria).
• When terminal arborizations of the axon meet the dendrites of another neuron to form a synapse they form synaptic knobs (= end plates).
• The synaptic knobs contain mitochondria and secretory vesicles. On muscle fibres and gland cells, the terminal arborizations end as motor end plates.
• Each axon may also possess lateral or branches called collateral fibres which are usually much finer than the main axonal process.
• The axon conducts nerve impulses away from the cell body, therefore, called an efferent process.
• A synapse is a site of junction between terminal arborizations of axon of one neuron and the dendrites of
• another neuron.

• Each neuron receives an impulse through its dendrites and passes it on to the next neuron through synapse.
• A fresh impulse is set up in the dendrites at the synapse with the help of chemicals called neurotransmitters, such as acetylcholine produced by the secretory vesicles of the synaptic knobs.
• Acetylcholine is the first neurotransmitter to be discovered.