Neurons (= Nerve Cells):
Neuron is a structural and functional unit of the neural tissue and hence the neural system. Certain neurons may almost equal the length of body itself. Thus neurons with longer processes (projections) are the longest cells in the body. Human neural system has about 100 billion neurons.
Majority of the neurons occur in the brain. Fully formed neurons never divide and remain in interphase throughout life. Shortly after birth, new neurons do not develop. Certain neurons have flask shaped cytons and are called Purkinje cells, which occur in the cerebellum of the brain. A neuron consists of main cell body and cytoplasmic processes arising from it.
(i) Cell body (= Cyton or Soma):
It varies in size and form. It may be up to 13.5 µm in diameter and may be irregular, spherical, oval, rounded, star-shaped or pyramidal. Like a typical cell it consists of cytoplasm, nucleus and cell membrane.
It has abundant cytoplasm, called neuroplasm and a relatively large spherical central nucleus with a distinct nucleolus. The cytoplasm has mitochondria, Golgi apparatus, rough endoplasmic reticulum, ribosomes, lysosomes, fat globules, pigment granules, neurofibrils, neurotubules and Nissl’s granules.
Presence of neurofibrils and Nissl’s granules is characteristic to all neurons. Neurofibrils play a role in the transmission of impulses. Neurotubules are in fact, microtubules which maintain the shape of the neuron.
The Nissl’s granules (also called Nissl’s bodies) are irregular masses of rough endoplasmic reticulum with numerous attached and free ribosomes and polysomes. The Nissl’s granules probably synthesize proteins for the cell.
Centrioles, formerly believed to be absent in mature neurons, have been described in neurons and may be associated with the production and maintenance of microtubules. The cytoplasm immediately surrounding the nucleus is loaded with protein-synthetic machinery, and is called perikaryon.
Previously the name perikaryon was given to the cyton (cell body or soma). Ageing neurons contain a pigment lipofuscin (made up of residual bodies derived from lysosomes). Cyton is concerned with metabolic maintenance and growth.
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 towards the cell body and are called afferent processes (= receiving processes).
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. The part of the sarcolemma (muscle plasma membrane) that lies beneath the axon terminals/nerve endings, is called motor end plate.
Each axon may also possess side 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.
There are two types of axon namely myelinated and non-myelinated. In myelinated nerve fibres Schwann cells form myelin sheath around the axon. The gaps between two adjacent myelin sheaths are called nodes of Ranvier.
Myelinated nerve fibres are found in cranial and spinal nerves. In non-myelinated nerve fibres Schwann cells do not form myelin sheath and are without nodes of Ranvier. They are commonly found in autonomous and somatic neural systems.
Types of Neurons on the Basis of Structure: edit
1. Non-Polar Neurons:
Each neuron has several branched processes (projections). These neurons are rare in vertebrates but occur in cnidarians (coelenterates) e.g., Hydra.
The body has only one axon. It is found usually in the embryonic stage.
3. Pseudo unipolar Neurons:
A single process arises from the cyton and then divides into axon and dendrite. They are found in dorsal root ganglia of spinal nerves.
Each bipolar neuron has one axon and one dendrite. They are present in the retina of eye.
5. Multipolar Neurons:
These neurons have several dendrites and an axon. They are found in cerebral cortex.