NEURON – ITS ANATOMY AND PARTS

A neuron is a specialized cell that receives, processes and transmits information throughout the body by utilizing electrical and chemical signals. They receive sensory stimulus from environment, process it and send motor commands to muscles.

Neuron is the structural and functional unit of nervous system. It converts electrical signals into chemical signal. When electrical impulse reaches axon terminal, it triggers release of chemical called neurotransmitter at the junction of two neurons (synapse). Impulse is then conveyed to the next neuron. There are about 86 billion neurons in the human brain alone. The human nervous system contains approximately 10¹² neurons.

Neuron consists of three main parts – cell body, dendrites and axon. Nerve cell with its processes (dendrites and axons) is called a neuron. There is only one axon for a nerve cell whereas dendrites can be one or more. Nerve cell is present in the grey matter while dendrites and axons are present in the white matter.

PARTS OF NEURON

CELL BODY ( SOMA / PERIKARYON )

The cell body is also known as soma or perikaryon and is central part of nerve cell. It contains nucleus, cytoplasm an organelles. Cell body connects dendrites with axon.

It integrates signals received by dendrites, performs protein synthesis and carries out cell’s metabolic functions.

SIZE – Varies from 5 micron (granule cell of cerebellum) to 130 micron (in anterior horn of spinal cord).

SHAPE – Varies depending upon the number of processes. Round in unipolar cells, oval in bipolar cells, star shaped in multipolar cells. They are pyramidal in cerebral cortex. Purkinje cells of cerebellum are flask shaped.

NUCLEUS – Spherical, large and centrally placed. It appears pale and empty looking with prominent nucleolus.

CYTOPLASM – It contains :

• Neurofibrils – Thread like cytoplasmic fibrils that stain with silver salts. They form closely meshed branching network inside cytoplasm extending into all processes. They provide mechanical support and stability to nerve cell.
• Nissl’s granules – These are basophilic granules. They are rough endoplasmic reticulum rich in RNA. They are present in cell body and dendrites but are absent from axon and axon hillock. Nissl’s granules are the sites of protein synthesis.
• Golgi body – Perinuclear in position.
• Mitochondria
• Inclusion bodies – Found in some neurons.

DENDRITES

Dendrites are afferent processes of the neuron that receive sensory impulse and transmit it to cell body. They can be one or more in number. Dendrites appear as short extensions of nerve cell which show tree like branching structure.

AXON

Axons are efferent processes of the nerve cell. They carry nerve impulse away from cell body to other neurons or to effector organs. They are long, slender and uniform in diameter. There is only one axon for a neuron.

Axon arises from axon hillock which is a small conical projection of cell body. The terminal branches of axon are known as axon terminals or telodendria. Axon terminals end in synaptic knobs. Branches called collaterals arise from axon at right angles. Collaterals and terminal branches serve to transmit impulses to other dendrites or to cell body of other axons.

Axon is surrounded by a membrane called axolemma. Its axoplasm contains neurofibrils and mitochondria, but is devoid of Nissl’s granules. The axon acquires myelin sheath after a short distance from its origin called initial segment of axon. The intervening segments of axon which are devoid of myelin sheath are called Nodes of Ranvier. Myelin sheath facilitates conduction of impulse and prevents cross stimulation of adjacent axons by its insulating property.

TYPES OF NEURONS BASED ON NUMBER OF PROCESSES

UNIPOLAR NEURONS

These have only one process which arises from spherical cell body and splits like letter T into two. They are also known as pseudo-unipolar neurons. Example : all sensory ganglia (except vestibular and cochlear ganglia). True unipolar neurons are those having only a single process. Example : neurons in mesencephalic nucleus of trigeminal nerve.

BIPOLAR NEURONS

These have two processes, an axon and a dendrite arising from spindle shaped cell body. Example : cochlear and vestibular ganglia, olfactory neuroepithelium, inner and outer nuclear layers of retina.

MULTIPOLAR NEURONS

They have many processes (one axon and many dendrites). Example : neurons in CNS and autonomic ganglia. Multipolar neurons are either motor or internuncial neurons.

TYPES OF NEURONS BASED ON FUNCTION AND ANATOMICAL RELATIONS

SENSORY / AFFERENT NEURONS

They carry information from sense organs to central nervous system, allowing sensory perception. Their cell bodies lie outside CNS in sensory ganglia of cranial nerves and dorsal root ganglia of spinal nerves.

INTERNUNCIAL / ASSOCIATION NEURONS / INTERNEURONS

They transmit nerve impulses from sensory to motor neurons and facilitate communication, processing and reflexes. They are found in CNS.

MOTOR / EFFERENT NEURONS

These carry commands from the CNS to effector organs, muscles and glands, resulting in physical and physiological response. Their dendrites lie in CNS and axons leave CNS to supply skeletal muscles.