THE NEURON

INTRODUCTION

The neuron is defined as the structural and functional unit of the nervous system.
It is having nucleus and all the organelles in cytoplasm.
It is different from other cells in two ways:

Neuron has branches (processes) called axon and dendrites.
Neuron does not have centrosome. So, it cannot undergo division.

Total estimated number of neurons in human brain is more than 10^{12 .}

CLASSIFICATION OF NEURON

Neurons are classified by 3 different methods.
A. Depending upon number of poles
B. Depending upon function
C. Depending upon length of axon.

A. DEPENDING UPON THE NUMBER OF POLES

Divided into 3 types:
1. Unipolar neurons
2. Bipolar neurons
3. Multipolar neurons.

1. Unipolar Neurons:

Have only one pole.
Both axon and dendrite arise from a single pole.
Present only in embryonic stage in human beings.

2. Bipolar Neurons:

Neurons with two poles.
Axon arises from one pole & dendrites arise from another pole.

3. Multipolar Neurons:

Have many poles.
One of the poles gives rise to axon & all other poles give rise to dendrites.

B. DEPENDING UPON THE FUNCTION

Classified into 2 types:
1. Motor neurons (Efferent).
2. Sensory neurons (Afferent).

1. Motor Neurons:

Carry motor impulses from CNS to peripheral effector organs like muscles, glands, blood vessels, etc.

2. Sensory Neurons:

Carry sensory impulses from periphery to CNS.

C. DEPENDING UPON THE LENGTH OF AXON

Divided into 2 types:
1. Golgi type I neurons
2. Golgi type II neurons.

1. Golgi Type I Neurons have long axons. Cell body is in different parts of CNS and their axons reach the remote peripheral organs.

2. Golgi Type II Neurons have short axons. present in cerebral cortex and spinal cord.

STRUCTURE

Neuron is made up of 3 parts:

Nerve cell body
Dendrite
Axon.

Dendrite and axon form processes of neuron.
Dendrites are short processes and axons are long processes.
Dendrites and axons are called nerve fibres.

1.NERVE CELL BODY (soma or perikaryon):

It is irregular in shape.
It is constituted by cytoplasm called neuroplasm, which is covered by a cell membrane.
Cytoplasm contains a large nucleus, Nissl bodies, neurofibrils, mitochondria and Golgi apparatus.
Nucleus: Each neuron has one nucleus, centrally placed which has one or two prominent nucleoli but it does not contain centrosome, cannot multiply like other cells.
Nissl Bodies: These are small basophilic granules in cytoplasm which are present in soma & dendrite but not in axon & axon hillock.They are responsible for spotted appearance of soma on staining.They contain ribosomes and are concerned with synthesis of proteins.
Neurofibrils: These are thread-like structures present in the form of network in soma & nerve processes, consist of microfilaments & microtubules.
Mitochondria: Present in soma & in axon.They form powerhouse of nerve cell, where ATP is produced.
Golgi Apparatus: Concerned with processing & packing of proteins into granules.