Neuron and Classification of Neuron

 Neuron or nerve cell is defined as the structural and functional unit of nervous system. Neuron is similar to any other cell in the body, having nucleus and all the organelles in cytoplasm. However, it is different from other cells by two ways:

1. Neuron has branches or processes called axon and dendrites

2. Neuron does not have centrosome. So, it cannot undergo division.

 

Neuron and Classification

CLASSIFICATION OF NEURON

Neurons are classified by three different methods.

A. Depending upon the number of poles

B. Depending upon the function

C. Depending upon the length of axon.

DEPENDING UPON THE NUMBER OF POLES

Based on the number of poles from which the nerve fibers arise, neurons are divided into three types:

1. Unipolar neurons

2. Bipolar neurons

3. Multipolar neurons.

1. Unipolar Neurons

Unipolar neurons are the neurons that have only one pole. From a single pole, both axon and dendrite arise. This type of nerve cells is present only in embryonic stage in human beings.

2. Bipolar Neurons

Neurons with two poles are known as bipolar neurons. Axon arises from one pole and dendrites arise from the other pole.

3. Multipolar Neurons

Multipolar neurons are the neurons which have many poles. One of the poles gives rise to axon and all other poles give rise to dendrites.

 

DEPENDING UPON THE FUNCTION

On the basis of function, nerve cells are classified into two types:

1. Motor or efferent neurons

2. Sensory or afferent neurons.

1. Motor or Efferent Neurons

Motor or efferent neurons are the neurons which carry the motor impulses from central nervous system to peripheral effector organs like muscles, glands, blood vessels, etc. Generally, each motor neuron has a long axon and short dendrites.

2. Sensory or Afferent Neurons

Sensory or afferent neurons are the neurons which carry the sensory impulses from periphery to central

nervous system. Generally, each sensory neuron has a short axon and long dendrites.

DEPENDING UPON THE LENGTH OF AXON

Depending upon the length of axon, neurons are divided into two types:

1. Golgi type I neurons

2. Golgi type II neurons.

1. Golgi Type I Neurons

Golgi type I neurons have long axons. Cell body of these neurons is in different parts of central nervous system and their axons reach the remote peripheral organs.

2. Golgi Type II Neurons

Neurons of this type have short axons. These neurons are present in cerebral cortex and spinal cord.

STRUCTURE OF NEURON

Neuron is made up of three parts:

1. Nerve cell body

2. Dendrite

3. Axon.

Dendrite and axon form the processes of neuron. Dendrites are short processes and the

axons are long processes. Dendrites and axons are usually called nerve fibers.

NERVE CELL BODY

Nerve cell body is also known as soma or perikaryon. It is irregular in shape. Like any other cell, it is constituted by a mass of cytoplasm called neuroplasm, which is covered by a cell membrane. The cytoplasm contains a large nucleus, Nissl bodies, neurofibrils, mitochondria and Golgi apparatus. Nissl bodies and neurofibrils are found only in nerve cell and not in other cells.

Nucleus

Each neuron has one nucleus, which is centrally placed in the nerve cell body. Nucleus has one or two prominent nucleoli. Nucleus does not contain centrosome. So, the nerve cell cannot multiply like other cells.

Nissl Bodies

Nissl bodies or Nissl granules are small basophilic granules found in cytoplasm of neurons and are named after the discoverer. These bodies are present in soma and dendrite but not in axon and axon hillock. Nissl bodies are called tigroid substances, since these bodies are responsible for tigroid or spotted appearance of soma after suitable staining. Dendrites are distinguished from axons by the presence of Nissl granules under microscope. Nissl bodies are membranous organelles containing

ribosomes. So, these bodies are concerned with synthesis of proteins in the neurons. Proteins formed in

soma are transported to the axon by axonal flow. Number of Nissl bodies varies with the condition of

the nerve. During fatigue or injury of the neuron, these bodies fragment and disappear by a process called chromatolysis. Granules reappear after recovery from fatigue or after regeneration of nerve fibers.

Neurofibrils

Neurofibrils are thread-like structures present in the form of network in the soma and the nerve processes. Presence of neurofibrils is another characteristic feature of the neurons. The neurofibrils consist of microfilaments and microtubules.

Mitochondria

Mitochondria are present in soma and in axon. As in other cells, here also mitochondria form the powerhouse of the nerve cell, where ATP is produced.

Golgi Apparatus

Golgi apparatus of nerve cell body is similar to that of other cells. It is concerned with processing and packing of proteins into granules

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