Neuron – Structure And Classification Of Neuron

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In this blog, I give you information regarding Neuron.Neuron or nerve cell is defined as the structural and functional unit of nervous system.We learn about structure and classification of neuron.

Neuron is similar to any other cell in the body.But it is different from other cells by two ways:

  1. Neuron has Axon and dendrites as you can see in image that is not present in other cells of body.
  2. Neuron does not have centrosome.So, it can not multiply.

STRUCTURE OF NEURON

Neuron is made up of three parts:

Neuron
  1. Nerve cell body(Soma)
  2. Dendrites
  3. Axon

(1) BODY (SOMA)

Nerve cell body also known as soma or perikaryon.
It is in irregular shape.
It is constituted by a mass of cytoplasm called neuroplasm, which is covered by a cell membrane.
Cytoplasm contains Nucleus,Nisslgranules,Mitochondria,Neurofibril,Gogli appartus

Nucleus

Nucleus not contain centrosome,so can not multiply like other cells of body.Each neuron has only one nucleus,which is placed in center.

Nissl Granules

Nissl bodies or Nissl granules are small basophilic granules found in cytoplasm of neuron.It is present only in soma and dendrites.It is not present in axon hillock and in axon.

Nissl granules contains ribosomes.So, it concerned with synthesis of protein in the neurons.Protein formed in soma are transported to the axon by axonal flow.

Mitochondria

It is power-house of neuron, where ATP is produced.Miyochondria are present in soma and in axon.

Nuerofibril

It is characteristic feature of neuron.It is thread like structures present in the form of network in the soma and the nerve processes.

Golgi Apparatus

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

(2) DENDRITES

dendrites

Dendrites is branched process of neuron.It looks like hairy projections as you can see in image.

In neuron dendrites may be present or absent.

If dendrites present than it may be one or many in number.

Dendrites transmits impulses towards the nerve cell body.Usually, the dendrites is shorter than axon.

(3) AXON

Axon is the longer process of nerve cell.Length of longest axon is about 1 meter.

Each neuron has only one axon.Axon arises from axon hillock of the nerve cell body and axon part not contain Nissl granules.

Axon transmits impulse away from the nerve cell body.

ORGANISATION OF NERVE

When axon (nerve fiber) of many neuron comes together, it is called one group of nerve fiber or fasciculus.

Each nerve is formed by many groups of nerve fibers or by many fasciculus.

When we do cross section of nerve, it looks like as shown in image.

Nerve has many fasciculus.Each fasciculus is covered by perineurium.Each nerve fiber is covered by endoneurium and whole nerve is covered by tubular sheath called epineurium.

NON-MYELINATED VS MYELINATED NERVE FIBER

As you can see in image that there are two types of axon.

One is simple axon, which has no myelin sheath called non-myelinated nerve fiber.Second one is covered with myelin sheath called myelinated nerve fiber.

MYELIN SHEATH

Myelin sheath is a thick lipoprotein sheath that insulates the myelinated nerve fiber.This sheath is not continuous as you can see in image, it is absent at regular interval.The area where sheath is absent is called node of Ranvier.Myelin sheath is responsible for white colour of nerve fibers.

FUNCTION OF MYELIN SHEATH

  1. Faster conduction
    Myelin sheath is responsible for faster conduction of impulse through the nerve fibers.Here impulse jump from one node to another node.This type of transmission of impulses is known as saltatory conduction.
  2. Insulating capacity
    Myelin sheath has a high insulating capacity.So, it can restrict the nerve impulse within single nerve fiber and prevents the stimulation of neighboring nerve fibers.Simply we can say that it can stop the massage going ahead.

CLASSIFICATION OF NEURON

  1. Depending upon the number of pole
  2. Depending upon the function
  3. Depending upon the length of axon

(1) DEPENDING UPON THE NUMBER OF POLE

Based on number of poles from which the nerve fiber arises,neuron divided into 3 types.

unipolar,bipolar,multipolar neuron
  1. Unipolar neurons
  2. Bipolar neurons
  3. Multipolar neurons

Unipolar neurons

This neurons having only one pole.From single pole, both axon and dendrites arises.This type of nerve cells is present only in embryonic stage in human being.

Bipolar neurons

Neurons having two poles known as bipolar neurons.Axon arise from one pole and dendrites arising from other pole.They are part of the sensory pathways for smell, sight, taste, hearing, touch.

Multipolar neurons

The neuron which have many poles is called multipolar neuron.One pole give rise to axon and all other poles give rise to dendrites.This type of neuron mostly found in brain and spinal cord.

(2) DEPENDING UPON THE FUNCTIONS

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

  1. Sensory or afferent neurons
  2. Motor or efferent neurons

Sensory neurons

Sensory neuron are the neurons which carry the sensory impulses from periphery(body) to central nervous system.Mostly sensory neuron has short axon and long dendrites.Long dendrites will help to pass sensation from body to brain.

Motor neurons

Motor neurons are the neurons which carry the motor impulses from central nervous system to peripheral organs like muscles,glands,blood vessels, etc.Generally, motor neuron has long axon so it can pass speedily message to necessary organs.

(3) DEPENDING UPON LENGTH OF AXON

Depending on length of axon, neuron devided into two types:

  1. Golgi type l Neurons
    Golgi type l Neurons have long axon.
  2. Golgi type ll Neurons
    Neurons of this type have short axons.