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What is a Neuron? – Definition, Structure, Classification, Functions

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  • Last Updated : 03 Jul, 2022
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Human body consists of several organs and functional units. Of them, the central nervous system is the major functional unit that helps human think, react, respond and reply. This nervous system functions due to neurons. Neuron is the structural and functional unit of the nervous system. Neurons are simply called as nerve cells or Neurones. Neurons are defined as “the building blocks” of the nervous system. The study of brain functions is known as Neuroscience.

What is a Neuron?

A neuron can be defined as a nerve cell that transmits information from one part of the body to another through neural impulses. Neurons receive and transmit signals to different parts of the body. This transmission process can be carried out both in physical and electrical forms.  Below mentioned are the types of neurons that are classified based on their activities,

Sensory Neuron

Sensory neurons are a major form of neurons that are present throughout the body and carry information from the sensory receptor cells to the brain. These neurons are Pseudo unipolar. For example, when you are walking suddenly if you step on a thorn, immediately the sensory cells that are present there senses the reaction and immediately send a signal to the brain/ nervous system about the information they have received. The data that is received by the sensory neurons can be chemical or physical, relative to our five senses. Thus physical input data received can be any of the things like sound, heat, fire, light, touch, etc. the chemical input data can be smell or taste. These chemical and physical inputs can be sent to the brain.

Motor Neuron

Motor neurons are the nerve cells that are connected from the brain to the body parts. These Neurons Carry the information that the brain responds to the data sent by the sensory neurons to the muscles/body parts. The neurons are multipolar. 

There are two types of motor neurons:

  1. Lower Motor Neuron The neurons that travel from the spinal cord to muscle are called lower motor neurons.
  2. Upper Motor Neuron The neurons that travel between the brain and spinal cord are called upper motor neurons.

Inter Neuron

These are multipolar. These neurons transmit data between various neurons (neurons – neurons)in the body. They connect the spinal motor and sensory neurons. Besides transferring signals between sensory and motor neurons,  interneurons also communicate with each other, forming circuits of various complexity. 

Since neurons are the structural and functional unit of the body, they contain the following parts. 1. Cell Body   2. Cell Processes

Cell Body(perikaryon) 

The cell body is also known as Soma. The cell body has a variable size and shape. The cell body consists of the nucleus which is enormous and is able to show its nucleolus. The cell body also consists of the Mitochondria, Golgi apparatus, pigment granules, neurofibrils, and Neuro tubules in the Cytoplasm which supports the Neuron. The rough endoplasmic reticulum is seen in Nissl granules which extend into dendrites, but an absent in axon Hillock. Neurons do not contain centrioles so they cannot divide. The plasma membrane of the cell body continues as axolemma. the plasma membrane shows many receptor sides.

Cell Processes( Neurites)

 

This is defined as any projection from the cell body of the neuron. The projection can be an axon or a dendrite. These are the extensions of the cell body. These contain the following components.

The Dendrites

Dendrites are tree-like structures (dendritic trees) that are designed to receive communications from other cells. They are 2micro meters in length. They receive signals from various neurons and process and transfer the signals to the cell body. It contains numerous ribosomes, smooth endoplasmic reticulum, and Golgi apparatus which are useful for high protein synthesizing activity during signal transmission. The surface of dendrites is with more number of receptors that become activated when it gets submitted to the neurotransmitters. These dendrites receive synaptic inputs from axons. These are many short branching extensions towards Nissl granules, and conduct impulses towards the cell body. They bear Spines(The small protrusions found on dendrites.) which are specialized for contact with other neurons.  

The Axon

This is the transmitting part of the neuron. These are the long thin structures (Axon length >1mm to <1m)  in which axon potentials are generated.  After initiation, action potentials travel down through the axons to cause the release of neurotransmitters. It is a single long process arising from the axon Hillock. It is cylindrical and does not contain Nissl granules, gives side branches (collateral), and conducts impulses away from the cell body. The axons show terminal branches with enlarged ends called buttons. The axons in the peripheral nervous system are ensheathed by cells of Schwann(Neurilemma) which are arranged along the length of the axon. The large Axons have a Myelin sheath produced by encircling cells of Schwann around them. Myelin consists of lipoproteins in the plasma membrane of Schwann cells.  The Nodes of Ranvier is a gap between two cells of Schwann where myelin is absent and the axon is exposed. The axons in the central nervous system are myelinated by Oligodendrocytes. 

Classification of Neurons

The neurons show variations in their structure and functions and can be classified as,

Functional classification

  • Sensory neurons or Afferent neurons: They conduct impulses from sensory receptors to CNS
  • A motor neuron or Efferent neurons: They conduct impulses from CNS to the effectors

Structural classification

According to the number of processes of Neurons, Neurones are divided as

  • Unipolar Neurones: show a single process bifurcating into dendrite and axon. These are produced by the fusion of two processes of original bipolar neurons example: cells of the dorsal route ganglia.
  • Bipolar neurons: Bipolar neurons have one axon and one dendrite example: bipolar cells of the retina and olfactory mucosa.
  • Multipolar Neurones: Multipolar Neurones have one axon and many dendrites. The majority of neurons in the CNS are multipolar.

Conceptual Questions

Question 1: Why are neurons called building blocks of the brain? 

Answer:

Neurons are called building blocks of the brain because each neuron consists of the cell body, dendrites, and axons which are connected to each other as blocks.

Question 2: What are the main functions of the neuron?

Answer:

The main function of the Neuron is to send and receive signals from the other sensory organs of the body.

Question 3: What is the shape of the Neuron?

Answer:

Neurons are star-shaped, spindle-shaped, and pyramid-shaped.

Question 4: What are the functions of Myelin sheath?

Answer:

Protects axon, Insulates axon from electrically charged ions, and helps in the repair of injured fibers.

Question 5: What happens when neurons stop working?

Answer:

Brain stroke occurs when neurons stop working, it also causes partial or full paralysis. sometimes death.

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