Neuron: Definition, Structure, Diagram, Parts & Functions

Neurons also known as nerve cells are the fundamental part and the longest cell in vertebrates and invertebrates. These cells generate action potentials and transmit messages from different parts of the body by releasing neurotransmitters.

What are Neurons?

Neurons are fundamental and structural units of the nervous system and are electrically excitable cells which are specialised to receive and transmit information to different parts of the body. 

Neurons are anatomically characterised into three types based on polarity.

• Unipolar: These neurons have a single short process and are found in the embryonic stage. Processes emerge from the cell body and divide into proximal and distal branches.

• Bipolar: These neurons are found in the retina of the eye and have two processes, one axon and another one dendrite which extend from opposite ends of the cell body.

• Multipolar: These neurons are most common in nature, found in the cerebral cortex. They have one axon and two or more dendrites arise from the two opposite ends of the cell body. 

Types of neurons

Neurons are classified on the basis of the direction of signal transmission into 3 parts.

• Sensory neurons: Sensory neurons' main function is to receive information from different sensory receptor cells present in different parts of the body and transmit it to the central nervous system. They are examples of afferent neurons.

• Motor neurons: Motor neurons collect response from the central nervous system and transmit it to the muscles. They are also called efferent neurons.
• Interneurons: The interneurons’ function is to transmit information between different neurons of the body. 

Basic functions of neurons

Some of the important functioning of the neurons are given below.

• Receive information from different parts of the body.
• Collect and Integrate all the incoming information from different receptors.
• Transmit information to required target cells or neurons or glands. 

Neuron Parts and their functions

Depending upon the function and location of the neuron its shape and size varies. All neurons have three main parts- Dendrites, Cell body and Axon.

Dendrites

These branch-like extensions are at the beginning part of the neuron to help in receiving signals and transmit them to the cell body of the neuron. The number of dendrites varies from the location of the cell. They receive signals in electrical or chemical form and send them to inwards parts of the neuron. 

Cell body

Cell body is the neuron’s core containing a cell nucleus and numerous cellular components which helps in performing different functions of the cell. It supports and maintains the functions of the cell body with a nucleus, Golgi body, endoplasmic reticulum, mitochondria and other components. The cell body is also called soma. It maintains the genetic structure, carries genetic information and provides energy to drive various activities.

Axon hillock

The position of axon hillock is found at the end of soma and it acts as a manager, summing the total inhibitory and excitatory signals. When the sum of all the signals received by dendrites exceeds a certain threshold value, the action potential is caused by changes in ion channels transmitted in the form of an electrical signal down the axon from the cell body. In a normal resting state, it possesses an internal polarization of approximately -70mV. 

Axon

Axon is a tube-like structure of the neuron which conducts signals from the cell body to the synaptic knob. For faster transmission of signals, a larger diameter of the axon is required. Axons are covered by Schwann cells and it secretes myelin sheath. The broken point is called nodes of Ranvier which helps in increasing the efficiency of transmission of signals. 

Terminal buttons and synapses

The location of terminal axons are at the end of the neuron and they are responsible for transferring signals from one neuron to other neurons. The gap at the end of the terminal neurons is known as a synapse in which neurotransmitters are responsible for the transmission of signals to other neurons. Terminal buttons contain some vesicles which hold neurotransmitters that convert the electrical impulses into chemical signals. 

Types of synapses

In neurons, signal transmission takes place by two types of synapses namely Chemical and electrical.

Chemical Synapse: These are the synapses formed between the two neurons during the transmission of signals with the help of neurotransmitters which may be excitatory or inhibitory in nature. 

Electrical Synapse: These are the type of synapses where neurotransmitters are not involved and transmission of signals takes place similarly as transmission takes place in a single axon as the terminal buttons of two synapses are closely associated with each other.

Things to Remember

• The neuron is a highly specialized cell that is responsible for the transmission of nerve impulses.
• It is classified into unipolar, bipolar and multipolar neurons.
• On the basis of transmission mechanism, neurons are classified into sensory, motor and interneurons.
• Dendrites, Cell body and Axon are the main parts of the neuron.
• The cell body is also called Soma.
• Electrical and chemical synapses are the two types of synapses.