Neurons

The neurons are essentially the body’s tiny messengers, the specialised cells that send chemical and electrical signals to the nervous system. These signals are the core of the nervous system functions, controlling everything from movement to touch to reflexes. Discovered in the late 19th century by contributions of several scientists. With the final and refined work on neuron structure and connectivity by Santiago Ramón y Cajal, brain science saw new frontiers in the controlled functioning of body and mind. 

1.0What are Neurons? 

Neurons are specialised cells that form the building blocks of the nervous system, conveying electrical and chemical messages throughout the body. They are distinct cells in contrast to most other body cells, as they are specifically made for quick and precise communication to ensure coordination between the brain, spinal cord, and the rest of the body.

Fun Fact: Neurons never divide or undergo cell division like any other normal cells. That is,  they only stretch with increasing age and size. 

2.0Structure of a Neuron:

The neurons are structured such that they receive and send electrical signals created by external stimuli. It consists of three core structures:

• Cell Body (Soma): The cell body contains a nucleus, a Golgi body, endoplasmic reticulum, mitochondria, and other cell organelles. It is also responsible for maintaining the health of the neurons. 
• Dendrites: Dendrites are the branch-like extensions of the cell body. This part of the neuron is specialised to receive signals from other neurons & transmit them toward the cell body. 
• Axon: The axon is the long, slender, thread-like projection that transmits the received signals from the cell body to other neurons by axon terminals. Most axons are covered with a fatty covering that insulates the axon and eventually speeds up signal transmission. This covering is also known as the myelin sheath. 

Synapses: The Connecting Link between Two Neurons

The synapses are the place where two neurons actually communicate. These are the junction or gap between the terminal end of the preceding neuron and the dendrite of the subsequent one. While neurons physically touch each other, they communicate through these synapses, which are filled with neurotransmitters such as acetylcholine. 

3.0Types of Neurons

Neurons can be categorised into two broad categories, which can be further subdivided into additional types. These include: 

Classification Based on Function

Based on the location of the signal transfer and function, neurons can be classified into three major categories: 

1. Sensory Neurons: These neurons are present at the location of sensory organs (skin, eye, ears, etc) and carry signals to the spinal cord and brain. That is, these neurons are the primary receptors of external stimuli. 
2. Motor Neurons: Motor neurons transmit signals from the brain or spinal cord to muscles to respond to external stimuli.
3. Interneurons: Interneurons process the information captured and transmitted by sensory neurons in the brain and spinal cord. Also, these form a link between motor and sensory neurons as well. 

All three types of neurons mentioned above work together to form a reflex arc or response system. That is, consider a stimulus of touching a hot object, the signal is carried by sensory neurons, which is interpreted by interneurons in the brain, and motor neurons send the signal back to pull the hand away. 

Classification Based on Structure 

Neurons are divided into four major types based on the number of extensions or processes from the cell body of neurons. These are: 

Types	Description
Unipolar Neuron	This neuron has only a single extension, splitting into two branches.
Bipolar Neuron	It has one axon and one dendrite each, found in special sensory organs like the retina of the eye.
Multipolar Neuron	The most common type of neuron. Predominantly forms the brain and spinal cord.
Pseudopolar Neuron	It seems to have one extension, but actually originates from the fusion of two.
Visual Representation

4.0Working of Neurons

The speed of working of neurons, although incredibly fast, gives ample time to detect, process, and react in real time. The step-by-step process includes: 

Stimulus Detection

Each message starts with a stimulus — for example, sound, touch, or temperature change. This stimulus travels to a sensory receptor, which creates an electrical signal in a sensory neuron.

Generation of Nerve Impulse (Action Potential) 

The neuron creates an electrical impulse, called an action potential. This is caused by the movement of ions (such as sodium & potassium) across the membrane of the neuron, producing a momentary voltage shift.

Impulse Transmission Down the Axon 

The action potential propagates down the axon in the form of a wave, heading towards the axon terminals. In myelinated neurons, the signal leaps from one Node of Ranvier to another, greatly enhancing the speed.

Transfer of Signal at Synapse

When the impulse arrives at the axon terminals, it cannot leap to the next neuron directly. It instead causes the release of neurotransmitters (such as acetylcholine) into the synaptic cleft — the space between neurons.

Activation of the Next Neuron

The neurotransmitters attach to receptor proteins on the postsynaptic neuron. This opening of ion channels creates a new action potential in the receiving neuron, propagating the signal.

Signal Termination

Once the signal is transmitted, enzymes destroy neurotransmitters, or they are taken up by the transmitting neuron in a reuptake process, preventing the signal from recurring indefinitely.

5.0Function of Neurons

Neurons facilitate a number of functions within the body, crucial for survival, movement, and higher-order processes. These functions include: 

1. Receiving Sensory Input: Neurons sense changes in the outside or inside world (such as temperature, light, or pain).
2. Transmitting Signals: Neurons transmit impulses from one location in the body to another.
3. Processing Information: In the spinal cord and brain, interneurons make sense of incoming information and decide on the proper response.
4. Controlling Muscles and Glands: Motor neurons stimulate muscles or glands according to processed signals.