18.3. Neuron as Structural and Functional Unit of Neural System

A neuron is a microscopic structure composed of three major parts, namely, cell body, dendrites and axon. The cell body contains cytoplasm with typical cell organelles and certain granular bodies called Nissl’s granules. Short fibres which branch repeatedly and project out of the cell body also contain Nissl’s granules and are called dendrites. These fibres transmit impulses towards the cell body. The axon is a long fibre, the distal end of which is branched. Each branch terminates as a bulb-like structure called synaptic knob which possess synaptic vesicles containing chemicals called neurotransmitters. The axons transmit nerve impulses away from the cell body to a synapse or to a neuro-muscular junction.

A neuron is essentially the basic building block of the nervous system. It has three key components:

• Cell Body: This is the main part of the neuron where the nucleus and organelles are located. It supports the metabolic activities of the neuron.
• Dendrites: These are short, branched extensions that receive signals from other neurons and transmit them to the cell body.
• Axon: This long fiber transmits impulses away from the cell body towards other neurons or muscles. The end of the axon has structures called synaptic knobs that release neurotransmitters to facilitate communication between neurons. Understanding these parts is crucial as they illustrate how neurons communicate and process information.

Think of a neuron like a communication network in a company. The cell body is akin to the headquarters, where decisions are made. Dendrites are like the email systems that receive messages from various branches (other neurons). The axon is like the postal service that sends out information and instructions to different branches and customers (other neurons or muscles) to maintain the company's operations.

Based on the number of axon and dendrites, the neurons are divided into three types, i.e., multipolar (with one axon and two or more dendrites; found in the cerebral cortex), bipolar (with one axon and one dendrite, found in the retina of eye) and unipolar (cell body with one axon only; found usually in the embryonic stage).

Neurons can be classified based on their structure into three main types:

• Multipolar Neurons: These neurons have one axon and multiple dendrites, allowing them to connect with many other neurons. They are primarily found in the brain's cerebral cortex, where complex processing occurs.
• Bipolar Neurons: These have one axon and one dendrite. They are less common and are mainly found in specific sensory organs, such as the retina of the eye, where they help transmit visual information.
• Unipolar Neurons: These consist of a single axon that branches into two directions. They are primarily found in the early stages of development (embryonic stage) and are involved in sensory information processing. This classification helps us understand how different neurons are adapted for specific functions.

Imagine the types of phones in a large office. Multipolar neurons are like smartphones, capable of handling multiple calls and notifications at once (many connections). Bipolar neurons can be compared to a traditional phone that can only make or receive one call at a time (one connection). Unipolar neurons are like emergency phones that connect directly to one point, typically used in critical situations (very specific functions).

There are two types of axons, namely, myelinated and non-myelinated. The myelinated nerve fibres are enveloped with Schwann cells, which form a myelin sheath around the axon. The gaps between two adjacent myelin sheaths are called nodes of Ranvier. Myelinated nerve fibres are found in spinal and cranial nerves. Unmyelinated nerve fibre is enclosed by a Schwann cell that does not form a myelin sheath around the axon, and is commonly found in autonomous and the somatic neural systems.

Axons can be either myelinated or unmyelinated:

• Myelinated Axons: These axons are covered with a fatty layer called the myelin sheath, produced by Schwann cells. The myelin sheath acts as insulation, which speeds up the transmission of nerve impulses along the axon. The myelin sheath is not continuous; it is interrupted at intervals by small gaps known as nodes of Ranvier, where the axonal membrane is exposed. This arrangement allows for rapid impulse conduction via saltatory conduction, where the impulse jumps from node to node.
• Unmyelinated Axons: In contrast, unmyelinated axons lack this insulating layer, resulting in slower impulse transmission. These are found in areas of the autonomic and somatic nervous systems where quick response is less critical. Understanding the difference between myelinated and unmyelinated axons is crucial because it impacts how quickly our bodies can respond to stimuli.

Think of myelinated axons like cars driving on a highway with express lanes (myelinated), allowing them to travel faster, especially when changing lanes at exits (nodes of Ranvier). Unmyelinated axons, on the other hand, are like cars on a city street: they move slower because they face more stops (no myelin) and have to navigate through traffic signals (impulse transmission happens more gradually).

Neurons are excitable cells because their membranes are in a polarised state. The electrical potential difference across the resting plasma membrane is called the resting potential. When a stimulus is applied at a site on the polarised membrane, the membrane at that site becomes freely permeable to Na+. This leads to a rapid influx of Na+ followed by the reversal of the polarity at that site, i.e., the outer surface of the membrane becomes negatively charged and the inner side becomes positively charged. This process is known as depolarisation.

The generation and conduction of a nerve impulse involve several steps:

1. Polarised State: Neurons maintain a resting potential due to concentrations of ions, primarily sodium (Na+) and potassium (K+). That means the inside of the neuron is negative compared to the outside.
2. Stimulus Application: When a stimulus occurs, it makes the neuron's membrane at that specific site become permeable to sodium ions (Na+).
3. Depolarisation: As Na+ rushes into the neuron, the internal charge shifts to positive. This change in charge is called depolarisation and creates an action potential.
4. Impulse Conduction: Once depolarised, the action potential triggers further depolarisations down the axon, allowing the nerve impulse to travel rapidly. Understanding this process is crucial for grasping how signals travel through our nervous system.

Think of this process like a line of dominos. The initial domino (the stimulus) falls over (depolarisation) and causes the next domino to tip (impulse conduction) and so on down the line. Just as the falling domino sets off a chain reaction, the initial change in the neuron's charge leads to an impulse that rapidly travels along the neuron.

A nerve impulse is transmitted from one neuron to another through junctions called synapses. A synapse is formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron, which may or may not be separated by a gap called synaptic cleft. There are two types of synapses, namely, electrical synapses and chemical synapses. At electrical synapses, the membranes of pre- and post-synaptic neurons are in very close proximity.

Synapses are critical for communication between neurons:

1. Synapse Structure: A synapse connects a pre-synaptic neuron (sending neuron) and a post-synaptic neuron (receiving neuron). There may be a small gap between them called the synaptic cleft.
2. Types of Synapses:
   • Electrical Synapses: These allow direct flow of electrical signals between neurons. The membranes are very close, permitting rapid transmission of impulses. They function similarly to impulse conduction along an axon. These are typically faster but less common.
   • Chemical Synapses: At these synapses, the neurons do not touch. Instead, the pre-synaptic neuron releases neurotransmitters into the synaptic cleft. These chemicals then bind with receptors on the post-synaptic neuron, generating a new potential that can lead to an action potential. Understanding how impulses are transmitted across synapses is essential, as it explains how signals propagate through complex neural networks.

Imagine synapses as two people passing a message through a wall. Electrical synapses are like passing a message through a hole in the wall (quick and direct), while chemical synapses are like writing the message down on a piece of paper (neurotransmitter) and handing it over. The second person reads the message and acts upon it (new potential) on their side.

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