A Brief Introduction to Neurons & Nerve Impulse
The nervous system is responsible for generating a response to external stimuli. In a broad sense, the nervous system has three components, namely, the brain, spinal cord and neurons.
In this article, you will find information about neurons and generation and conduction of nerve impulse. In turn, it will help you understand related concepts more effectively.
That being said, read on to find more!
What is Neuron?
A neuron is a structural and functional unit of the nervous system. Collectively, neurons can identify, receive and transmit different kinds of stimuli.
So, in case you are wondering, “what is an impulse in Biology?” Remember that nerve impulse in biology is simply how neurons communicate with one another. It occurs owing to a disparity in electrical charges in a neuron’s plasma membrane.
Notably, neurons interact with one another at designated junctions known as synapses. Now, they are either chemical (interacting through chemical messengers) or electrical.
These are the Components of Neurons –
Dendrite
Axon
Axon terminal
Soma
Node of Ranvier
Schwann cell
Myelin Sheath
Nucleus
This figure below offers a pictorial representation of a neuron and its components.
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State as True or False: There are 3 parts in a Neuron.
Generation and Conduction of Nerve Impulse
Generation of nerve impulse is dependent on the strength of a stimulus, which in turn, triggers both chemical and electrical changes in neurons. Notably, the neural membrane harbours several ion channels which are selectively porous to different ions.
For instance,
A. Generation of Nerve Impulse: Resting Membrane
When a specific neuron is resting, its axonal membrane is relatively porous to potassium ions. Alternatively, it is quite non-porous to sodium ions (Na+) and other negatively charged proteins in axoplasm.
This axoplasm comprises a high concentration of potassium ion (K+), negatively charged ions and also a low concentration of sodium ion. Likewise, the fluid present outside axon comprises a low concentration of potassium ion but a high concentration of sodium ion.
Resultantly, it forms a concentration gradient. Now, these ionic gradients in a resting membrane are kept stable through the active transmission of ions.
As the sodium-potassium exchange pump transmits 3Na+ out to draw 2 K+ into the cell, the interior of the axonal membrane becomes negatively charged. Similarly, the membrane’s exterior becomes positively charged and through the course gets polarised.
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B. Generation of Nerve Impulse: Active Potential
Regardless in the event of generation of nerve impulse, the porousness of the cell membrane changes. The above mentioned sodium ions now flow inside, while the potassium ions flow outside. This reserves the charges and depolarises the cell.
In turn, an action potential occurs which further drives the nerve impulse across the axon. Also, the said depolarisation takes place throughout the nerve.
It is noteworthy that a series of reactions take place where sodium ions flow out and potassium ions move into a cell. Consequently, the process again leads to the polarisation of cells with the restoration of their initial charges.
Subsequently, neurotransmitters release chemicals when a nerve impulse makes its way at the end of the axon. These chemicals diffuse in the synaptic gap and may be transmitted either by the chemical synapse or the electrical synapse.
This figure below represents the generation conduction of nerve impulse.
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Test Your Knowledge:
Which part of a neuron receives a nerve impulse first?
What is Axoplasm?
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FAQs on Neurons and Nerve Impulse
1. What are the primary components of a neuron and their specific functions?
A neuron, the basic unit of the nervous system, consists of three main parts, each with a specific function:
- Cell Body (Soma): This contains the nucleus and is the neuron's metabolic centre. It integrates incoming signals.
- Dendrites: These are short, branched extensions that receive electrochemical signals (nerve impulses) from other neurons and transmit them towards the cell body.
- Axon: This is a single, long extension that carries the nerve impulse away from the cell body to other neurons, muscles, or glands. It ends in axon terminals which release neurotransmitters.
2. What is the fundamental difference between a nerve and a neuron?
The main difference lies in their structural level. A neuron is a single, individual nerve cell responsible for transmitting electrical and chemical signals. In contrast, a nerve is a bundle of axons from multiple neurons, all enclosed in a protective connective tissue sheath. Think of a neuron as a single wire and a nerve as a thick cable containing many of those wires.
3. What is a nerve impulse and how is it generated?
A nerve impulse is a wave of electrical and chemical change that travels along the membrane of a neuron. It is also known as an action potential. Generation begins when a neuron receives a stimulus (e.g., from another neuron or a sensory receptor) that is strong enough to reach a threshold level. This triggers a rapid change in the neuron's membrane potential, causing sodium ions (Na+) to rush into the cell, which initiates the impulse.
4. Can you explain the difference between resting potential and action potential in a neuron?
Resting potential is the electrical potential difference across the plasma membrane of a neuron when it is not stimulated or excited. The inside of the neuron is negatively charged compared to the outside. An action potential, on the other hand, is the rapid, temporary reversal of this potential that occurs when a neuron is stimulated. It's the 'firing' of the neuron, where the inside briefly becomes positive, allowing the nerve impulse to travel.
5. What are the key steps involved in the transmission of a nerve impulse across a synapse?
The transmission of an impulse across a synapse (the junction between two neurons) involves several key steps:
- An action potential arrives at the axon terminal of the pre-synaptic neuron.
- This triggers the opening of voltage-gated calcium channels, and calcium ions (Ca²⁺) enter the terminal.
- The influx of calcium causes synaptic vesicles, which contain neurotransmitters, to fuse with the pre-synaptic membrane.
- Neurotransmitters are released into the synaptic cleft and diffuse across to the post-synaptic neuron.
- They bind to specific receptors on the post-synaptic membrane, causing a new electrical signal (either excitatory or inhibitory) in the next neuron.
6. Why is the transmission of a nerve impulse described as an electrochemical event?
A nerve impulse is called an electrochemical event because it involves both electrical and chemical processes. The 'electro' part refers to the movement of ions (like Na+ and K+) across the neuron's membrane, which creates an electrical charge difference (the action potential). The 'chemical' part refers to the process at the synapse, where chemical substances called neurotransmitters are released to transmit the signal from one neuron to the next.
7. What is the importance of the myelin sheath in nerve impulse conduction?
The myelin sheath is a fatty insulating layer that covers the axons of many neurons. Its primary importance is to dramatically increase the speed of nerve impulse conduction. It does this by allowing the action potential to jump from one gap in the sheath (Node of Ranvier) to the next, a process called saltatory conduction. This is much faster than the continuous conduction that occurs in unmyelinated axons.
8. What does the 'all-or-none' principle mean for neuron firing?
The 'all-or-none' principle states that if a stimulus is strong enough to reach the threshold potential, the neuron will fire completely and generate a full action potential of a fixed size. If the stimulus is below the threshold, no action potential will be generated at all. There is no 'half' or 'weak' firing; the neuron either fires at its maximum strength or does not fire at all, similar to a light switch being either fully on or fully off.
