Generation and Conduction of Action Potential

 Introduction

Imagine a locality with telephones in every house connected via cables. The communication system in our body is identical to this. Our nervous system is responsible for controlling and coordinating the actions of the entire body. Nerves play the role of wires to transmit signals or commands from the brain to different locations of our body.These signals are in the form of electrical impulses. These impulses carry several electrical impulses to muscles. 

Conduction of Action Potential in neurons

As we know, the transmission of information from neurons to different parts of the body is an electrochemical process. These electrochemical signals propagate through the length of the neuron in the form of an action potential.An action potential occurs whenever a neuron sends information via an axon. Spike and impulse are other terms for the action potential. The communication through nerves takes place with the electrochemical process.

Neurotransmitters are chemical messengers that stimulate action potential in cells to help the signal travel across the body.

Neurons act in response to the importance or urgency of the necessary action. Some signals must travel faster because of an emergency. There are specific nerves that transmit information faster in case of urgency. These nerves possess a larger diameter and the myelin sheath as unique attributes.

Effect of size of axons on the speed of action potential

The creation of action potential and the subsequent propagation is like a cascade. The entry of positive ions into the cell body causes trans-membrane channels to open up and allow more positive ions. It stimulates nearby channels to accept more positive ions.

The process continues further and creates action potential in neurons. Axons with a larger diameter can send signals rapidly because of the minimum resistance to the flow of ions. The axons will continue to move at higher speeds because of no resistance. Moreover, the larger diameter will cause more ions to travel in the appropriate direction.

Effect of the sheath on the action potential in cellular membrane

Insulation with sheath is another way to accelerate the process of action potential in the cellular membrane.  These cells wrap around axons in peripheral and central nervous systems to create multiple insulation layers.

The myelin sheath prevents the loss of ions during the passage of action potentials. Ions may escape during their exit from the cell as they cross the membrane. The presence of myelin sheath reduces the number of negative ions near the membrane. The propagation of action potential becomes easy as there are lesser numbers of negative ions to counter the depolarization.  

Beginning of action potentials in nerves

The action potential  in neurons is a rapid sequence of events that cause changes in the voltage across a membrane. Neuronal action potential begins at the axon hillock because of intense polarization. In sensory neurons, the action potential begins at the axon’s distal terminal.

Conduction of action potential propagates from one neuron to another via the synapses. Synapses are junctions between neurons. Following are the two types of neurons:

• Electrical synapse- Flow of electrical current from one neuron to another is possible because the pre and postsynaptic neurons are close to each other 

• Chemical synapse- A synaptic cleft separates pre and postsynaptic neurons to cause transmission of action potential because of neurotransmitters

Action potential involves the resting stage of nerve fiber, which is nothing but polarization. There is a positive charge surrounding the axonal membrane and a negative charge inside the membrane. There is an exactly opposite situation during depolarisation.

Propagation of the action potential occurs through the axon, synapse and neuromuscular junction during depolarisation. There is a refractory period between polarisation and depolarisation.  

Beginning of action potential in neurons

Chemicals in our body produce an electrical signal. The electrically charged chemicals in our bodies are ions. The nerve cells have a semi-permeable membrane that allows the exchange of these ions because of the difference in their electric charges. The neurons work on the principle of all-or-none because the size of all action potentials remains the same.

Following are the steps involved in the  action potential in neurons:

1. The presence of several sodium ions outside the neuron results in their influx into the neuron as the sodium channels open up in response to a stimulus.

2. Depolarisation of neurons occurs because the incoming sodium ion has a positive charge.

3. Potassium channels take longer to open. There is a reversal of depolarisation following the exit of potassium ions.

4. There is a simultaneous closing of sodium channels leading to repolarisation.

5. Potassium channels stay open for a longer time to allow hyper-polarisation

6. There is a gradual reversal of ion concentrations to resting levels.

   Conclusion

   The potential action occurs as an explosion of electrochemical activity in response to a depolarising current in the neuron. The potential resting moves toward 0 mV following a stimulus. A neuron releases action potential as the depolarization reaches -55 mV. This stage is a threshold stage. There is no release of action potential if the neuron fails to reach the threshold. Action potential in neurons of a fixed size will be generated whenever a neuron reaches the threshold.  Action potential in nerves depends on the size of the axon and the presence of myelin sheath.