Understanding resting and action potentials

Neurons are specialised cells in the nervous system that transmit information through electrical signals. These signals rely on differences in electrical charges across the neuron's membrane, which create what is known as a potential.

Key types of neuronal potentials

• Resting potential - This is the stable electrical state of a neuron when it is not transmitting a signal.
• Action potential - This is a rapid change in the electrical charge across the neuron's membrane, allowing the neuron to send a signal along its axon (the long, slender projection that conducts impulses away from the cell body).

The transition from resting potential to action potential happens only when a specific level of stimulation is reached, ensuring that signals are transmitted efficiently.

The neural threshold and the all-or-nothing principle

For a neuron to fire a signal, incoming stimuli must be strong enough to trigger a change in its electrical state. This process is governed by a key threshold and operates on a strict principle.

The neural threshold

The neural threshold is the minimum level of electrical stimulation required to shift a neuron from its resting potential to an action potential. If the stimulation is below this threshold, no action potential occurs, and the neuron remains at rest.

The all-or-nothing principle

Action potentials follow an all-or-nothing rule, meaning that once the neural threshold is reached, the neuron fully commits to firing the action potential - there are no partial responses. If the threshold is not met, nothing happens, but if it is, the action potential travels the full length of the axon without weakening.

The process of depolarisation and repolarisation

When the neural threshold is reached, a sequence of electrical changes occurs along the axon's membrane. This involves shifts in polarity, which refers to the condition of having positive and negative charges separated across the membrane.

Steps in generating an action potential

1. Stimulation and depolarisation - A stimulus causes ions (charged particles) to flow across the membrane, reversing the polarity. The inside of the cell becomes positively charged relative to the outside, a process called depolarisation. This change spreads along the axon as the action potential travels.
2. Peak and repolarisation - Once depolarisation is complete, ion channels adjust to restore the original charge. This repolarisation resets the membrane back to its negative internal state.
3. Return to resting potential - After repolarisation, the neuron then fully returns to the resting potential, ready for the next stimulus.

As a result of this cycle, the action potential propagates down the axon, changing the membrane's polarity section by section.

Factors influencing the speed of action potential conduction

The speed at which an action potential travels along an axon varies based on structural features of the neuron. Faster conduction allows for quicker responses in the nervous system, which is crucial for reflexes and coordination.

Role of axon diameter and resistance

• Axon diameter - Wider axons conduct action potentials faster because they offer less resistance to the flow of electrical current.
• Resistance to current leak - Axons with lower resistance prevent the electrical signal from dissipating, maintaining its strength over distance.

These factors work together to influence conduction speed, with larger, low-resistance axons enabling rapid signal transmission.

The importance of myelin and nodes of Ranvier

• Myelin is a fatty insulating layer that wraps around many axons, formed by specialised cells.
• Myelin acts as an electrical insulator, preventing ions from leaking out of the axon.
• This reduces signal loss and increases conduction speed by allowing the action potential to travel more efficiently.
• Nodes of Ranvier are small, regularly spaced gaps in the myelin sheath along the axon. At each node, ion channels open, regenerating the action potential. The signal effectively "jumps" from one node to the next.

Without nodes of Ranvier, the action potential would spread continuously but much more slowly, as it would need to regenerate at every point along the axon.