Action Potential

Stimulus-response Model in Action Potential

First, let's quickly recap how a nerve impulse travels to produce a response in the effector cells. A stimulus-response model can be used to describe this.

1. A receptor detects a stimulus.
2. If the stimulus reaches above a certain threshold, the receptor will transform the stimulus into a nerve impulse.
3. The nerve impulse travels to the CNS.
4. The CNS initiates a response (involuntary to voluntary) to the stimulus which is passed down to effector cells.

Now that you are familiar with the process, let's use an example of a stimulus response.

Steps in generating action potential

Action potentials describe the change from negative to positive membrane potential. For this change to occur, the stimulus must surpass the threshold value, which usually ranges from -50 to -55 mV. As a result, action potentials follow the all-or-nothing principle in which an action potential is only generated when the threshold value is reached. If the stimulus reaches a value below this threshold, an action potential is not generated.

Stages of an action potential

An action potential consists of four main stages:

1. Depolarisation: the membrane potential rapidly rises to about +40 mV. This causes sodium voltage-gated channels to open in the membrane, and sodium ions (Na+) enter the cell.
2. Repolarisation: when the potential difference reaches +40 mV, the sodium voltage-gated channels close and potassium ion channels open. This reaction causes a large efflux of potassium ions (K+) out of the cell, reducing the membrane potential.
3. Hyperpolarisation: the efflux of K+ causes an overshoot of the potential difference, causing the membrane potential to be more negative than the resting state, which is around -75 mV.
4. Resting state: the neurone returns to its resting membrane potential where no action potential is generated. This value is around -70 mV.

Refractory period

During hyperpolarisation, a refractory period occurs where no action potential can be generated. This occurs due to the lag in the closure of potassium ion channels and the inactivation properties of voltage-gated sodium channels. In this way, the number of action potentials is limited.

This is important because sodium ions diffuse in one direction along the neurone to depolarise the next region. This allows for discrete and unidirectional action potential transmission.

There are two types of refractory periods:

• Absolute refractory period: this period occurs during depolarisation and repolarisation. New action potentials cannot be generated during these stages because sodium channels are inactive.
• Relative refractory period: this period occurs during hyperpolarisation. A second action potential can be initiated; however, a greater stimulus is required, i.e., a higher threshold value.

The resting potential and the sodium-potassium pump

The resting potential describes the difference in ion concentrations of Na⁺ and K⁺ on either side of the neurone membrane when no action potential is generated. This potential is usually -70 mV, so when the neurone is at 'rest', the inside is more negative than the outside. This occurs because the neurone membrane is naturally more permeable to K⁺ due to more open K⁺ channels that allow K⁺ leakage out of the neurone more quickly than Na⁺ can enter the neurone!

This resting potential is also maintained by the Na⁺/K⁺ ATPase pump. This transmembrane protein uses active transport to pump 3 Na⁺ ions out of the neurone for every 2 K⁺ ions pumped into the neurone. As more cations are kept outside the neurone, this maintains a negative resting potential. Don't be tricked into thinking the neurone is simply at 'rest' when it is at its resting potential. The neurone is still very much active due to the activity of the Na⁺/K⁺ ATPase pump, which is a highly active process as it requires ATP!

The generation of a cardiac action potential

In neurones, action potentials are generated through nerve activity. However, cardiac cells are slightly different because they have specialised cells that generate action potentials without a stimulus! These specialised cells are called pacemaker cells, and the action potentials they generate spread to other cardiac cells to allow the heart to contract.

Pacemaker cells are found in:

• The sino-atrial node (SAN)
• The atrioventricular node (AVN)

The SAN is the primary location of pacemaker cells. Unlike neuronal action potentials characterised by Na⁺ movement, SAN pacemaker cells use Ca²⁺ to generate action potentials! Cells located in the AVN, on the other hand, are described as secondary pacemaker cells as they are used in case of SAN failure.

Types of transmission of action potentials

The propagation of action potentials across an axon can occur in two ways:

• Continuous conduction
• Saltatory conduction

Continuous conduction occurs in unmyelinated axons, while saltatory conduction occurs in myelinated axons.

Continuous conduction

In unmyelinated axons, the action potential moves along the whole length of the axon in continuous conduction. This is a slower mechanism that requires more energy as it employs a greater number of ion channels to change the neurone's resting state. These ion channels take time to open and close!

Saltatory conduction

The sites where Schwann cells are absent in myelinated axons are known as Nodes of Ranvier. The presence of these nodes allows the action potential to 'jump' from one node to another. As a result of this myelination, the action potentials' propagation is faster as fewer ion channels are needed.