Neurons are special cells that help your brain and spinal cord communicate with your organs and muscles. They do this by using a process called action potentials. This process happens when the electrical charge inside a neuron changes from negative to positive because of special chemicals called ions.

There are three stages to an action potential: depolarisation, repolarisation, and hyperpolarisation. These stages describe how the charge changes inside the neuron. The charge is influenced by the different amounts of ions inside and outside the cell.

So, in short, action potentials are important because they help neurons talk to each other and control body's response to the world you.

Stimulus-response Model in Action Potential

To understand how our body responds to different situations, we use a stimulus-response model. A stimulus is a change in the environment that our body can detect, like heat or sound. Effector cells, like muscles and glands, produce a response to the stimulus.

When a receptor detects a stimulus, it sends a signal to the central nervous system (CNS). The CNS responds to the stimulus and sends a message to the effector cells to produce a response.

For example, on a hot summer day, the receptors on our skin detect the heat. The CNS sends a message to our effector cells to help us cool down. The involuntary response is sweating, which happens when our arteries widen (vasodilation) to increase water evaporation from our skin. The voluntary response could be moving to the shade or sitting near an air conditioner, which.

in summary, our body responds to different situations using a stimulus-response model. This helps us to react to changes in our environment to keep ourselves safe and comfortable.

Steps in generating action potential

Action potentials are important because they describe the change in membrane potential from negative to positive. To generate an action potential, the stimulus must surpass threshold value, which is between50 - this threshold, there will be no action potential.

After neurotransmitters have crossed the synaptic cleft and bound to receptors on the post-synaptic membrane, the action potential is passed on to the next neuron. But, usually, just one action potential is not enough to produce a response. Several incoming action potentials are needed, which is called summation.

There are two types of summation: spatial and temporal. In spatial summation, signals from several-synaptic neurons are sent to one post-synaptic neuron. In temporal summation, a single pre-synaptic neuron sends signals in quick succession to one post-synaptic neuron.

So, in summary, action potentials are only generated when the threshold value is reached. Summation is needed to produce a response, which can occur through spatial or temporal summation.

Stages of an action potential

An action potential consists of four main stages: depolarisation, repolarisation, hyperpolarisation, and the resting state. Depolarisation is the initial stage of an potential, where membrane potential rapidly rises to about +40 mV, causing sodium voltage-gated channels to open in the membrane and sodium ions (Na+) to enter the cell. Repolarisation is the second stage, where the potential difference reaches +40 mV and the sodium voltage-gated channels close, allowing potassium ion channels to open and cause a large efflux of potassium ions (K+) out of the cell, reducing the membrane potential. Hyperpolarisation is the third stage, where the efflux of K+ causes an overshoot of the potential difference, making the membrane potential more negative than the resting state, which is around -75 mV. The fourth and final stage is the resting state, where the neuron returns to its resting membrane potential and no action potential is generated, which is around -70 mV.

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Action potentials can be generated in both neurones and skeletal muscle. The difference is that the membrane potential in skeletal muscle is more negative due to a greater K+ and Cl- gradient and greater membrane permeability to Cl-. Otherwise, the action potential diagram is similar to that of a neurone.

Refractory period

During hyperpolarisation, a refractory period occurs where no action potential can be generated. This period happens because of the lag in the closure of potassium ion channels and the inactivation properties of voltage-gated sodium channels. This refractory period is essential in limiting the number of action potentials.

The reason for this limitation is because sodium ions move in one direction along the neuron to depolarise the next region. This allows for discrete and unidirectional action potential transmission.

There are two types of refractory periods: absolute and relative. The absolute refractory period occurs during depolarisation and repolarisation, which means new action potentials cannot be generated during these stages because sodium channels are inactive. The relative refractory period occurs during hyperpolarisation, and a second action potential can be initiated; however, a greater stimulus is required, i.e., a higher threshold value.

The resting potential is the difference in ion concentrations of Na+ and K+ on either side of the neuron's membrane when no action potential is generated. This potential is usually -70 mV, which means that when the neuron is at rest, the inside is more negative than the outside. This occurs because the membrane more to more open that K leakage out neuron more quickly than Na+ can enter the neuron.

The resting potential is also maintained by the Na+/K+ ATPase pump, which is a transbrane protein that uses active transport to pump 3 Na+ ions out of the neuron for every 2 K+ ions pumped into the neuron. This active transport of ions maintains the concentration gradient across the neuron membrane, which is essential for the generation of action potentials. The Na+/K+ ATPase pump requires ATP to function and is a highly active process that keeps more cations outside the neuron, maintaining its negative resting potential.

The generation of a cardiac action potential

Pacemaker cells are specialised cells that generate action potentials without a stimulus. These cells are found in two main locations in the heart: the sino-atrial node (SAN) and the atrioventricular node (AVN). The SAN is the primary location of pacemaker cells and they use calcium (Ca2+) to generate action potentials, rather than sodium (Na+) like neuronal action potentials. The AVN is a secondary location of pacemaker cells and is used in case of SAN failure.

Types of transmission of action potentials

The of action potentials in axons can occur through two mechanisms: continuous conduction and saltatory conduction. Continuous conduction occurs in unmyelinated axons and is slower as it requires a greater number of ion channels to change the neuron's resting state. Saltatory conduction occurs in myelinated axons and is faster as the action potential can jump from one node of Ranvier to another. The presence of nodes of Ranvier allows for a faster propagation of action potentials as fewer ion channels are needed.

Key takeaways from action potentials include the fact that they occur when the membrane potential of a neuron shifts from negative to positive in response to a stimulus. An action potential is only generated when the stimulus reaches the threshold value. The stages of an action potential include depolarisation, repolarisation, hyperpolarisation, and the resting state. Refractory periods, both absolute and relative, allow for discrete and unidirectional action potential transmission. In cardiac cells, action potentials are generated by pacemaker cells located in the SAN and AVN and do not require a stimulus to be generated.

Action Potential

What is the first event of an action potential?

The first event of an action potential is depolarisation. This describes the stage in which the membrane potential becomes more positive (+40 mV) due to the influx of Na+ through sodium voltage-gated channels.

How are action potentials generated?

Action potentials are generated when the stimulus passes the threshold value. This is usually -50 to -55 mV. Due to this threshold value, action potentials follow the all-or-nothing principle, in which an action potential is only generated when the threshold value is reached. 

What is an action potential?

An action potential describes a change in a neurone's membrane potential, from negative to positive. This is in response to a stimulus and is driven by the flow of Na+ and K+ ions.

What is action potential propagation?

Action potential propagation describes the way in which the impulse travels across the axon. In unmyelinated axons, propagation occurs via continuous conduction. In myelinated axons, propagation occurs via saltatory conduction.

What is the main function of an action potential?

To transmit signals to target cells/tissues.

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