Neurones are specialised cells that coordinate your central nervous system (CNS), which is made of your brain and spinal cord, to your organs and muscles to respond to environmental changes. So how exactly do our neurones communicate with other neurones? The answer is through action potentials!
Action potentials occur when the membrane potential of a neurone shifts from negative to positive due to the flow of Na+ and K+ ions. The stages of an action potential can be described as depolarisation, repolarisation and hyperpolarisation.
The membrane potential of a neurone describes the electrical potential difference between the inside and outside of the cell. This electrical potential is influenced by the presence of more or fewer ions on either side of the cell.
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.
A stimulus describes a detectable change in the internal or external environment, such as heat, pressure and sound. Effector cells produce the response to the stimulus, such as muscles and glands.
- A receptor detects a stimulus.
- If the stimulus reaches above a certain threshold, the receptor will transform the stimulus into a nerve impulse.
- The nerve impulse travels to the CNS.
- 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.
Let's say it is a hot summer's day. Receptors found on your skin detect this heat and will send signals to your brain to initiate a cooling response. The involuntary reaction to this includes sweating, which occurs when your arteries dilate (vasodilation) to increase the rate of water evaporation from your skin. The voluntary response includes going into the shade or sitting by an air conditioner by consciously moving your muscles.
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.
The threshold potential is a specific value that needs to be reached or surpassed to produce an action potential. This value is usually in the range of -50 to -55 mV.
After neurotransmitters have diffused across the synaptic cleft and bound to receptors on the post-synaptic membrane, the action potential is continued on the next neurone (the one that receives/binds to the neurotransmitters). However, signals from only one action potential are usually not enough. The addition of several incoming action potentials is needed, known as summation.
Two types of summation can lead to the depolarisation of the neighbouring neurone:
- Spatial summation: several pre-synaptic neurones provide signals to one post-synaptic neurone.
- Temporal summation: a single pre-synaptic neurone provides signals in quick succession to one post-synaptic neurone.
Stages of an action potential
An action potential consists of four main stages:
- 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.
- 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.
- 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.
- Resting state: the neurone returns to its resting membrane potential where no action potential is generated. This value is around -70 mV.
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 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 Na+/K+ ATPase pump is a transmembrane protein that uses active transport to pump 3 Na+ out of the neurone for every 2 K+ pumped into the neurone.
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.
Action Potential - Key takeaways
- Action potentials occur when the membrane potential of a neurone shifts from negative to positive due to the flow of ions in response to a stimulus.
- An action potential is only generated when the stimulus reaches above the threshold value from -50 to -50 mV.
- The stages of an action potential include; depolarisation, repolarisation, hyperpolarisation and the resting state.
- Refractory periods (absolute and relative) allows for discrete and unidirectional action potential transmission.
- Cardiac action potentials are generated by pacemaker cells located in the SAN and AVN. These cells do not need a stimulus to generate an action potential.
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