Electrical Signaling in Nerves

This post will discuss the second part of our Nerves and Synapses topic in Veterinary Physiology 1. I'll be discussing the differences between graded potentials and action potentials, refractory periods, and myelination and its effect on the speed of conduction of an electrical signal. 

Graded Potentials (GPs) and Action Potentials (APs)

Graded potentials are input signals and are found in dendrites or cell body of the neuron. They occur in response to a stimulus acting on the cell. The stimulus opens gated ion channels causing a flow of ions into or out of the cell. This produces a graded potential, which is a small change in Vm (see previous post). The sum of GPs determine if an action potential will fire - but more on that later. Now that you know a bit about GPs, there are some important properties you need to know. 

1. GPs can be hyperpolarising (making Vm more negative than RMP) or depolarising (making Vm more positive than RMP). This depends on which gated ion channels open and consequently which ions flow into or out of the cell. For example, if sodium channels open, then sodium will flow into the cell causing depolarisation. If potassium channels are opened then K+ will flow out the cell and hyperpolarisation will occur. 
2. GP amplitude depends on stimulus amplitude. This is because a more intense stimulus opens more gated channels causing more ions to enter or exit the cell. 
3. Amplitude decreases with distance travelled. This is because ions passively flow out the cell through leak channels. 
4. GPs can summate (add together). This can occur in two ways:

• Temporal summation: The adding together of 2 or more GPs generated by the same input close together in time. 
• Spatial summation: Adding together of 2 or more GPs generated by different neurons.     

Action potentials, on the other hand, cannot summate, their amplitude doesn't decrease with distance, their amplitude is always the same, and they are always depolarising. 

Action potentials are output signals and travel along the cell's axon. A number of steps are followed by the cell every time an action potential is fired. It is important to note that an AP will only fire if the sum of the GPs is higher than the stimulus threshold. At the RMP, voltage gated sodium channels are inactivated. Once threshold has been reached three things are triggered. Voltage gated sodium channels are opened (this occurs quickly), the inactivation gates of these channels are closed (this happens slowly) and voltage gated potassium channels are opened (this happens slowly). This causes the following to occur.   

1. Voltage gated Na channels open. This causes a rapid influx of Na+ ions, depolarising the cell. These channels then close.
2. Potassium voltage gated channels open. This causes an efflux of K+ cells causing the membrane to repolarise (the Vm returns to the RMP)
3. A delay in closing the K+ channels causes a period of hyperpolarisation
4. The normal concentrations of ions inside and outside the cell is returned by the sodium- potassium pump. 

 
Propagation of an Action Potential

In order for an AP to fire, voltage gated sodium channels need to open to allow depolarisation to occur. When these channels open they allow Na ions to flow into the cell and close shortly afterwards. This prevents more ions from entering the cell at that particular section of the cell membrane. The channel will remain closed for some time, allowing the proteins of the voltage gated sodium channel to return to their original polarity. Meanwhile, the Na+ ions which have entered the cell begin to passively diffuse in both directions along the cell membrane. The Na+ ions which diffuse down the axon cause that part of the cell membrane to depolarise, causing it to become positive with respect to the outside of the cell. This triggers another AP there, causing an influx of Na+ and so on. This repeats, causing the AP to be propagated down the axon. The Na+ ions which diffused up the axon (backwards) don't initiate another AP because at that point in time the voltage gated potassium channels of the first AP are open causing K+ efflux and repolarisation of the cell membrane. 


I know this sounds confusing, but looking at a diagram will make this seem much simpler. This website has an excellent explanation as well as some good diagrams which I found helpful.

Refractory Periods 

After an action potential has fired there is a period of time during the AP and a short time after the AP in which another AP cannot be fired. This is called a refractory period. A refractory period occurs when the voltage gated sodium channels are inactivated. This prevents a further influx of Na+ and the firing of an AP in the wrong direction. There are two types of refractory periods:

• Absolute Refractory periods: when the voltage gated Na+ channels are inactivated.
• Relative refractory period: an AP can be fired but only if the stimulus is considerably greater than the threshold.

Myelination 

Neurons can either be myelinated or unmyelinated. Myelination refers to the fatty material which surrounds and electrically insulates sections of the axon. In the peripheral nervous system Schwann cells wrap the axon in myelin. In the central nervous system, oligodendrocytes produce the myelin.  The gaps between the myelin sheaths are called the Nodes of Ranvier. Myelinated axons conduct impulses much faster than unmyelinated axons.

The diagram below explains why:  

The Effect of Myelination on Electrical Signalling in Nerves

The top picture represents an unmyelinated axon that has only one voltage gated sodium channel (VGSC). The stimulus occurs at the start of the axon. At the stimulus a relatively high voltage is present. This is because the AP has just been fired and Na+ have just entered the cell to depolarise the membrane. As the AP travels along the axon the voltage decreases quickly. This is because of the presence of sodium leak channels in the cell membrane which cause Na+ ions to leak out the cell. 

The second picture represents an axon that has many VGSC spread along the length of the membrane. Sodium leak channels are still present but the VGSCs "top up" the Na+ concentration, allowing the AP to be propagated along the axon. 

The third picture represents a myelinated axon. Leak channels are still present so sodium ions still leak out the cell. However the myelin sheaths act as an electrical insulator and prevent most of the Na+ ions from leaking out the cell, allowing the Na+ ions to travel further without having to be "topped up". This allows the AP to 'jump' from Node to Node. This dramatically increases the speed of conduction in an axon.