A2 Biology - Topic 8 - All You Need To Know - Part Two
This is the second hub of the series 'A2 Biology - All you need to know'. For the first part - Click Here.
This hub will cover the following sub-topics in this order:
- What happens when you stimulate a nerve
- What actually causes the action potential
- How the impulse is passed along the axon
- Impulse Sizes
- Impulse Speeds
- How the impulses transfer to other neurones
- Inhibitory Synapses
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1. What happens when you stimulate a nerve
There is a threshold level for change in the charge of an axon.
That is to say, only after a certain strength of electrical current will the potential difference of an axon's membrane change. It is important to know that no matter how far over the threshold you go, the same change will occur to the axon's membrane.
The change is that the axon reverses its polarisation. Where it used to be negative on the inside (-70mV) it becomes positive (+40mV); where it used to be positive on the outside it becomes negative. This switching charge is known as 'depolarisation'.
The time at which the axon membrane actually remains in this depolarised state is very short (about 3 milliseconds). After this, it returns back to its resting (normal) state of -70mV. Returning back to its resting state of -70mv is called 'repolarisation'.
The great change of voltage in the membrane is called 'action potential'.
Note: It is important for the axon to return to its normal state as soon as possible so that the neurone may conduct more impulses if needed to and so fulfill its function as a neurone cell.
2. What Actually Causes the Action Potential?
After enough electrical current has been used to stimulate the axon membrane (once threshold stimulation occurs), the action potential (change in voltage) occurs because of changes in the permeability of the axon membrane to sodium and potassium ions.
These permeability changes occur via opening and closing of voltage dependent sodium and potassium channels.
At rest, these channels are blocked by "gates" which prevent the flow of sodium and potassium ions flowing through them. When the voltage changes, the gates open and allow sodium and potassium through.
The Three Stages
- Depolarisation - A neurone is stimulated, a small amount of depolarisation occurs. The change in the potential difference triggers the sodium ion gates to open, allowing sodium ions to flow in through the voltage-dependent sodium channels. As the positive sodium ions flow into the axon, the axon becomes more positive and so becomes even more depolarised, triggering more sodium ion gates to open.
This is an example of positive feedback wherein a change results in more change of the same sort (and so on).
This positive feedback means that once depolarisation starts, it will continue until all of the sodium gates are open and depolarise to its full extent. This is referred to as an "all-or-nothing" event because if the action potential threshold is reached the effect occurs, and if not then no effect occurs - there is no middle ground.
- Repolarisation - After approximately 0.5 ms the voltage-dependent sodium ion channels suddenly close and sodium permeability is no longer present in the cell. Because of the depolarisation that occurred, potassium ion voltage-dependent channels opened allowing potassium ions to leave the axon because of the electrochemical gradient present in the cell surface membrane. Since the outside of the membrane is now negative, the positive potassium ions are now attracted to the outside of the membrane. The concentration gradient also increases the rate at which potassium ions leave the cell.
The leaving potassium ions mean that the inside of the cell once again becomes more negative than the outside, and in fact becomes more negative than it was at resting potential. This is known as hyperpolarisation.
- Restoring Resting Potential - In order to retrieve the required -70mV charge inside the axon, potassium ions are given time to diffuse back into the axon via normal potassium channels (not voltage dependent ones which have now closed).
3. How is the impulse passed along the axon?
After one part of the membrane becomes depolarised, the sodium ions (who have a positive charge) flow to the adjacent space next to them which is charged negatively. This will depolarise this new area, and trigger a new sodium ion gate, which will allow more sodium ions into the cell to deploarise the next adjacent area. This continues along the axon all the way down and is what is known as a "nerve impulse".
For about 5 milliseconds, a new action potential cannot be generated in the same membrane section. This short time interval of inactivity is called the "refractory period" and occurs because the resting potential is necessary to attain before another action potential is generated. This wait means that the impulse can only travel in one direction, since in that 5 milliseconds, the sodium will always choose to go in the way of part of the membrane that is not in its refractory period.
4. Impulse Sizes
The size of the impulse itself never varies. It either occurs or it does not, as soon as the threshold level has been met, the same action potential as every other will be generated.
A strong light will produce the same sized action potential as a dim light in your eye.
In order for us and our brains to distinguish the intensity of stimuli however, the brain allows for the size of the stimulus to affect the frequency of impulses created, and reading the number of neurones in a bundle of neurones (a nerve) that are actually creating impulses or not (due to the strength of the stimulus). More stimulus of course results in a higher frequency of impulses being created, as well as more neurones in a nerve carrying the impulse to the brain, which then calculates the strength of the stimulus.
5. Impulse Speeds
The speed of nervous conduction is determined by the diameter of the axon in which it occurs.
Larger diameters result in faster conduction.
Mammals like humans overcome not being able to have such large axon diameters by having myelin sheaths around their axons.
Myelin sheaths acts as electrical insulators and allow for something called 'saltatory conduction'. This simply means the jumping of impulses across the nodes of Ranvier which are just gaps between Schwann cells. Saltatory conduction increases the speed at which the impulses travel down the axon.
6. How do the impulses transfer to other neurones?
When an action potential reaches the presynaptic membrane (the end of an axon) it stimulates the release of neurotransmitter into the synaptic cleft (the gap between one synapse and the next).
The neurotransmitter diffuses across the synaptic cleft and then causes (you don't need to know how) the depolarisation of the postsynaptic cleft membrane (the start of the next axon). Then the impulse is carried down this new axon just like it did the last until it reaches its target destination (brain, effector cell etc.)
Summation is the process of many impulses being sent to one neurone which then uses the mix of inhibitory and excitatory responses to either depolarise enough to reach the threshold potential and send an impulse somewhere else or not depolarise enough to reach the threshold and not send an action potential down its axon to stimulate an impulse to somewhere else.
This type of summation involves many neurones sending impulses to a single neurone.
This involves many impulses coming in short succession along a single neurone to another. If the time interval between each impulse and the actual quantity of impulses is enough, enough depolarisation will occur to reach threshold potential and generate an action potential down the axon.
8. Inhibitory Synapses
These are of course the opposites of excitatory synapses and work to decrease the likelihood of an action potential being generated in other neurones.
These synapses work by making the inside of neurones more negative. This process of becoming more negative than at rest potential is as you should already know called 'hyperpolarisation'.
The neurotransmitter from post synaptic synapses cause the opening of potassium and chloride ion channels. Chloride ions are of course negatively charged an potassium ions are positively charged. Since there are more Potassium ions outside the cell, potassium will diffuse there taking its positive charge with it.
Since there are more chloride ions on the outside of the cell, these ions move into the celIs via diffusion down the concentration gradient, making the inside of the cell even more negative.
The inside of the axon being more negative means that more excitatory impulses will be necessary to depolarise the cell. In this way inhibitory synapses inhibit a new action potential in recipient axons.