Formation and Transmission of Impulses
Formation and Transmission of Impulses
In the surface membrane of a cell there are protein carriers.
These actively pump Na+ (Sodium) ions out of the cytoplasm to the outside of the cell. At the same time, K+ (Potassium) ions are pumped from the outside in.
This active pumping of Na+ and K+ ions requires energy (in the form of ATP) because the ions are being moved against their concentration gradients (from where they are at a lower concentration to where they are at a higher concentration). K+ and Na+ ions diffuse back down their concentration gradient but K+ diffuses back out of the cell faster than Na+ can diffuse back in.
This means there is a net movement of positive ions out of the cell making the inside of the cell negatively charged, relative to the outside.
This charge is the resting potential of the cell and is about -70mV.
When a receptor is stimulated, it will create a positive environment inside the cell.
This is caused by a change in the concentrations of Na+ and K+ ions in the cell and happens in a number of steps:
There is a change in permeability (the ability of the cell membrane to let ions through it) to Na+ and K+ in the cell surface membrane at the area of stimulation, which causes Na+ channels in that area to open.
Na+ therefore floods into the cytoplasm down the concentration gradient.
As this happens the membrane depolarises (this means that the resting potential of the cells starts to decrease). If this depolarisation reaches a certain level, called the threshold level (about -55 to -50 mV), then an action potential has been generated and an impulse will be fired. If it does not reach this level, nothing will happen.
Once +40mV is reached the Na+ channels close and K+ channels open. K+ floods out of the cytoplasm so that the overall charge inside goes back down. This stage is called repolarisation.
The K+ channels then close, the sodium-potassium pump restarts, restoring the normal distribution of ions either side of the cell surface membrane and thus restoring the resting potential.
An example of an action potential being reached would be pressure receptors cells in the skin which. If your hand was squashed, the pressure receptors cells in your skin would be would be pressed out of shape (this would be the external stimuli).
In response to this the Na+ channels in that area would open up, allowing Na+ ions to flood into the cell and thus reducing the resting potential of the cells. If the resting potential of the cell drops to the threshold level, then an action potential has been generated and an impulse will be fired.
The above has only described one area of the neurone and not how the impulse is carried along the neuron, this happens by another chain reaction.
Once an impulse is made, a local current is set up between the area where there is an action potential and the resting area next to it. The flow of some Na+ sideways towards the negative area next to it causes the Na+ channels in that area to open and depolarisation to occur there. That way, the action potential is moved down the neurone.
There is a length of time called the refractory period when the resting potential is being re-established. During this time no new action potential can be generated.
In this way the action potential can only travel in one direction down the neurone because the area behind the action potential is in a state of recovery.
Generally cells are covered in a fatty myelin sheath and therefore the Na+ and K+ cannot flow through this. This means that the ions can only flow through unprotected cell-surface membrane.
In the case of a myelinated neurone, the ions can only move in and out of the cytoplasm at the nodes of Ranvier.
Because of this, the action potential will 'jump' from one node to the next, a process called saltatory conduction, and so will travel much faster than in an unmyelinated neurone.
Other factors that affect the speed of conduction are diameter of the axon (the bigger, the faster) and temperature (up to 40°C, the higher the faster).
Action potentials themselves do not change size as they move down the neurone. All stimuli, as long as they cause the threshold level to be reached, cause an action potential of +40mV, no more or less. The speed of conduction is not altered by the intensity of the stimulus either.
If the stimulus is large, it will produce a greater frequency of impulses. Another one will very quickly follow the previous action potential (i.e. the intensity is frequency modulated).
Another consequence of an intense stimulus is that more than one neurone is likely to be affected. That way the brain, receiving more action potentials from more neurones, will interpret the stimulus as being strong.