NB: The electrical currents of membrane potentials are formed from the net flux of charged particles that move across the membrane.
Resting Membrane Potentials (RMP)
The RMP arises as a small build up of negative charges forming just inside the neurone, there is also an EQUAL build up of positive charges on the outside of the neurone.
The RMP is measured in millivolts (mV) and typically lies between -40 and -90 mV.
The RMP arises from differences in the ion concentrations between the cell (neurone) and the extracellular fluid.
Outside the cell = ,
Inside the cell =
The membrane is 100 times more permeable to than it is to therefore the fluid that is next to the inner surface of the neurone becomes more negative as the leaves.
Action Potential (aka nerve impulse)
A stimulus of sufficient size will change the membrane potential to the critical level, this will then lead to the threshold potential - this is the potential at which a response is produced. this threshold potential then triggers a rapid and continuously increasing depolarisation until it become briefly positive inside this is the action potential and continues to become inside positive until it reaches +10 - +50mV, this brief period of inside positive is called the overshoot - refer to 2a on figure 6 booklet 1, this therefore creates an inward current of excitable cells along the neurone.
NB: The stimulus must be depolarising not hyperpolarizing.
Deploarising is the decrease in the electrical gradient causing channels to open resulting in an inflow of ions into the cell therefore the potential becomes less negative resulting in the threshold being reached.
Sequence of events in an action potential - Summary.
1. Rapid opening of channels causing depolarisation.
2. Membrane polarity reverses therefore inside becomes more positive than the outside
3. Opening of the channels and then the closing of the channels, leading to repolarisation and the recovery of RMP.
NB: Therefore in depolarisation comes into the cell making the inside more positive and
repolarisation is therefore leaves the cell resulting the inside to become more positive.
Figure 21 in booklet 1 shows the equipment used in the measurements of membrane potentials.
Figure 26 shows the molecular physiology of action potentials and the role of voltage.
The refectory period -RP (aka the undershoot phase) Figure 6 booklet 1
During this period both the gates are closed therefore a second depolarising stimulus arriving can not trigger the Action Potential as the gates are shut.
This is a period of insensitivity to depolarisation.
The RP sets the maximum rate of Action Potential generated and is therefore very important.
Action Potentials Travel along the Axon
The neurone is stimulated at the dendrite or cell body at a specific point ( The action
potential is a localised membrane depolarisation).
A signal must travel along the axon to the other end of the neurone, however the action potential does not travel along the axon as it is regenerated along the axon instead ( it self propagates like toppling dominoes and is often refered to as the domino effect).
Depolarisation of the Action potential causing neighbouring neurone regions to become depolarised above there threshold. Therefore a new Action potential is then triggered at a new position once the threshold is exceeded. Refer to figure 17.
NB: It is the RP that prevents the Action potential from travelling in a backward direction