- Once sodium channels open up and an influx of
sodium ensues, this sodium influx would influence
neighbouring sodium channel resulting in their
opening. Thus the sodium channels open in
sequence one after the other.
- The opening of the sodium channels will also have
an affect on the opening of the potassium
channels.
- Thus, an action potential (depolarisation
followed by repolarisation) travels accross the
neural membrane in all directions.
- On examining the changes underlying
depolarisation, repolarisation and hypolarisation
certain basic facts become apparent
- If the stimulus does not reach a treshold level,
the external gates do not open.
- If the external gates do not open then the action
potential is not initiated.
- Once the sodium gates open, then a positive loop
ensues and an action potential results.
- This forms the basis of the all or none
rule. This means that once a minimal
threshold level is reached, the maximal effect
results.
- While the external sodium gates are open or while
the internal gates are closed, no stimulus of
whatever strength can cause a new action
potential. This is considered to be the Absolute
Refractory Period.
- During the last part of the repolarisation phase,
some of the sodium channels have returned to the
initial resting phase, a stimulus that is greater
than normal, can cause the opening of these
sodium channels and thus induce a new action
potential. This phase is called the Relative
Refractory Period.
- Whilst the action potential can travel in all
directions accross the cell membrane, it can
travel only in one direction accross a synapse.
Thus synapses act as one way valves, and are
responsible for the direction of nervous signals.
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