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Neurophysiology; Chemical Synapses Part 1 (Action potential ("All or…
Neurophysiology; Chemical Synapses
Part 1
Action potential
Wave of electrical discharge that travels
along the membrane of a cell
Stimulation:
Causes the membrane to become
momentarily permeable to Na+ ions
Cell reverses charge to become positive inside
by about +45 millivolts
Phases in action potential:
1 Depolarization
2 Repolarization
3 Hyperpolarization
4 Refraction
"All or nothing" response
If the stimulation is insufficient and below the
-50mV treshold no action potential will be elicited
Depolarisation and Repolarisation
Occur due to coordinated opening and closing
of Na+ and K+ channels
Na+ channels allow Na+ to enter the cell
K+ channels allow K+ to escape the cell
Depolarization:
Opening of voltage-sensing Na+
channels in the membrane of the axon
Na+ ions rush inward along their concentration gradient
Repolarization:
Transfer of large numbers of K+ to the
outside immediately after the influx of Na+ ions
Hyperpolarization
The resting potential of the membrane
is exceeded for a few seconds
Caused by:
Slower closing of the K+ channels and over the
longer period of action of the Na+/K+ pump in
recharding the membrane
Refractory Period:
A second action potential cannot occur while the
membrane is still depolarised from the preceding action potential
1) Absolute Refractory Period
1 msec
Period during the activity of the Na+ and K+ channels
2) Relative Refractory Period
1-10 msecs
Threshold is higher than normal
Propagation
Action Potential cuases a further depolarization in the neighbouring area of the fibre and a nerve impulse travels along
Include Action Potential Diagram