Action Potentials + Refractory Periods

Action potential

Brief reversal of membrane potential of around 100 mV

for long distance signals → non decremental with constant magnitude

All or non principal of action potential

If threshold is reached, full action potential happens

if not, it doesn’t → no half action potential etc

Four Phases of an Action Potential

Resting stage, depolarization, repolarization, hyperpolarization

Resting stage of action potential

resting membrane potential → -70

only leakage K+ and Na+ channels are open

gated channels are closed

positive outside negative inside

neuron is polarized

Depolarization stage of action potential

voltage gated Na+ channel opens → Na+ into cell → membrane potential becomes less negative (-70 → 30)

Repolarization stage of action potential

voltage gated Na+ channels close and voltage gated K+ channel opens → K+ exits cell → membrane potential becomes negative again

Hyperpolarization stage of action potential

K+ channels remain open for too long → too much K+ leaves so membrane potential is more negative than resting 70 mV

All action potentials are alike and are independent of

stimulus intensity

– How do we tell the difference between hot and light touch?

– How does CNS tell difference between a weak stimulus and a

strong one?

Based on frequency not magnitude

strong stimulus → lots of action potential close together

weak stimulus → few action potentials

Refractory period and what are the two types

period during or after an action potential when firing another action potential is difficult

absolute and relative

Absolute Refractory Period → when and what

During depolarization - from opening of Na+ channels till close → another action potential cannot be fired at all

Relative refractory period

while repolarization (late repolarization and hyperpolarization) is happening → threshold is slightly elevated → making it hard to fire another action potential

action potential can still be generated with a strong enough stimulus

Can action potential travel in more than one way and why

No, the signal can only move forward because of the refractory period since it cannot fire immediately again

Multiple Sclerosis

axon loss → Myelin sheath in CNS is damaged so action potential is slower or blocked