Action potentials do/are NOT - Proportional to the stimulus size - Act locally - Attenuate with distance - Spread in both directions - Take place in many types of cells

Action potential can be generated and propagated ONLY in: -Neurons (only at the axon) -Muscles Why only there? Function follows form

Axon Cell body Dendrites Axon hillock (trigger zone) Voltage - gated channels are found mainly on the axon and the axon hillock

Both gates open Activation gate closed Sodium voltage - gated channels have two gates Inactivation gate closed Activation gate Inactivation gate Potassium channel Outside Inside

It looks a little more complex than this:

At a resting state -Na + activation gate: closed -Na + inactivation gate: open -K + gate: closed Upon depolarization -Gates change their status The key players K+K+ Na + -Na + activation: a fast gate -Na + inactivation and K + : slow gates (a 0.5 ms delay)

1) Channels are closed (inactiv. Na gates are open) Resting membrane potential 2) Na act. gates open depolarization 4) Na act. close, K open Hyperpolarization 3) Na inact. gates close, K open Repolarization Stimulus Na K (1) (2) (3) (4)

And all of this: <3ms

The Na + voltage-gated channels provide a positive feedback cycle

stimulus - Once the potential reaches a threshold (opening the Na + activation gate), it is an all-or-none process Threshold - The action potential is not proportional to the size of the stimulus A positive feedback loop

Action potential propagates along the axon

Remember the graded potential - Once Na + ions entered the cell, they spread locally and depolarize the nearby regions - Graded potential die out - When it comes to the axon, Na + voltage-gated channels allow the action potential to proceed

A single action potential does not actually move through the cell Formation of AP in one site, generates AP in the next site, and so forth

Let’s start with the trigger zone

Absolute refractory period Relative refractory period Refractory periods Na + channels have not yet reset to their resting positions. A new AP can NOT be generated Some Na + channels have reset to their resting positions

Propagation of the action potential

Action potentials do/are NOT - Proportional to the stimulus size - Act locally - Attenuate with distance - Spread in both directions - Take place in many types of cells

Nodes of Ranvier Myelin sheaths provide faster propagation Saltatory conduction (Saltare = “to leap”)

Saltatory conduction Conduction along myelinated axons Continuous conduction Conduction along unmyelinated axons

Multiple sclerosis Jacqueline du Pré is considered to be one of the greatest cellist in our times. Her career was cut short by Multiple sclerosis, which forced her to cease performing at the age of 28, and led to her premature death at the age of 43 What happened?

Size Does Matter! The larger the diameter, the lower the resistance, the faster the impulse

Summary The plasma membrane is polarized in a resting state Chemically-gated channels generate local, graded potentials Voltage-gated channels allow generation of action potentials Action potentials propagate along the axons Conduction speed is dependent on size and degree of myelination