Action Potential

Information in the brain

• Signals can travel electrically - wiith cells capable of generating and conducting nerve impulses or APs have excitable membranes

• They can travel chemically - via Neurotransmitters

Membrane Potential

Voltage across the membrane

Resting potential

At rest, the inside of the neuronal membrane is electrically negative relative to the outside.

Vm= -70mV

Equilibrium Potential

• Many ions inside and outside the cell: K+, Na+, Cl- and Ca2+

• All ions have a different equilibrium potential, so are added up

• Most influential are K+ (80mV) and Na+ (62mV)

  • Cell membrane is 40X more permeable for K than Na

  • Resting potential is close to equilibrium potential

Sodium-Potassium pump

• Exchanges internal Na for external K, against their concentration gradients

• Energetic cost > ATP

• 70% total ATP produced in the brain

Action Potential

Signal conveying information over distances

• AKA a spike, nerve impulse or discharge

Generation of an AP

• Threshold for generation of an action potential, i.e. small random fluctuations in the membrane potential are not interpreted as useful information

• All-or-none law ensures full size of action potential

• Generator potential - causes depolarisation of the membrane

Absolute Refractory Period

Period in which it is impossible to initiate another AP (around 1msec)

Relative refractory period

Amount of current required to depolarise the neuron to AP threshold is elevated above normal.

Firing rate

Number of APs per time interval

• Maximum is 1000Hz

Action Potential Generation

Voltage-gated sodium channel

• Opens with little delay, makes rising phase occur quickly

• Stay open for about 1 msec and then deactivate, partly explaining why the AP is so brief

• Cannot be opened again by depolarisation until the AP returns to a negative value near threshold (refractory period)

Propagation of action potential

Propagation of the Action Potential

• APs propagate at different speeds (0.1m/sec to 100m/sec)

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Increase velocity by increasing the diameter

• the thicker an axon, the more ions move down an axon inside the membrane

• AP-adjacent Na-channels can be opened ealier, makes AP propagation faster

• Squids have giant axons → up to 1mm thick

• In human brain not enough space for such thick axons

Increasing velocity through myelin

• AP ‘jumps’ from Node of Ranvier to Node of Ranvier - saltatory conduction

• Myelinated axons are much faster than non-myelinated axons

• Myelinated: up to 100m/sec; unmyelinated up to 1m/sec