NMS Lecture 3 - Action Potential

What determines the resting membrane potential

The relative permeability of the membrane to K+ and Na+

How is the RMP disrupted

A specific stimuli causes the ion selective channels to open

What is the physical stimuli in the stretch reflex

Patellar tendon tap

When a stimuli activates the cell, permeability of what ion increases initially

Sodium

2 main types of ion channels (other than leak channels)

1. Voltage-gated ion channels - open at a specific membrane voltage
2. Ligand-gated ion channels - involved in neurotransmitter release and synaptic transmission

Action potential

Large change of membrane potential from the resting membrane potential of -70mV to + 30mV, approaching the equilibrium potential Na+ (+55mV)

Time of action potential

2 to 3 milliseconds

How is afferent neuron activated in stretch reflex

Muscle stretch results in increased opening of specialized Na+ receptors. Na+ enters efferent fiber and depolarizes the afferent neuron

Threshold potential (-55mV)

Minimum membrane potential that must be reached for voltage-gated sodium channels to open, firing the action potential

How does potential reach the threshold potential

Sodium entering through pores

What channels are open/closed at RMP

Leak channels are open, passively moving Na+ and K+. Voltage-gated ion channels are closed

At threshold potential which channel opens

Voltage-gated sodium ion channel opens up, sodium rushes into cell, increasing potential to +30mV

At +30 mV, which channels open/close

Voltage-gated sodium ion channel closes, votage-gated potasium ion channel opens, potassium rushes into the cell, decreasing potential.

Absolute refractory period

Time during which an excitable membrane cannot generate an action potential in response to any stimulus

What causes the absolute refractory period

Inactivation of voltage-gated sodium channels

Relative refractory period

Time during which an excitable membrane will generate an action potential only with a stimulus greater than the usual strength

Stage 1 of action potential

Resting membrane potential. Voltage-gated ion channels are clsoed

Stage 2 of action potential

Stimulus causes depolarization to threshold potential. Sodium moves into cell via pores

Stage 3 of action potential

Depolarization - Voltage-gated sodium channels open. Sodium rushes into the cell, depolarizing the cell to +30mV

Stage 4 of action potential

Repolarization - Voltage gated potassium channels open, sodium channels close. Potassium rushes into the cell, repolarizing the cell

Stage 5 of action potential

Hyperpolarization - Voltage gated potassium channels are open, sodium channels are completely closed. Cell moves closer to equilibrium potential of potassium (-90mV).

Stage 6 of action potential

Resting membrane potential - All voltage- gated channels are closed

How does membrane conductance change throughout the action potential

Sodium conductance increases during the depolarization stages, but decreases during repolarization. Potassium conductance decreases during depolarization, but increases during repolarization

The movement of an action potential along a neuron is related to ________

The relative permeabilities of Na+ and K+

Steps of action potential transmission

1. Axon at RMP
2. Activation of stretch receptor opens voltage-gated Na+ channels
3. Local depolarization of membrane causes adjacent voltage-gated Na+ channels to activated
4. New action potential generated in adjacent membrane
5. Action potential only travels in one direction due to refractory period

Why does action potential propagation only occur in one direction?

Refractory period limits which cells can be stimulated

Electrotonic conduction

Spread of current inside axon, proceeding in only one direction.