Physiology - Ion channels & AP

Key properties of membrane ion channels

• permeability

• selectivity

• gating

*ion channels are proteins, so properties are based on AA sequences

channelopathy

disease causes by mutations in ion channel genes

when is an ion channel considered voltage gated?

not volted = constant conductance slope

volted = conductance slope changes

what is the ball & chain model?

an inactivation mechanism can plug the channel pore even while the activation gate is open, a channel can therefore be activated but unavailable

graded potential

• slow, analog, variable amplitude, usually generated at synapses in dendrites or cell body

• vary in size

action potentials

• fast digital short duration fixed amplitude usually generated in axons

• stronger stimulation can increase the number/frequency of AP

what is the relationship between graded potential & action potential?

as GP gets fired, the number of AP increases

the voltage clamp technique

• keeps Vm constant while measuring the current required to maintain the voltage

• Bc Vm is constant, changes in measured current reflect changes in conductance

• helps real different ionic currents associated with Na+ & K+

Na & K during Action Potential - Channel blockers

• Na channels activate rapidly during depolarization

• K channels activate more slowly and remain open longer

• Help produce sequence of events underlying the AP

Sequence of Ion channel states during AP

1. Na & K channels are close and deactivated

2. Na & K channels open

3. Na close and become inactive, K open

4. Na begin to de-inactive & K begins to close

• the exact timing of inactive/de-inactive can cry substantially among different axon types

Refractory Period

• after large depolarization, Na channels remain inactive for some time

• Na inactivation plus longer activation of K produce the refractory

• period places a upper limit on spike frequency

• Na channels must recover from inactivation before they can be activated normally again

• hyper polar helps remove Na inactive

Why is AP all or nothing

• associated with high concentration of Na channels & positive feedback during Na activation

• Na active is initially fast

• once ap threshold is reached, response becomes all or nothing

• sufficiently strong depolarization input can push the membrane to threshold and trigger spike

AP Propagation

• since they’re all or none they can travel long distances without simply becoming smaller along the way

• when an ap is generated, current flows both ways along axon and across membrane

• only regions that have not yet experienced ap can generate it age

• recently activated regions cannot immediately generate action portent bc their Na channels aren’t ready

What determines propagation speed?

• low axial resistance & low membrane capacitance allow AP to propagate faster

• increasing diameter, decreases resistance, and increases propagation speed

• decreasing capacitance also increases propagation speed

why does lower capacitance speed up propagation?

• lower mem cap means less charge required to change membrane voltage, which means voltage can change more quickly as current flows

Salutatory Conduction

• ap are generated at nodes of ranvier

Demyelination Disease

• demyelination impairs ap propogation

• multiple sclerosis