5.1 Ion Channels and Action Potentials

In a resting neuron, voltage gated Na+ and K+ channels are __________

closed

In a resting neuron, K+ leak channels ___________________________

enable the flow of K+ ions across the PM

What is an ungated channel?

leak channel that is open in resting neurons

What is a voltage gated channel?

channel that will open with a change in voltage, found in axon membranes

What is a ligand gated channel?

channel that opens when a ligand is bound, found at synapse

What is a mechanically gated channel?

channel opened by physical force, in sensory receptors

What is an example of a mechanically gated channel?

the hair cells in the inner ear

What is the process of opening the mechanically gated channel in the inner ear?

1. sound waves moves fluid in inner ear

2. fluid pushes on stereocilia

3. when stereocilia move, spring like protein attached to channel stretches

4. stretched protein pulls open channel

5. K+ enters cells and depolarizes cells, creating potential that will be translated into AP in auditory nerves

What type of proteins are voltage gated ion channels?

integral membrane proteins

How many types of voltage gated ion channels are there?

over 140

Voltage gated ion channels are related by ____________ and __________ motifs

structural and functional

What are the three aspects of the functional elements of the ion channel superfamily?

1. ion conductance

2. pore gating

3. regulation

What is A-type currents?

fast activating and fast inactivating potassium channels

Voltage gated k+ channels have ____________ which guarantees there will always be an available source of current for repolarization (return to resting)

variability

A single ion channel may have multiple _______ ______ that respond to changes in membrane potential in opposite ways or at diff rates

internal gates

What is the bottom line of ion channels?

there are many types of ion channels, and they do not always open and close independently

The rate of ionic flow through a channel is determined by ________________ and _____________________

the maximum channel conductance (how easily ions pass through) and the electrochemical driving force for that ion (electrochemical gradient)

What is an action potential?

abrupt and quick change in membrane potential that occurs when a neuron or other type of electrically excited cell conducts and electrical impulse

What are the steps of an AP?

1. stimulus causes Na+ charges to flow into neuron

2. membrane potential becomes less neg (depolarized)

3. depolarization occurs slowly until the membrane potential reaches the threshold (usually 10-20mV more +)

4. sudden increase in membrane potential due to rapid influx of Na+ ions

5. at peak of AP potential K+ channels open, letting K+ out

6. membrane potential falls, usually below resting potential

7. membrane potential returns to resting potential

What does it mean that APs are all or nothing?

once threshold is reached, depolarization will occur, and AP will fire regardless of strength of stimulus

What are the states the Na+ channels go through from open to close?

channels go from open to inactive, then to closed

The magnitude of an AP stays the ______ as it travels down the axon.

same

What does is mean that APs can propagate?

although APs are generated on patches on excitable membrane, the currents that result can trigger APs on neighboring parts of membrane

What is the absolute refractory period?

excitable membrane CANNOT generate AP in response to stimulus

What is the relative refractory period?

excitable membrane will produce AP but only to a stimulus of greater strength than usual threshold strength

When does the absolute refractory period occur?

occurs when voltage gated Na+ channels are already open or inactive

*inactive gate that blocks these channels must be removed by repolarizing the membrane and closing the pore

What is the main basis for the absolute refractory?

Na+ channels inactive

What is the main basis for the relative refractory period?

voltage gated potassium channels are open and also hyperpolarized

When does the relative refractory period occur?

occurs when some Na+ channels are closed, some K+ channels still open

A new stimulus can create an AP during relative refractory period only if ___________________ and _______________________

stimulus is large enough to open Na+ channels and stimulus lasts long enough to pass the relative refractory period

Why is the AP evoked during relative refractory smaller than normal?

still some K+ channels open, so K+ is still leaving cell, and some Na+ channels still inactivated

The intensity of a stimulus is indicated by the ___________ of action potentials

frequency