Lecture 7: Ionic Basis of Action Potentials

What are the basic functions of a neuron?

• generate intrinsic activity through voltage-dependent membrane properties and internal second-messenger mechanisms

• receive synaptic inputs

• integrate signals by combining synaptic responses with intrinsic membrane activity

• encode output patterns in graded potentials or action potentials

• distribute synaptic outputs

How does a neuron collect, distribute, and integrate information?

neuronal signals transmitted over a long distance as the generation and regeneration of action potentials with different frequencies and patterns

True or False: The cytosol has a negative charge relative to extracellular space at rest. An AP is the rapid reversal of this situation such that for an instant, the inside of the membrane becomes positively charged with respect to the outside.

true

Where is an action potential generated?

the axon hillock

What is an AP caused by?

depolarization of the membrane beyond action potential threshold

True or False: APs are “all-or-none” because until the depolarization reaches the AP threshold, the AP won’t fire.

true

How can an AP be artificially generated?

inject current into a neuron using a microelectrode and depolarize just to threshold

What is firing frequency?

the number of APs per second

What does firing frequency reflect?

the magnitude of the depolarizing current

True or False: The action potential firing rate increases as the depolarizing current increases.

true

What is the typical spontaneous firing rate of mammalian neurons?

0.1 - 50 Hz

Is there a limit to the rate at which a neuron can generate an AP?

yes, the maximum is about 1000 Hz

What is the absolute refractory period?

no stimulus can initiate another AP (about 1 msec)

What is a relative refractory period (several msec)?

another AP can be initiated, but only be a larger stimulus than required in the resting state

Why was Hogdkin and Huxley’s experiment important?

• intracellular recording from the giant squid axon

• first time to measure the resting membrane potential

• direct measurement of V_m from inside of the cell

What did the Hodgkin and Huxley experiment suggest? How did Hodgkin and Katz test this?

• the membrane might become selectively permeable to Na+ ions—bringing the membrane close to E_Na

• Hodgkin and Katz replaced the fraction of NaCl in sea water with choline chloride, glucose, or sucrose (which doesn’t pass through Na channels)

What did the Hodgkin and Katz experiment show?

AP rose less steeply and propagated less rapidly

What did Hodgkin and Huxley hypothesize about the ionic basis of the AP? How did they directly test this hypothesis?

• hypothesized that the AP is generated through a rapid increase in the conductance of the membrane to Na+

• tested the hypothesis with a voltage clamp (clamp membrane potential at any chosen value to isolate current)

What did Hille do in 1970?

verified the roles of Na+ and K+ current in AP by using voltage-gated Na+ channel blocker tetrodotoxin and K+ blocker tetraethylammonium

What is rapidly activating and inactivating I_Na?

sodium conductance serves to initiate action potential

What is the delayed rectifier I_K?

potassium conductance serves to rectify or reset membrane potential

Compare and contrast Na+ channels and K+ channel gates.

• both open in response to depolarization

• K+ channel gates open later than Na+ channel gates

What is the rising phrase?

transient increase in g_Na influx of Na+ ions

What is the falling phase?

prolong increase in g_K,efflux of K+ ions

What is the molecular basis behind the absolute refractory period?

Na+ channel inactivation/closure

What is the molecular basis behind the relative refractory period?

• recovery from sodium inactivation isn’t yet complete

• elevated K+ conductance due to opening of voltage-gated or calcium-activated K+ channels makes it difficult for an entering Na+ current to exceed exiting K+ current to trigger a new AP

What are the functional properties of voltage-gated Na+ channels?

• open with little delay

• stay open for about 1 msec

• cannot reopen by depolarization

• absolute refractory period (channels are inactivated)

Describe the structure of voltage-gated Na+ channels.

• large subunit with 24 transmembrane domains

• 4 main units

• S1-S4: voltage sensing domain

• S4: main voltage sensor

• S5-pore-S6: central pore conduction domain

True or False: In voltage-gated Na+ channels, water accompanies the Na+ ions as they pass through the channel.

true

True or False: The cryogenic EM structure of Na_vPaS was determined with an overall resolution of 3.8 A.

true

Describe the structure of voltage-gated potassium channels.

• 6 transmembrane domains

• 1 unit

• S1-S4: voltage sensing domain

• S4: main voltage sensor

• S5-pore-S6: central pore conduction domain

• heterotetrametric channels

Contrast TTX and TEA.

• TTX binds to a selectivity filter

• TEA extracellularly blocks the KcsA K channel