lecture 4

Question 1

how to distinguish early and late currents using pharmacological blockers?

Answer 1

TTX can block Na channels

TEA can block K channels

Question 2

how do early and late current change with voltage?

Answer 2

early inward current first gets bigger, then smaller, and eventually reverses direction at +65mV
delayed outward current gets bigger with increasing voltage.

Question 3

what is the ball & chain model of inactivation?

Answer 3

1. open-inactivate-close-open
2. after Na+ pass through the channel, the ball goes into the channel and bind it.
3. when the channel closes, it forces the ball out.
4. pronase: it can prevent ball from entering the channel.

Question 4

what is the use of channel inactivation?

Answer 4

1. introduces directionality of information flow
2. allows higher frequency of signal conduction (if an action potential can open all sodium channels, the channels would be constantly activated. it would be hard for cell membrane to reset to fire another action potential)

Question 5

which two types of current are required for action potential propagation?

Answer 5

the passive flow of current(does not require the movement of Na+ along the axon but instead occurs by a shuttling of charge)
as well as active current flowing through Na+ current channel

Question 6

how does long-distance signaling achieved by means of action potential?

Answer 6

1. a depolarizing stimulus locally depolarizes the axon, thus opening the voltage-sensitive Na+ channel in that region.
2. the opening of Na+ channels causes inward current of Na+, and the resultant depolarization of the membrane potential generates an action potential at that site.
3. some of the local current generated by the action potential will passively flow down axon.
4. local depolarization causes neighboring Na+ channels to open and generates an action potential here.
5. upstream Na+ channels inactivate, while K+channels open. membrane potential repolarizes and axon is refractory here.
6. this process is repeated, propagating the action potential along the axon.

Question 7

why is action potential self-supporting?

Answer 7

• this mechanism of action potential generation represents a positive feedback loop.
  1. activating the voltage-dependent Na+ conductance increases Na+ entry into the neuron, which makes the membrane potential depolarize, which leads to the activation of still more Na+ conductance, more Na+ entry, and still further depolarization.
  2. positive feedback continues until Na+ conductance inactivation and K+ conductance activation restore the membrane potential to the resting level.