Action potentials Flashcards
All cells have a resting membrane potential, but only two cell types alter their membrane potential in response to stimuli. What cells are they and what is this process called?
Muscle cells and Neurons, such ability is excitability or irritability.
What are ion currents?
What limits where they can flow?
Ions will diffuse down their electrochemical gradient once the membrane is permeable, this flow of ions is called an ion current.
The limit to where these currents can flow is whether there are the correct ion channels to allow ion flow in a given area. (in other words ion channels do not appear everywhere in the cell, where they do not appear there can be no action potentials.
When voltometers are measuring the potential, and it becomes less negative, is the inside or outside becoming less negative?
The inside of the cell (influx of Na+)
Define:
Hyperpolarization:
Depolarization:
Repolarization:
Hyperpolarization: inside the cell becomes more negative, inhibitory signal.
Depolarization: inside the cell becomes more positive, excitatory signal.
Repolarization: A return to the resting membrane potential
What is an oscilloscope:
A device which allows measure of varying voltage.
Is the magnitude of all cells resting potentials the same? If not give counter examples.
No.
Neuron on average -70 mV
Cardiac cell on average -85 mV
Explain the structure of the voltage gated ion channel (example being a Na+ channel)
The channel itself spans the membrane, and can be in an open and closed formation (independent of the ball and chain mechanism, the part which opens and closes are called the gates)
A ball and chain also exists which is capable of blocking the bottom of the channel.
What are two causes of hyperpolarization:
Negative charges entering the cell.
Positive charges leaving the cell.
Voltage gated channels exist in the axons, true or false?
True, the conduct action potentials. The gate is the part of the channel, internal, which can open and close.
What is a non-gated ion channel called?
A leakage channel
Are channels for K predominately gated or ungated?
Are channels for Na predominately gated or ungated?
This is a bad question. The majority of potassium channels are still probably gated, but there are some leakage channels. This is why the membrane is more permeable to K+ then Na+.
Na channels are all gated, and the gates are closed at membrane potential.
Na channels are all gated and these gates are closed at resting membrane potential, but Na still gets into the cell. How?
Sodium gated channels occasionally flicker open, allowing some entry of sodium.
What does the slight inward movement of Na result in?
A resting membrane potential which is slightly more positive then the equilibrium potential for Potassium.
Why are cable properties not a reliable way of relaying charge down an axon?
Internal resistance (there are things in the way of the charges, an analogous situation would be pouring water through a tube filled with sand vs one filled with gravel, there will be a greater equivalent radius of the tube with gravel better enabling flow) and charges leak out of the membrane (this is affected by channel proteins and is mitigated by myelin sheaths, which decrease flow out of the cell), capacitance is also a factor but we will not focus much on it (myelin also decreases capacitance).
What example threshold does he use to trigger an action potential? What cell type is this most indicative of (from those you’ve learned about)
-55 mV is threshold. Resting potential is -70 mV, which is indicative of neurons (though not all neurons)
What happens when a neuron reaches threshold in the axon initial segment?
Na+ voltage gated channels open (the gate mechanism within the channel opens)
How is depolarization achieved in an action potential?
How is repolarization achieved in an action potential?
Open Na+ channels
Open K+ channels (letting K+ out)
What does the term electrochemical gradient refer to?
The combined electrical (voltage) gradient and the chemical (concentration) gradient acting on an ion.
Why can the neuron be quickly refired if it has just allowed Na+ in and K+ out?
Because it is operating in a relatively small area at the surface of the membrane (where the capacitance is occurring), the majority of the cell is not undergoing any real change in charge and enough Na and K are allowed to flow to just change this small region.
What are the steps/configurations for a Na channel depolarization?
Channel closed: (resting membrane potential) Channel Open: depolarization (action potential) Channel inactivated: Possible ball and chain mechanism blocks channel, lasts ~ 1 millisecond (refractory period, channel is technically open but nothing can pass because of the ball/chain)
Describe an action potential:
Depolarization: Na channels open, cell temporally gains + charge.
Depolarization: Na channels are inactivated by the ball and chain mechanism, K+ channels open (voltage gated, activated by depolarization of the cell), the cell quickly loses its positive charge.
Is any ATP necessary to fire an action potential?
No ATP is directly used within an action potential, but ATP was used to set up the conditions (Na+ and K+ conc.) and it will be used to return the cell to its normal conditions.
What is termed the overshoot?
What is termed the undershoot?
The membrane potential continues to drive itself towards an increasingly positive number past a voltage of zero, termed the overshoot.
The undershoot refers to a hyperpolarization of the membrane on the downstroke of an action potential.
Why does the cell not reach Na+ equilibrium potential during an action potential?
Why does a cell not stay at K+ equilibrium potential as its resting potential?
Na+ inactivation (ball and chain, this is positive feedback and they would stay open without this mechanism) occurs prior to reaching +66 volts.
Some leakage of Na+ occurs even at resting potential, resulting in a potential slightly more positive then the resting potential for Na.
