Excitable Cells Part 2 (6/19/15) Flashcards
Describe the 6 steps of establishing a resting membrane potential….
- There is a concentration gradient across the plasma membrane for K+.
- Permeability for K+ is greater so K can leave the cell (more K inside the cell to begin with)
- K+ ions move down concretion gradient leading to the development is a diffusion potential.
- Equilibrium is reached when the force in the concentration gradient for K+ ions is balanced out by the opposing force in electrical gradient which favors keeping the K+ ions IN the cell.
- At this point, membrane potential is very close to the equilibrium potential for K+. Resting membrane potential does not = Equilibrium potential for K+ because Na+ still affects it some. (Na is diffusing into the cell down both its concentration and electrical gradient, keeping the resting membrane potential slightly lower than EK.)
- Therefore, K+ ions diffuse out of the cell and Na+ ions diffuse into the cell at rest!
True for False, Resting membrane potential is not steady.
False, it is! It is not a transient diffusion potential.
What does the Na/K pump do?
Uses ATP energy to pump K+ ions back into the cell and Na+ out of the cell, when the cells are at rest.
What happens if the Na/K pump gets poisoned?
A slow decrease in the resting membrane potential (moves toward 0 mV)
_______ moves toward 0 mV.
Depolarization (Rising phase)
_______ moves toward resting potential.
Repolarization (Falling phase)
_______ increase in membrane potential.
Hyperpolarization
______ is a membrane portion found within almost all cells.
Na/K pump
Na/K pump is an example of _____.
Homeostasis
What is an Action Potential?
A large transient change in membrane potential (Neurons and muscle cells)
Where is the Action potential initiated?
in the Initial segment of the neuron (never in cell body or dendrite)
What are the steps to CREATING an action potential?
Stimulus –> depolarizes membrane (dendites of neuron) –> Depolarizes cell body –> AP first develops at initial segment –> action potential propagates down axon.
What are the steps DURING the development and Time course of an Action potential?
- Increase in membrane permeability to Na+ ions.
- Na+ ions enter through Na channels driven by the concentration and electrical gradient, carrying + charges with them. *These Na channels are voltage regulated!
- The membrane potential move closer to 0 mV (rising phase/depolarization)
- If depolarization is sufficient, threshold point will be reached and many more Na channels will open and more Na+ enters the cell!
- Now the membrane potential move VERY RAPIDLY toward 0 mV and overshoots, reaching a peak @ around +40 mV.
- K channels open (increasing K+ permeability) because they too are voltage aged and K+ ions cross over to outside of cell.
- Membrane potential abruptly reverses direction and rapidly returns to resting value as K+ ions diffuse out of the cell. *When 0 mV is hit during the falling phase the electrical gradient will be lost for K+ which will inhibit K+ moving out of the cell. In other words, K+ will move out of cell via electrical gradient as long as potential is +).
- Overshoots Hyperpolarization
T or F, a relatively small number of ions can result in a large change in membrane potential.
True!
What is the latent period?
A very brief period between applying a stimulus and beginning of depolarization during the rising phase of an AP.
The latent period precedes the ________.
Foot of the AP which is the portion of the rising phase before the threshold is reached.
Why does the peak of the AP stop at approx. 40 mV?
B/C peak of AP is approaching Eq of NA (58 mV) which it cannot reach unless K+ channels do not open.
What is the relationship between the concentration gradient and electrical gradient of Na+ at peak of AP?
The electrical and Concentration gradients are nearly equal at Peak AP.
The End of the repolarization phase (The resting membrane potential) is close to _______.
equilibrium K (eK) *B/C permeability of K is very high during the falling phase of AP until the concentration and electrical gradient are nearly balanced at end of AP.
________ is insufficient to take membrane to threshold.
Sub-threshold stimuli
________ is just sufficient to take membrane to threshold.
Threshold stimulus
________ is Greater than a threshold stimulus.
Supra-threshold stimulus
Events resulting from a sub threshold stimuli is confined to where?
The immediate region of the membrane
________ are small depolarizations or hyper-polarizations that do not move along the membrane.
Local responses
_____ responses are proportional to stimulus strength.
Graded responses
What happens if threshold is reached?
An AP WILL occur, it is automatic at that point.
What is an Absolute refractory period?
Brief period when a second threshold stimulus cannot elicit a second AP.
What is a Relative Refractory period?
For a longer period after and AP a supra-threshold can elicit another AP, but a threshold stimulus cannot.
*Basically it imposes an upper limit on the frequency at which cells can fire AP’s. This is important in the heart.
_______ binds to Na channels and blocks sodium influx.
Tetrodotoxin (TTX) from pufferfish
What is the stimulus strength-duration curve?
Curve that relates strength of an instantaneous stimulus to the duration of that same stimulus that is required to elicit a response.
A Drug that enhances stimulus sensitivity would shift the stiumuls strength-duration curve to the _______.
Left!
A Drug that enhances stimulus sensitivity would shift the stimulus strength-duration curve to the ______.
Right!
What is Rheobase?
The magnitude of least intense stimulus that can elicit a response.
What is Utilization time?
duration required to elicit response by a stimulus with a Rheobase magnitude.
What is Chronaxie?
duration required to elicit a response by a stimulus TWICE the strength the magnitude of the Rheobase stimulus *used to compare the excitability of different cells.
_____ are passive currents (do not propagate) that occur at immediate site of stimulation.
Local currents
T or F, AP’s only go in one direction.
True, due to Na+ channel inactivation.
_____ wrap extensive layers of plasma membrane around axon except at periodic nodes (nodes of ranvier)
Schwann cells
Action potential in a nerve bundle is known as _______.
Compound action potential
Axons with larger diameters have ______ velocities.
Higher velocities
