Flashcards in Chapter 5 - Guyton Deck (27):
Membrane potentials are caused by what?
Define diffusion potential.
ion concentration difference on the two sides of the membrane, ex. potassium concentration greater within a nerve fiber membrane than on the outside on the membrane, strong tendency for potassium to diffuse out, eventually the diffusion potential is great enough to block further net potassium diffusion)
Normal potential difference required in mammalian nerve fiber for potassium is:
94mV (negative inside the fiber)
Normal potential difference required in mammalian nerve fiber for sodium is:
61 mV (positive inside the fiber)
diffusion potential across a membrane that exactly opposes the net diffusion of a particular ion through the membrane, determined by ratio of concentrations of specific ion on two sides of the membrane
When membrane is permeable to different ions, the diffusion potential depends on these factors:
1) polarity of the electrical charge of each ion; 2) permeability of the membrane to each ion; 3) concentrations of the respective ions on the inside and outside membrane
gives the calculated membrane potential on the inside of the membrane when two univalent positive ions (sodium and potassium) and one univalent negative ion (chloride) are involved
Sodium, potassium, and chloride are the most important ions in membrane potential development. The degree of importance is determined by:
it is proportional to the membrane permeability of each ion, the resting potential is closest to the equilibrium potential for the ion with the highest permeability (ex. if potassium and chloride can't get through then Nernst equation of sodium will determine entire membrane potential)
What is primarily responsible for signal transmission in neurons?
rapid changes in sodium and potassium permeability
What instrument is used to measure the membrane potential?
Resting membrane potential
-90mV (inside is 90mV more negative than outside)
Leakage of potassium through nerve membrane
far more permeable to potassium than sodium (100 times more) but can leak through, key factor in determining the level of normal resting membrane potential
Which contributes more to the resting membrane potential, sodium or potassium?
potassium (K+ Nernst potential = -94mV)
Using Goldman equation because sodium and potassium interact, potential inside the membrane would be -86mV. Why is it -90mV?
contribution of the sodium-potassium pump, more sodium pumped out than potassium in, causing continual loss of positive charge in the cell
Resting stage of action potential
before the action potential begins, membrane is polarized at -90mV
membrane becomes permeable to sodium, allowing it to diffuse interiorly, depolarization overshoots and the membrane potential becomes positive in large fibers and gets close to 0 in smaller fibers
sodium channels begin to close and potassium channels open more than normal, potassium influx re-establishes normal negative resting membrane potential
When is activation of the sodium channels occurring?
between -70 and -50mV, sodium channels can't open again without first repolarizing because inactivation gate won't reopen until the membrane potential returns to near the original resting potential
When does the voltage-gated potassium gate open?
between -90 and 0mV, efflux of potassium
Why does depolarization overshoot?
the inactivation gate closes than the activation gates open so sodium is able to get in
What method of feedback is the action potential?
positive feedback, voltage increase opens up channels which increases voltage more and opens up even more channels until all open
Threshold for action potential
What re-establishes the ion concentrations on both sides of the membrane after the action potential?
sodium-potassium pump (requires ATP, stimulated by excess sodium accumulating inside the cell membrane)
What would cause a plateau in the action potential (prolonged period of depolarization)?
voltage-gated sodium channels (fast) and voltage gated calcium-sodium channels (slow), slow channels are responsible for the plateau
Where in the body does a prolonged period of depolarization naturally occur?
heart muscle fibers, causing contraction of heart muscle to last for longer period
Rhythmical discharges cause:
beat of heart, peristalsis in intestines, neuronal events such as breathing