Electrochemical Gradients and Action Potentials Flashcards
electrochemical gradient
electrical gradient + chemical gradient
equilibrium potential
- function of concentration inside and out, and the charge of the ion (as well as temperature and physics based constants)
- EP = electrical potential (electrical energy) necessary to balance a concentration gradient
Nernst Equation
EPion = 61/z * log(Cout/Cin) z = valence charge Cout = extracellular concentration of ion Cin = intracellular concentration of ion equation is valid at 37 deg celsius
— numbers of ions can create concentration gradients
very small
-negative charges on inside of cell line-up with positive charges on the outside of the cell
Reasons inside of the cell is negative
- 3 Na+ go out for every 2 K+ that come in
- more channels for letting potassium back out to leave more negative charges in
- negative proteins inside cell
Goldman-Hodgkin-Katz Equation
-Nernst only works for one ion
-GHK tells you membrane voltage
-use for +1 valence
-pos ion: Cout/Cin
-neg ion: Cin/Cout
Vm = 61 log ( (Pion1C1out + PionC2out….) / (Pion1Cin1 + Pion2Cin2…) )
Creation of Electrochemical Gradient in Cell
- Na/K pump creates concentration gradient
- 3Na:2K ratio yields a small electrical gradient
- K has higher permeability so large flux of K out
- This leads to a greater electrical gradient, with neg on inside and pos on out
- Steady leak of Na in and K out.
- Now at resting membrane potential: -70 mv
Chloride
- no active transport, but has open channels.
- Cl goes out of cell because it is neg and outside is pos. it follows the electrical gradient
- always in equilibrium, adjusts itself according to the membrane potential
if permeability of Na, K, or Cl ions were to increase.
- Na: more out than in, so would flow in, destroy electrochemical gradient of cell
- K: more in than out, so would flow out, change electrochemical gradient
- Cl: flows in and out according to membrane potential, so would flow more, electrochemical gradient would be more stable
Graded Potential
- localized
- cations come in only whee the channel is open and can only disperse so quickly, like a crowd through a door
- see notebook for pic
- vary in size depending on stimulus strength
- but are smaller than action potentials
- decrease with distance from site of depolarization (decremental)
- no threshold - no cutoff in any functional sense
- summation: add graded potentials together for bigger potential
Action Potential
- long distance signaling
- threshold for stimulus exists
- all or nothing response
- refractory period exists
- action potential moves in one direction and stops
- moves down axon without decrement (doesn’t get weaker)
Action Potentials Created By
- Voltage gated channels
- open/close (=shape change) based on voltage (membrane potential)
- Na+ voltage gated: 3 states - closed, open, inactivated
- K+ voltage gated: 2 states - open, closed
- see notebook for Na channel pictures and for sequence of channel activation
Change in Membrane Potential of Cell
- resting potential is -70mv
- action potential depolarizes membrane - makes more positive
- overshoot - the charge is above needed threshold
- the member repolarizes - goes back to -70mv
- hyperpolarizing is bringing the potential below this
- the cell undershoots - membrane under -70 - and then goes back up to -70, resting potential
a review of the types of channels
- what happens during an excitatory graded potential?
- what happens during an action potential?
-see notebook
Refractory Period
=drives the directionality of the action potential
-channels are resting and cannot communicate during this period
-therefore the action potential can only continue down the membrane, cannot come back
=Absolute: all voltage gated Na+ channels are open (or already inactivated and must be repolarized)
=Relative: some Na+ channels are back to resting state (closed) and some K+ channels are still open
Action Potential Gunpowder analogy
- AP doesn’t move, but it sets off the next wrong
- can’t go backward
Amplitude: Action v Graded
Action: all or none, once membrane is depolarized to threshold, amplitude is independent of the initiating event
Graded: varies with size of the initiating event
Summation: Action v Graded
Action: cannot be summed
Graded: can be summed
Threshold: Action v Graded
Action: has a threshold that is usually about 15 mV depolarized relative to resting potential
Graded: has no threshold
Refractory Period: A v G
Action: yes
Graded: no
Decrement: A v G
Action: is conducted without decrement, the depolarization is elevated to a constant value at each point along the membrane
Graded: is conducted with decrement, amplitude decreases with distance
Duration: A v G
Action: duration is constant for a given cell type under constant conditions
Graded: duration varies with initiating conditions
(Hyper/De)polarization: A v G
Action: is only a depolarization
Graded: can be a depolarization or a hyperpolarization
Initiated By…: A v G
Action: initiated by a graded potential
Graded: initiated by environmental stimulus (receptor), NT (synapse) or spontaneously
Mechanism depends on…: A v G
Action: mechanism depends on voltage gated channels
Graded: mechanism depends on ligand gated channels or other chemical or physical changes
Saltatory (jumping) conduction of action potentials
- myelin prevents leakage
- local current physically extends farther
- faster signal
- fewer ions moving so metabolically cheaper
- see notebook for picture
