Block 1 - Membrane Potentials (L3-5) Flashcards
Cells are always low in —– and high in —–
low in Na+, high in K+
There are way more —- channels than any other channel
K+
Why does K+ create a membrane potential?
K+ is charged, so diffusion across the membrane establishes an electrical voltage and a membrane potential
What forces affect the movement of K+ ions?
chemical force of the concentration gradient pushes K+ outward (since cellular concentration is high, ions want to move toward lower concentration)
electrical force of the membrane potential pulls K+ inward (Na+/K+ pumps create a negative charge inside the cell, pulls K+ in)
In most cells, the —– force is balanced, which results in an overall ——-
the electrical force is balanced, resulting in an inside-negative membrane potential as K+ is pulled outward by the concentration gradient
What is a typical cell membrane potential/resting potential?
-50 to -100 mV
How do cells change their membrane potential?
they open or close ion channels
How is a cell generally polarized or depolarized?
if K+ channels are open, membrane voltage is negative and the cell is polarized
if Na+ channels are open, the membrane voltage is positive and the cell is depolarized
What are Wchem and Welec?
Wchem is the energy associated with moving N moles of an ion from concentration X1 to concentration X2
Welec is the energy associated with moving N moles of an ion in an electrical field of V volts
What does the Nerst equation represent?
the equilibrium potential of an ion, or the membrane potential when an ion is at equilibrium
(shows that the voltage is a function of concentrations on one side vs. the other side)
What does the Goldman equation reflect?
the real situation, where multiple ions contribute to the membrane potential and ion permeabilities determine the relative influence of each of those ions
What are the two important parameters of the Goldman equation?
ion permeabilities and ion gradients
If a membrane was only permeable to K+, the Goldman equation would also be —–
the Nernst equation
What ion always has the highest permeability?
K+
Why is Vm normally negative even though V Na+ is so positive?
becuase both Vcl and Vk are highly negative, and have greater influence on the Vm since there are very few Na+ channels and so many K+ channels
What are the two critical factors of regulating ion permeability?
- establishing and maintaining Na+ and K+ gradients
- varying the activity of specific ion channels
Describe the Cl- concentration in cells
normally close to equilibrium, resting potential tends to push Cl- out of the cell but it is balanced by the concentration gradient pushing Cl- into the cell
How are voltage gated channels opened?
depolarization (when the membrane gets less negative inside)
How are chemical/ligand gated channels opened?
when a signal molecule binds to the channel protein
How are mechanically gated channels opened?
when the membrane gets stretched
How do the strength of a stimulus and the size of the voltage response correlate?
stronger stimulus leads to a larger voltage response
When a second stimulus closely follows the first stimulus, what happens?
the magnitude of voltage response may be greater (the responses are additive)
Where do graded potentials occur?
dendrites and cell body of neurons
What are the important characteristics of graded potentials?
- size varies with strength of stimulus
- multiple stimuli are additive
- opposing stimuli cancel each other out
What happens when a membrane is depolarized to a threshold?
voltage gated Na+ channels open, Na+ enters the cell, more depolarization, more Na+ channels open, etc.
(positive feedback cycle)
right after Na+ channels open they snap closed, but the depolarization also slowly opens K+ channels
with these effects combined, the membrane repolarizes to the resting value
What are the important characteristics of action potentials?
- all or none
- a second potential can not start until the refractory period is over
- not additive
What are the absolute and relative refractory periods?
absolute: first refractory period, cannot be restimulated because sodium channels need to be reset
relative: second refractory period, takes a larger stimulation than normal because potassium channels need to be reset
What is the significance of refractory periods?
- influence the max rate of action potential firing
- influence the characteristic of action potential propagation
Do graded potentials spread?
Yes, but the magnitude diminishes with distance
Because they propagate, action potentials can…
communicate signals over long distances
What begins the propagation of an action potential?
Describe how it spreads.
entry of Na+ produces a local current that spreads and depolarizes nearby areas
if they depolarize to threshold, another action potential is triggered
as the voltage spreads it becomes weaker, and when it gets too weak it does not cause more channels to open (so those channels are not recruited by the depolarization)
but, the channels that were recruited will produce another signal that then further propagates and recruits more channels
What 2 factors influence velocity of the action potential propagation and how?
- size (diameter) of the axon = larger have a higher velocity because they can send signals faster, electrical resistance decreases as axon diameter increases
- myelination speeds the conduction velocity
How is an axon myelinated? Describe the structures.
- Schwann cell (in PNS) and Oligodendrocytes (in CNS) wrap around the axon
- the neurolemma of the cell develops into the myelin sheath
- the ends of the axon that poke out of the cell or in between multiple cells are called Nodes of Ranvier
Describe continuous vs saltatory conduction?
Continuous:
1. Na channels open
2. electronic conduction flows down and opens more Na channels the entire way
3. signal moves more slowly
Saltatory:
1. Na channels at a node open
2. electronic conduction through the myelinated part of the axon
3. Na channels at the next node open
4. etc, electronic signal jumps from node to node and is faster
What are some reasons that demyelination can occur? What do they do exactly?
multiple sclerosis (central) or Guillan-Barre (peripheral)
disrupt the myelin sheath
since there are only Na channels at the nodal regions but the myelin is now gone, the conduction gets blocked at the nodes and cant make it to depolarize the next node (so nerve conduction fails)
