Download What is a membrane potential?

What is a membrane potential?
How do we know a cell is “alive”?
10/17
and 10/19
How do we calculate Vm when many different ions and
permeabilities exist? GHK-Equation
How do environmental changes affect cellular Vm?
Why are patch clamps useful for studying Vm?
What are the properties of voltage-gated channels?
What is “self-propagation” and why is this property important
with regards to a cellular membrane potential?
What is saltatory conduction and why is it so fast?
How do gap junctions create an electric syncitium in the heart?
How do action potentials use ligand-gated channels to cross
synaptic clefts between cells and create new APs in adjacent
cells?
Good practice problems: 13-1, 2, 4, 5, 6, 7 and 8
It is important to remember that membranes are “selectively”
permeable and that this property generates “gradients” which represent
potential energy and even voltage (ions).
2
Remember that there are considerable differences within organs and across species,
these are “ballpark” figures. There will be a TQ asking for typical intra and
extracellular ion concentrations for a mammalian cell.
3
Because membranes are permeable to many different ions at
different relative permeabilities, we need a more complicated
equation to determine the true membrane potential!
Goldman-Hodgkin-Katz Eq. Predicts Membrane Potential (voltage)!
Equation lets us predict how different ions and permeabilities
interact for a predictable Memb.Pot!
Applications: Toxicity, Environmental Quality, Nutrition,
Human/Vet.Med…uses are innumerable!
• Vm=2.303RT/F log Pk(K)o+Pna(Na)o+Pcl(Cl)i
Pk(K)i+Pna(Na)i+Pcl(Cl)o
F=FaradayConstant= 23,062 cal/Volt-mole
RT as before:
R= GasConstant=1.987 cal/mole-degrees Temp=298K (20oC)
P=permeability relative to most permeable ion (K+)
100%=1.0 10%=0.1….not used as an absolute value
P values: K=100% Pk=1.0; Cl=45% Pcl=.45; Na=4% Pna=.04
• Ca++: negligible perm. at cellular rest, so folks generally
ignore its contribution!
Example: How to calculate Vm given ion concentrations
What is the Vm for a normal neuron in your brain?
• Vm=2.303RT/F log Pk(K)o+Pna(Na)o+Pcl(Cl)i
•
Pk(K)i+Pna(Na)i+Pcl(Cl)o
i=intracellular (inside)
o=extracellular (outside)
• Vm=2.303RT/F log 1(.005)o+0.04(0.14)o+0.45(.01)i
•
1(.14)i+0.04(0.01)i+0.45(.125)o
• Vm=_____volts or _____millivolts
• Vm= Try the math for the test! What happens to Vm if the
Na+ permeability changed to from 0.04 to 1??
• Why does [K+]normally matter most for Vm at rest?
Why does [Na+] determine Vm during action potential when a
temporarily OPENING of Na-channels occurs?
• Why is the permeability for chloride ions ONE (1)?
What happens to Vm values of a cardiac muscle cell if
you eat a banana (rich in KCl) or some salty chips
(NaCl)? Which causes a heart attack Na+ or K+?
•
•
•
•
•
•
•
•
•
Banana: assume plasma K+ goes to 15 mM/Cl goes to 135mM
Vm=2.303RT/F log 1.0( )o+0.04(
)o+0.45( )i
1.0( )i+0.04(
)i+0.45( )o
Salty Chips: assume plasma Na+ goes to 160mM/Cl=145 mM
Vm=2.303RT/F log 1.0( )o+0.04( )o+0.45( )i
1.0( )i+0.04( )i+0.45(
)o
What happens to salt if a toxin increases Pna to 0.40?
Vm=2.303RT/F log 1.0( )o+0.40( )o+0.45(
)i
1.0( )i+0.40( )i+0.45(
)o
• What are some environmental conditions that could cause these
effects?
• Why does plasma calcium have little effect on Vm for a healthy cell?
• What happens to Vm when a toxin changes the permeability?
How are Transmembrane Channel types characterized?
• Some Physical Properties to consider:
– Membrane potential: mV difference across membrane
– Channel Current: Amperes..means ions are moving!
• Channel type?
• Ion type?
• Ions/second passing through?
– Conductance: movement of ions when a specific voltage is
applied across the membrane
• 4 Membrane Ion Channels: “4 ways to open”
– 1) Ligand Gated Channel:
– 2) Mechanosensitive Channel:
– 3) Signal Gated Channel:
• Any of these three can initiate an action potential!
– 4) Voltage Gated channel (VGC): V.I.P.!!
• This is the channel that “Propogates” an action potential!
These Channels can be in OPEN or CLOSED state
Question: is channel open or closed and for how long?
There are four different types of channel
(mechanosenitive not shown below).
Remember that pores and porins are always open!
What are some properties specific to the VGC?
Function of VGC: Convert a change in Vm into a change
in ion permeability! Create Action Potential!
Three Main Types of VGC: Na, K, and Ca
VGCs have an amino acid segment that changes
conformation if the membrane potential changes!
VGCs have excellent ion selectivity:
– Related to size of charge and atomic mass!
– Related to size of sphere of hydration? MOST IMPORTANT
VGCs have channel gate(s):
VGCs can be opened(activated)/closed (deactivated)
VGCs modulate ACUTE CHANGES in permeability!
What do K+-VGC look like? Why is the sphere of
hydration important? Which is bigger with respect to
its atomic mass and sphere of hydration? Na+ or K+
The sodium VGC actually has two gates! Some books simplify it a bit by
calling it one gate. Often times environmental toxins work by disabling
these VGC! Some classic Na-VGC neurotoxins from pufferfish(tetrodotoxin
or TTX) and red tide(dinoflagellates) work at this channel (fatal).
What would these toxins do to action potential formation and “breathing”?
Patch clamps are fantastic tools for isolating channels and evaluating each
type of membrane channel for its contribution to total membrane ion
permeability. If the scientist is “lucky” they hope to get a patch with just one
channel protein (voltage and amps current).
Patch Clamps measure “flickers” created by the opening and closing of an individual
channel or channels! We often times determine the nature of a toxin by examining
effects on membrane potentials with patch clamps.
In the example below, the patch contains TWO channels, each opening at different
times, and occasionally at the same time (doubling the current)!
VGC have a unique amino acid sequence (letters): some parts are variable
between different VGCs (not shaded below), and some (in pink below) give each
type of VGC a specificity for a single ion: Na+, K+ or Ca++
Why does similarity/dissimilarity explain why some drugs affect only certain ion
permeabilities, people or different species of animal?
What does an action potential look like as it starts self propagating?
What happens to Na, K, Na-VGC, K-VGC and Vm over time?
Can you draw/describe this for a test?
• Lets sketch this on the membrane below:
•
•
•
•
•
Extracellular
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------Intracellular
Side
•
The key is that when the VGC open at “A”, it changes the ions overlying the VGC at
“B” causing them to open too, this causes the VGC at “C” to open and the AP can
self-propagate on down the membrane!
Dendrites carry the AP towards the cell body and axons carry the
AP away from cell body to the next cell! If the stimulus occurs
artificially in the middle of an axon, the AP can travel both ways!
Once the membrane is depolarized how do we get its depolarized
membrane potential back to its original hyperpolarized state?
Solution:1) We limit the amount of time the VGC stay open!
2) Hyperpolarize Na-VGC before they can open again!
What are the terms used to describe the events/changes in membrane
potential (mV) and time (msec) on the last diagram?
• #1Resting Membrane Potential
• “Sub-threshold Depolarization”
•
•
•
•
#2“Depolarization Phase”
“Overshoot”
#3“Repolarization Phase”
#4“Undershoot/Hyperpolarization Phase”
• Refractory Period: V.I.P.
• “Absolute” Refractory Period- can channels open?
• “Relative” Refractory Period- will high P-K+ let membrane
depolarize? Would an electric shock open channels?
The changes in Vm occur because the many many gates open or close.
In the end the Na/K-ATPase cleans up the rest to get back to the original 140
mM/10mM concentrations! Conductance describes permeability
Once a local depolarization that exceeds threshold is created the AP
spreads in either direction from the origin.
Normally this is from the cell body towards the synapse!
What happens to the AP-velocity if we wrap layers of electrical insulation
(sphingomyelin) in a sheath between pockets of VGC?
Extremely Rapid Saltatory Conduction Results!
• Unmyelinated Velocity is about 0.5 meters/sec
• Myelination increases velocity to up to 70-100 meters/sec
Blue Whale: How long will it take to go from brain to tail?
30 Meters: With Myelination:__
Without Myelinationation:__
How tall are you head to tail?
Why does sphingomyelin help?
What is a Node of Ranvier?
Why no VGC between nodes?
Why does Saltatory Conduction use less Glucose/ATP?
The myelin sheath is created by cells to GREATLY increases Action Potential velocity!
Oligodendrites in the central nervous system
Schwann Cells in the peripheral nervous system
Gaps between cells are called Nodes of Ranvier
Synaptic Gap Junctions (if present) permit the incoming Na+ from the
action potential in one cell to diffuse INTO the next cell creating a
threshold potential opening its Na-VGC: new APcreated!
This creates an Electrical Syncitium! (T.Q.)
This synaptic device isn’t too common!
Poor regulation!
Importance to heart?
Why can the heart fibrillate?
Acetylcholine is a ubiquitous ligand that opens a sodium ligand gated channels. How
does ACH do this? When is ACH released at a synapse? Why is acetylcholinesterase
important in regards to limiting the duration of the Increase in Permeability? Why do
some toxins work by modifying the activity of ACHase?
25
The action potential can also cross between cells by causing chemicals to
be released at the pre-synaptic cell that find/open Ligand Gated
Channels on the post synaptic cell and change the Na-permeability of
the second cell to create a new AP on (or change in) the second cell!
Costs of membrane potential with respect to ATP use and
production. What organelles “could” facilitate this and why
are neurons so sensitive to hypoxia?
Neurons maintain a “FINE” balance between ATP Supply and
Use, why is this so?
• Perfusion of the brain
• Na+/K+-ATPase function at rest and when cell is “active”
• Long-term potentiation in neuronsmemory
• Neuron packing and space for mitochondria
• Glycolysis and the cytosol (lactate?) ATP yield
• Mitochondria ATP yield
• Phosphocreatine  ATP backup
• If blood supply stops: Why do you pass out in 20 second and
why do brain cells begin to undergo infarct in 5 minutes?