Lecture 2: Membrane and action potentials Flashcards
Does Na+ want to flow into the cell or outside?
Wants to flow inside the cell
Does K+ want to flow inside the cell or outside
outisde
What is the definition of Resting Membrane Potential (Vm)
Electical potential difference across the membrane
What do you call it when you there is a potential difference across the membrane?
POLARIZED
What would happen if the membrane was ONLY permeable to K+/Na+
K+/Na+ would diffuse down its concentration gradient until the electrical potential (diffusion potential) across the membrane counters the K+ diffusion
If membrane was only permeable to K+, when would diffusion stop?
When the concentration difference=electrical difference
What is another name for NERST potential?
equilibrium potential?
Define Nerst Potential (equilibirum potential)
the electrical (diffusion) potential across a membrane that exactly opposes the net diffusion of a particular ion through the membrane
What is the nerst equation
±61 log(𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑖𝑛𝑠𝑖𝑑𝑒 (𝐾𝑖)/(𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑢𝑡𝑠𝑖𝑑𝑒 (𝐾0)
How do you determine the sign for the nerst potential equation?
-The sign of the potential is positive if the ion diffusing is negative
and it is negative if the ion is positive
The electrical potential that counters net diffusion of Na+ is called…
The Na+ equilibrium potential OR
Nerst potential for Na+
What does the Goldman ( Goldman-Hodgkin-Katz ) equation assess
Equilibrium Potential of Membrane Permeable to Several Different Ions
What 3 factors does the Goldman ( Goldman-Hodgkin-Katz ) equation depend on?
1) CONCENTRATION: of respective ions on the inside (i) and outside (o) of the membrane
2) PERMEABILITY: of membrane (P) to each ion
3) polarity of the electrical charge of ions positive or negative
Why is the resting membrane potential so close to the equlibrium potential for K+ in comparison to Na+
The membrane is far more permeable to K+ than to Na+
What is the equlibrium potential for na+ and k+
na+= 61 mV K+= -94mV
What are the 2 Transport Properties of Resting Nerve Membrane
1) Leak channels much more permeable to K+ than Na+ (~ 100 times)
2) Na+/K+ pump: Net increase of 1 positive charge outside: More Negativity inside
What is the resting membrane potential for large nerve fibers? for small neurons?
- 90mV for large nerve fibers (e.g. motoneurons)
- 70 mV for small neurons of nervous system
Resting membrane potential of nerves depends on what 3 factors?
- Diffusion of potassium and sodium ions (K+ : Inside to Outside,
Na+: Outside to Inside (less degree) - Permeability of ions through leak channels
P to K+»_space; P to Na+ (~ 100 times). - Na+/K+ Pump
Creates more negativity inside (transports 3 Na+ ions to outside vs 2 K+ ions to inside).
The resting membrane potential is closer to the equilibrium potential for the ion with the BLANK permeability (i.e. K+).
HIGHEST PERMEABLITY
What is action potential?
AP is a very rapid change in the membrane potential from “-“ to “+“ values and return back to initial resting potential level
True or false and explain: AP is an all or nothing event?
TRUE: either it occurs fully or it does not occur at all (amplitude of AP never change)
Where does AP propagate
along nerve fiber until it comes to the end of the fiber
What are the 3 functions of AP
1) transmitting nerve signals
2) rapid transmission over distance
3) encoding information (neuronal language)
Explain the 3 functions of AP
1) Transmitting nerve signals
- Transfer all sensory information from the periphery to central nervous system (CNS)
- Transfer all motor information from CNS to periphery (e.g. muscles, endocrine system…etc)
- Transfer information between different parts of CNS
2) Rapid Transmission over distance
Note: speed of transmission (conduction velocity) depends on fiber size (how big the axon is) and whether it is myelinated
3) Encoding Information (neuronal language)
- The frequency of APs encodes information
What does speed of transmission of AP depend on?
depends on fiber size (how big the axon is) and whether it is myelinated
In which of these examples the AP will travel at the highest speed?
a) Twisting your ankle…ouch! It hurts!!!
b) Feeling the position of my limbs in space
c) Feeling a pressure on my skin
Feeling the position of my limbs in space
What encodes information? the frequency or the amplitude of the AP
THE FREQUENCY
What are the 5 sequence of events for AP
1) resting state
2) depolarization
3) repolarization
4) undershoot
5) back to resting
be able to identify the sequence of events for Ap on a diagram
/
Explain resting stage of nerve AP
-90 mV, membrane is balanced
Explain depolarization state
-90 to 35 mV
0.1 ms period (very rapid)
Caused by sudden opening of Na+ channels, Na+ ions flow to inside the cell
explain repolarization
+35 to -90 mV
Na+ channels begin to close, Na+ ions stop to flow inside,
K+ channels open slowly.
K+ ions start to flow outside
Re-establish negative resting potential
explain “overshoot” of AP
Overshoot occurs because some Na+ will still get in the membrane after the channels close
(goes from 0-35 because while the gate is closing a few ions will still come through)
explain afterpotential (undershoot)
Less than -90 mv
Caused by K+ channels that remain open for few milliseconds after repolarization of membrane is completed.
explain back to resting stage
closing of K+ channels
What causes the different stages of action potential
Different stages of the action potential are due to the activation of two special types of protein channels on the nerve membrane
voltage gated Na+ and K+ channels
What are the 2 protein channels on the nerve membrane that affect AP
Voltage-gated Na+ channels
Voltage-gated K+ channels
Does voltage gated channels open due to simple diffusion?
yes, sensitive to change in membrane potential
What are the 2 gates of the sodium gated channel and where are they located
activation gate: exterior
inactivation gate: interior
What are the 3 states of the gated sodium channels?
Resting state: Activation closed and inactivation open
Activated state: Both open (depolar)
Inactivated state
Activation open and inactivation closed (sodium stops entering the cell)
Explain resting state of voltage gated sodium channels
Activation gate is closed
Inactivation gate is open
No entering of Na+ ions
explain activated state (depolarization)
Sudden opening of activation gate is triggered by the shifting of membrane potential towards + values
Opening threshold: between -70 and -50 mV
Permeability to Na+ increases to 500 to 5000 times
Sodium ions flow inside (MASSIVE FLOW OF Na+)
What is the opening threshold for the sodium gated channel
between -70 to -50
True or false: when in activated state, the permiability to sodium increases?
TRUE
explain inactivated state
Inactivation gate is activated when membrane potential increases above 0 mV (overshoot).
Na+ ions stop to flow inside and the membrane potential recovers back toward resting state
Inactivation gates do not re-open until membrane returns to resting potential (Refractory Period)
Where is the gate for the gated potassium channel?
one gate in the interior of the membrane
What are the 2 states of the gated potassium channel
Resting state
Slow activation state
explain the resting state
GATE IS CLOSED
explain the slow activation state (repolarization)
- K+ channels are activated when membrane potential increases above 0 mV (overshoot)
- Gates open slowly at the same time when Na+ channels (inactivation gates) begin to close
-Potassium ions flow outside
It accelerates repolarization toward resting potential
Explain undershoot
K+ channels remain open for few milliseconds after repolarization of membrane is completed.
Excess K+ ions flow out of cell
Because of the function that mirror neurons play, Dr. Ramachandran referred to them as:
empathy neurons
explain threshold for initiation
Threshold is the level of membrane potential at which the positive-feedback cycle is created (point of no return)
When does the threshold occur for AP
Occurs when the number of sodium ions entering the nerve becomes greater than the number of potassium ions leaving the nerve (want membrane to become less negative)
~ - 65 mv for a Nerve with Vm = -90 mV
what are the 3 causes for initial increase of membrane potential
1) Electrical stimulation
2) Mechanic stimulation
3) Chemical stimulation
The amplitude of an AP is ________________ of the intensity of the stimulus that evokes it
INDEPENDANT
The frequency of firing (number of APs) is ____________ on the intensity of the stimulus.
.DEPENDANT
explain absolute refractory period and why
Period during which a new AP cannot occur as long as the membrane is still depolarized from the preceding AP
WHY:
- Na+ channels are inactivated
- Membrane has to return to near original resting potential to allow inactivation gates to reopen
explain relative refractory period
Period that follows the AbsoluteRefractory Period during which a new AP can occur in response to a strong stimulus (greater than normal
why:
- Some Na+ channels start to go back to resting state and others are still inactivated
- K+ channels are still open and the membrane is hyperpolarized (during the undershoot)
why can you not generate new AP in absolute refractory period?
Na+ channels are inactivated
-Membrane has to return to near original resting potential to allow inactivation gates to reopen
explain why you can generate AP during the relative refractory period
- Some Na+ channels start to go back to resting stae and others are still inactivated
- K+ channels are still open and the membrane is hyperpolarized (during the undershoot)
what is the axon surrounded by
myelin sheaths
what is myeline produced as an extension of what cells>
Myelin is produced as an extension of particular cells (glial cells):
Schwann cells (in peripheral nervous system)
Oligodendrocytes (central nervous system
where do you find shwann cells?
peripheral nervous system
where do you find oligodendrocytes ?
CNS
Where does propogation of AP occur
myelinated nerve fibers
Can ions flow significantly through the myelin sheath
Ions cannot flow significantly through thick myelin sheath, which thus insulates nerve fibers.
About once every 1-3 mm, myelin sheath is interrupted by …
interrupted by a node of Ranvier
what are the two types of neuron fibers
myelinated and unmyelinated
Explain saltatory conduction
APs can only occur at the nodes of Ranvier and are conducted from node to node.
Saltatory conduction conserves energy for axon (little metabolism required to re-establish ionic concentration differences)
Conduction velocity varies from little as 0.25 m/s in unmyelinated fibers to as great as 120 m/s in large myelinated fiber
where are voltage gates channels found?
nodes of ranvier
what are the difference in conduction velocities of myelinated vs unmyelinated?
.Conduction velocity varies from little as 0.25 m/s in unmyelinated fibers to as great as 120 m/s in large myelinated fiber
explain propagration of AP on unmyelinated fibers
Action potential excites adjacent portions of membrane, resulting in propagation of the action potential.
Na+ ions flow to adjacent area:
Increase voltage of adjacent area to threshold level.
Initiate adjacent action potential
0.25 ms
be able to identify propagation visuals
..
what is MS and some symptoms of it
MS is an immune-mediated inflammatory disease that causes the demyelination of the central nervous system.
Muscle weakness
Loss of sensation
Death
what does MS cause?
conduction blockage
