Lecture 1 - Neuronal Action Potential Flashcards
Which vertebrate cell has the longest duration?
- Motor
- Skeletal
- Cardiac Ventricle
Cardiac ventricle
- duration is fast, about 200 milliseconds
- heart has long AP to protect it from additional AP’
- thus cannot be activated as quickly
What is the term for the response that does NOT generate an AP because the voltage is not large enough?
Sub threshold response
- decreases in amplitude with distance
- change in membrane potential not sufficient for action potential
What is the length constant?
Distance over which a sub threshold depolarization (local response) will spread & influence the next membrane
- larger length constant = MORE RAPID CONDUCTION
(fires an AP)
When you decrease Rm, what happens to the space constant? What about decreasing Ri?
- decrease Rm = decrease length constant
2. decrease Ri = INCREASE length constant
What is Rm and Ri?
Rm = membrane resistance Ri = internal resistance (or Ra)
With a large axonal diameter, is the length constant large or small?
length constant is LARGE since Ri is LOW(internal resistance)
-
As distance increases from the current source, what happens to the sub threshold voltage?What manner does this decay (graph wise)
DECREASES (decays) with increasing distance from source
- decays EXPONENTIALLY
Larger diameter axons have lower or higher membrane and internal resistance? (rm and Ri)
Large diamter = LOWER resistance than smaller axon
- but since Ri is squared, it decreases more than rm
Large diameter results in _____ conduction
Faster conduction velocity
What determines how easily an axon can conduct an AP?
Space constant
What determines the space constant?
membrane Resistance
internal Resistance
What occurs once an area of membrane is activated by an action potential?
REVERSAL OF MEMMBRANE POLARITY
inside becomes more positive than outside
- originally NEG inside
Local current flow occurs between activated (depolarized) and inactive regions, activating ______ channels to depolarize the next segment of membrane and initiating an AP/
NA channels!!!
A stimulus is a ____ or ____ response.
All or none
Upstroke of the action potential graph is caused by what? Repolarization is caused by what?
- Na activation (opening of M channels)
2. delayed increase in K+ conductance and inactivation of Na channels (closing of h gates)
Where is the only place AP’s occur?
Nodes of Ranvier
An action potential occurs between the E of what two ions?
Ena and Ek
Repolarization turns off what channel conductance completely?
K+ (deactivated)
Describe the gating properties of Na channel at the following stages:
- Resting
- Activated
- Inactivated
- resting: M closed, h open
- Activated: both open, NA INFLUX (h slowly closing)
- Inactivated: M open, h closed
What is the time required to reset the channel from inactivated state back to resting state?
Recovery from Inactivation
Recovery from inactivation requires what?
Repolarization
Gating kinetics for Na and K both depend on what?
Time and Voltage
What gates due NOT have inactivation gates?
K+
What channels respond RAPIDLY to depolarization?
Na+ channels
What occurs during regenerative depolarization?
- Na+ moves into the cell down its electrical & conc gradient
- DEPOLARIZE membrane to Ena
- increase Na permeability
- causing FURTHER DEPOLARIZATION
What channels close rapidly at depolarized (positive) voltages?
Na+
- inactivation is time and voltage dependent
What is the basis for refractory periods?
Voltage-dependent inactivation of Na+ channels
Recovery from inactivation requires what?(to reach resting state)
- time
2. Negative voltages (repolarization)
What channels remain open with maintained depolarization of the membrane? When do these channels completely deactivate?
K+!!!
- no inactivation
- completely deactivate with DEPOLARIZATION
What flows into the cell, what flows out?
NA in, K+ out (down its conc gradient)
Na moves down its electric and chemical gradient
Depolarization of the membrane potential toward Ek causes the potential to go more negative than RMP, called:
HYPERPOLARIZATION
What is the time during which a stimulus CANNOT elicit a regenerative response? When does this occur?
ABSOLUTE refractory period
- occur during Na+ inactivation during depolarization and increase in membrane potential to more positive values
What is the time during which a stimulus CAN elicit a regenerative response/AP? When does this occur?
- Relative Refractory Period
- when membrane potential RE-polarizes, Na+ channels recover from inactivation and reach RESTING state
HYPERPOLARIZATION
As the voltage becomes more positive, the number of Na channels/conductance increases or decreases?
Decreases!
- at -60mV 50% of sodium channels open
BUT
at -90 –> 100% are open
Conduction can be slowed causing muscle weakness. This would be due to loss of what channels?
Sodium channels
Refractory periods in nerve (and heart) are solely based on voltage-depends characteristic of what channels?Which specific gate?
- SODIUM!
- H gate!!!
- inactivated at positive voltages
re-polarization = Na+ channels recover
What cation modulates Sodium channel activity?
CALCIUM!
If membrane bound calcium increases, what happens to sodium channels?
BECOME INACTIVATED
- believe that area has become more positive
- H gates CLOSE!
What occurs to the following during HYPERCALCEMIA:
- calcium concentration
- Na+ threshold
- Membrane excitability
- increases
- increases
- decreases membrane excitability
How can hypercalcemia be tested clinically?
REFLEX TEST
- slower/muted reflexes are a sign
How can hypocalcemia be tested clinically?
REFLEXES
- hyper-reflexivity is a result of increased membrane excitability due to low calcium conc.
What occurs to the following during HYPOCALCEMIA:
- calcium concentration
- Na+ threshold
- Membrane excitability
- decreases
- decreases
- INCREASES membrane excitability
What occurs to the following during Hyperventilation:
- CO2 concentration
- respiratory state
- Membrane excitability
What is a sign?
- decreased CO2
- Respiratory Alkalosis
- Increased membrane excitability
(irritable)
What occurs to the following during Hypoventilation:
- CO2 concentration
- respiratory state
- Membrane excitability
What is often the cause?
- more CO2
- Respiratory acidosis (co2 binds to water to make acid)
- decreased membrane excitability
ex: barbituates, foreign object in trachea
What occurs during Hyperkalemia to the following:
- RMP
- Na+ channels
- inward Na+ current & conduction
- SIGNS & SYMPTOMS
- more positive resting membrane potential
(increase K+)
- Na+ channels inactivated
- inward Na+ DECREASED = slower conduction
- Slow mentation, muscle weakness
What is the normal serum potassium level?
3-5mmol
- higher = hyperkalemia
Potassium levels often change with what diseases?
Renal Failure
- Hyperkalemia occurs
What allows us to have significantly greater conduction velocity?
Myelination of axons
What increases conduction more, myelination or axon diameter?
MYELINATION
- unmyelinatd axons 100 times greater in diameter
What is the only place not covered by Schwann cells?
Node of Ranvier
Myeline sheath increases what? This increases or decreases the length constant and conduction velocity?
Rm
- increases length constant
- increases conduction velocity
What is internal distance between 2 nodes of Ranvier?
1-2 mm
Na+ channels are concentrated where? What is found here? This type of conduction is called:
- Nodes of Ranvier
- Action POtentials
- SALTATORY CONDUCTION
What diseases destroy myelination and slow neuronal conduction?
Diabetic neuropathies
Multiple Sclerosis
