Section 1 Flashcards
Describe the different configurations of neuronal, skeletal, and cardiac action potentials.
- Motor neuron action potential: 2 ms in length, RMP = -70 mV, slight period of hyperpolarization 2. Skeletal muscle action potential: 5 ms in length, RMP = -90 mV 3. Cardiac ventricle action potential: 200 ms in length, RMP = -90 mV
Why are there different action potential waveforms for different types of tissue?
To accommodate the function of the tissue
Describe how various ion conductances affect the nerve action potential.
- Increase in Na+ channel conductance causes the upstroke of the action potential (depolarization) 2. Delayed increase in K+ channel conductance and decreased Na+ channel conductance cause repolarization of the AP 3. K+ channel conductance is turned off by repolarization of the membrane potential.
What are the 2 Na+ channel gates?
- m activation gate 2. h inactivation gate
Describe the activity of Na+ channels during an action potential.
- Resting: m gate closed, h gate open - Na+ cannot enter 2. Activated: m gate opens, h gate remains open - Na+ influx, depolarization 3. Inactivated: m gate remains open, h gate closes - Na+ cannot enter 4. Recovery from inactivation: reset to m gate closed, h gate open
How does recovery from inactivation occur?
Repolarization and time
Describe the voltage-dependence of Na+ channels.
At a resting membrane potential (-90 mV), the Na+ channels (h gates) are very available. As depolarization occurs, they become less available
Na+ channels activate rapidly in response to ___. They are dependent on what two factors?
Depolarization; time and voltage
Describe the regenerative depolarization of Na+ channels.
Na+ moves rapidly into the cell down its electrical and concentration gradients to depolarize the membrane potential. Depolarization increases Na+ permeability, opening more Na+ channels, which causes further depolarization (positive feedback).
On what is inactivation of Na+ channels dependent?
Time and voltage (voltage dependence is the basis for refractory periods)
What is the absolute refractory period?
The time during which a stimulus cannot elicit a regenerative responses (AP); many Na+ channels are voltage-inactivated
What is the relative refractory period?
The time during which a stimulus can elicit a regenerative response (AP); requires a much higher stimulus
Describe how Na+ channels create the absolute and relative refractory periods.
At more positive voltages, Na+ channels inactivate (absolute refractory period). As the membrane potential repolarizes, Na+ channels recover from inactivation (relative refractory period).
How are K+ and Na+ channels different?
K+ channels do not have inactivation gates and thus remain open with maintained depolarization of the membrane
What activates the opening of K+ channels and where does it flow?
Depolarization/AP; K+ flows out of the cell down its concentration gradient
Outward K+ current causes ___; the membrane potential becomes more ___ than the resting membrane potential - this is known as hyperpolarization.
Repolarization; negative
True or false - voltage-dependent activation of K+ channels is much faster than activation of Na+ channels.
False - it is much slower
True or false - K+ channels deactivate when the membrane repolarizes.
True - there is no inactivation parameter
What is hyperkalemia?
Abnormally elevated extracellular K+ (normal is 4.5 mM)
Describe the effects of hyperkalemia on the membrane potential.
Hyperkalemia causes the RMP to become more positive as K+ flows into the cell. At more positive voltages, Na+ channels become less available (inactivate). Inward Na+ current decreases and conduction slows.
What are two signs of hyperkalemia?
Slow mentation and muscle weakness
What are some possible causes of hyperkalemia?
Patients on dialysis, kidney failure, hypertension drugs (ACE inhibitors), lethal injection, arrhythmia, ventricular fibrillation
How does Ca2+ modulate Na+ channel activity?
Ca2+ alters membrane surface charge and has the same effect as K+ without changing the membrane potential.
What is hypercalcemia and what effect does it have?
Abnormally elevated extracellular Ca2+; raises threshold for Na+ channel activation, decreases membrane excitability
