11.23 C Flashcards
What is the sodium channel activation curve?
a curve showing the relationship between membrane potential and open probability of sodium activation gates
At what membrane potential do sodium activation gates begin opening?
roughly -50 mV
Describe the kinetics of the sodium activation and inactivation gates.
the activation gates open rapidly and the inactivation gate closes slowly
Define threshold.
the point at which the depolarizing current slightly exceeds the repolarizing current
Describe the kinetics of the potassium delayed rectifier channel.
it opens and closes slowly, beginning at roughly -50mV
What is an absolute refractory period?
a period of time during which a cell cannot generate an AP regardless of the strength of the depolarizing stimulus
What is a relative refractory period?
a time during which inactive channels are being converted to resting channels and the cell can generate an AP but only if a stronger depolarizing stimulus is given than would normally be necessary
The number of resting sodium channels is dependent on what two factors?
- the amount of time given between depolarizing stimuli
- the extent of repolarization such that the sodium inactivation gate has a high probability of returning to the open state
Why would hypokalemia decrease the number of resting sodium channels and thus diminish the ability of the cell to generate a sodium current?
because it would diminish the flow of potassium ions and thus the repolarizing current, which sodium channels rely on in order to reopen their inactivation gate
The “All or None” Law of action potentials is dependent on what two factors?
- sodium and potassium concentrations remain the same
- there has been sufficient time during repolarization to recover resting sodium channels
What does the “All or None” Law of action potentials state?
that if a depolarizing stimulus reaches threshold, the resulting AP will have the same characteristics
The magnitude of the depolarizing current during the upstroke of the action potential will determine what four things?
- threshold potential
- amplitude of the AP
- rate of rise of the AP
- conduction velocity
What is conduction velocity?
how quickly an AP propagates down the tissue
Threshold potential, amplitude of the AP, rate of rise of the AP, and conduction velocity are all dependent on what other aspect of the AP?
the magnitude of the depolarizing current during the upstroke
Conduction velocity is dependent on what factor of an AP?
the magnitude of the depolarizing current during the upstroke of the AP
Why does magnitude of the depolarizing current during the upstroke of an AP influence the conduction velocity?
because the greater the magnitude, the faster cations accumulate inside the cell and spread to adjacent regions
What conditions will reduce sodium current?
- increased intracellular Na will decrease E(na)
- drug blockage of sodium channels
- reduced number of resting channels due to hyper or hypokalemia
- depolarizing shift of activation curve due to hypercalcemia
What is the most common life threatening disorder associated with cancer?
hypercalcemia
How does cancer lead to hypercalcemia?
malignant cells release factors that cause calcium to be released from the bone
What are some symptoms of hypercalcemia?
- muscle weakness is the hallmark
- tiredness, confusion
- unstable gait
- increased urination and bone pain
An individual presents with muscle weakness, tiredness, unstable gait, and increased urination. What is a likely diagnosis?
hypercalcemia
What does hypercalcemia due to the excitability of neurons and muscle cells?
it reduces the excitability
How does hypercalcemia reduce the excitability of neurons and muscle cells?
by shifting the sodium activation curve in a positive direction, thus making them less likely to open and for the cell to be less likely to reach threshold
What effect does raising the resting potential of a cell have on excitability?
it will lower the excitability
Why would raising the resting potential of a cell lower the excitability?
because more sodium inactivation gates are closed and there is a smaller sodium current
