The Action Potential - I Flashcards
Three types of cells with APs?
Neurons, Muscle, and Neuroendocrine cells
How was resting potential determined?
Cell kept in an electrolyte solution. Using glass micropipettes as electrodes, measure on either side of the cell membrane. When one is inserted, steady -60mV is read.
What is resting potential?
-60mV
What will adding a negative current do to the membrane potential?
Hyperpolarization – Electrotonic Hyperpolarization, proportionally
What will adding a positive current do to the membrane potential?
Inside of cell become positive (depolarization).
Small enough, its proportional
If sufficient, you hit threshold and trigger AP
Amplitude of the voltage changes with stimulus is proportional to…. (2 things)
Applied Current
Input Resistance
T of F. An AP is graded in size depending on the size of the stimulus.
F. An AP is an all-or-none phenomenon
List the 5 characteristics of an Action Potential.
- Triggered By Depolarization
- All or None
- Regenerative and Propagates
- Have Overshoot and Undershoot
- Has Absolute Refractory Period
While the amplitude of action potentials is constant, the _________ of APs can vary from stim to stim.
Frequency.
What do I mean when I say a potential is regenerative and propagates.
At the end of the line its the same signal it was at the beginning
What is the Overshoot of an AP?
At the peak of AP whn the mem pot reverses sign and becomes positive
What is the undershoot of an AP?
As the potential comes back down it becomes more negative than normal.
What is the absolute refractory period?
A period in which it is impossible to make the neuron fire another AP.
What did Hodgkin and Huxley do in their studies?
Use the voltage clamp to dissect and characterize the permeability changes associated with AP
What does a voltage clamp let you do?
Control the membrane potential at a desired level while measuring current necessary to maintain potential.
How did Hodgkin and Huxley’s experiments work?
Place an electrode inside the cell to measure mem pot. and compare to command voltage. Amount of current necessary to maintain mem pot at the command volt is measured.
How might Hodgkin and Huxley’s experiments be modernized?
Cell patch clamp recording
(Electrode sealed to cell membrane
Vm and I for Vc is recorded/injected
What is a capacitive current?
A brief redistribution of charge across the membrane.
When Vm is hyperpolarized, what is seen?
A brief capacitive current, then very little current flows
When Vm is depolarized, what is seen?
What does this establish?
A brief capacitive current, then rising inward ionic current (+ entering cell), then a delayed outward current.
Establishes that membrane permeability is voltage dependent
Describe the shape of the early current.
First increases, then decreases as depolarization increases. Current reverses back at higher membrane potentials.
Why is there no longer any inward current in the cell when Vm is clamped at 55mV? What does this mean?
According to Nernst, 55 is Na’s equilibrium potential. At 55 there will no longer be a net flux of Na across the membrane.
Strongly suggests that inward current is carried by Na.
What did the ionic substitution experiment entail? What did it show?
90% reduction in external sodium eliminates the early inward current. It enforces the idea that Na carries the early inward current.
How have pharmacological studies reinforced the belief in the roles of K and Na
Using toxins that specifically block Na show a loss of inward current. Toxins that blow K show a loss of outward current.
What nerotoxin blocks Na+ currents?
Tetrodotoxin
What neurotoxin blocks K+ currents?
Tetraethylammonium
Describe the cellular mechanisms associated with seizure disorders.
Leaky Na channels causing fluctuating voltage cause strings of unwanted AP
How do you treat seizures?
Dilantin
What is hyperkalemic Periodic Paralysis?
Heightened K causes Na channels to shut down.
Treat with Glucose and Ins to drive K into the cells
What is hypokalemic periodic paralysis?
Can occur with rest after strenuous activity. Decreased K means No AP possible.
