Lecture 2 - The Action Potential Flashcards
What are three pieces of evidence that let us know that sensory stimulus evokes electrical impulses?
- Local anesthesia
- Stroke (when neurons for a certain part of the body are damaged, that part of the body stops functionning)
- Electrical stimulation (the firing of action potentials lets us know that something is touching us)
What was Johannes Muller’s theory and why was it wrong?
He said that we would never be able to measure the speed of an action potential because it is much too fast like in the case of the speed of light. This was wrong since the Action Potential conduction is actually slower than the speed of light
What is the rule concerning distance of stimulus, and the time needed to perceive that stimulus?
- If the stimulus is close to the brain, perception occurs with little delay
- If the stimulus is far from the brain, perception occurs with longer delay
What did Hermann von Helmholtz do?
- He calculated the motor nerve conduction speed in frogs by shocking the frogs at different distances from the muscle, and seeing how long it took the frog to react/have a muscle contraction
- He also measure the sensory nerve conduction speed in humans by shocking their skin and different distances from the brain and seeing how long it took them to react (clench their teeth, etc.)
What are the three different “times” involved in perceiving stimulus?
- Afferent: Time for signal to move towards the NS
- Efferent: Time for signal to move away from NS
- Intra-cortical: Time for instruction to be given the body and cause a response
What solution can be made in order to calculate the conduction speed of a sensory AP?
Disregard efferent and intra-cortical times because they are the same for both the ankle and the shoulder.
The equation is usually:
Conduction speed = (Distance traveled from skin to brain) / (Time)
But we can have it be:
Speed = (Distance from ankle to brain - Distance from shoulder to brain) / (Ankle time - Shoulder time)
What is the Goldman-Hodgkin-Katz (GHK Equation) and how does it differ from the Nernst equation?
The Nernst equation gives use the equilibrium potential of ions, as it is hypothetical and considers the membrane to be permeable to just one ion
The GHK equation is much more realistic, and considers that the membrane is permeable to multiple ions
The equation is:
What is the proportion of PK to PNa during an Action Potential?
Before threshold: PK = 24PNa
During the rising phase: PK=10PNa
End of rising phase: PNa=4PK
What are the three steps leading up to the opening of voltage-gated sodium channels?
- Sensory receptor activity
- Depolarization
- Voltage-gated sodium channels open
What are the three possible phases that a voltage-gated sodium channel can have?
- Closed (inactivation gate is opened, activation gate is closed)
- Opened (inactivation gate is opened, activation gate is opened)
- Inactivated (inactivation gate is close, activation gate is opened)
What does the Rising Phase loop look like?
Fast positive feedback loop
DEPOLARIZE MEMBRANE POTENTIAL –> Voltage gated sodium channels open –> Na+ rushes into the cell (depolarizes) –> …
What does the End of the Rising Phase look like?
Fast positive feed back loop ends
DEPOLARIZE MEMBRANE POTENTIAL –> Voltage gated sodium channels inactivate –> Na+ no longer rushes into the cell (depolarizes) –>…
What are the three steps leading up to the opening of voltage-gated potassium channels?
- Na+ entry into the cell (rising phase)
- Depolarization
- Creaky door i.e. the voltage gated potassium channel opens letting K+ out
What do the End of the Action Potential/Falling Phase loops look like?
There is the End of Rising Phase loop, that connects to the Falling Phase loop through the DEPOLARIZES MEMBRANE POTENTIAL:
DEPOLARIZES MEMBRANE POTENTIAL –> Voltage-gated potassium channels open –> K+ slowly leaves the cell (hyperpolarizes) –>…
Why, even at its peak, is the membrane potential of an AP not the equilibrium potential of sodium?
This is because, as the GHK equation explains to us, the neuron is never exclusively permeable to sodium and the potassium leak channels always have potassium leaving the cell, pulling the membrane potential away from ENa
What does a refractory period create?
It creates a “one way street” for action potentials, making them have a unidirectional conduction
What is an Absolute Refractory Period?
- Sodium channels are completely inactivated
- No new AP can be triggered at all
What happens if the VgNaC are blocked?
An action potential cannot fire
What is a Relative Refractory Period?
- Some of the sodium channels have returned to their resting state, but not all of them, still leaving a possibility for another action potential to get fired with a strong enough stimulus
- The constant potassium efflux that causes an AHP/undershoot, makes a strong stimulus necessary
What happens if the VgKC are blocked?
An action potential’s falling phase back to resting potential will be much slower, and there will be no undershoot
What is the benefit of a refractory period?
Allows a unidirectional action potential conduction away from point of origin
What is the consequence of a refractory period?
It limits the amount of Action Potentials that can be fired (upper limit on AP firing rate)
What part of the refractory period causes the unidirectional conduction?
Refractory Wake
What happens if we inject sodium ions in the middle of a cell?
- Depolarizations spreads on either side of the injection site
- Two action potentials are fired moving in opposite ways,
- Given that their RPs are behind them they can only move forward and not backwards towards the injection site
What happens when two action potentials moving in opposite direction meet?
Each action potential will meet the refractory period of the other action potential and they will essentially cancel each other out (they cannot continue conducting further).
Who invented the Voltage Clamp technique? And what did they discover?
Hodgkin and Huxley, and they discovered the ionic basis on an action potential
What animal was used in the Voltage Clamp technique?
Hodgkin and Huxley used a squid giant axon
How does a Voltage Clamp work?
Investigators will clamp the voltage of a squid giant axon to whatever they want and study the changes in K+ and Na+ conductance
What does the rapid inward current represent?
Sodium entering (rising phase)
What does the slow outward current represent?
Potassium leaving (falling phase)
At +52 mV why did Huxley and Hodgkin notice that their voltage amplifier did not need to inject negative current?
+52 mV is the sodium equilibrium potential in the squid (so there was no sodium coming in)
How was the current being injected discovered to be related to the ions conduction in the cell?
When negative current needed to be injected to match the desired voltage (Vc), it meant that the neuron was gaining positive charge
When positive current needed to be injected to match the desired voltage (Vc), it meant that the neuron was losing positive charge
How did Huxley and Hodgkin verify their theory?
They manipulated the Na+ extracellular concentration to make sure that the inward current was a result of sodium entering the cell
What are three examples of VGNaC blockers?
- Local anesthetics likes lidocaine
- TTX (found in pufferfish liver + ovaries), that cause paralysis of the respiratory system
- STX (found in butter clams) causing numbness, paralysis, paresthesia (pins and needle sensation on skin)
What is the difference between an Action Potential and a Receptor Potential?
An action potential is all or none whereas a receptor potential (like the ones in Merkel cells) is graded and does not have a threshold (gradually changes based on the intensity of the stimulus)
