Lecture 1 - Chapter 2 : Electrical Signals of Nerve Cells

Question 1

What type of currents can be recorded by micro-electrodes?

Answer 1

• Receptor potentials (in skin touch)
• Synaptic potential (upon activation of single synapse)
• Action potential (when threshold potential is reached)

Question 2

An experiment is performed where two micro-electrodes are put on a neuron. One electrode is able to record and measure membrane potential and the other electrode is there to stimulate and inject a current.

What is first measured by the recording micro-electrode, when the other electrode is not yet injecting a current?

Answer 2

The micro-electrode than measures a resting membrane potential of approximately -65 mV.

Question 3

An experiment is performed where two micro-electrodes are put on a neuron. One electrode is able to record and measure membrane potential and the other electrode is there to stimulate and inject a current.

What happens when a negative current is injected into the neuron?

Answer 3

It induces hyperpolarization, where the membrane potential decreases even further.

Question 4

An experiment is performed where two micro-electrodes are put on a neuron. One electrode is able to record and measure membrane potential and the other electrode is there to stimulate and inject a current.

What happens when a positive current is injected into the neuron?

Answer 4

It induces depolarization, where the membrane potential increases.

Question 5

An experiment is performed where two micro-electrodes are put on a neuron. One electrode is able to record and measure membrane potential and the other electrode is there to stimulate and inject a current.

What is the threshold potential?

Answer 5

The threshold of the membrane potential that needs to be reached (via depolarization) in order to initiate an action potential. The threshold is usually around -50 mV.

Question 6

What’s meant by the fact that action potentials are all-or-none events?

Answer 6

An action potential is always the same size, there are no action potentials that are bigger/smaller than other action potentials. As a consequence, it’s either that the neuron reaches its threshold and initiates an action potential or the neuron does not. So in other words, all-or-nothing.

Question 7

What determines the stimulus intensity?

Answer 7

The frequency of action potentials.

Question 8

What do action potentials make possible?

Answer 8

Long range signal transduction → long range transport of electrical signals.

Question 9

As already stated: action potentials allow long range transport of electrical signals.

Explain why single injection of a current by an electrode that doesn’t cause the threshold potential to be reached, is not sufficient for long range transport of electrical signals.

Answer 9

If you were to inject a current that is not sufficient to surpass the threshold potential, no action potential is generated and the signal wears off as it is transported through an axon.

Question 10

As already stated: action potentials allow long range transport of electrical signals.

Explain why single injection of a current by an electrode that does cause the threshold potential to be reached, is sufficient for long range transport of electrical signals.

Answer 10

If the threshold potential is reached, an action potential can be initiated. This action potential is constant over distance

Question 11

An action potential is generated by the electrochemical gradient of Na+ and K+. Describe the situation during the resting membrane potential.

Answer 11

Here, Na+ concentration is highest outstide the cell and K+ concentration is highest inside the cell. This generates a membrane potential of -70 mV.

Question 12

What can cause a resting membrane potential to reach its threshold?

Answer 12

A stimulus from a sensory cell or other neuron causes the cell to depolarize toward the threshold potential.

Question 13

What needs to happen for the action potential to occur (after the threshold potential is reached)?

Answer 13

• Na+ channels open and Na+ ions travel from the outside of the cell to the inside. The inside gets more positive, which is called depolarization of the membrane.
• If +30 mV is reached, the peak of the action potential is also reached.
• At this point, K+ channels open and Na+ channels close. Here, K+ ions leave the inside of the cell and travel to the outside.
• The membrane becomes more negative (repolarization) and even hyperpolarizes.

Question 14

What happens after the action potential is reached?

Answer 14

At this point, K+ channels open and Na+ channels close. K+ ions leave the inside of the cell and travel to the outside. The membrane becomes more negative (repolarization) and even hyperpolarizes.

Question 15

The hyperpolarized membrane is a refractory period where the neuron cannot fire again. What happens during this period in order to recover towards its resting membrane potential?

Answer 15

K+ channels close and the Na+/K+ transporter restores the resting potential.

Question 16

What is the Nernst equation?

Answer 16

A formula that can be used to predict the equilibrium potential (electrial potential generated across the membrane at electrochemical equilibrium).

Question 17

Explain how an electrochemical equilibrium of K+ can be reached.

Answer 17

If the concentration inside and outside the cell is equal, the potential measured is 0 V. But if the concentration K+ is 10 mM inside and 1 mM outside, K+ will move to the outside of the cell. As this happens, the potential slowly changes from 0 to around -58 mV. This potential that is generated over the membrane tends to impede further flow of K+, by repelling positive K+ ions that would otherwise move across the membrane.

Thus, as the outside of the cell becomes more positive relative to the inside, the increasing positivity makes the outside less attractive to positive K+ ions. The net movement of K+ will then stop at the point (i.e. equilibrium) where the membrane potential halts ion movement.

Question 18

What does this picture explain?

Answer 18

• That there’s variable permeability to Na+ and K+, for example at rest the membrane is more permeabel to K+ compared to Na+.
• The right picture explains how Na+/K+ permeability is important for the generation and recovery of an action potential.

Question 19

Fill in or choose:

• During a resting membrane potential, the permeability for K+ is *higher/lower* than it is for Na+.
• During a resting membrane potential, the equilibrium potential is *positive/negative* and equilibrium potential is for *Na+/K+*.
• During depolarization, the permeability for *Na+/K+* increases.
• During an action potential, the permeability for K+ is *higher/lower* than it is for Na+.
• During a action potential, the equilibrium potential is *positive/negative* and equilibrium potential is for *Na+/K+*.
• During repolarization, the permeability for *Na+/K* decreases.

Answer 19

• During a resting membrane potential, the permeability for K+ is *higher* than it is for Na+.
• During a resting membrane potential, the equilibrium potential is *negative* and equilibrium potential is for *K+*.
• During depolarization, the permeability for *Na+* increases.
• During an action potential, the permeability for K+ is *lower* than it is for Na+.
• During a action potential, the equilibrium potential is *positive* and equilibrium potential is for *Na+*.
• During repolarization, the permeability for *Na+* decreases.

Question 20

What is the Goldman equation?

Answer 20

As seen in previous questions, it’s also important to consider membrane permeability, especially since permeability is different for different ions and also changes dependent on the electrochemical state of a neuron. Therefore, you need an equation that takes different concentration gradients of different ions and the relative permeability into account. The Goldman equation takes these things into account.