BIO210Y5 Lecture Notes - Lecture 7: Depolarization, Action Potential

Studies on the squid giant axon were instrumental to our current understanding of how action potentials are generated. You decide to do some experiments on the squid giant axon yourself.

A. You remove the cytoplasm in an axon and replace it with an artificial cytoplasm that contains twice the normal concentration of K+ by adding KOAc, where OAc– is an anion to which the membrane is impermeable. In this way, you double the internal concentration of K+ while maintaining the bulk electrical balance of the cytoplasmic solution. Will this make the resting potential of the membrane more or less negative?

B. You add NaCl to the extracellular fluid and effectively double the amount of extracellular Na+ ions. How does this affect the action potential?

C. You replace half of the NaCl in the extracellular fluid with choline chloride. (Choline is a monovalent cation much larger than Na+. Note that the presence of choline will not impede the flow of Na+ through its channels.) How will this affect the action potential?

Studies on the squid giant axon were instrumental to our current understanding of how action potentials are generated. You decide to do some experiments on the squid giant axon yourself.

A. You remove the cytoplasm in an axon and replace it with an artificial cytoplasm that contains twice the normal concentration of K+ by adding KOAc, where OAc– is an anion to which the membrane is impermeable. In this way, you double the internal concentration of K+ while maintaining the bulk electrical balance of the cytoplasmic solution. Will this make the resting potential of the membrane more or less negative?

B. You add NaCl to the extracellular fluid and effectively double the amount of extracellular Na+ ions. How does this affect the action potential?

C. You replace half of the NaCl in the extracellular fluid with choline chloride. (Choline is a monovalent cation much larger than Na+. Note that the presence of choline will not impede the flow of Na+ through its channels.) How will this affect the action potential?

1.4 An interesting feature of the action potential is this: the amplitude is always the same. That is, whether the nerve is communicating something important or boring, complicated or simple, the amplitude of the pulse moving down the axon is exactly the same. The shape and speed of the pulses are also always the same! Given these constraints, how might action potentials be used to convey more than one piece of information?

1.5 The membrane of the axon cell controls the action potential and therefore determines what we sense, how we think, and much more. The membrane potential is an example of a Nernst potential, described by the equation: delta V = (k_B T / q) ln (c₁/c₂). How might the cell membrane actually control the electric potential that exists between the inside and outside of the cell and what quantities could be measured to test your proposed mechanism? Your answer should refer explicitly to the physics of the Nernst potential. The best answers will relate the state of the membrane to one or more of the phases of the action potential, but please do not write a book. No more than one paragraph is called for here.

1.6 Nerve impulses are transmitted from the end of one axon to the beginning of the next by the release ofneurotransmitters. These are chemicals that cause a modest change in the potential across any nearby axon membrane, including that of the signaling nerve. Consider the features of the action potential. The neurotransmitters released by the signaling nerve do not trigger a return action potential that would run back along the same axon from which the signal came. In light of this observation, how much of a change in potential can these neurotransmitters induce, and for how long? Your answer should be based solely on the model and information provided here, and not from the much more complicated scenarios described on the internet, in articles, and in books.