Membrane potential and action potentials
In this section, we’ll learn how neurons generate electrical signals—how membrane potentials arise from ion gradients and permeability, and how action potentials propagate those signals rapidly along axons.
1) Start with these videos:
3) Read these articles for some historical context:
History of Neuroscience - Luigi Galvani
History of Neuroscience - Hodgkin and Huxley
4) Watch these videos for a quick refresher:
5) Finally, answer these questions to test your understanding:
1. The resting membrane potential of a typical neuron is closest to:
- A. +40 mV
- B. 0 mV
- C. -70 mV
- D. -90 mV
Show Answer
Correct answer: C
The resting membrane potential of a typical neuron is about -70 millivolts (mV), meaning the inside of the neuron is negatively charged relative to the outside. The negative resting potential is essential for the neuron’s ability to generate action potentials when stimulated.
2. When a neuron is at rest (i.e., not in the process of firing an action potential), where is the concentration of sodium ions highest?
- A. inside the cell
- B. outside the cell
- C. in the nucleus of the cell
- D. sodium ions are evenly distributed on both sides of the cell membrane
Show Answer
Correct answer: B
At rest, a neuron maintains its resting membrane potential by keeping more sodium ions (Na⁺) outside the cell and more potassium ions (K⁺) inside. Thus, sodium concentration is much higher outside the neuron when it’s at rest.
3. What is the term for the membrane potential at which depolarization opens voltage-gated sodium channels and triggers an action potential?
- A. threshold
- B. resting membrane potential
- C. refractory phase
- D. depolarization point
Show Answer
Correct answer: A
When neurotransmitters interacting with receptors causes repeated depolarization of the neuron, eventually the neuron reaches what is known as its threshold membrane potential. When threshold is reached, a large number of sodium channels open, allowing positively charged sodium ions into the cell and triggering an action potential.
4. In the timeline of an action potential, which event below happens LAST?
- A. Sodium (Na+) channels close, and potassium (K+) begins leaving the cell
- B. The neuron reaches threshold
- C. The neuron becomes hyperpolarized
- D. Sodium (Na+) channels open, and sodium rushes into the cell
Show Answer
Correct answer: C
After sodium (Na+) has rushed into the cell in the rising phase of the action potential, the sodium channels close and potassium (K+) begins to leave the cell. This brings the neuron back to its resting membrane potential. The potassium channels, however, close gradually and allow enough positively-charged potassium to leave the neuron that the cell becomes briefly hyperpolarized. This creates a "relative refractory period," when it's less likely for the neuron to fire another action potential.
5. Alan Hodgkin and Andrew Huxley's experiments on the giant axons of this animal helped us to understand how neurons generate and transmit action potentials:
- A. elephant
- B. whale
- C. squid
- D. giraffe
Show Answer
Correct answer: C
Hodgkin and Huxley performed their experiments on the giant axon of the squid. This axon is unusually large, which made it easier to insert electrodes and measure electrical changes, allowing them to uncover how action potentials are generated and transmitted.
