Lecture transcription, with study notes built in

Key concepts

Resting membrane potential

The voltage difference across a cell's membrane when it is not firing, about negative seventy millivolts for a typical neuron, with the inside negative relative to the outside.

Sodium-potassium pump

A protein machine in the membrane that, for every cycle, pushes three sodium ions out and pulls two potassium ions in, burning one molecule of ATP. It builds the ion gradients the cell later spends to fire.

Electrogenic transport

Movement that carries net charge. Because the pump sends out three positive ions and brings back only two, each cycle makes the inside a little more negative and so directly contributes to the voltage.

Potassium leak and the resting voltage

The pump builds the gradients, but the membrane is far more permeable to potassium at rest, so potassium leaking back out is what pins the resting potential close to potassium's own equilibrium. The pump is the bank; the leak is where the money actually moves.

Action potential

The fast voltage spike when sodium channels fly open and sodium rushes in down the gradient, flipping the voltage positive in under a millisecond. It is the cheap, fast release of energy the pump slowly stored.

Outline

• Hook: a cell spends a huge slice of its energy budget pushing salt around, and that is why you can think
• Definition: the resting membrane potential is the voltage across the membrane when it is not firing, about negative seventy millivolts
• The pump: three sodium out, two potassium in, one ATP per cycle
• Why it is electrogenic: net charge moves, so each cycle makes the inside more negative
• Student question: if the pump sets the voltage, why do textbooks credit potassium?
• The resolution: the pump builds the gradients, but potassium leak sets the resting voltage
• Mental model: the pump charges a battery, the leak channels slowly drain it, and the balance is negative seventy millivolts
• The payoff: sodium rushes in and the action potential fires because the pump pre-paid for it
• Why the brain is an energy hog: about a fifth of resting calories keep the pumps running
• Assignment: read voltage-gated channels and trace one action potential from threshold to reset

Takeaways

• The resting membrane potential is roughly negative seventy millivolts, with the inside negative relative to the outside.
• The sodium-potassium pump moves three sodium out and two potassium in per ATP, and because net charge leaves, it is electrogenic.
• The pump builds the gradients, but it is the potassium leak that sets the resting voltage near potassium's equilibrium.
• The action potential is the fast release of energy the pump slowly stored, the way a wound spring releases the work put into it.
• Roughly a fifth of your resting calories go to keeping these pumps running, which is why the brain is such an energy hog.

Study questions

• Define the resting membrane potential and state its approximate value for a typical neuron, including which side of the membrane is negative.
• Explain why the sodium-potassium pump is described as electrogenic, referring to how many sodium and potassium ions move per cycle.
• The pump builds the gradients, but the leak sets the voltage. Reconcile that with the claim that potassium determines the resting potential.
• Trace what happens to sodium and to the membrane voltage during an action potential, and explain in what sense the pump pre-paid for it.
• Why does the brain consume so much energy at rest? Tie your answer to the cost of running the sodium-potassium pumps.