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What determines the resting potential of a cell?

The resting potential is determined largely by the difference in K⁺ concentration between the inside and outside of the cell, along with a small Na⁺ permeability. The Goldman-Hodgkin-Katz (GHK) equation accurately predicts the resting potential.

What is the typical resting membrane potential of a neuron?

The resting membrane potential is about -70 mV, with the inside of the cell being more negative than the outside.

Why can't neurons transmit signals over long distances with only K⁺ channels?

Signals would die out quickly because positive charges injected into the axon are attracted to negative charges lining the membrane but dissipate quickly; depolarization reduces the negativity inside the cell, decreasing the electrical force holding K⁺ in, causing K⁺ efflux and restoring the resting potential.

What are voltage-gated ion channels?

Voltage-gated ion channels are proteins in the cell membrane that open or close in response to changes in membrane potential. They include voltage-gated sodium (Na⁺) channels and voltage-gated potassium (K⁺) channels, which generate action potentials.

What are the two gates in voltage-gated sodium (Na⁺) channels?

The activation gate is closed at rest and opens with depolarization; the inactivation gate is open at rest and closes shortly after the activation gate opens.

What happens when the activation gate of a voltage-gated Na⁺ channel opens?

Na⁺ ions flow into the cell, driven by both their concentration gradient and the electrical attraction of the negatively charged interior, causing rapid depolarization of the membrane potential.

What is the role of the inactivation gate in voltage-gated Na⁺ channels?

The inactivation gate closes shortly after the activation gate opens, stopping the influx of Na⁺ ions, ensuring the channel remains open for only about 1 ms.

What is the sequence of events during an action potential?

Depolarization: Na⁺ channels open, allowing Na⁺ influx and driving the membrane potential to +55 mV; Repolarization initiation: inactivation gates begin to close; Rapid repolarization: K⁺ channels open, causing K⁺ efflux; Undershoot: membrane potential becomes more negative than resting potential; Resetting: Na⁺ and K⁺ channels reset.

What is the refractory period, and why does it occur?

The refractory period is the time after an action potential during which another action potential cannot be generated because the Na⁺ activation gate is closed, the inactivation gate is plugged, and voltage-gated K⁺ channels remain open.

What is meant by the 'all-or-none' nature of action potentials?

An action potential is an all-or-none event: If the membrane potential reaches the threshold (about -55 mV), an action potential is triggered. Subthreshold depolarizations do not produce action potentials.

What is Ohm's Law, and how is it applied to ion channels?

Ohm's Law relates current (I), conductance (g), and driving force (Eion - Vm): I=g×(Eion−Vm). It describes how ions flow through channels based on the electrical potential difference.

What is the voltage clamp technique, and why is it important?

The voltage clamp technique holds the membrane potential constant while recording ion currents, allowing researchers to measure Na⁺ and K⁺ currents separately.

How did Hodgkin and Huxley use the voltage clamp to study ion channels?

Hodgkin and Huxley held the membrane potential constant, measured Na⁺ and K⁺ currents, and used substitution experiments to separate these currents.

What is the role of tetrodotoxin (TTX) in studying ion channels?

TTX is a neurotoxin that selectively blocks voltage-gated Na⁺ channels, eliminating the inward Na⁺ current and proving that the early inward current during an action potential is carried by Na⁺.

What is the role of tetraethylammonium (TEA) in studying ion channels?

TEA selectively blocks voltage-gated K⁺ channels, eliminating the delayed outward K⁺ current and proving that the outward current during an action potential is carried by K⁺.