Action potentials

What determines the influence of an ion on membrane potential?

1. The ion’s concentration gradient across the membrane

2. The membrane’s permeability to that ion
   Since the membrane is more permeable to K⁺ than other ions, K⁺ largely determines the membrane potential.

What is the Nernst equation used for in neuroscience?

It calculates the equilibrium potential (Eₓ) for an ion—the membrane voltage at which there’s no net movement of that ion across the membrane.

Why does K⁺ reach equilibrium across the membrane?

Electrical attraction pulls K⁺ into the cell, but diffusion pushes it out. When these forces balance, K⁺ stops moving overall—this is the K⁺ equilibrium potential (Eₖ).

What is the value of Eₖ (K⁺ equilibrium potential), and what does the negative sign mean?

Eₖ ≈ -90 mV. The negative sign indicates the inside of the cell is more negative than the outside.

What happens if the membrane potential becomes more or less negative than -90 mV?

• More negative: K⁺ is pulled into the cell

• Less negative: K⁺ diffuses out of the cell
  Thus, -90 mV keeps K⁺ concentrations stable inside and outside.

Why is the resting membrane potential not exactly -90 mV?

Because not all K⁺ channels are open (only ~30%) and other ions slowly leak across, the resting potential is usually around -70 mV, between -65 to -85 mV.

Action Potential

A rapid shift in a neuron’s or muscle cell’s membrane potential, crucial for transmitting nerve impulses along an axon.

What triggers the start of an action potential?

Mechanical or ligand-gated ion channels open in response to a stimulus, changing the resting potential; if the depolarization reaches the threshold (around -55 mV), voltage-gated Na⁺ channels open.

What happens when voltage-gated Na⁺ channels open?

Na⁺ rapidly enters the cell, making the membrane potential rise quickly and become less negative, approaching sodium’s equilibrium potential.

Sodium Equilibrium Potential (ENa)

Predicted by the Nernst equation to be around +66 mV, based on Na⁺ being more concentrated outside (145 mM) than inside (12 mM) the cell.

Steps of an action pitentual

1. Resting State: The neuron’s membrane is at -70 mV, maintained by the sodium-potassium pump and leaky potassium channels.

2. Threshold Reached (-55 mV): A stimulus depolarizes the membrane to -55 mV, triggering voltage-gated sodium channels to open.

3. Rising Phase: Rapid sodium influx causes the membrane potential to rise sharply toward +30 mV.

4. Peak: Sodium channels close and potassium channels open, stopping Na⁺ entry and starting K⁺ exit.

5. Repolarization: Potassium ions leave the cell, making the inside more negative and returning the membrane potential toward resting.

6. Hyperpolarization: Potassium channels close slowly, causing the membrane potential to briefly become more negative than resting.

7. Return to Resting: The sodium-potassium pump restores the original ion concentrations and membrane potential returns to -70 mV.

Refractory Periods: No new action potential can start during the absolute refractory period, and a stronger stimulus is needed during the relative refractory period.

What is the relative refractory period?

It's the phase after an action potential when the membrane is hyperpolarized and requires a stronger-than-normal stimulus to trigger another action potential.

What role does the Na⁺/K⁺ ATPase pump play after an action potential?

It restores original ion concentrations by pumping 3 Na⁺ out and 2 K⁺ in using ATP, re-establishing the resting membrane potential and ion gradients.

All-or-None Law

Once the threshold is reached, an action potential fires fully or not at all. The size (~+30mV peak) and duration of the action potential do not depend on stimulus strength.

Does a stronger stimulus produce a bigger or longer action potential?

No. The action potential’s size and duration are always the same once triggered, regardless of stimulus strength.

Propagation of the Action Potential

After initiation at the axon hillock, the action potential travels down the axon by sequentially depolarizing adjacent membrane segments, like a domino effect.

Why can action potentials only travel forward along the axon?

Because the recently activated section enters a refractory period, making it temporarily unresponsive to new stimuli, preventing backward propagation.

Action Potential Conduction in Muscle Fibers

Skeletal muscle action potentials resemble those in neurons (triggered by Na⁺ influx). However, Cl⁻ permeability plays a bigger role in resting potential and recovery, unlike neurons where K⁺ dominates.

What muscle condition is caused by defective chloride channels?

Myotonia — difficulty relaxing muscles after contraction.

Saltatory Conduction

The rapid transmission of action potentials in myelinated axons, where electrical current jumps between nodes of Ranvier instead of spreading continuously.

What role do Schwann cells play in saltatory conduction?

hey wrap axons in an insulating myelin sheath, preventing ion leakage and enabling faster signal transmission.

Why is saltatory conduction faster than conduction in unmyelinated axons?

Because the action potential “jumps” from node to node, avoiding slow continuous current spread along the entire axon membrane.

How does saltatory conduction save energy?

Ion exchange happens only at the nodes of Ranvier, so fewer Na⁺ and K⁺ ions cross the membrane, reducing the workload for the Na⁺/K⁺ ATPase pump.