Resting & Action Potentials

Resting Membrane Potential

• The cell membrane has many proteins that move ions from one side to the other.

• At “rest” (not sending a nerve impulse), the membrane has an excess of charge on one side of the membrane → The membrane is polarized.

• The flow of charge in and out of the cell creates electrical energy that allows the cell to do work.

• The electric potential can be measured (inside → outside) at - 65 mV.

  • This is proven with experiments with axons from giant squid.

• Large negatively charged ions and proteins are attracted to the inner membrane face, which creates a greater charge separation.

→ At rest, the electrical potential difference between the two faces of the cell membrane is - 65 mV.

• The membrane resting potential is achieved by 3 membrane proteins.

Na⁺/K⁺ pump

• Na⁺ is actively transported out of the cell.

• K⁺ is actively transported into the cell.

• For 3 Na⁺ to be pumped out of the cell, 2 K⁺ are pumped into the cell.

→ Maintains the inside negatively charged relative to the outside.

Na⁺ channel

• Not very permeable.

• Allows a backflow of Na⁺ to diffuse into the cell.

→ Maintains a high external [Na⁺].

K⁺ channel

• Quite permeable

• Allow some K⁺ to diffuse out of the cell.

→ Makes the outside even more positive.

Nerve Impulse

• A nerve impulse occurs when a neuron is active, NOT at rest, in response to a stimulus.

• When the neuron is activated, Na⁺ and K⁺ channel gates open in response to electrical stimulation (= a change in voltage) → Rapid diffusion.

Action Potential

1. Dendrite stimulation

• Dendrites receive stimulation from the receptor or the previous neuron.

• Causes the membrane to leak a little Na⁺ in and K⁺ out of the cell.

• Reduces the polarization (less difference between the inside & the outside).

• When the potential reaches the threshold level, - 55 mV, the two gates open up, initiating the action potential.

2. Na⁺ gates open quickly

• Causes a rapid flow of Na⁺ into the cell through Na⁺ channel gates.

• Leads to depolarization of the membrane (no more membrane potential difference).

• The membrane depolarizes, exceeding 0 mV, to ~ 35 - 40 mV.

3. Na⁺ gates close

• Na⁺ gates start to close once the voltage reaches 0 mV.

• It takes time for them to fully close, which occurs by the time the voltage reaches 40 mV.

• At this time, K⁺ gates fully open.

4. K⁺ gates open quickly

• A lot of K⁺ diffuses out of the cell through the K⁺ channel gates.

• Causes repolarization of the membrane, returning to the negative charge inside the cell.

• At - 65 mV, the membrane undergoes hyperpolarization
  → Some excess K⁺ diffuses out, decreasing the voltage to below - 65 mV.

5. Refractory period

• Refractory period: When a section of membrane depolarizes, it cannot transmit another impulse (action potential).

• After the hyperpolarization, both Na⁺ and K⁺ are on the wrong sides, and this must be corrected before another action potential occurs.

• The Na⁺/K⁺ pump is responsible for returning the membrane to the resting state (high Na⁺ outside, high K⁺ inside).

• While the resting membrane is re-establishing, an action potential only occurs if the stimulus is stronger than usual.

  • This is because an action potential occurs in an all-or-none fashion.

  • It only occurs when the membrane reaches the threshold of - 55 mV
    → The membrane will depolarize maximally to 40 mV, and repolarize to - 65 mV.

  • If the threshold is not reached, no action potential occurs.