BIOL LECTURE (02/07/25)

Resting Membrane Potential

• The resting membrane potential is the difference in voltage across the cell membrane at rest.

• Typical resting membrane potential is around -70 millivolts.

• This potential is maintained by the concentration gradients of ions across the membrane.

Ion Movement and Concentration Gradients

• Ions move down their concentration gradients through open channels:

  • Sodium (Na+): higher concentration outside the cell, thus moves into the cell.

  • Potassium (K+): higher concentration inside the cell, thus moves out of the cell.

• Membrane potential can change due to the movement of these ions.

Types of Ion Channels

• Voltage-Gated Ion Channels: Open in response to membrane depolarization, allowing specific ions to flow.

• Ligand-Gated Ion Channels: Open in response to binding of a molecule (ligand), allowing ions to flow.

Graded Potentials and Action Potentials

• Graded potentials occur when stimulus strength varies, affecting the degree of depolarization:

  • Weak stimulus results in a small change in membrane potential.

  • Strong stimulus leads to a larger change in membrane potential.

• If a graded potential reaches the axon hillock and meets threshold (-55 to -50 mV), it can trigger an action potential.

Depolarization Process

• Depolarization is the process of making the inside of the cell less negative:

  • Sodium ions rush into the cell, causing a significant increase in membrane potential.

  • Voltage-gated sodium channels open rapidly after graded potentials reach the axon hillock.

• This influx of sodium causes the membrane to become more positive.

Hyperpolarization

• Following depolarization, potassium channels may remain open longer, causing hyperpolarization:

  • Hyperpolarization makes the interior of the cell even more negative than the resting potential.

Returning to Resting Membrane Potential

• After hyperpolarization, both sodium and potassium voltage-gated channels close:

  • This allows the membrane potential to return to resting state.

• The sodium channels transition from inactive back to the closed state allows future action potentials to occur.

Propagation of Action Potentials

• Action potentials propagate along the axon in one direction:

  • Triggering one action potential at the axon hillock leads to the activation of adjacent sodium channels (action potential two).

  • This chain reaction ensures actions potentials only move toward the axon terminals, preventing backward flow.