Understanding Membrane Potential and Ion Movement

Lecture Notes on Membrane Potential

Summary

In today's lecture, we discussed membrane potential, focusing on the factors affecting the ion movement across cell membranes and how these contribute to membrane voltage. The lecture emphasized the role of ion permeability, concentration gradients, and electrostatic forces in creating membrane potential. We also explored the calculation of equilibrium potential and the implications of ion distribution in resting neurons.

Key Concepts

• Membrane Potential:
  • Defined as the difference in electric charges across a cell membrane.
  • Usually negative in most cells, indicating more negative charges inside relative to the outside.

Rules Governing Ion Movement

1. Concentration Gradient:
   • Ions move from areas of higher to lower concentration.
2. Electrostatic Forces:
   • Ions move away from like charges and toward opposite charges.
3. Permeability of the Cell Membrane:
   • Ion movement is also controlled by the membrane's permeability, which can vary with the physiological state of the cell.
   • Ion channels aid in regulating permeability by opening or closing.

Example to Illustrate Ion Movement and Membrane Potential

• Scenario with Equal Permeability:

  • If a membrane is permeable to both sodium and chloride ions, they diffuse to equalize concentration without creating a membrane potential.

• Scenario with Selective Permeability:

  • If only sodium is allowed to pass, it moves to the side with less concentration, causing an accumulation of positive charge and creating a membrane potential.
  • This leads to an equilibrium (equilibrium potential) where the diffusion force and electrostatic force counteract each other.

Equilibrium Potential

• Defined as the voltage at which the net movement of ions stops because the concentration gradient and electrostatic forces are balanced.
• Can be calculated based on the concentration gradient of the ions.

Ion Distribution in Neurons

• Sodium (Na+):

  • Higher concentration outside the neuron.
  • Positive equilibrium potential to drive sodium out when neuron is active.

• Potassium (K+):

  • Higher concentration inside the neuron.
  • Negative equilibrium potential.

• Chloride (Cl-):

  • Similar to sodium in movement direction.
  • Needs a negative internal environment to be pushed out.

• Resting Membrane Potential:

  • Typically around -70mV in neurons.
  • Chloride is in equilibrium at resting potential, while sodium and potassium are not due to active transport mechanisms.

Sodium-Potassium Pump

• Continually operates to maintain the resting membrane potential.
• Pumps sodium out and potassium in, against their respective concentration gradients.
• This active transport is crucial for maintaining the negative resting potential and for the generation of action potentials.

Conclusion

• The creation and maintenance of membrane potential are crucial for the proper functioning of cells, particularly neurons, enabling them to respond to stimuli and conduct impulses.