Biophysics for Dentistry (PHY113) Lecture 6

Biophysics for Dentistry (PHY113) Lecture 6

• Course covers 15 weeks, with lecture 6 focusing on membrane potential.

• Lecture content includes introduction, cell structure, cell dynamics, passive and active transport across cell membranes, membrane potential, Nernest potential, electrochemical driving force of ions, action potential, neuron and signal transmission, muscle contraction, electromyogram, and electrocardiogram.

Movement of Ions Across the Cell Membrane

• Ions move from higher to lower concentration areas.
• Ions move away from like charges and towards opposite charges.
• Cell membrane permeability defines ion movement.
• Key ions include cations (K+, Na+, Ca++) and anions (Cl-, Protein-).
• Electrical Gradient: Gradient based on charge.
• Chemical (concentration) Gradient: Gradient based on concentration.

Membrane Potential

• The membrane potential is the difference in electrical potential between the inside and outside of the cell membrane.
• The potential is due to the unequal distribution of ions across the plasma membrane.
• Resting Membrane Potential: A resting neuron has a voltage across its membrane.
• Action Potential: A rapid rise and fall in voltage (spike) across a cellular membrane, in a specific pattern.

Membrane Potential Determinants

• Membrane potential is determined by both ion concentration gradients and membrane permeability to each ion type.
• The largest contributors are Na+ and Cl- at high extracellular concentrations, and K+ alongside large intracellular protein anions.

Permeability of Cell Membranes

• Cell membranes are impermable to Na+, which typically stays highly concentrated outside.
• Cell membranes are permeable to K+, allowing easier passage.
• Anion proteins are largely inside the cell because they cannot easily cross the membrane.

How Ions Move Across the Membrane

• Charged ions cannot directly pass through the hydrophobic lipid regions of cell membranes.
• Specialized channel proteins provide hydrophilic tunnels for ion passage.
• Leak channels are open in resting neurons.
• Other channels open in response to signals.

Membrane Potential: Leak Channels

• Cell membranes have various leak channels permitting constant ion movement.
• Ions travel through leak channels following their concentration gradient.
• K+ ions move freely through leak channels from inside to outside.
• Anions inside the cell also follow the cations.
• Intracellular negatively charged proteins don't easily diffuse out.
• Cell membranes are significantly leakier to K+ than to Na+.
• Na+'s inward diffusion doesn't balance K+'s outward diffusion, creating net positive outside and negative inside charges .

Membrane Potential: Polarization

• Cell membranes are polarized due to the present potential difference.
• Depolarization: Membrane potential shifts more positive.
• Hyperpolarization: Membrane potential shifts more negative.

Sodium/Potassium Pump

• The Na+/K+ pump maintains the concentration gradients of sodium and potassium (and, thus, the resting membrane potential).
• The pump actively moves Na+ and K+ against their electrochemical gradient.
• A protein called the Na+/K+ ATPase maintains the concentrations.

Calculation of Resting Membrane Potential

• The resting membrane potential generally arises from ion contributions.
• In many cell bodies, critical contributors to membrane potential are K+, Na+, and Cl-.
• Their collective transmembrane movements create the membrane potential.
• The Goldman-Hodgkin-Katz (GHK) equation calculates the membrane potential when more than one channel is present.

Permeability Factor

• Permeability reflects how easily ions cross a membrane.
• Permeability is proportional to the number of open channels.
• PK is often higher in resting neurons than PNa and PCI, as a reference value of 1 for calculating relative permeabilities.
• If ion channels are closed, corresponding relative permeability values are set to zero.
• Membrane potential is rarely situated at equilibrium potential for multiple ion contributors.

Solved Example

• Given cell membrane parameters, calculate the membrane potential value (in mV).

Summary

• Resting potential is a cell's membrane voltage in a non-signaling state.
• It's based on both ion concentration gradients across membranes and the membranes' permeability of each ion type.
• For resting neurons, Na+ and K+ concentration gradients exist across membranes.
• Membrane permeability to K+ is greater than that to Na+, influencing the resting potential.

This quiz on Lecture 6 of Biophysics for Dentistry focuses on membrane potential and the movement of ions across the cell membrane. It covers essential concepts such as electrical and chemical gradients, key ions involved, and the implications of membrane potential in cellular functions. Test your knowledge on these critical biophysical principles relevant to dentistry.