Potentials Membrane agus Pòlaichean Gnìomh

Potentials of the Membrane and Action Potentials

• Electrical potentials exist across membranes in almost all body cells.
• Nerve and muscle cells create rapid electrochemical impulses for signal transmission.
• Other cell types – such as glandular cells, macrophages, and ciliated cells – also experience changes in membrane potential affecting cellular functions.
• Electrical potentials arise from ion concentration differences across selectively permeable membranes.

Basic Membrane Potentials

• Potassium (K+) ion concentration is high inside nerve fibre membranes but low outside.

• If the membrane is only permeable to K+, K+ ions diffuse out, creating positive charge outside and negative charge inside (because of trapped anions).

• A diffusion potential arises, preventing further net K+ outflow.

• In mammals, this resting potential is roughly -94mV.

• Sodium (Na+) ions have a high concentration outside and a low concentration inside a nerve fibre membrane.

• If the membrane is only permeable to Na+, positive Na+ ions diffuse inside creating a positive membrane potential inside.

• A diffusion potential is created.

• In mammals, this potential is about +61mV.

• The Nernst Equation can calculate the diffusion potential for a particular ion at a particular temperature, depending on concentration differences across a membrane.

• EMF (millivolts)= ±61/Z × ln(internal concentration/external concentration)

Goldman Equation

• When a membrane is permeable to multiple ions, the Goldman Equation, or Goldman-Hodgkin-Katz equation calculates the membrane potential. This takes into account ion valence, membrane permeability, and ion concentration on either side of the membrane.
• EMF(millivolts)=-61 × ln((P Na+[Na+]o +(P K+[K+]o+ (P Cl−[Cl−]i)/(P Na+[Na+]i +(P K+[K+]i−(P Cl−[Cl−]o)))

Resting Membrane Potential

• Varied resting membrane potentials across different cell types, ranging in mammals from -85 to -90mV.
• The exact potential depends on the specific combination of ions and their permeabilities.
• Key ions include sodium, potassium, and chloride.

Electrochemical Driving Force

• When multiple ions contribute to a membrane potential, there is an electrochemical driving force for each ion, the difference between the membrane potential and the ion's equilibrium potential.