Learning Objective: By the end of this section, learners should be able to: Explain the principles governing excitability in nerve and muscle cells. Describe membrane potential, equilibrium potential, electrochemical gradients, conductance, and net driving force for ions. Relate ion concentrations to membrane excitability and ion flux.

Excitable Tissue: Key Principles

Ion Concentrations:

• Excitable cells have specific intracellular and extracellular concentrations of key electrolytes (Na⁺, K⁺, Cl⁻, Ca²⁺).
• Intracellular proteins contribute a net negative charge inside the cell.

1. Membrane Potential (Em)

• Separation of charge across the cell membrane creates a membrane potential.
• Measured in volts (mV), it represents stored potential energy available to do work.

2. Electrochemical Gradient

• Combines a chemical (concentration) gradient and an electrical gradient.
• Ions move down chemical gradients (high → low) and are attracted/repelled by electrical forces.

3. Equilibrium Potential (E_ion)

• The membrane potential at which there is no net diffusion of a specific ion.
• Calculated using the Nernst equation:

• R = gas constant, T = absolute temperature, z = ion charge, F = Faraday constant.
• Balances the chemical and electrical forces for the ion.

4. Conductance (g)

• Describes the flow of an ion through membrane channels.
• Open channels → high conductance; closed channels → low conductance.

5. Net Driving Force (DF)

• Indicates how far an ion is from equilibrium.
• Calculated as:

Activity