Membrane Potential

Neurophysiology: The Membrane Potential and Action Potential

Page 1: Introduction

  • Course: Neurophysiology 197

  • Instructor: Dr. Andrew A. Sharp

Page 2: Membrane Potential

  • Definition:

    • The potential difference across a cell membrane.

    • Measured in volts (V).

  • Resting Membrane Potential (RMP):

    • Approximately -70 mV in an unstimulated cell.

  • Key Points:

    • Ion Concentrations:

      • Extracellular: High concentrations of [Na+] and [Cl-].

      • Intracellular: High concentrations of [K+] and negatively charged proteins.

Page 3: Achieving Membrane Potential

  • Membrane potential arises from ion concentration gradients inside and outside the cell.

Page 4: Active Transport of Ions

  • Ion transporters use ATP for movement across the membrane.

  • Na+/K+-ATPase:

    • Most critical ion pump in neurons.

    • Transports 3 Na+ ions out and 2 K+ ions into the cell.

    • Requires ATP (uses ~70% of a neuron's ATP).

Page 5: Facilitated Diffusion

  • The plasma membrane's selective permeability affects ion movement.

    • Ions cannot freely diffuse; facilitated diffusion is necessary.

  • Types of Ion Channels:

    • Leak Channels:

      • Always open and selective for specific ions.

Page 6: Relative Permeability

  • Not all ions pass through leak channels equally.

    • Ion channels selectively allow passage of specific ions.

  • Potassium (K+) permeability is greater than Sodium (Na+) through leak channels.

Page 7: Resting Membrane Potential Determination

  • Result of ion pump activities and ion flow through leak channels, resulting in a net charge balance.

Page 8: Electrochemical Gradient

  • The movement of ions influenced by:

    • Chemical Gradient: Concentration across the membrane.

    • Electrical Gradient: Difference in ionic charge (membrane potential).

  • Key Ions:

    • Potassium (K+), Sodium (Na+).

Page 9: Equilibrium Potential of an Ion

  • Defined as the potential at which net flux is zero if only one ion is permeable.

  • Nernst Equation:

    • Calculates based on ion concentration and charge (z) — concept only, not equation recall needed.

  • Equilibrium Potentials:

    • K+: ~-90 mV

    • Na+: ~+55 mV.

Page 10: Potassium Ion Gradients

  • At resting potential, K+ face opposing chemical and electrical gradients.

    • Chemical gradient tends to move K+ out, but electrical gradient opposes this.

Page 11: Potassium Ion Gradient Continuation

  • If permeable to K+, outflow would continue until -90 mV equilibrium is reached, similar to resting potential.

Page 12: Sodium Ion Gradients

  • At resting potential, both chemical and electrical gradients drive Na+ into the cell.

Page 13: Sodium Ion Gradient Continuation

  • If permeable to Na+, inflow would continue until +66 mV equilibrium is reached, notably different from resting potential.

Page 14: Gated-Ion Channels

  • Classification: Ion channels that require a stimulus to open.

    • Types include:

      • Chemically gated (ligand-gated)

      • Voltage-gated

      • Mechanically gated.

Page 15: Chemically Gated Ion Channels

  • Open upon binding specific chemicals (e.g., neurotransmitters).

    • Example: ACh receptors at neuromuscular junction.

    • Found in neurons, muscles, glands.

Page 16: Voltage-gated Ion Channels

  • Open/close in response to membrane potential changes.

    • Primarily found on electrically excitable cells.

    • States: Closed, Open (activated), Inactivated.

Page 17: Mechanically Gated Ion Channels

  • Open due to mechanical deformation of the plasma membrane.

    • Common in sensory receptors and certain muscle types.

Page 18: Graded (Local) Potentials

  • Activation of ion channels changes local ion concentration, affecting membrane potential.

    • Changes dissipate with distance from activation site.

Page 19: Types of Potential Changes

  • Depolarization: Less polarized, e.g., positive ions enter.

  • Hyperpolarization: More polarized, e.g., positive ions exit.

  • Repolarization: Return to resting potential due to facilitated diffusion and ion pumps.

Page 20: Graded Potential at a Synapse

  • Example: Neuromuscular Junction

    • Release of ACh leading to Na+ entry, causing local depolarization and potential action.

Page 21: Definition of Action Potential

  • Triggered by sufficient depolarization reaching a threshold potential.

    • Involves activation of voltage-gated Na+ and K+ channels.

    • Action potentials are all-or-nothing, lasting about 1 ms.

Page 22: Action Potential Generation

  • If synaptic potential exceeds the threshold at the trigger zone, action potentials occur via activated Na+ channels.