PPT

• Lecture Objectives:
• Lecture Objectives


  • Know the basic principles of electricity
  • Define resting membrane potential
  • Understand graded and action potentials
  • Compare and contrast graded and action potentials
• Physiological processes are dictated by the laws of chemistry and physics
• Electrolyte composition


  • ECF - sodium and chloride ions
  • ICF - potassium ions and ionized non-penetrating molecules (phosphate compounds and proteins with negatively charged side chains)
• Charges play a significant role in signal integration and cell-to-cell communication
• Resting Membrane Potential:
• All cells under resting conditions have a potential difference (voltage difference between two points) across their plasma membranes
• Inside of the cell is negatively charged with respect to the outside
• Cells at Rest exist because of a tiny excess negative charge inside the cell and excess of positive ions outside
• Membrane or Transmembrane Potential:
• Steady transmembrane potential of a cell that is not producing an electrical signal
• Differences in specific ion concentration in the ICF and ECF
• Differences in membrane permeabilities to the different ions reflect the number of open channels for the different ions in the plasma membrane
• Different cells have different resting membrane potentials
• Electrical Potential: Negative charge inside repels K+ from moving out
• Chemical Potential: Concentration gradient favors diffusion of Na+ inside and K+ outside
• Equilibrium Potential: Voltage difference across a membrane that produces a flux of a given ion species that is equal but opposite to the flux due to the concentration gradient of that same ion species
• Nernst Equation: Describes the equilibrium potential for any ion
• Nernst Potential for various ions
• Nernst Potential: Consider a neuron at RMP (-70 mV) and conductance for sodium increases?
• Nernst Potential


  1. The membrane will be depolarized to +55 mV

  2. This will equilibrate the ion concentration inside and out of the cell

  3. What happens if K or Cl conductance increases?
• Expanded version of the Nernst equation

  Takes into account individual ion permeabilities
• Goldman-Hodgkin-Katz (GHK) Equation: 'V& = 61log(P'K$% + P()Na$% + P*+ P'Cl$# / P'K!# + P()Na!# + P*+ P'Cl!# = -70mV'
• Action of the Na+/K+-ATPase pump

  1. Sets up the concentration gradients for Na+ and K+

  2. Balances the rate at which the ions leak through open channels

  3. There is a greater flux of K+ out of the cell than Na+ into the cell

  4. Greater number of open K+ channels than there are Na+ channels at RMP

  5. Significant negative membrane potential develops (approaches K+ equilibrium potential) - more positive efflux than influx

  6. There is a small but steady leak of Na+ into the cell and K+ out of the cell
• Depolarization is the potential moving from RMP to less negative values
• Repolarization is the potential moving back to the RMP
• Hyperpolarization is the potential moving away from the RMP in a more negative direction
• Graded Potential: Potential change of variable amplitude and duration that is conducted decrementally and has no threshold or refractory period
• Characteristics of Graded Potentials: Occurs in an active area of the membrane
• Characteristics of Graded Potentials: Magnitude varies directly with the magnitude of the stimulus
• Characteristics of Graded Potentials: Spread decrementally by local current flow
• Characteristics of Graded Potentials: Flow is between the active area and adjacent inactive areas
• Characteristics of Graded Potentials: Die out over a short distance
• Examples of Graded Potentials: Synaptic Potential, Receptor potential, Pacemaker potential
• Action Potential: Brief all-or-none depolarization of the membrane, which reverses polarity in neurons, has a threshold and refractory period and is conducted without decrement over long distances
• Action Potential


  Brief all-or-none depolarization of the membrane, which reverses polarity in neurons, has a threshold and refractory period and is conducted without decrement over long distances
• Characteristics of Action Potentials

  • Large alterations in the membrane potential
  • Generally very rapid (as brief as 1 to 4 ms) and may repeat at frequencies of several hundred per second
  • Membrane potential at which an active potential is initiated (“all or none”)
• Voltage-Gated Ion Channels


  Action potentials are generated when voltage-gated sodium and potassium channels are activated at threshold
• Action Potential Mechanism


  1. Step

  2. Voltage-Gated Ion Channels

  3. Ion Permeability

  4. Action Potential Curve

  5. Resting State

  6. Depolarization

  7. Repolarization

  8. Hyperpolarization
• Feedback Control of Gated Channels

  • Positive Feedback of Sodium Channels
  • Negative Feedback of Potassium Channels