Membrane Physiology

• Plasma membrane Na+/K+-ATPase pumps maintain low intracellular Na+ concentration and high intracellular K+ concentration by active transport
• At rest, leak channels in the plasma membrane are much more available for K+ than Na+, so the membrane potential is close to the K+ equilibrium potential
• Membrane potential can be calculated by the GHK equation as long as the concentrations of all ions that can cross the membrane through channels are known, as well as their relative permeabilities
• The Na+/K+-ATPase pumps directly contribute a small component of the potential because they are electrogenic
• Graded potentials are changes in membrane potential that are confined to a relatively small region of the plasma membrane
• Graded potentials are usually produced when some specific change in the cell’s environment acts on a specialized region of the membrane
• Graded potentials are given various names related to the location of the potential or the function they perform—for instance, receptor potential, synaptic potential, and pacemaker potential are all different types of graded potentials
• Terms describing the membrane potential

  • Potential or potential difference
  • Membrane potential
  • Equilibrium potential
  • Resting membrane potential
• Membrane potential


  The voltage difference between the inside and outside of a cell
• Equilibrium potential


  The 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
• Resting membrane potential


  The steady potential of an unstimulated cell
• Graded potential


  A potential change of variable amplitude and duration that is conducted decrementally; has no threshold or refractory period
• Action potential


  A brief all-or-none depolarization of the membrane, which reverses polarity in neurons; has a threshold and refractory period and is conducted without decrement
• Synaptic potential


  A graded potential change produced in the postsynaptic neuron in response to the release of a neurotransmitter by a presynaptic terminal; may be depolarizing (an excitatory postsynaptic potential or EPSP) or hyperpolarizing (an inhibitory postsynaptic potential or IPSP)
• Receptor potential


  A graded potential produced at the peripheral endings of afferent neurons (or in separate receptor cells) in response to a stimulus
• Pacemaker potential

  A spontaneously occurring graded potential change that occurs in certain specialized cells
• Threshold potential

  The membrane potential at which an action potential is initiated
• Charge flows between the place of origin of a graded potential and adjacent regions of the plasma membrane when a graded potential occurs
• Depolarization spreads to adjacent areas along the membrane when a graded potential occurs
• Graded potentials can occur in either a depolarizing or a hyperpolarizing direction
• The magnitude of graded potentials is related to the magnitude of the initiating event
• Change in membrane potential decreases as the distance increases from the initial site of the potential change
• Local current is decremental, the flow of charge decreases as the distance from the site of origin of the graded potential increases
• Graded potentials and the local current they generate can function as signals only over very short distances
• Summation can occur if additional stimuli add to the graded potential from the first stimulus before it has died away
• Summation

  1. Particularly important for sensation

  2. Will be discussed in Chapter 7

  3. Will be revisited later in this chapter
• Graded potentials

  • The only means of communication used by some neurons
  • Initiate a type of signal that travels longer distances
• Action potentials

  • Large alterations in the membrane potential
  • Membrane potential may change by as much as 100 mV
  • Rapid (as brief as 1–4 milliseconds)
  • May repeat at frequencies of several hundred per second
• Propagation of action potentials down the axon

  Mechanism the nervous system uses to communicate from cell to cell over long distances
• Properties of ion channels allow them to generate large, rapid changes in membrane potential
• Action potentials are propagated along an excitable membrane
• Voltage-Gated Ion Channels


  • Give a membrane the ability to undergo action potentials
  • Vary by which ion they conduct (e.g., Na+, K+, Ca2+, or Cl−)
  • Behave differently as the membrane voltage changes
• Focus on voltage-gated Na+ and K+ channels that mediate most neuronal action potentials
• Voltage-gated Na+ channels respond faster to changes in membrane voltage compared to K+ channels
• Voltage-gated Na+ channels have an inactivation gate that limits the flux of Na+
• Explanation of how action potentials occur integrating channel properties with basic principles governing membrane potentials
• During an action potential, transient changes in membrane permeability allow Na+ and K+ to move down their electrochemical gradients
• Illustration of the steps that occur during an action potential
• Action potential


  1. Step 1: Resting membrane potential is close to the K+ equilibrium potential due to more open K+ channels than Na+ channels

  2. Step 2: Depolarization becomes a positive feedback loop as Na+ entry causes depolarization, opening more voltage-gated Na+ channels

  3. Step 3: Rapid depolarization of the membrane potential overshoots, making the membrane positive on the inside and negative on the outside

  4. Step 4: Na+ permeability declines as inactivation gates block open Na+ channels

  5. Step 5: Voltage-gated K+ channels open, increasing K+ flux out of the cell and repolarizing the membrane

  6. Step 6: Afterhyperpolarization occurs as voltage-gated K+ channels close slowly, transiently hyperpolarizing the membrane

  7. Step 7: Resting membrane potential is restored
• If extracellular [Na+] is elevated, the resting potential and action potential of a neuron would change