AVS 271 Anatomy and Physiology

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AVS 271 Anatomy and Physiology

• Handout 9
• September 29, 2008
• Nervous System
• Chapter 8

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Resting Membrane Potential

• Transient changes in the membrane potential
  from its resting level produce electric
  signals
• The most important way neurons process and
  transmit information
• Two forms of electric signals
• Graded potentials
• Action potentials

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Terminology Changes in Membrane Potential

• Depolarize
• Overshoot
• Repolarize
• Hyperpolarize

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Graded Potential

• Changes in membrane potential confined to a
  relatively small region of the plasma
  membrane
• Die out within 1 to 2 mm of their
  site of origin
• Usually produced by some specific change in
  the cells environment acting on a
  specialized region of the plasma membrane
• The magnitude of the potential change can
  vary (i.e., is graded)
• All cells can develop a graded membrane
  potential
• Function at afferent nerve receptors,
  synapses, and other specialized areas
• May increase or decrease the likelihood of
  an action pot.

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Action Potential

• Rapid (1 msec), large alterations in the
  membrane potential
• e.g., change from -70 mV to 30 mV
• Only occur in excitable membranes
• Nerve, muscle, some specialized cells
• Propagation of action potentials is the
  mechanism used by the nervous system to
  communicate over long distances

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Action Potential

• Result from a transient change in membrane
  permeability, which allows selected ions to
  move down their concentration gradient
• In the resting state, primarily potassium
  channels are open
• resting membrane potential of -70 mV
• The depolarizing phase of an action
  potential is due to the opening of voltage
  gated sodium channels
• The repolarizing phase is due to the
  opening of voltage gated potassium channels

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Action Potential
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Positive Feedback Cycle
Generally starts with a localized graded
potential
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Threshold Potential

• Not all membrane depolarizations trigger the
  positive feedback cycle that leads to an
  action potential
• The positive feedback cycle starts once
  sodium influx exceeds potassium efflux
• i.e., the net movement of positive charge
  is inward
• The membrane potential at which this occurs
  is the threshold potential
• Approximately 15 mV less negative than the
  resting membrane potential for most
  excitable membranes

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All or None Response

• Action potentials either occur maximally or
  they do not occur at all
• If a stimulus causes the membrane to reach
  the threshold potential, an action
  potential will occur
• If a stimulus does not cause the membrane
  to reach the threshold potential, an action
  potential will not occur
• Because they are all or none, an action
  potential cannot convey information about
  the magnitude of the stimulus that
  initiated it

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Threshold and The All or None Response
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Refractory Periods

• Absolute refractory period
• Relative refractory period
• Play a key role in determining the
  direction of action potential propagation

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Absolute Refractory Period

• During an action potential, a second
  stimulus, no matter how strong, will not
  produce a second action potential
• Voltage gated sodium channels enter a
  closed, inactive state at the peak of the
  action potential

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Relative Refractory Period

• After the absolute refractory period, there
  is an interval during which a second
  action potential can be produced, but only
  if the stimulus strength is greater than
  usual
• Continued inactivation of sodium channels
• Excess potassium channels are open
• Membrane is hyperpolarized

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Action Potential Propagation
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Action Potential (movie)
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