Action Potential

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Action Potential

• Lecture Presented to AM Physiological Psychology
  class 9-9-99

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Resting Potential

• Ion movement rather than electron
• Na, Cl-, K, A-
• Semipermeable membrane
• Na pump
• Electrical Chemical Gradients
• Why a Resting Potential?

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Action Potential (Spike)

• General Characteristics
• Results from reversal of the resting potential.
• K and Cl- are in equilibrium Na has both
  chemical electrical forces acting to push into
  cell.
• It is kept outside by the cell membrane.
• The action potential results because the Na
  permeability of the membrane is changed allowing
  Na to rush into the cell.
• resting potential altered in 1 of 4 ways.
• hyperpolarization - increase in resting potential
  (inhibitatory).
• hypopolarization - reduce the resting potential.
• depolarization - abolish resting potential.
• action potential - complete polarity reversal

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Graded vs Action Potentials

• Graded Action
• dendrites cell bodies Axon
• decreases with time distance self
  propagating
• amplitude function of intensity same amplitude
• temporal or spatial summation all-or-none
• hypopolarization
• axon hillock - 10 mv a spike is initiated in the
  axon.
• permeability for Na is immediately lowered
• Na ions rush in locally cancel resting pot.
• Threshold - point where membrane no longer offers
  resistance - (area of approx. 1 cm).
• voltage inside the cell reverses to 30
• K and Cl- ions are no longer in equilibrium
• change is so brief involves so few particles.
• Na pump's operation then works to exchange Na
  K ions.

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Characteristics of Action Potential

• Passive - heat liberated during recovery.
• Ion movement is in out not down the axon.
• Cl- movement is negligible
• Ion movement studied by use of voltage clamp
• device (feedback) which can prevent
  depolarization of axon.
• works by delivering current which would exactly
  equal flow of ions
• across membrane.
• allows this required voltage to be recorded.

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Excitability of Neuron

• excitability - reciprocal of the stimulation
  threshold.
• Absolute refractory period - 1 msec
• period when Na is rushing in.
• Relative refractory period (negative after
  potential)
• higher threshold
• K moving out
• repeated stimuli can prolong
• Supernormal Period - 12 msec
• increased excitability following relative RP
• Subnormal period
• reduced excitability
• tetanus increases time of occurrence
• Stimuli can be timed to inhibit activity during
  subnormal period.

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Terms

• Action potential
• Hyperpolarization
• Hypopolarization
• Depolarization
• Polarization
• Temporal/Spatial summation
• Threshold
• Period of latent addition
• Absolute refractory period
• Relative refractory period
• Axon Hillock