Exam 3 Review:  Chapter 11:  Action Potentials

action potential = nerve impulse - A momentary change in electrical voltage potential (a wave of depolarization) on the surface of a nerve or muscle cell or gland cell which takes place when the cell is stimulated at or above its threshold level; the all-or-none action potential is capable of transmitting information (inhibition or excitation) to the next cell(s) in the communication pathway.

depolarization - The change in membrane voltage potential which triggers the action potential or nerve impulse; the change is due to the opening of gated Na⁺ channels which allow Na⁺ ions to enter the cell cytoplasm causing the resting potential of -70 mV to change to +30 mV in the cell interior; similar events are observed when muscle cells are stimulated to contract and when glandular cells are stimulated to secrete.

threshold - The point that must be exceeded to begin producing a given effect or result or to elicit a response; in the case of excitable cells, the amount of stimulus, in whatever form (light, heat, mechanical activity, pH change, binding of ligand to receptor, voltage change, etc.) required to trigger the action potential.

suprathreshold stimulus - An amount of stimulus, in whatever form (light, heat, mechanical activity, pH change, binding of ligand to receptor, voltage change, etc.) greater, usually markedly greater, than the minimum stimulus required to trigger the action potential.

repolarization - The restoration of a polarized (resting) state across a membrane, as in a muscle fiber following contraction, a gland cell following secretion, or a neuron following transmission of a nerve impulse; the change is due to the closing of gated Na⁺ channels and opening of gated K⁺ channels which allow K⁺ ions to exit the cell cytoplasm causing the resting potential of -70 mV to be restored from the +30 mV in the cell interior during depolarization.

all-or-none principle (of action potential) - The term describing the fact that certain physiological processes, including the generation of an action potential and the triggering of a muscle fiber contraction utilizing the sliding filament mechanism, proceed in the same fashion, at the same rate, at the same strength, every time they occur; once stimulated, the process completes itself without significant variation in performance.

List:

9. Sequentially, the molecular events occurring during an action potential at a single location in the axon membrane.

          I.  Resting state:
                 a)  all voltage-gated and chemically-gated ion channels are closed
                 b)  membrane potential approximately -70 mV
                 c)  the Na⁺ K⁺ ATPase pumps are operating (3 Na⁺ out/2 K⁺ in per ATP hydrolysis)

         II.  Depolarization phase:
                 a)  a stimulus of threshold strength or greater causes the gated Na⁺ ion channels to open; Na⁺ ions diffuse into the cytoplasm
                 b)  membrane potential reverses from approximately -70 mV to approximately +30 mV
                 c)  the Na⁺ K⁺ ATPase pumps are operating (3 Na⁺ out/2 K⁺ in per ATP hydrolysis)

        III.  Repolarization phase:
                 a)  the gated Na⁺ ion channels slowly close; Na⁺ ions can no longer diffuse into the cytoplasm
                 b)  meanwhile, the gated K⁺ ion channels open; K⁺ ions diffuse out of the cytoplasm
                 c)  membrane potential reverses again, dropping from approximately +30 mV to approximately -70 mV
                 d)  the Na⁺ K⁺ ATPase pumps are operating (3 Na⁺ out/2 K⁺ in per ATP hydrolysis)

         IV.  Hyperpolarization phase:
                 a)  the gated Na⁺ ion channels have closed; Na⁺ ions can no longer diffuse into the cytoplasm
                 b)  meanwhile, the gated K⁺ ion channels slowly close; K⁺ ions can no longer diffuse out of the cytoplasm
                 c)  membrane potential becomes more negative than the resting state, dropping to approximately -80 mV
                 d)  the Na⁺ K⁺ ATPase pumps are operating (3 Na⁺ out/2 K⁺ in per ATP hydrolysis) and will restore the resting state conditions

10. Two types of action potential (= nerve impulse) conduction. Which type is fastest? List the other factors that affect conduction speed.

          1) continuous conduction (unmyelinated axons) - slower
          2) saltatory conduction (myelinated axons) - faster

          Conduction speed is also influenced by axon diameter (larger fibers are faster) and degree of myelination of the axon (more myelination yields faster conduction).

Sketch and Label:

5. A graph of a neuron's action potential. Explain the phases of the action potential and how they correspond to the opening and closing of specific ion channels.

See the phases of the action potential and how they correspond to the opening and closing of specific ion channels in the answer to question 9 above.