Neuroscience 2610: Chapter 4.3: Generation and Propagation of the Action Potential

  • action potential

    • large, brief reversal in the polarity of an axon membrane

    • occurs when a large concentration of Na+ first and the K+ crosses the membrane rapidly

  • depolarization phase of the action potential is due to Na+ influx

  • hyperpolarization phase of the action potential is due to K+ efflux

  • threshold potential

    • action potential is triggered by the opening of Na+ and K+ voltage activated channels

    • membrane charge undergoes a remarkable change with no additional stimulation

  • if an axon memebrane is stimulated electrically while the solution surrounding the axon contains the chemical TEA

    • the result is a smaller than normal ion flow due to an Na+ influx

  • if an axon’s membrane is stimulated electrically while the solution surrounding the axon contains TTX

    • the result is a slightly different ion flow due to the efflux of K+

  • voltage activated channels

    • gated potential channel that opens or closes only at specific membrane voltages

    • closed when an axon’s membrane is at its resting potential, ions can’t pass through them

    • when the membrane reaches the threshold voltage, they open briefly which enable ions to pass through then close again to restrict the flow

  • dendrites and dendritic branches do not have many voltage activated channels and do not produce action potentials

  • inputs close to the initial segment are much more influential than those occurring some distance away and inhibitory as well, creating IPSPs

  • giant depolarizing potentials

    • some cells in the developing hippocampus can produce addition action potentials when the cell would ordinarily be refractory

  • back propagation

    • reverse movement of an action potential into the soma and dendritic field of a neuron

    • plays a role in the plastic changes that underlie learning

  • optogenetic

    • transgenic technique that combines genetics and light to excite or inhibit targeted cells in living tissue

  • ChR2

    • light activated channel that absorbs blue light

    • opens briefly to allow the passage of Na+ and K+

    • the resulting depolarization excites the cell enough to generate action potentials

  • NpHR

    • light driven ion pump fr Cl- ions

    • found in archaea or halobacteria

    • when illuminated with green-yellow light, the NpHR pumps Cl- anions into the cell, hyper polarizing if and inhibiting its activity

  • absolutely refractory

    • state of an axon in repolarizing period, during which a new action potential cannot be elicited because gate 2 of Na+ channels is closed

    • unable to produce an action potential while its repolarizing

  • relatively refractory

    • state of an axon in the later phase of an action potential, during which higher intensity electrical current is required to produce another action potential

    • a phase during which K+ channels are still open

  • nerve impulse

    • propagation(to give birth) of an action potential on the membrane of an axon

  • total voltage change during an action potential is 100 mV

  • membrane’s resting state is -70mV

  • action potential threshold levels is -50mV

  • an action potential does not dissipate

    • either generated completely or not generated at all

  • refractory periods

    • determined by the positions of the gates that mediate ion flow in the voltage activated channels

  • the action potential’s refractory phase has 2 uses for nerves that are conducting information

    • the maximum rate at which action potentials can occur is about 200 per second, the sensitivity of voltage activated channels affects firing frequency

    • refractory periods prevent the action potential from reversing direction and returning to its point of origin

  • action potential cannot occur where myelin is wrapped around an axon because the myelin is an insulating barrier that blocks ionic current flow

  • glial cells play a role in speeding nerve impulses in the vertebrate nervous system

  • nodes of ranvier

    • tiny gaps in the myelin sheath that is not covered in myelin

  • myelin has 2 important consequences for propagating action potentials

    • propagation becomes energetically cheaper, since action potentials are regenerated only at the nodes of ranvier and not along the axon’s entire length

    • myelin improves the action potential’s conduction speed

  • multiple sclerosis(MS)

    • disorder resulting from the loss of myelin sheath produced by oligodendroglia around axons in the CNS

    • initial symptoms

      • loss of sensation in the face, limbs or body

      • loss of control over movements

      • or both

  • autoimmune diseases

    • results from the loss of the immune system’s ability to discriminate between foreign pathogens in the body and the body itself

  • the cell body does not contain voltage activated ion channels but if summed inputs excite the initial segment to a threshold level, action potentials are triggered and then propagated as they travel a long the axon as a nerve impulse

  • nerve impulses travel more rapidly on myelinated axons because of saltatory conductions

  • saltatory conduction

    • action potentials leap between the nodes separating the glial cells that form the axon’s myelin sheath