Lab 2: Excitable Cell Membrane

• Resting membrane potential difference (resting potential) - voltage difference across the cell membrane at rest
  • Artificially consider the outside of the cell to be 0mV and consider the difference to be the voltage inside for simplicitiy
  • Higher [K+] ICF
  • Higher [Na+] ECF
  • Usually -40 to -80 mV, -70 mV in example neuron
• Stimulus →graded potential → action potential → exocytosis of ACh from axon terminal
• Graded potential - depolarization of soma and dendrites via influx of Na+
  • Membrane receptors - gated Na+ channels lining the soma and dendrites that respond to stimulus (ACh, other chemical ligand, mechanical force on cell membrane, etc.)
  • Strength of graded potential determined by strength of stimulus
  • Number of membrane receptors opened
  • Length of time membrane receptors stay open
  • Amount of Na+ allowed to influx = strength of potential
  • Summation - accumulation of Na+ from multiple graded potentials
  • No distinct repolarization process in soma, takes time for Na+ to dissipate or be pumped back out after influx, can have multiple action potentials from a single graded potential and/or summation
  • Temporal summation - from multiple graded potentials over short period of time from the same origin (separated by time)
  • Spatial summation - from multiple graded potentials arriving at about the same time from different origins (separated by space)
• Action potential - depolarization along length of axon
  • Threshold - voltage required to initate action potential
  • Axon hillock - connection between soma and axon
  • Voltage gated Na+ channel
  • Activiation gate opens at -55 mV (threshold in example neuron)
    • Allows influx of Na+ from ECF causing depolarization
  • Inactivation gate closes at about the same time axon reaches +30 mV (in example neuron, timed from threshold)
    • Does not respond to voltage again after closing (under normal circumstances), takes time to reset.
  • Voltage gated K+ channel starts opening at -55 mV (threshold in our example neuron)
  • Allows efflux of K+ from ICF causing repolarization, hyperpolarization
  • Only has activation gate
  • Slower to open than Na+ channel activation gate
  • Timed to be open about the same time the Na+ inactivation gate closes (cell reaches +30 mV based on normal concentration gradient and influx rate of Na+)
  • “All or nothing” due to nature of the channels
  • Absolute refractory period - time during which another action potential cannot be initiated
  • Time between activation of the voltage gated Na+ channel and when it resets to resting state
  • Starts at threshold when voltage gated Na+ channel opens
  • Continues while inactivation gate of voltage gated Na+ channel is closed
  • Keeps action potentials as separate, distinct depolarizations rather than allowing axon to depolarize for extended period of time
  • Relative refractory period - time during which it is more difficult than normal to initiate an action potential
  • Cell is hyperpolarized so it takes a greater amount of Na+ from the soma to push the axon hillock to threshold
  • Frequency coding - frequency or number of action potentials codes for strength of signal or strength of stimulus
  • strength of stimulus → strength of graded potential → number or frequency of action potentials → amound of ACh exocytosed at axon terminal → stimulus for target cell
  • Transduction rate
  • Axon diameter - walls cause resistance to flow, more distance from walls = faster rate; threfore larger axons allow for faster transduction (less efficient but faster)
  • Myelination - prevents ion leakage, if Na+ leaks back out along axon → [Na+] inside axon decreases → diffusion rate decreases → transduction rate decreases
    • Fewer gated channels involved, Na+ diffuses down axon through myelin covered section (no other path available), less Na+ required to be moved for process = more efficient
    • Fast pain = myelinated axon
    • Slow pain = non-myelinated axon
  • Compound action potential - nerve segment is bundle of axons, generally not possible to separate a single one to be tested (except for squid giant axon); the sum of action potentials travelling on parallel axons at the same time, will vary in strength as number of axons depolarizing at one time changes
• Exocytosis of neurotransmitter
  • Depolarization at axon terminal triggers voltage gated Ca2+ channels (Ca2+ messenger)
  • The influx of Ca2+ triggers the release of the synaptic vesicles from the cytoskeleton, their movement toward the membrane, and exocytosis.

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