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.