Neuroscience: Synaptic Potentials Quiz

Synaptic Potentials

• Excitatory Postsynaptic Potential (EPSP):

  • Causes partial depolarization of the postsynaptic membrane.
  • Reaches maximum after 1-1.5 milliseconds.
  • Lasts 2-5 milliseconds.
  • Ionic basis: excitatory neurotransmitter binding to receptors opens ligand-gated Na+ (or Ca++) channels, allowing Na+ (or Ca++) influx, creating a small depolarization.
  • Summation is required to reach threshold for an action potential.
  • Temporal summation occurs when the presynaptic neuron is stimulated repeatedly in quick succession.

• Inhibitory Postsynaptic Potential (IPSP):

  • Causes partial hyperpolarization of the postsynaptic membrane.
  • Reaches maximum after 1.5-2 milliseconds.
  • Lasts 3 milliseconds.
  • Ionic basis: inhibitory neurotransmitter binding to receptors opens ligand-gated Cl- (or K+) channels, allowing Cl- influx (or K+ efflux), creating a small hyperpolarization, moving the membrane away from the firing level.
  • IPSPs can also be produced by closing Na+ or Ca++ channels.
  • Summation is required to affect membrane potential.
  • Temporal and spatial summation are similar to EPSPs.

• Grand Postsynaptic Potential (GPSP):

  • Sum of all EPSPs and IPSPs occurring at the same time in one postsynaptic neuron.
  • Four possible outcomes:
    • Balance between facilitation and inhibition: no change to membrane potential.
    • Facilitation: EPSP > IPSP, no action potential.
    • Action potential: EPSP >> IPSP, membrane reaches firing level.
    • Inhibition: IPSP > EPSP, hyperpolarization.

Action Potential vs. Postsynaptic Potential

• Action Potential:

  • All-or-none law.
  • Cannot be graded.
  • Cannot be summed.
  • Propagated.
  • Has absolute refractory period.
  • Blocked by anesthesia.
  • Duration: 1 millisecond.

• Postsynaptic Potential:

  • Does not follow all-or-none law.
  • Graded (amplitude varies).
  • Can be summed.
  • Not propagated.
  • No absolute refractory period.
  • Not blocked by anesthesia.
  • Duration: 20 milliseconds.

Presynaptic Potentials

• Presynaptic Inhibition:

  • Third neuron (inhibitory) releases an inhibitory transmitter.
  • Mechanisms:
    • Closure of Ca++ channels.
    • Opening of K+ or Cl- channels.
    • Reduced Ca++ entry into the presynaptic knob, decreasing neurotransmitter release.
  • Slow development, lasting minutes to hours.

• Presynaptic Facilitation:

  • Third neuron (excitatory) releases an excitatory transmitter (e.g., serotonin).
  • Serotonin/cAMP increases in the presynaptic knob.
  • Phosphorylates a protein in K+ channels.
  • Closes K+ channels, prolonging depolarization.
  • Increased Ca++ influx causing sustained neurotransmitter release.

Synaptic Transmission Characteristics

• Forward Direction (Unidirectional): Impulses travel from presynaptic to postsynaptic neuron.
• Synaptic Delay: Time taken for an impulse to travel across a synapse, typically 0.5 milliseconds.
• Fatigue: Decreased rate of impulse discharge from the postsynaptic neuron due to repetitive stimulation (exhaustion of synaptic vesicles).

Synaptic Plasticity

• Short-Term Inhibition (Habituation): Gradual loss of response to a benign stimulus repeated at intervals (inactivation of Ca2+ channels reducing neurotransmitter release).
• Short-Term Facilitation (Post-tetanic Potentiation): Brief, high-frequency stimulation of a presynaptic neuron; brief, intense stimulation leading to sustained release of neurotransmitter. Ionic basis is accumulation of Ca2+ in the presynaptic neuron, weakening Ca2+ pump.
• Sensitization: Prolonged augmented response of the postsynaptic neuron due to application of a noxious stimulus accompanying a benign stimulus (Pre-synaptic facilitation).