Neuroscience Chapter on Synaptic Potentials

Synaptic Potentials

• Excitatory Postsynaptic Potential (EPSP):

  • Partial depolarization of the postsynaptic membrane.
  • Reaches maximum after 1-1.5 ms.
  • Duration: 2-5ms.
  • Ionic Basis: Excitatory neurotransmitter binds to receptors, opening ligand-gated Na⁺ (or Ca²⁺) channels. Influx of Na⁺ (or Ca²⁺) causes a small depolarization and brings the membrane closer to threshold.
  • Summation is necessary to reach threshold. Temporal summation occurs when presynaptic neuron stimulates repeatedly, closely timed EPSPs summate.

• Inhibitory Postsynaptic Potential (IPSP):

  • Partial hyperpolarization of the postsynaptic membrane.
  • Reaches maximum after 1.5-2 ms.
  • Duration: 3 ms.
  • Ionic Basis: Inhibitory neurotransmitter binds to receptors, opening ligand-gated Cl⁻ (or K⁺) channels. Influx of Cl⁻ (or efflux of K⁺) causes hyperpolarization, moving the membrane further from threshold.
  • IPSPs can also be caused by the closure of Na⁺ or Ca²⁺ channels.
  • Similar to EPSPs, IPSPs can summate temporally and spatially.

• Grand Postsynaptic Potential (GPSP):

  • Sum of all EPSPs and IPSPs occurring at the same time in one postsynaptic neuron.
  • Four possible outcomes:
    • Balance: EPSPs = IPSPs, no significant change.
    • Facilitation: EPSPs > IPSPs (membrane does not reach firing level).
    • Action Potential: EPSPs >> IPSPs (membrane reaches firing level).
    • Inhibition: IPSPs > EPSPs (hyperpolarization).

Action Potential vs. Postsynaptic Potential

Presynaptic Potentials

• Presynaptic Inhibition:

  • Third neuron releases inhibitory neurotransmitter (e.g., GABA).
  • Possible mechanisms:
    • Closure of Ca²⁺ channels in the presynaptic neuron.
    • Opening of K⁺ or Cl⁻ channels.
  • Reduced Ca²⁺ influx decreases neurotransmitter release.
  • Slow onset, lasting minutes to hours.

• Presynaptic Facilitation:

  • Third neuron releases excitatory neurotransmitter (e.g., serotonin).
  • Increases neurotransmitter release.
  • Possible mechanism:
    • Serotonin → cAMP → phosphorylation of K⁺ channels. Closed K⁺ channels and prolonged depolarization.
    • Prolonged depolarization increases Ca²⁺ channel activation, resulting in more transmitter release.

Synaptic Transmission Characteristics

• Forward Direction: Impulses flow from pre- to postsynaptic neuron.
• Synaptic Delay: Time taken for an impulse to cross a synapse (0.5 ms). It depends on the number of synapses.
• Fatigue: Decreased impulse discharge rate from postsynaptic neuron due to repetitive stimulation. Causes include depletion of synaptic vesicles and receptor inactivation.

Synaptic Plasticity

• Ability of synapse to change function according to demand.

• Short-term Inhibition (Habituation):

  • Gradual loss of response to repeated stimuli.
  • Mechanism: gradual inactivation of Ca²⁺ channels, reducing intracellular Ca²⁺, and neurotransmitter release.

• Short-term Facilitation (Post-tetanic Potentiation):

  • Brief intense stimulation leads to increased neurotransmitter release, longer-lasting response.
  • Mechanism: accumulation of Ca²⁺ in the presynaptic terminal due to weakened Ca²⁺ pumps, leading to continuous transmitter release. This is involved in memory formation.

Explore the intricacies of excitatory and inhibitory postsynaptic potentials in this quiz. Understand the ionic basis of these processes and their significance in neural communication. Test your knowledge on EPSPs and IPSPs, their characteristics, and how they contribute to synaptic transmission.