Synaptic EPSPs & IPSPs

Overview of Synaptic Signalling

  • The lecture revisits key electrical events in neurophysiology:
    • Action potentials (all-or-none spikes that travel long distances).
    • Graded potentials (local, variable-magnitude changes in membrane voltage).
    • Chemical synapses as the junction where information is passed from a presynaptic neuron to a postsynaptic neuron via neurotransmitters.
  • Neurotransmitters can be either excitatory or inhibitory in their effect on the postsynaptic membrane.

Neurotransmitter Release and Binding

  • Sequence of events:
    1. Presynaptic neuron releases neurotransmitter into the synaptic cleft.
    2. Neurotransmitter diffuses across the cleft.
    3. It binds to specific receptor proteins on the postsynaptic membrane.
    4. Receptor activation produces a graded potential in the postsynaptic neuron.
  • Two critical variables at the synapse determine the size of that graded potential:
    • Amount of neurotransmitter released.
    • Duration the neurotransmitter remains in the cleft and available to bind.

Graded Potentials: General Properties

  • Can vary continuously in amplitude (unlike the all-or-none action potential).
  • Sign can be positive (depolarizing) or negative (hyperpolarizing).
  • Magnitude ∝ (quantity of neurotransmitter) × (time neurotransmitter present).

Excitatory Postsynaptic Potentials (EPSPs)

  • Definition: A graded depolarization generated on the postsynaptic membrane when an excitatory neurotransmitter binds its receptor.
  • Depolarization = membrane potential moves closer to zero (becomes less negative).
  • Main features stressed in the lecture:
    • Still a graded event—size can be small or large depending on transmitter quantity/duration.
    • Because it is depolarizing, it increases the likelihood that the neuron will reach threshold and fire an action potential.
  • Conceptual reminder: Many people intuitively focus on excitation because it “drives” neural activity, but inhibition is equally important.

Inhibitory Postsynaptic Potentials (IPSPs)

  • Definition: A graded hyperpolarization produced when an inhibitory neurotransmitter activates its receptor.
  • Hyperpolarization = membrane potential becomes more negative, i.e. moves farther from zero.
  • Consequence: The neuron is moved farther from threshold, making it harder to trigger an action potential.
  • Key variables (same as EPSPs): amount of transmitter and its dwell time in the cleft set the IPSP magnitude.

Threshold, Depolarization, and Hyperpolarization (Contextual Connections)

  • While not numerically specified in the clip, typical values from foundational material:
    • Resting potential ≈−70 mV\approx -70\,\text{mV}.
    • Threshold potential ≈−55 mV\approx -55\,\text{mV}.
  • EPSPs that cumulatively bring the membrane from −70-70 to −55 mV-55\,\text{mV} initiate an action potential.
  • IPSPs that move the membrane even a few mV below resting can effectively veto nearby EPSPs, illustrating the push-pull logic of neural computation.

Why Inhibition Matters (Philosophical & Practical Implications)

  • Although the lecture only hints at it ("You'd be surprised"), inhibition:
    • Prevents runaway excitation and seizures.
    • Sharpens signal processing by creating contrast (e.g., lateral inhibition in sensory systems).
    • Enables complex logic computations (e.g., NOT, AND-NOT circuits) in neural networks.
    • Balances excitatory drive to maintain homeostasis in neural circuits.

Take-Home Points

  • EPSP = depolarizing graded potential; IPSP = hyperpolarizing graded potential.
  • Both are produced by neurotransmitter–receptor interactions; neither is all-or-none.
  • The magnitude of either potential depends on how much and how long neurotransmitter is present.
  • EPSPs move the membrane toward action-potential threshold; IPSPs move it away, making threshold harder to reach.
  • Effective neural signalling relies on the dynamic interplay of excitation and inhibition, not solely on excitation.