Lecture Notes: Excitatory and Inhibitory Synaptic Potentials

Key Concepts

• EPSP (Excitatory Post Synaptic Potential):

  • Typically not strong enough to reach threshold voltage on its own.
  • A single EPSP is usually around 0.5 mV.
  • Threshold voltage for action potential is -55 mV (from a resting potential of -70 mV).

• Neuronal Threshold:

  • Action potentials occur once threshold is reached.
  • Membrane potential is influenced by thousands of synapses, both excitatory and inhibitory.

Synaptic Integration

• The membrane potential of a post synaptic cell results from all synapses acting on it.
• Different pre-synaptic axons have varying influences on post-synaptic charge.

Summation

• Summation: The cumulative effect of synaptic potentials on a post synaptic neuron.

Types of Summation

• Temporal Summation:

  • Occurs when multiple signals from a single neuron are received in quick succession.
  • Greater depolarization occurs due to lack of time for ion exchange to fully return to resting state.
  • No refractory period in ligand-gated channels, unlike voltage-gated sodium channels.

• Spatial Summation:

  • Involves simultaneous signals from multiple presynaptic neurons.
  • Results in greater depolarization due to increased ion flow.

Combination of Summations

• Temporal and Spatial Summation Combined:
  • Can result in significant depolarization sufficient to reach threshold for action potential.

Inhibitory Synapses

• Inhibitory Neurons (e.g., Neuron C):
  • Open potassium or chloride channels leading to hyperpolarization.
  • Can neutralize excitatory effects when simultaneously stimulated with excitatory neurons.

Action Potential Dynamics

• Initial Segment (Axon Hillock):

  • Has a threshold of -55 mV.
  • Higher density of voltage-gated sodium channels makes it sensitive to depolarizations.

• Voltage-Gated vs. Ligand-Gated Channels:

  • Ligand-gated channels generate synaptic potentials.
  • Voltage-gated sodium channels initiate action potentials.

Synaptic Potential Characteristics

• Graded Potentials:

  • Rapidly dissipate; non-decremental.
  • Presynaptic neurons closer to axon hillock have greater influence.

• Synaptic Potentials Last Longer:

  • Longer duration compared to action potentials.
  • Allow for bursts of action potentials in post synaptic neurons.

Practical Implications

• Action Potentials in Bursts:
  • Synaptic potentials can cause rapid sequences of action potentials rather than isolated events.
  • Facilitates continuous signaling until repolarization occurs.

These notes provide an overview of key topics on neuronal synaptic activity, focusing on EPSP, IPSP, and the mechanics of summation in generating action potentials.