Lecture Notes on Postsynaptic Potentials

Introduction

• Postsynaptic Potential: Graded potentials occurring in postsynaptic cells, like neurons.
• Neurotransmitter Role: Released from presynaptic neuron, binds to receptors on postsynaptic membrane.
• Ion Permeability: Changes in ion permeability lead to graded potentials.

Graded Potentials

• Influences:
  • Amount of neurotransmitter affects graded potential size.
  • Duration neurotransmitter remains in synaptic cleft affects potential size.
• Depolarization vs. Hyperpolarization:
  • Excitatory Postsynaptic Potentials (EPSPs): Depolarizing current, primarily due to sodium influx.
  • Inhibitory Postsynaptic Potentials (IPSPs): Hyperpolarizing current, due to potassium efflux or chloride influx.

Mechanism of EPSPs and IPSPs

• EPSPs: Sodium and potassium channels open; sodium influx greater than potassium efflux, leading to cell excitation.
• IPSPs: Potassium exits or chloride enters, making the cell voltage more negative.

Summation of Potentials

• Temporal Summation: Multiple EPSPs or IPSPs from one synapse add together over time.
• Spatial Summation: Simultaneous inputs from multiple synapses sum to create net effect.

Action Potentials

• Threshold Requirement: EPSPs need to be strong enough to reach action potential threshold at axon hillock.
• Propagation: EPSPs degrade over time and distance; must reach threshold to propagate action potential.

Synaptic Potentiation and Learning

• Synaptic Potentiation: Repeated use of a synapse increases the postsynaptic cell's responsiveness (akin to a positive feedback loop).
• Long-Term Potentiation (LTP): Strengthening of synaptic connections with repeated use, crucial for learning and memory.

Presynaptic Inhibition

• Inhibition Mechanism: An excitatory neuron can be inhibited by another neuron via an axo-axonal synapse.

Comparison: Graded Potentials vs. Action Potentials

• Graded Potentials:
  • Varied size (graded), decay over time and distance.
  • Stimulated by various stimuli, including light, pressure, temperature.
• Action Potentials:
  • All-or-none response, do not decay over distance.
  • Triggered by depolarization at axon hillock.
  • Positive feedback cycle propagates signal.
  • Repolarization involves voltage-gated channels.

Ionic Movements

• EPSPs: Sodium ions flow in, depolarizing the cell.
• IPSPs: Potassium flows out or chloride flows in, hyperpolarizing the cell.
• Action Potentials:
  • Rising phase: Sodium influx.
  • Falling phase: Potassium efflux.

These notes capture the key points of the discussion on postsynaptic potentials, emphasizing the mechanisms behind graded potentials and their role in neural communication.