Lecture Notes on Temporal and Spatial Summations

Key Concepts

• Excitatory Post-Synaptic Potentials (EPSPs)

  • Voltage fluctuations promoting membrane depolarization.
  • Increase membrane potential, making it easier to reach threshold.

• Inhibitory Post-Synaptic Potentials (IPSPs)

  • Voltage fluctuations preventing depolarization.
  • Make membrane potential more negative, harder to reach threshold.

• Neurotransmitters

  • Excitatory: Glutamate
    • Increases permeability to Na+ and K+, making potential more positive.
  • Inhibitory: GABA
    • Increases permeability to Cl- ions, making potential more negative.

Summation

• Summation
  • Sum of voltage changes from multiple potentials.
  • Occurs at the axon hillock.
  • EPSPs and IPSPs decrease in voltage from synapse to axon hillock.

Types of Summation

Temporal Summation

• Occurs when multiple action potentials arrive in rapid succession at a single presynaptic neuron.
• EPSPs add up at the axon hillock if they arrive within a short time.
• Time-dependent: Needs action potentials in quick succession to summate.
• Example: If EPSPs are too spaced out, they won’t summate properly.

Spatial Summation

• Occurs when action potentials from multiple presynaptic neurons arrive simultaneously at the postsynaptic neuron.
• Simultaneous EPSPs from various neurons result in a large compound EPSP.
• Results in sufficient voltage to reach the threshold and trigger an action potential.

Comparison:

• Temporal Summation

  • Successive action potentials in one neuron.
  • Short time frame for summation.

• Spatial Summation

  • Multiple neurons synapsing on one postsynaptic neuron.
  • Simultaneous action potentials for summation.

Visual Representation

• Temporal summation shows reduced amplitude from synapse to axon hillock due to voltage loss over distance.
• Spatial summation results in a compound EPSP sufficient for reaching the threshold.