Synaptic Transmission Study Notes

Synaptic Transmission between Neurons and Post-synaptic Integration

Overview of Synaptic Information Flow

  • Convergence and Divergence: Key concepts in the organization of neuronal connections.
    • Convergence: The phenomenon where multiple presynaptic neurons synapse onto a single postsynaptic neuron.
    • Divergence: The branching of one presynaptic neuron to innervate multiple postsynaptic neurons.
  • Most neurons receive hundreds to thousands of synaptic inputs that must be integrated before responding with an action potential.

Transfer of Information at Chemical Synapses

  • The process of synaptic transmission involves transmitter binding to post-synaptic receptors which can affect:
    • Membrane potential of the postsynaptic neuron.
    • Biochemistry of the postsynaptic neuron.
  • These effects can occur at different speeds and can be classified based on the mechanisms of receptor binding:
    1. Transmitter-gated ion channels: Directly alter ion flow across the membrane.
    2. G-protein linked channels: Activate G proteins leading to slower biochemical changes.
    3. Second messengers: Can further affect ion channels and various proteins, leading to more complex responses.

Types of Ion Channels Involved in Synaptic Transmission

Transmitter-gated Ion Channels
  • Selectivity: Less selective compared to voltage-gated channels and often react to multiple ions.
  • Types:
    • Excitatory:
    • Activate depolarization by allowing Na+ influx and K+ efflux through the same channel.
    • Inhibitory:
    • Induce hyperpolarization primarily through Cl- influx and K+ efflux via different channels.

Fast and Slow Synaptic Inputs

  • Neurons can process both fast and slow synaptic inputs:
    • Enables rapid response to immediate stimuli as well as adjustments in responsiveness to future inputs over time.

Post-synaptic Membrane Potential Changes

  • Changes can result in:
    • Depolarization: Creates an Excitatory Post-Synaptic Potential (EPSP); it occurs when the membrane potential moves closer to the action potential threshold.
    • Hyperpolarization: Creates an Inhibitory Post-Synaptic Potential (IPSP); it makes the interior of the neuron more negative, moving it further from the threshold.
  • Representation:
    • EPSP reaches around -55 mV, while IPSP is governed by the dynamics of K+ and Cl- ions.

Characteristics of Post-synaptic Potentials versus Action Potentials

Post-synaptic Potentials
  • Involves: Transmitter-gated or second messenger-gated channels.
  • Features:
    • Graded response (varying in amplitude based on input strength).
    • Long-lasting (>1-2 ms, variable duration) and can sum since there is no refractory period.
    • Amplitude decreases passively with distance (“electrotonically”).
Action Potentials
  • Involves: Voltage-gated channels.
  • Features:
    • All-or-none response, brief duration (around 1-2 ms).
    • Has refractory periods, preventing summation.
    • Amplitude is consistent (does not adapt) and is propagated without a loss of strength.

Spatial and Temporal Summation of Inputs

Spatial Summation
  • Simultaneous excitatory inputs from different locations add together to reach the threshold for an action potential.
Temporal Summation
  • Repeated excitatory inputs close in time add together to reach the action potential threshold.
  • With separate stimuli, EPSPs do not sum if spaced too far apart.

Factors Influencing Synaptic Transfer

  1. Presynaptic Activity Modulation:
    • Transmitter release is dependent on the size & duration of depolarization in the presynaptic terminal.
    • Increased presynaptic depolarization enhances Ca++ entry, increasing transmitter release and subsequently EPSP amplitudes.
  2. Historical Activity at the Synapse:
    • Prior neuronal activity can influence current synaptic performance and effectiveness.
  3. Post-Synaptic Adaptation:
    • The postsynaptic neuron can also regulate its own response dynamics, influencing how it interacts with incoming signals.