Synaptic Signalling Study Notes

Overview of Synaptic Signalling

References Used

  • Bear, Connors, Paradiso: 'Neuroscience- Exploring the Brain'

  • Berne & Levy: 'Principles of Physiology'

  • Haines: 'Fundamental Neuroscience'

  • Kandel, Schwartz, Jessell: 'Principles of Neural Science'

  • Dr. Kevin Whitehead's contact: k.j.whitehead@bham.ac.uk

Key Concepts

  • Synaptic Diversity: Various types of synapses can influence neural communication and behavioral outcomes.

  • Receptor-mediated Effects on Excitability:

    • Ionotropic Receptors: These are receptor-operated/ligand-gated channels that mediate rapid responses.

    • Metabotropic Receptors: These are G-protein coupled receptors that mediate slower but longer-lasting effects.

Synaptic Integration of Information

  • Spatial Summation: This refers to the summation of post-synaptic potentials occurring at multiple synapses simultaneously.

  • Temporal Summation: This pertains to the summation of post-synaptic potentials that occur at the same synapse in rapid succession.

Frequency Coding of Synaptic Signalling

  • Action potentials communicate information through the frequency rather than amplitude. The more frequent the action potentials, the greater the stimulus intensity.

The Chemical Synapse

  • Structure:

    • Presynaptic Active Zone: Site for vesicle docking and exocytosis.

    • Postsynaptic Density: Location for receptor expression and the intracellular signalling machinery.

Synaptic Arrangements in the CNS

  • Types of Synapses:

    1. Axodendritic: Synapse between an axon and a dendrite.

    2. Axosomatic: Synapse between an axon and a soma, often more influential due to proximity.

    3. Axoaxonic: Synapse between two axons.

Influence of Point of Synaptic Contact

  • Proximity to Axon Hillock:

    • Synapses closer to the axon hillock exert a greater influence on action potential generation.

    • Inhibitory synapses are often found on the soma and near the axon hillock, providing a crucial mechanism for controlling neuron excitability.

  • A typical neuron may have between 1,000 to 10,000 synapses influencing its activity.

Receptor Signalling Mechanisms

  1. Ionotropic Receptors:

    • When a transmitter binds, it causes a conformational change that opens the channel, allowing ion movement.

    • A typical activation might involve an excitatory post-synaptic potential (EPSP).

  2. Metabotropic Receptors:

    • Transmitter binding leads to a conformational change, activating a G-protein that subsequently activates effector systems, leading to indirect effects such as opening or closing ion channels or stimulating/inhibiting enzymes and secondary messenger systems.

Electrical Effects of Receptor Activation

  • Post-Synaptic Membrane Potential:

    • Recording shows a resting membrane potential around -70 mV.

    • Following activation, small EPSPs and IPSPs can occur, altering the electrical state of the neuron.

Action Potential Generation

  • Threshold Potential: For an action potential to occur, the neuron must reach a threshold where voltage-dependent Na+ channels open.

    • The action potential is characterized as an all-or-nothing event, meaning its amplitude does not vary with stimulus strength.

  • Refractory Periods:

    • Absolute Refractory Period (ARP): Time frame during which no new action potential can be generated regardless of stimulus strength.

    • Relative Refractory Period (RRP): A period in which a stronger-than-usual stimulus is necessary to generate an action potential.

Summation Mechanisms

Spatial Summation

  • EPSPs from multiple synaptic inputs combine to increase overall membrane potential, enhancing the chance of generating an action potential.

  • Best co-operation of inputs happens when the coinciding EPSPs are close enough in time and space.

Temporal Summation

  • Rapid successive activation of the same synapse allows for increased local potential, which can serve to push the neuron over the threshold for firing.

Communication of Neurons

  • Neuronal communication results from integrating all input signals to send a frequency-encoded message:

    • The frequency of action potentials relates directly to the intensity of the stimulus, rather than using amplitude modulation.

    • Maximum firing rates can reach up to 200–300 Hz, primarily limited by the durations of the ARP and RRP.

Main Points Recapped

  • The influence of synapses varies depending on location: axodendritic versus axosomatic connections.

  • Different receptor types (ionotropic vs metabotropic) result in different signaling speeds and durations.

  • Ion fluxes, like Na+ for depolarization and Cl- for hyperpolarization, are critical for controlling membrane polarization and cell excitability.

  • Information integration occurs through spatial and temporal summation, leading to action potentials that encode signals through frequency modulation.