Psychology 2: Synaptic Communication

Synaptic Communication: Comprehensive Notes

The Reflex Arc

  • Components:

    • Receptor: Detects stimuli.

    • Sensory Neuron: Carries sensory information to the central nervous system.

    • Interneuron (in Gray Matter): Connects sensory and motor neurons, allowing for integration (e.g., in the spinal cord).

    • Motor Neuron: Carries motor commands from the CNS to the effector.

    • Effector: Muscle or gland that responds to the motor command.

    • Dorsal Root Ganglion: Contains cell bodies of sensory neurons.

    • Ventral Root: Contains axons of motor neurons.

    • Mixed Nerve: Contains both sensory and motor fibers.

    • Cerebrospinal Canal: Central canal within the spinal cord.

Properties of Synapses (Observed via Reflexes)

Three important points about reflexes that highlight synaptic properties:

  1. Slower Conduction: Reflexes are significantly slower than the speed of action potential conduction along a single axon. This delay occurs at the synapse due to the time required for chemical transmission.

  2. Antagonistic Muscle Excitation/Relaxation: As one set of muscles involved in a reflex becomes excited and contracts, another antagonistic set of muscles simultaneously relaxes (e.g., flexor-extensor reflex). This indicates both excitatory and inhibitory synaptic processes.

  3. Summation: Several weak stimuli presented at slightly different times (temporal summation) or slightly different locations (spatial summation) can produce a stronger reflex than a single weak stimulus. This demonstrates the integrative nature of synapses.

Chemical or Electrical Messages? The Otto Loewi Experiment

  • Otto Loewi's Discovery: Demonstrated that neurons communicate via chemical messengers, not solely electrical signals.

  • Experiment Setup:

    • Loewi isolated two frog hearts.

    • Heart 1 (donor heart) had its vagus nerve (which slows heart rate) stimulated.

    • The fluid bathing Heart 1 was then transferred to Heart 2 (recipient heart), which did not have its vagus nerve stimulated.

  • Results:

    • Stimulating the vagus nerve in Heart 1 caused its heart rate to slow down.

    • The fluid transferred from Heart 1 to Heart 2 also caused Heart 2's heart rate to slow down.

  • Conclusion: A chemical substance, later named Vagusstoff, was released by the stimulated vagus nerve into the fluid, carrying the message to slow the heart rate.

  • Key Discoveries:

    • Vagusstoff (Acetylcholine): Reduced heart rate.

    • Accelerenzstoff (Epinephrine): Increased heart rate.

    • This seminal experiment established the principle of chemical neurotransmission.

Chemical Synaptic Neurotransmission: The Five Stages

The process of chemical communication at the synapse involves a series of sequential events:

  1. Synthesis: Neurotransmitters are manufactured.

  2. Packaging: Neurotransmitters are stored in vesicles.

  3. Release: Neurotransmitters are discharged into the synaptic cleft.

  4. Receptor Action: Neurotransmitters bind to receptors on the postsynaptic neuron.

  5. Inactivation: Neurotransmitters are removed from the synaptic cleft to terminate their action.

1. Synthesis

  • Classical Neurotransmitters (e.g., small-molecule neurotransmitters):

    • Synthesized in the terminal button of the presynaptic neuron.

    • Precursor molecules are converted into neurotransmitters by synthetic enzymes.

  • Peptide Neurotransmitters (Neuropeptides):

    • Synthesized in the soma (cell body) of the neuron.

    • Begin as larger precursor peptides, which are then processed by the rough endoplasmic reticulum and Golgi apparatus.

    • Active peptide neurotransmitters are then packaged into secretory granules.

2. Packaging

  • Classical Neurotransmitters:

    • Synthesized in the terminal button.

    • Transporter proteins embedded in the synaptic vesicle membrane actively pump the neurotransmitter molecules into synaptic vesicles.

  • Peptide Neurotransmitters:

    • Synthesized in the soma.

    • Packaged into larger secretory granules (often called dense-core vesicles) by the Golgi apparatus. These granules are then transported down the axon to the terminals.

3. Release (Exocytosis)

This intricate process leads to the discharge of neurotransmitters into the synaptic cleft:

  1. Action Potential Arrival: An action potential arrives at the presynaptic terminal.

  2. Voltage-Gated Ca^{2+} Channel Opening: The depolarization caused by the action potential opens voltage-gated Ca^{2+} channels in the presynaptic membrane.

  3. Ca^{2+} Influx: Ca^{2+} ions rush into the presynaptic neuron from the extracellular fluid, as the intracellular Ca^{2+} concentration is typically very low.

  4. Ca^{2+} Action on Vesicles: The influx of Ca^{2+} acts on a cluster of protein molecules located in the membrane of the synaptic vesicle and the presynaptic membrane.

  5. Vesicle Docking and Fusion: Ca^{2+} causes the docked synaptic vesicles to fuse with the presynaptic cell membrane. Special protein clusters facilitate this docking.

  6. Fusion Pore Formation: