Chemical Events at the Synapse

Study Section: Core Concepts & Signal Processing

1. Properties of the Action Potential and Related Terms

  • All-or-None Principle:

    • The rule that the size and speed of an action potential remain constant once the threshold is reached, regardless of stimulus intensity.

  • Graded Potentials:

    • EPSP (Excitatory Postsynaptic Potential): A graded depolarization that occurs when neurotransmitters open sodium gates, bringing the neuron closer to threshold.

    • IPSP (Inhibitory Postsynaptic Potential): A graded hyperpolarization that occurs when neurotransmitters open potassium or chloride gates, moving the neuron further from threshold.

  • Summation Mechanisms:

    • Temporal Summation: The combination of effects from multiple stimuli arriving at a single synapse in rapid succession.

    • Spatial Summation: The combination of effects from stimuli arriving at multiple synapses on the same neuron at the same time.

2. Comparison: Ionotropic vs. Metabotropic Receptors

  • Ionotropic Receptors:

    • Speed: Immediate and fast-acting (< 1 ms).

    • Mechanism: Directly opens ion-gated channels (e.g., sodium or chloride).

    • Duration: Brief, used for sensory information and rapid movement.

  • Metabotropic Receptors:

    • Speed: Slower onset (hundreds of ms up to seconds).

    • Mechanism: Activates a G-protein and second messengers (e.g., cAMP) to influence metabolic activity or gene expression.

    • Duration: Long-lasting, affecting enduring states like mood, hunger, or attention.

3. Neurotransmission: Order of Events

  1. Synthesis: The neuron manufactures neurotransmitter molecules from precursors supplied by the diet.

  2. Conduction: An action potential travels down the axon to the presynaptic terminal.

  3. Calcium Influx: Voltage-gated Ca^{2+} channels open; calcium enters the terminal.

  4. Exocytosis: Calcium triggers vesicles to fuse with the membrane, releasing neurotransmitters into the synaptic cleft (20-30 nm).

  5. Binding: Neurotransmitters diffuse across the cleft and bind to postsynaptic receptors, inducing EPSPs or IPSPs.

  6. Deactivation: The neurotransmitter is removed via enzymatic degradation (e.g., acetylcholinesterase) or reuptake by transporters.

  7. Negative Feedback: Autoreceptors on the presynaptic neuron detect excess neurotransmitters and signal to inhibit further release.

Summary of Material

  • Chemical neurotransmission primarily mediates synaptic communication; Otto Loewi's experimentation was pivotal in this discovery.

  • Various compounds function as neurotransmitters and modulate synaptic activity based on their receptor interactions.

  • Ionotropic effects produce rapid responses, whereas metabotropic effects initiate slower but prolonged changes, involving second messengers.

  • Several psychoactive substances act on specific receptors altering behavior and perception.

  • Reuptake systems and inactivation mechanisms determine neurotransmitter concentrations and prolong signaling effects.

  • Hormonal activity complements synaptic transmission, impacting broader physiological changes throughout the organism.

Quiz Preparation

  • Key study topics:

    • Properties of action potentials

    • Differences between ionotropic and metabotropic receptors

    • Events in neurotransmission sequence and associated terms.