AL

Synapses

Nervous System I: Basic Structure & Function

Synapses

  • Types of Synapses: There are two primary types of connections between electrically active cells: chemical synapses and electrical synapses.

    • Chemical Synapses:

      • Involve the release of neurotransmitters, which are chemical signals.

      • Transmit information chemically, influencing the receiving cell.

      • An example: Neuromuscular Junction (NMJ), where a neurotransmitter released affects muscle contractions.

    • Electrical Synapses:

      • Feature a direct connection between two cells.

      • Allow ions to pass directly from one cell to another, causing depolarization if one cell becomes depolarized.

Common Characteristics of Synapses

  • Synapses have several common characteristics:

    • Presynaptic Element: The part of the neuron that releases neurotransmitters; for NMJ, this is the motor neuron's axon terminals.

    • Neurotransmitter: The chemical signal; in NMJ, this is acetylcholine.

    • Synaptic Cleft: The small gap between cells where neurotransmitters diffuse.

    • Receptor Proteins: Proteins that receive neurotransmitters; NMJ uses nicotinic acetylcholine receptors.

    • Postsynaptic Element: The receiving cell component; for NMJ, this is the sarcolemma of the muscle cell.

    • Neurotransmitter Elimination: Involves the breakdown or reuptake of neurotransmitters; NMJ utilizes acetylcholinesterase for acetylcholine elimination.

Release and Interaction of Neurotransmitters

Neurotransmitter Release

  1. Action Potential Arrival: When an action potential reaches axon terminals, voltage-gated Ca²⁺ channels open.

  2. Ca²⁺ Influx: The concentration of Ca²⁺ increases inside the end bulb, associating with proteins on neurotransmitter vesicles.

  3. Exocytosis: This association facilitates the merging of vesicles with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft through exocytosis.

Neurotransmitter Interaction

  • After release, neurotransmitters diffuse across the synaptic cleft and bind to specific receptor proteins on the postsynaptic membrane.

  • Each receptor is specific to a particular neurotransmitter, functioning like a key and lock.

  • This specificity ensures precise signal transmission between neurons, allowing only specific binding events to occur.

Terms to Know

  • Neurotransmitter: A chemical substance that transmits signals between neurons or from neurons to target cells; typically stored in vesicles in the presynaptic terminal.

  • Neuronal Reuptake: The process of taking neurotransmitters back into the presynaptic terminal for reuse after their release.

  • Glial Reuptake: The removal of neurotransmitters from the synaptic cleft by nearby glial cells (e.g., astrocytes), helping to regulate neurotransmitter levels.