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.
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.
Action Potential Arrival: When an action potential reaches axon terminals, voltage-gated Ca²⁺ channels open.
Ca²⁺ Influx: The concentration of Ca²⁺ increases inside the end bulb, associating with proteins on neurotransmitter vesicles.
Exocytosis: This association facilitates the merging of vesicles with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft through exocytosis.
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.
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.