What occurs when presynaptic neurons are depolarized, and how do they release neurotransmitters into the synapse?

SouravNovember 9, 2024

Answer

When presynaptic neurons are depolarized, a series of events occurs that leads to the release of neurotransmitters into the synapse. This process is essential for communication between neurons and other cells, such as muscle cells or glandular cells. Here’s a detailed overview of the mechanism involved:

Depolarization of Presynaptic Neurons

1. Action Potential Arrival:
   • The process begins when an action potential (an electrical impulse) travels down the axon of the presynaptic neuron and reaches the axon terminal. This action potential is characterized by a rapid change in membrane potential, leading to depolarization.
2. Opening of Voltage-Gated Calcium Channels:
   • The depolarization at the presynaptic terminal causes voltage-gated calcium (Ca2+2+) channels in the presynaptic membrane to open. This allows Ca2+2+ ions, which are present in higher concentrations outside the neuron, to flow into the presynaptic terminal.
3. Calcium Influx:
   • The influx of Ca2+2+ ions is a critical trigger for neurotransmitter release. The increase in intracellular calcium concentration initiates a cascade of events that prepares synaptic vesicles for fusion with the presynaptic membrane.

Release of Neurotransmitters

1. Vesicle Docking and Priming:
   • Synaptic vesicles, which contain neurotransmitters, are located near the presynaptic membrane at specialized sites known as active zones. Proteins called SNARE proteins (including syntaxin and SNAP-25 on the membrane and synaptobrevin on the vesicle) facilitate the docking and priming of these vesicles, preparing them for rapid fusion.
2. Fusion of Vesicles with Membrane:
   • The rise in intracellular calcium is sensed by a protein called synaptotagmin, which interacts with the SNARE complex and promotes membrane fusion. This fusion event allows the vesicle membrane to merge with the presynaptic membrane, resulting in exocytosis—the release of neurotransmitters into the synaptic cleft.
3. Neurotransmitter Diffusion:
   • Once released, neurotransmitters diffuse across the synaptic cleft (the small gap between the presynaptic and postsynaptic membranes) and bind to specific receptors on the postsynaptic cell.
4. Receptor Activation:
   • The binding of neurotransmitters to postsynaptic receptors can lead to various effects, such as depolarization (excitatory postsynaptic potential, EPSP) or hyperpolarization (inhibitory postsynaptic potential, IPSP) of the postsynaptic neuron, depending on the type of receptor activated.
5. Endocytosis and Recycling:
   • After neurotransmitter release, synaptic vesicles are quickly recycled through endocytosis, allowing them to be refilled with neurotransmitters and prepared for subsequent rounds of release.