How is synaptic transmission initiated?

How is synaptic transmission initiated?

• A. Nernst potential is reached.
• B. Myelin is formed.
• C. An action potential reaches the axon terminal, opening voltage-gated Ca2+ channels and triggering vesicle exocytosis.
• D. Na+ channels in soma open spontaneously.

Answer: (C)

The event that starts synaptic transmission is the arrival of an action potential at the presynaptic terminal. When the depolarization reaches the terminal, voltage-gated calcium channels open and Ca2+ enters the neuron. The rise in intracellular calcium triggers the fusion of neurotransmitter-filled vesicles with the presynaptic membrane, a process driven by calcium-sensing proteins and SNARE complexes, causing vesicle exocytosis. The released neurotransmitter then crosses the synaptic cleft and binds to receptors on the postsynaptic cell, propagating the signal. This mechanism isn’t about the ion gradients themselves (the Nernst potential) or myelin’s insulating role. Myelin speeds conduction along the axon but doesn’t initiate release at the terminal. Simply opening Na+ channels in the soma won’t trigger vesicle release at the synapse unless an action potential actually reaches the presynaptic terminal to cause Ca2+ influx.

The event that starts synaptic transmission is the arrival of an action potential at the presynaptic terminal. When the depolarization reaches the terminal, voltage-gated calcium channels open and Ca2+ enters the neuron. The rise in intracellular calcium triggers the fusion of neurotransmitter-filled vesicles with the presynaptic membrane, a process driven by calcium-sensing proteins and SNARE complexes, causing vesicle exocytosis. The released neurotransmitter then crosses the synaptic cleft and binds to receptors on the postsynaptic cell, propagating the signal.

This mechanism isn’t about the ion gradients themselves (the Nernst potential) or myelin’s insulating role. Myelin speeds conduction along the axon but doesn’t initiate release at the terminal. Simply opening Na+ channels in the soma won’t trigger vesicle release at the synapse unless an action potential actually reaches the presynaptic terminal to cause Ca2+ influx.