An action potential arriving at the presynaptic terminal triggers a series of events that result in the release of neurotransmitters into the synaptic cleft. These events include the opening of voltage-gated calcium channels, the influx of calcium ions, the fusion of synaptic vesicles with the presynaptic membrane, and the exocytosis of neurotransmitters.
Structure of an Action Potential Arriving at Presynaptic Terminal
When an action potential reaches the presynaptic terminal, it triggers a series of events that lead to the release of neurotransmitters. These neurotransmitters then travel across the synaptic cleft to bind to receptors on the postsynaptic neuron, causing a change in the membrane potential. The specific structure of an action potential is crucial for this process to occur effectively.
Key Features of an Action Potential
- Resting membrane potential: Before an action potential occurs, the neuron is at rest with a negative membrane potential, typically around -70 millivolts (mV).
- Depolarization: When a stimulus reaches the neuron, it causes the membrane to become less negative, known as depolarization.
- Threshold of excitation: When the depolarization reaches a critical level, called the threshold of excitation, it triggers an action potential.
- Rising phase: The membrane potential rapidly depolarizes to about +40 mV.
- Overshoot: The membrane potential then slightly overshoots the threshold of excitation, reaching a peak of about +50 mV.
- Falling phase: The membrane potential repolarizes, returning to the resting membrane potential.
- Hyperpolarization: After repolarization, the membrane potential briefly hyperpolarizes, becoming more negative than the resting potential.
- Refractory period: During the refractory period, the neuron is unable to generate another action potential due to the inactivation of voltage-gated sodium channels.
Calcium Influx and Neurotransmitter Release
The arrival of an action potential at the presynaptic terminal opens voltage-gated calcium channels, allowing calcium ions to flow into the cell. This increase in calcium concentration triggers the fusion of synaptic vesicles with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft.
Table: Key Parameters of an Action Potential
| Parameter | Value |
|---|---|
| Resting membrane potential | -70 mV |
| Threshold of excitation | -55 mV |
| Peak membrane potential | +50 mV |
| Absolute refractory period | 0.5-1 ms |
| Relative refractory period | 2-3 ms |
Factors Affecting Neurotransmitter Release
The amount of neurotransmitter released is affected by several factors, including:
- Frequency of action potentials: High-frequency firing leads to more calcium influx and higher neurotransmitter release.
- Duration of action potential: Longer action potentials allow more time for calcium influx and neurotransmitter release.
- Presynaptic calcium channels: The number and sensitivity of voltage-gated calcium channels influence the amount of calcium influx.
- Synaptic vesicle number and size: The availability of synaptic vesicles filled with neurotransmitter affects the amount released.
Question 1:
What does the arrival of an action potential at the presynaptic terminal cause?
Answer:
The arrival of an action potential at the presynaptic terminal causes the opening of voltage-gated calcium channels.
Question 2:
What is the effect of opening voltage-gated calcium channels at the presynaptic terminal?
Answer:
The opening of voltage-gated calcium channels at the presynaptic terminal allows an influx of calcium ions, triggering the release of neurotransmitters from synaptic vesicles.
Question 3:
How does the release of neurotransmitters from synaptic vesicles affect the postsynaptic neuron?
Answer:
The release of neurotransmitters from synaptic vesicles at the presynaptic terminal activates receptors on the postsynaptic neuron, leading to depolarization or hyperpolarization of the postsynaptic neuron.
And there you have it, folks! The journey of an action potential through the synapse, from arrival to neurotransmitter release. It’s a complex but fascinating process that underpins everything we do, from thinking to moving. Thanks for reading, and be sure to check back for more brain-busting adventures in the future!