Neurotransmitter Release Enables Communication Between Neurons
- The release of neurotransmitters from the presynaptic membrane is a key step in synaptic transmission.
- This enables communication between neurons and target cells.
- Imagine trying to send a package to a friend across a river.
- You can’t toss it directly, so you place it in a boat that ferries it across.
- In the nervous system, neurons face a similar challenge: how to transmit signals across the tiny gap between them, known as the synaptic cleft.
- Neurotransmitters—chemical messengers—act as the "boat," carrying the signal from one neuron to the next.
Calcium Ions Trigger Neurotransmitter Release at the Axon Terminal
- The release of neurotransmitters begins with an electrical signal called an action potential traveling down the axon of a neuron.
- When this signal reaches the end of the axon, or the axon terminal, it triggers a series of events that lead to neurotransmitter release.
The axon terminal is also known as the presynaptic membrane because it is the part of the neuron that sends the signal across the synapse.
1. Depolarization of the Presynaptic Membrane
- An action potential travels along the axon and reaches the axon terminal of the presynaptic neuron.
- The arrival of an action potential causes the presynaptic membrane to depolarize.
- This means the membrane potential becomes less negative, opening voltage-gated calcium channels.
Definition
Depolarization
A change in the membrane potential of the presynaptic neuron, making it more positive.
2. Influx of Calcium Ions
- Calcium ions (Ca²⁺) are more concentrated outside the neuron than inside.
- Calcium ions (Ca²⁺) flow into the presynaptic neuron due to their concentration gradient (high extracellular and low intracellular concentration).
- This influx of calcium is a critical step in neurotransmitter release.
- Without it, the process cannot occur.
3. Calcium as a Signaling Molecule
- Inside the neuron, calcium ions act as a signaling molecule.
- They bind to proteins that trigger the movement of synaptic vesicles - tiny sacs filled with neurotransmitters- toward the presynaptic membrane.
Think of calcium ions as a key that unlocks the vesicles, allowing them to move and release their contents.
4. Exocytosis: Releasing Neurotransmitters into the Synaptic Cleft
- Once the synaptic vesicles reach the presynaptic membrane, they fuse with it in a process called exocytosis.
- This fusion creates an opening through which neurotransmitters are released into the synaptic cleft.
- Think of the synaptic vesicles as delivery trucks carrying neurotransmitters.
- The calcium ions act like the driver telling the truck when to unload its contents at the synaptic cleft (delivery site).
