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).
- Imagine a balloon filled with water.
- When you squeeze the balloon, the water is forced out through a small opening.
- Similarly, exocytosis pushes neurotransmitters out of the vesicle and into the synaptic cleft.
Definition
Exocytosis
Exocytosis is a process that occurs when a cell moves large materials from inside the cell to the outside of the cell using small spheres of membrane called vesicles.
5. The Journey of Neurotransmitters
- After being released, neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane (the membrane of the next neuron or an effector cell, such as a muscle or gland).
- This binding triggers a response in the postsynaptic cell, such as the opening of ion channels, which can lead to the generation of a new action potential.
- Students often confuse the roles of calcium ions and sodium ions.
- Remember, calcium ions are involved in neurotransmitter release, while sodium ions are critical for generating action potentials.
Calcium Ions Are Ideal for Triggering Neurotransmitter Release
- Calcium ions are uniquely suited for this role because:
- They have a strong electrochemical gradient, ensuring a rapid influx when channels open.
- They can bind to specific proteins, triggering precise cellular responses.
- How does the specificity of calcium ions as a signaling molecule illustrate the relationship between structure and function in biology?
- Can you think of other examples where a molecule’s properties make it uniquely suited for its role?
Resetting the Synapse Prepares Neurons for the Next Signal
- After neurotransmitter release, the system must be reset to prepare for the next signal.
- This involves several key steps:
- Calcium Removal: Calcium ions are quickly removed from the axon terminal by calcium pumps and exchangers. This ensures that the signaling process is tightly controlled and does not continue indefinitely.
- Vesicle Recycling: The synaptic vesicles are reformed and refilled with neurotransmitters, ready for the next round of release.
- Neurotransmitter Clearance: Neurotransmitters in the synaptic cleft are broken down by enzymes or reabsorbed by the presynaptic neuron. This prevents continuous stimulation of the postsynaptic cell.
- Acetylcholine, a common neurotransmitter, is broken down by the enzyme acetylcholinesterase in the synaptic cleft.
- The breakdown products are then reabsorbed and used to synthesize new acetylcholine molecules.
The Bigger Picture: Communication in the Nervous System
- The release of neurotransmitters is a fundamental part of how neurons communicate with each other and with other cells.
- This process underlies everything from muscle contraction to complex cognitive functions like learning and memory.
How does the study of neurotransmitter release inform our understanding of consciousness and decision-making? Could disruptions in this process explain certain neurological disorders?
Self review- What triggers the opening of calcium channels in the presynaptic neuron?
- How does calcium function as a signalling chemical in the neuron?
- What role do synaptic vesicles play in neurotransmitter release?
- What would happen if calcium channels in the presynaptic membrane were blocked?
