What is the role of neurotransmitter at a chemical synapse?

At a synapse, neurotransmitters (chemical messengers) play a crucial role in facilitating communication between neurons (nerve cells) by transmitting signals across the synaptic cleft, a minute gap that separates two neurons. The general process involves the following steps:

1. Neurotransmitter Synthesis: Neurons synthesize neurotransmitters. Different neurons may produce different neurotransmitters based on their specific functions and the neural pathway in which they are involved.

2. Neurotransmitter Storage: Synthesized neurotransmitters are stored in membrane-bound vesicles within the presynaptic neuron (the neuron sending the signal).

3. Action Potential Arrival: When an action potential (an electrical signal) reaches the presynaptic terminal (the end of the neuron), it triggers a series of events leading to neurotransmitter release.

4. Calcium Ion Influx: The arrival of the action potential causes voltage-gated calcium channels in the presynaptic membrane to open. Calcium ions flood into the presynaptic neuron from the extracellular space.

5. Vesicle Fusion: The influx of calcium ions prompts the fusion of neurotransmitter-carrying vesicles with the presynaptic membrane. This fusion is a critical step in releasing neurotransmitters into the synaptic cleft.

6. Neurotransmitter Release: The fusion process leads to the exocytosis of neurotransmitter molecules into the synaptic gap, resulting in their diffusion across the cleft.

7. Binding to Postsynaptic Receptors: On the postsynaptic side (the neuron receiving the signal), there are receptor molecules embedded in the postsynaptic membrane. These receptors are specific for certain neurotransmitters. When neurotransmitter molecules bind to their respective receptors, a conformational change occurs.

8. Channel Opening or Closure: The conformational change usually leads to the opening of ion channels associated with the receptors, allowing certain ions (such as sodium, potassium, or chloride) to flow in or out of the postsynaptic neuron.

9. Postsynaptic Potential: The resulting influx or efflux of ions due to receptor binding generates an electrical signal in the postsynaptic neuron, known as a postsynaptic potential (PSP).

10. Signal Integration: Depending on the type of neurotransmitter and its inhibitory or excitatory effect, PSPs either make it easier (excitatory) or harder (inhibitory) for the postsynaptic neuron to generate an action potential. Multiple PSPs combine to determine whether the neuron reaches the threshold potential.

11. Action Potential Generation: When the cumulative effect of PSPs reaches a certain threshold at the postsynaptic neuron, an action potential may be generated, propagating the signal further along the neuron.

In summary, neurotransmitters play a critical role by transmitting chemical signals across synapses, allowing communication and signal processing between neurons. The interplay of neurotransmitters, receptors, and their impact on postsynaptic electrical potentials forms the basis for neuronal communication and information transmission in the brain.