シナプスシグナル伝達

Neurons communicate through chemical signaling at synapses, specialized junctions where the axon terminal of a presynaptic neuron meets a postsynaptic cell. When an action potential occurs, neurotransmitter-filled synaptic vesicles fuse with the cell membrane and release their contents into the synaptic cleft. Neurotransmitters diffuse across this narrow space and bind to postsynaptic receptors, triggering a response in the receiving cell.

Synaptic vesicles are membrane-bound organelles in the axon terminal that store neurotransmitter molecules. When an action potential arrives, voltage-gated calcium ion channels open, allowing Ca2+ to enter the presynaptic cell. This calcium influx triggers the fusion of secretory vesicles with the plasma membrane, releasing neurotransmitters into the synaptic cleft for signal transmission.

Neurotransmitter binding to postsynaptic receptors triggers chemical changes in the receiving cell, such as opening or closing ion channels. These changes alter the membrane potential of the postsynaptic neuron, potentially enabling it to fire an action potential. This mechanism allows neurons to transmit information to other cells, both near and far.

The synaptic cleft is the narrow space between presynaptic and postsynaptic cells where neurotransmitter diffusion occurs. After release from synaptic vesicles, neurotransmitters rapidly populate the synaptic cleft and bind to receptors on the postsynaptic neuron. This spatial separation allows for controlled, directional signal transmission between neurons.

Chemical synapses use neurotransmitters to transmit signals and can amplify or transform presynaptic signals. Electrical synapses are narrower and transfer ions directly between neurons, enabling faster signal transmission but without signal amplification. Electrical synapses synchronize neuronal activity, which is favorable for controlling rapid, invariable signals such as escape responses.

Synaptic signaling ends through multiple mechanisms: neurotransmitters in the synapse are degraded by enzymes, reabsorbed by the presynaptic cell, diffused away, or cleared by glial cells. These processes remove neurotransmitters from the synaptic cleft, stopping receptor activation and allowing the postsynaptic cell to return to its resting state.

Neurons receive signals from many other neurons simultaneously. The integration of numerous inputs received by postsynaptic cells ultimately determines their action potential firing patterns. This convergence of signals allows postsynaptic neurons to process complex information and generate appropriate responses based on the combined effect of all incoming synaptic signals.