化学シナプス

A chemical synapse consists of three key structures: the axon terminal of the presynaptic neuron containing synaptic vesicles filled with neurotransmitters, the postsynaptic cell membrane with neurotransmitter receptors, and the synaptic cleft, a fluid-filled space typically 20-50 nanometers wide separating the two membranes. These components work together to enable communication between neurons or between neurons and other cell types.

When an action potential reaches the axon terminal, membrane depolarization opens voltage-gated calcium channels, allowing calcium ions to rush into the presynaptic cell. This calcium surge initiates a signaling cascade that causes synaptic vesicles to fuse with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft. The strength of the calcium response determines how many vesicles release their contents.

Chemical synapses exhibit an approximately one millisecond delay because neurotransmitters must be released from synaptic vesicles, diffuse across the synaptic cleft, and bind to postsynaptic receptors before ion channels open. This synaptic delay is inherent to chemical transmission and differs from electrical synapses, which transmit signals directly and instantaneously between cells.

The type of neurotransmitter and the specific ion channels it opens on the postsynaptic membrane determine whether the response is excitatory or inhibitory. When neurotransmitters bind to postsynaptic receptors, they open ligand-gated ion channels that allow specific ions to enter, either depolarizing the membrane to trigger an action potential or hyperpolarizing it to prevent one.

The synaptic cleft, typically 20-50 nanometers wide, provides the extracellular space through which neurotransmitters diffuse from the presynaptic to postsynaptic membrane. This narrow gap ensures efficient neurotransmitter delivery while maintaining the separation necessary for unidirectional signal transmission. The cleft width also influences the time required for neurotransmitter diffusion and receptor binding.

Neurotransmitter removal from the synaptic cleft allows the postsynaptic membrane to reset and become ready to receive new signals. By regulating neurotransmitter availability, the synapse can fine-tune the strength and timing of neuronal communication. Without clearance, neurotransmitters would continue binding to receptors, preventing the postsynaptic cell from responding to subsequent stimuli.

In Lambert-Eaton myasthenic syndrome, antibodies target voltage-gated calcium channels, reducing acetylcholine release and causing muscle weakness. In myasthenia gravis, autoantibodies block acetylcholine receptors on the postsynaptic membrane, preventing neurotransmitter binding and inhibiting muscle contraction. Both conditions demonstrate how disrupting synaptic components impairs neuromuscular communication.