Synapses Chimiques

A chemical synapse consists of three key structures: the axon terminal of the presynaptic neuron containing neurotransmitter-filled synaptic vesicles, the postsynaptic cell 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 chemical communication between neurons.

When an action potential reaches the presynaptic axon terminal, it depolarizes the membrane and opens voltage-gated calcium channels. Calcium ions rush into the cell, initiating a signaling cascade that causes synaptic vesicles to fuse with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft.

When neurotransmitters diffuse across the synaptic cleft and bind to specific receptors on the postsynaptic membrane, they open ligand-gated ion channels. This allows ions to enter the postsynaptic cell, producing excitatory and inhibitory effects of neurotransmitters that either increase or decrease the postsynaptic membrane potential.

Chemical synaptic transmission is unidirectional because neurotransmitters are released only from the presynaptic neuron's axon terminal and receptors are located only on the postsynaptic membrane. This structural arrangement ensures signals travel in one direction, from the presynaptic to the postsynaptic cell.

The synaptic delay is approximately one millisecond between when an action potential reaches the presynaptic terminal and when neurotransmitters open postsynaptic ion channels. This delay occurs because neurotransmitters must be released from vesicles, diffuse across the synaptic cleft, and bind to receptors before the postsynaptic response begins.

Depending on the signal, few or many neurotransmitter vesicles may be released into the synaptic cleft. This variable release regulates neurotransmitter availability, allowing the synapse to fine-tune the neuronal signal and produce either stronger or weaker excitatory or inhibitory postsynaptic responses.

In Lambert-Eaton myasthenic syndrome, antibodies target voltage-gated calcium channels, reducing neurotransmitter release and causing muscle weakness. In myasthenia gravis, autoantibodies block acetylcholine from binding postsynaptic receptors at the neuromuscular junction, preventing muscle contraction and causing facial weakness and diminished expression.