sünapsides

Sünapsides, known in English as synapses, are specialized junctions through which neurons communicate with other neurons or effector cells. They typically consist of a presynaptic terminal, a synaptic cleft, and a postsynaptic membrane, though electrical synapses with gap junctions provide direct, rapid coupling without a cleft. Most brain signaling occurs at chemical synapses.

In chemical synapses, an action potential opens calcium channels in the presynaptic terminal, triggering neurotransmitter release. The molecules diffuse across the cleft and bind to receptors on the postsynaptic membrane, producing excitatory or inhibitory postsynaptic potentials. Receptors include ionotropic, ligand-gated ion channels, and metabotropic receptors that act via signaling cascades. Termination occurs by reuptake, enzymatic degradation, or diffusion.

Electrical synapses use gap junctions formed by connexins to allow ions to flow directly between cells. They enable fast, bidirectional signaling and synchronization, though they are less common in the mammalian brain than chemical synapses.

Synapses are plastic; their strength can change with activity through long-term potentiation and long-term depression, via altered neurotransmitter release, receptor density, and structural remodeling. This plasticity underlies learning and memory and helps circuits adapt to experience.

Developmentally, synapses form (synaptogenesis) and are later refined by pruning. Disruptions in synaptic function or plasticity are linked to various conditions, including Alzheimer's disease, autism spectrum disorders, and neuromuscular junction disorders such as myasthenia gravis.