Synapsikalvon

Synapsikalvon, Finnish for the synaptic membrane, denotes the membranes that form the active interface of neuronal communication at chemical synapses. It comprises the presynaptic membrane of the transmitting neuron and the postsynaptic membrane of the receiving neuron, separated by the synaptic cleft. In contrast to electrical synapses, which connect cells via gap junction channels, chemical synapses rely on neurotransmitter release and receptor binding at these membranes.

Presynaptic side: The presynaptic terminal contains an organized active zone with voltage-gated calcium channels. An action potential arrival opens these channels, causing influx of calcium that triggers exocytosis of synaptic vesicles. Vesicles fuse with the presynaptic membrane through SNARE proteins (such as syntaxin-1, SNAP-25, and synaptobrevin), releasing neurotransmitters into the synaptic cleft. Neurotransmitter reuptake and vesicle recycling, including clathrin-mediated endocytosis, help terminate signaling and maintain vesicle pools.

Postsynaptic side: The postsynaptic membrane hosts a variety of neurotransmitter receptors, including ionotropic receptors (e.g., AMPA and NMDA receptors for glutamate; nicotinic acetylcholine receptors; GABA_A receptors) and metabotropic receptors. These receptors are concentrated in the postsynaptic density, a protein-rich region that coordinates signaling cascades and receptor trafficking. Binding of neurotransmitters alters membrane conductance or initiates intracellular signaling, shaping neuronal responses.

Plasticity and maintenance: Activity-dependent changes in receptor number, subunit composition, and signaling complexes at the synaptic membrane underlie synaptic plasticity, such as long-term potentiation and long-term depression. Receptor trafficking, lateral diffusion, and remodeling of the postsynaptic density are central to these processes and to learning and memory.

Clinical relevance: Alterations in synapsikalvon structure or function are linked to neurological disorders, including neurodegenerative diseases and developmental conditions. Understanding vesicle cycling and receptor dynamics at the synaptic membranes remains a major focus in neuroscience.