Sünapsid

Sünapsid, or synapses, are the junctions through which neurons communicate with other neurons or with effector cells such as muscles or glands. They come in two main types: chemical synapses, where signals are transmitted via neurotransmitters, and electrical synapses, where ionic current passes directly through gap junctions. In chemical synapses, the presynaptic terminal releases neurotransmitters into the synaptic cleft in response to an action potential. Calcium ions entering through voltage-gated calcium channels trigger the fusion of synaptic vesicles with the presynaptic membrane. Neurotransmitters diffuse across the cleft and bind to receptors on the postsynaptic membrane, producing excitatory or inhibitory postsynaptic potentials. Depending on receptor type, the response can either depolarize or hyperpolarize the postsynaptic cell.

Electrical synapses use gap junction channels, formed by connexin proteins in vertebrates, to enable direct electrical coupling. These junctions permit rapid, bidirectional transmission and are important for synchronous activity in networks.

Synapses exhibit plasticity, allowing their strength to increase or decrease with activity. Long-term potentiation and long-term depression are cellular mechanisms underlying learning and memory. The neuromuscular junction is a well-studied chemical synapse that links motor neurons to skeletal muscle fibers, illustrating the bridge between nervous and muscular systems.

The concept of the synapse was developed in the early 20th century; Santiago Ramón y Cajal described contact points between neurons, and Charles Sherrington coined the term synapse. Understanding synapses is central to neuroscience and to explaining how experience shapes nervous system function.