<unk> s are specialized junctions through which the cells of the nervous system signal to each other and to non-neuronal cells such as those in muscles or glands.<unk> s allow the neurons of the central nervous system to form interconnected neural circuits. They are thus crucial to the biological computations that underlie perception and thought. They provide the means through which the nervous system connects to and controls the other systems of the body. A<unk>  between a motor neuron and a muscle cell is called a neuromuscular junction; this type of<unk>  is well-understood.
The human brain contains a huge number of<unk> s; young children have about 1016<unk> s (10 quadrillion). This number declines with age, stabilizing by adulthood. Estimates for adults vary from 1015 to 5 × 1015 (1-5 quadrillion)<unk> s.
The word<unk> " comes from "synaptein", which Sir Charles Scott Sherrington and his colleagues coined from the Greek "syn-" ("together") and "haptein" ("to clasp").<unk> s are not the only type of biological<unk> : electrical and immunological<unk> s exist as well. Without a qualifier, however,<unk> " commonly refers to a<unk> .
Chemical synapse
Excitatory synapse
Cell junction
Blood-brain barrier
Growth cone
Dendrite
Sensory receptor
Extracellular matrix