Chemical synapses are biological junctions through which neurons' signals can be sent to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow neurons to form circuits within the central nervous system. They are crucial to the biological computations that underlie perception and thought. They allow the nervous system to connect to and control other systems of the body. At a chemical synapse, one neuron releases neurotransmitter molecules into a small space (the synaptic cleft) that is adjacent to another neuron. The neurotransmitters are contained within small sacs called synaptic vesicles, and are released into the synaptic cleft by exocytosis. These molecules then bind to neurotransmitter receptors on the postsynaptic cell. Finally, the neurotransmitters are cleared from the synapse through one of several potential mechanisms including enzymatic degradation or re-uptake by specific transporters either on the presynaptic cell or on some other neuroglia to terminate the action of the neurotransmitter. The adult human brain is estimated to contain from 1014 to 5 × 1014 (100–500 trillion) synapses. Every cubic millimeter of cerebral cortex contains roughly a billion (short scale, i.e. 109) of them. The number of synapses in the human cerebral cortex has separately been estimated at 0.15 quadrillion (150 trillion) The word "synapse" was introduced by Sir Charles Scott Sherrington in 1897. Chemical synapses are not the only type of biological synapse: electrical and immunological synapses also exist. Without a qualifier, however, "synapse" commonly refers to chemical synapse. Structure Synapses are functional connections between neurons, or between neurons and other types of cells. A typical neuron gives rise to several thousand synapses, although there are some types that make far fewer. Most synapses connect axons to dendrites, but there are also other types of connections, including axon-to-cell-body, axon-to-axon, and dendrite-to-dendrite. Synapses are generally too small to be recognizable using a light microscope except as points where the membranes of two cells appear to touch, but their cellular elements can be visualized clearly using an electron microscope. Chemical synapses pass information directionally from a presynaptic cell to a postsynaptic cell and are therefore asymmetric in structure and function. The presynaptic axon terminal, or synaptic bouton, is a specialized area within the axon of the presynaptic cell that contains neurotransmitters enclosed in small membrane-bound spheres called synaptic vesicles (as well as a number of other supporting structures and organelles, such as mitochondria and endoplasmic reticulum).