<P> Synapses may be electrical or chemical . Electrical synapses make direct electrical connections between neurons, but chemical synapses are much more common, and much more diverse in function . At a chemical synapse, the cell that sends signals is called presynaptic, and the cell that receives signals is called postsynaptic . Both the presynaptic and postsynaptic areas are full of molecular machinery that carries out the signalling process . The presynaptic area contains large numbers of tiny spherical vessels called synaptic vesicles, packed with neurotransmitter chemicals . When the presynaptic terminal is electrically stimulated, an array of molecules embedded in the membrane are activated, and cause the contents of the vesicles to be released into the narrow space between the presynaptic and postsynaptic membranes, called the synaptic cleft . The neurotransmitter then binds to receptors embedded in the postsynaptic membrane, causing them to enter an activated state . Depending on the type of receptor, the resulting effect on the postsynaptic cell may be excitatory, inhibitory, or modulatory in more complex ways . For example, release of the neurotransmitter acetylcholine at a synaptic contact between a motor neuron and a muscle cell induces rapid contraction of the muscle cell . The entire synaptic transmission process takes only a fraction of a millisecond, although the effects on the postsynaptic cell may last much longer (even indefinitely, in cases where the synaptic signal leads to the formation of a memory trace). </P> <Table> <Tr> <Th> Structure of a typical chemical synapse </Th> </Tr> <Tr> <Td> Postsynaptic density Voltage - gated Ca channel Synaptic vesicle Neurotransmitter transporter Receptor Neurotransmitter Axon terminal Synaptic cleft Dendrite </Td> </Tr> </Table> <Tr> <Th> Structure of a typical chemical synapse </Th> </Tr> <Tr> <Td> Postsynaptic density Voltage - gated Ca channel Synaptic vesicle Neurotransmitter transporter Receptor Neurotransmitter Axon terminal Synaptic cleft Dendrite </Td> </Tr>

What are the structures that make up the nervous system