Here four options are available to the fibers: (1) they can run up the chain and synapse, (2) they can synapse at the level of entry, (3) they can pass straight through and synapse elsewhere - such as in the case of T5-12 (the splanchnic nerves), or (4) they can enter the chain and descend to synapse. It is this ability to move superiorly and inferiorly along the chain that results in the mass response to the sympathetic nervous system. A preganglionic fibre may synapse to 15-20 postganglionic fibres. The postganglionic neurons extend across most of the body.