Most ganglia of the sympathetic system aggregate to form chain-like structures that run along each side of the spinal cord. Enveloped in loosely organized connective tissue, these ganglia comprise the sympathetic trunk. Paravetebral ganglia, so named because of their close proximity to the vertebral column, typically appear as lumps that occur midway between adjacent spinal nerves. The central nervous system is connected to ganglions situated in the synaptic trunk by two sets of specialized fibers. The fist type of neuron is the preganglionic nerve, which originates from the brain stem or spinal cord and projects short myelinated (insulated) axons. Upon reaching ganglia masses, the preganglionic axons connect to a second specialized neuron called the postganglionic nerve by forming a synapse. Preganglionic axons may also send branched projections called dendrons to form synapses with other postganlionic neurons that are either in the same or nearby masses. Having long, non-myelinated axons, postganglionic fibers project to and synapse with tissues that influence such target organs as the heart, kidneys, or stomach. Other ganglia located outside the sympathetic trunk but also comprising the sympathetic system are referred to as collateral or prevertebral neurons.