15.1 Divisions of the autonomic nervous system  (Page 2/61)

The sympathetic division of the autonomic nervous system influences the various organ systems of the body through connections emerging from the thoracic and upper lumbar spinal cord. It is referred to as the thoracolumbar system    to reflect this anatomical basis. A central neuron    in the lateral horn of any of these spinal regions projects to ganglia adjacent to the vertebral column through the ventral spinal roots. The majority of ganglia of the sympathetic system belong to a network of sympathetic chain ganglia    that runs alongside the vertebral column. The ganglia appear as a series of clusters of neurons linked by axonal bridges. There are typically 23 ganglia in the chain on either side of the spinal column. Three correspond to the cervical region, 12 are in the thoracic region, four are in the lumbar region, and four correspond to the sacral region. The cervical and sacral levels are not connected to the spinal cord directly through the spinal roots, but through ascending or descending connections through the bridges within the chain.

A diagram that shows the connections of the sympathetic system is somewhat like a circuit diagram that shows the electrical connections between different receptacles and devices. In [link] , the “circuits” of the sympathetic system are intentionally simplified.

Connections of sympathetic division of the autonomic nervous system

To continue with the analogy of the circuit diagram, there are three different types of “junctions” that operate within the sympathetic system ( [link] ). The first type is most direct: the sympathetic nerve projects to the chain ganglion at the same level as the target effector    (the organ, tissue, or gland to be innervated). An example of this type is spinal nerve T1 that synapses with the T1 chain ganglion to innervate the trachea. The fibers of this branch are called white rami communicantes    (singular = ramus communicans); they are myelinated and therefore referred to as white (see [link] a ). The axon from the central neuron (the preganglionic fiber shown as a solid line) synapses with the ganglionic neuron    (with the postganglionic fiber shown as a dashed line). This neuron then projects to a target effector—in this case, the trachea—via gray rami communicantes    , which are unmyelinated axons.

In some cases, the target effectors are located superior or inferior to the spinal segment at which the preganglionic fiber emerges. With respect to the “wiring” involved, the synapse with the ganglionic neuron occurs at chain ganglia superior or inferior to the location of the central neuron. An example of this is spinal nerve T1 that innervates the eye. The spinal nerve tracks up through the chain until it reaches the superior cervical ganglion    , where it synapses with the postganglionic neuron (see [link] b ). The cervical ganglia are referred to as paravertebral ganglia    , given their location adjacent to prevertebral ganglia in the sympathetic chain.