A boundary string ganglion is an approx. centimetre-sized accumulation of postganglionic sympathetic neuronal bodies located right next to and next to the vertebral column and connected to a spinal nerve.
The border string ganglia are located on both sides of the vertebral column. The border ganglia are often named after the level of the vertebral column at which they are located and speak of cervical ganglia, thoracic ganglia, lumbar ganglia and sacral/pelvic ganglia. With few exceptions, there is a ganglion for each spinal nerve.
Among the more notable exceptions is the grouping of the cervical ganglia. Here, the four uppermost ganglia (C 1,2,3,4) have merged into the "upper cervical ganglia" (ganglion cervicale superius). The next two ganglia (C 5,6) form the "middle cervical ganglia" (ganglion cervical medium). Fusing of the ganglia C 7,8 produces the "lower cervical ganglia" (ganglion cervicale inferius). The latter is usually more or less completely fused with the uppermost thoracic ganglia to form the so-called "stellar ganglia" (ganglion stellatum or ganglion cervicothoracicum)
The boundary string ganglia are joined together to form the sympathetic boundary string, one such string on the right side and one on the left side. Each spinal nerve is connected to a borderline ganglion by means of a "gray junction branch" (ramus communicans griseus, RCG).
The spinal nerves T 1 to L 2-3 give off a so-called white junction branch (ramus communicans albus - RCA) to their connected cord ganglia.
The white connective branches contain myelinated preganglionic sympathetic nerve fibers. These start from the sympathetic nucleus of the spinal cord and are led with the help of the forward roots T 1 to L 2-3 from the vertebral canal out into the corresponding spinal nerves and from there, in the form of white connecting branches, into the border cord. NOTE that it is RCA that adds the sympathetic boundary cord to preganglionic nerve fibers. Once inside the boundary cord, some preganglionic filaments will turn up or down in the boundary cord, thus reaching the ganglia connected to the spinal nerves C 1 to C 8 and L3-4 to Cocc 1
Via RCG, the spinal nerves are supplied with postganglionic nerve fibers of three main functional types:
1/ Vasomotor filaments intended to innervate smooth muscle in the blood vessels of the skin and skeletal muscle. The vasomotor signal flow controls the blood flow in the skin and muscles.
2/Sudomotor filaments to innervate the sweat glands of the skin.
3/ Pilomotor filaments designed to innervate the smooth muscle cells present in the skin which, when activated, travel up the hairs on the skin; gives "goosebumps".