Elongated accumulation of sympathetic preganglionic neuronal bodies, one on the right and one on the left side, in the lateral horn of the gray spinal cord. The nucleus is limited to segments, levels T1 through L2-3, and gives rise to the so-called preganglionic sympathetic nerve fibers. These bundles of thin myelinated nerve fibers find their way out of the spinal cord with the help of the front roots T1 through L2-3 and further out into the corresponding (T1 through L2-3) spinal nerves. Once outside the vertebral canal, the preganglionic filamentous bundles leave the spinal nerves and, in the form of the white connecting branches (ramus communicans albus, RCA), find their way into the sympathetic boundary cord.

NOTE that the boundary cord thus only receives preganglionic nerve fibers via the spinal nerves T1 to L2-3!! From the boundary string level T1 to L2-3, preganglionic threads are then distributed up and down in the boundary string.

Once the preganlionic threads get into the sympathetic boundary string, they have two alternative end goals:

1/ They connect (form synapses) with the postganglionic neurons that have their cell bodies gathered in the border string ganglia. The unmyelinated postganglionic nerve fibers then leave the boundary cord and then have two different paths to choose from.
(a) either the postganglionic nerve fibres enter the spinal nerves via the grey junction branches (RCGs) and spread with them to the skin and to the musculoskeletal system, in particular the skeletal muscle blood vessels;
b) or bundles of postganglionic nerve fibers leave the border cord and find their way towards the front of the vertebral column where, together with parasympathetic nerve fibers, they form the so-called prevertebral braids - the prevertebral plexa.

2/ They pass through the boundary strand and then leave it in the form of so-called intestinal branches which find their postganglionic recipient neurons in a number of different places in the body.
The large and small intestinal nerves (n. splanchnicus major & minor) are examples of such intestinal branches. Here, the postganglionic target groups are the so-called abdominal ganglia.