The Sympathetic System


The Sympathetic SystemThe nerve cells connected with the organs of nutrition i.e. organs of digestion, of circulation, and of respiration present an interesting variation from those belonging to the supporting and pro mechanisms. Their withdrawal from the organs to a central axis is incomplete, small ganglia being present in the walls of the organs and along the pathways of their nerves towards the brain and spinal cord (Fig. 30). (Although this may be regarded as the story of the evolution of these ganglia, yet during their actual development in the embryo the nerve cells forming them travel outwards from the neural crest to the regions where they are ultimately found.)

For practical purposes the ganglia may be grouped into four series:

(1) Peripheral ganglia, embedded in the viscera.
(2) Irregular groups of ganglia associated with large sympathetic nerve plexuses in the thorax and abdomen.
(3) The lateral ganglionic chains.
(4) The sympathetic nerve cells in the brain and spinal cord.

The lateral ganglionic chains are composed of small ganglia linked by fibres to adjacent ganglia. They extend, on either side of the vertebral column, from the base of the skull to the end of the column, and each ganglion is connected to a spinal nerve by communicating branches (rami communicantes). If the segmental arrangement of these ganglia were as regular as that of the spinal nerves then every spinal nerve would have one ganglion connected with it. In the thoracic region and to a lesser extent in the lumbar and sacral regions, this arrangement is found; in other regions, adjacent ganglia have fused with one another so that one ganglion may be connected with several spinal nerves. In the cervical region, for instance, there are three ganglia on either side instead of eight; the uppermost is made up of four ganglia fused together, and each of the others similarly consists of two.

Typically two rami communicantes connect a sympathetic ganglion with a spinal nerve. The two rami differ slightly from one another in colour; one is termed the white ramus communicans, the other the grey ramus communicans. The white ramus consists chiefly of medullated fibres which give the white colour to the ramus ; they are fibres which emerge from the spinal cord and pass to the ganglion. The grey ramus is composed chiefly of non-medullated fibres which originate as axons of cells of the sympathetic ganglion and run out to join the spinal nerve.

Structure of a Complete Spinal Nerve. - The nerve fibres running to and from the supporting. and protective structures are usually designated somatic in contradistinction to the sympathetic or visceral fibres to the organs of nutrition. A complete spinal nerve contains fibres from all the neurons essential to a complete nervous system, i. e. the fibres necessary for four reflex pathways namely:

I. A somatic sensory motor are.
II. A sympathetic sensory motor arc.
III. A somatic sensory sympathetic motor are.
IV. A sympathetic sensory somatic motor are.

Unfortunately the sympathetic system is very difficult to study both anatomically and experimentally so that many of its precise interactions are still problematical. Thus it is yet doubtful whether the sympathetic system can carry out a reflex action without the interposition of the cerebro-spinal system; and, further, the exact connections of the afferent fibres from the viscera are but imperfectly understood.