Page 411 - Clinical Anatomy

ECA6 7/18/06 6:54 PM Page 396

396 The central nervous system

Visceral afferents
As well as the efferent system, there are afferent visceral fibres which
are concerned with the afferent arc of autonomic reflexes and with the
conduction of visceral pain stimuli. These nerves have their cell stations in
the dorsal root ganglia of the spinal nerves or of the ganglia of the cranial
nerves concerned with the autonomic system. The fibres from the viscera
ascend in the autonomic plexuses; those from the body wall are conveyed
in the peripheral spinal nerves. The afferent course from any structure is
therefore along the same pathway as the efferent autonomic fibres which
supply the part.
The afferent fibres ascend centrally to the hypothalamus and thence to
the orbital and frontal gyri of the cerebral cortex along as yet indeterminate
pathways. Normally, we are unaware of the afferent impulses from the
viscera unless they become sufficiently great to exceed the pain threshold
when they are perceived as visceral pain, e.g. the pain of coronary
ischaemia or intestinal colic.

The sympathetic system
The efferent fibres of the sympathetic system arise in the lateral grey
column of the spinal cord (see Fig. 275) from segments T1 to L2. From each
of these segments small medullated axons emerge into the corresponding
anterior primary ramus and pass via a white ramus communicans into the
sympathetic trunk.
The spinal segments responsible for the sympathetic innervation of the
various parts of the body are approximately as follows:
•◊◊head and neck, T1–2;
•◊◊upper limb, T2–5;
•◊◊thoracic viscera, T1–4;
•◊◊abdominal viscera, T4–L2;
•◊◊pelvic viscera, T10–L2;
•◊◊lower limb, T11–L2.
Stimulation of a single white ramus communicans would thus obvi-
ously have widespread effects — the anatomical basis of the ‘mass action’
response of sympathetic stimulation.

The sympathetic trunk

The sympathetic trunk on each side is a ganglionated nerve chain which
extends from the base of the skull to the coccyx in close relationship to the
vertebral column, maintaining a distance of about 1 inch (2.5cm) from the
midline throughout its course. Commencing in the superior cervical gan-
glion beneath the skull base, the chain descends closely behind the posterior
wall of the carotid sheath, enters the thorax anterior to the neck of the first rib,
descends over the heads of the upper ribs and then on the sides of the bodies
of the last three or four thoracic vertebrae. The chain then passes into
the abdomen behind the medial arcuate ligament of the diaphragm and