Superior Mediastinum and Root of Neck LO9
9. Describe the basic structure of the autonomic innervation of the thoracic viscera.
There are three major types of autonomic plexuses within the thorax, including:
- Cardiac plexuses,
- Pulmonary plexuses, &
- Esophageal plexus.
These plexuses contain sympathetic and parasympathetic fibers, largely sourced from either the cervical and thoracic sympathetic trunks, or the vagus nn. (CN X).
Control of the cardiac cycle is intrinsic, or myogenic, and arises in the sinu-atrial (SA) node of the heart as will be discussed in later sessions. Extrinsic control of the SA node occurs through autonomic stimulation. Sympathetic stimulation increases heart rate and dilates the coronary arteries, whereas parasympathetic stimulation decreases heart rate and maintains resting tone of the walls of the coronary aa.
The cardiac autonomic plexus is typically conceptualized in two parts: a superficial part which nestles in the inferior concavity of the arch of the aorta, and a deep part which is anterior to the tracheal bifurcation (and the anterior pulmonary plexuses).
The superficial cardiac plexus receives fibers from the left superior cervical sympathetic ganglion and the left vagus n. (CN X).
The deep cardiac plexus receives fibers from cervical and thoracic sympathetic trunks, vagus nn. (CN X), and recurrent laryngeal nn.
Fibers from the superficial and deep parts of the cardiac plexus coalesce to form sub-plexuses that serve specific parts of the heart and its major arteries, chiefly the:
- Left coronary plexus, serving the left coronary a. (LCA),
- Right coronary plexus, serving the right coronary a. (RCA), & the
- Atrial plexuses, serving the left and right atria.
The cardiac plexuses are contiguous with one another and with the pulmonary plexuses. Given the coordinated functions of the cardiovascular and respiratory systems, well-orchestrated responses from and between these systems is advantageous.
There are two anterior and two posterior autonomic pulmonary plexuses which nestle on either surface of the hila of the lungs.
Anterior pulmonary plexuses receive fibers from the vagus nn. (CN X) and the cervical sympathetic trunks via the superficial cardiac plexus.
Posterior pulmonary plexuses receive fibers from the vagal trunks and the thoracic sympathetic trunks. The left posterior plexus often will receive fibers from the left recurrent laryngeal n. As well. Sympathetic responses cause bronchodilation through relaxing smooth muscle of the conductive airways, whereas parasympathetic stimulation maintains resting tone of the smooth muscle.
The esophageal plexus is predominantly formed from fibers originating from the vagal trunks, which are loosely homologous to the left (becomes the bulk of the anterior trunk) and right (becomes the bulk of the posterior trunk) vagus nn. (CN X). Some vagus fibers (especially for those of the proximal esophagus) arise from the recurrent laryngeal nn. The sympathetic portions of the esophageal plexus are derived from the thoracic and cervical sympathetic trunks.
Visceral pain affecting the esophagus may be difficult to distinguish from visceral cardiac pain, because the visceral afferents run with sympathetic fibers, and both organs are served from fibers associated with the thoracic spinal cord.