|Autonomic Nerves in the Thorax|
Blood vessels are usually heavily innervated by the sympathetic nervous system, but generally do not receive a parasympathetic innervation.
Similarly the ventricles of the heart have a strong sympathetic innervation and little in terms of parasympathetic nerves.
The sympathetic nerves cause the heart to beat faster and more strongly, and they contract (constrict) the blood vessels (apart from the capillaries).
The main influence of the parasympathetic system on the cardiovascular system is to slow the heart rate, by its action of the sinu-atrial node.
The large airways contain smooth muscle and mucous glands.
Sympathetic nerves relax these smooth muscles.
Parasympathetic nerves to the respiratory tract constrict the airways and cause the secretion of mucus.
|Autonomic Nerves in the Abdomen|
The sympathetic supply to the abdomen is derived from the mid and lower thoracic segments of the cord. The preganglionic axons are longer and reach the sympathetic pre-vertebral ganglia within the abdominal cavity, such as the coeliac ganglia, and the adrenal medulla - in reality a modified pair of ganglia which release hormones into the circulation.
The sympathetic post-ganglionic neurones that arise from prevertebral ganglia are generally long, and run to the viscera alongside the blood vessels. In addition to innerating muscle, they have actions on the enteric nervous system.
The preganglionic fibres are contained in the splanchnic nerves, which also contain visceral afferent axons.
The parasympathetic nerve supply to the gastrointestinal tract runs in the vagus (the tenth cranial nerve), which branches after it enters the abdomen to enter the abdomen and upper two-thirds of the gastointestinal tract. The pelvic nerves contain the sacral parasympathetic axons and innervte the lower thrd of the gastrointestinal tract and the urogenital systems.
Gastrointestinal and Urinary Systems
The smooth muscles of the gastrointestinal and urinary tracts receive a supply from both divisions of the autonomic nervous system, and have antagonistic effects on these muscles.
The parasympathetic nerves contract most of the smooth muscles of the gastrointestinal and urinary tracts, and the sympathetic systems has opposing effects.
Parasympathetic nerves generally promote glandular secretion in the gastrointestinal tract; sympathetic nerves reduce this activity possibly because of a reduction in blood flow to the glands, reducing the amount of fluid that can be secreted.