The Human Brain : From Neurone to Nervous System

THE AUTONOMIC NERVOUS SYSTEM (ANS)


  1. The sympathetic nerves originate from the lateral horns of segments T1 to L2 of the human spinal cord ('the thoraco-lumbar outflow') and synapse within sympathetic ganglia; the sympathetic chain occupies a para-vertebral position, alongside the vertebral column.

  2. The parasympatheetic nerves arise for the lateral horns of the sacral spinal cord (S2-4) and synapse in parasympathetic ganglia situated close to the organs they innervate (the lower third of the intestine and the pelvic viscera). In addition parasympathetic nerves travel in the cranial nerves, so the parasympathetic is sometimes called the 'cranio-sacral outflow'.

  3. In some tissues, these two systems have opposing actions.

  4. Sympathetic nerves regulate the activity of internal organs, including the heart, stomach and intestines. We are usually unaware of most autonomic functions because they are involuntary - not under the control of the will. The main task of the ANS system is the involuntary (automatic) control and maintenance of a stable bodily internal environment (homeostasis), in response to both fluctuations in internal conditions and externally applied stimuli. Postganglionic sympathetic nerves are long and travel to targets such as blood vessels and sweat glands within spinal nerves: the innervate structures throughout the body - from the top of the head to the soles of the feet.

  5. The sympathetic chains also innervate the eyes and other structures within the head.

  6. Internal organs have separate, pre-vertebral, sympathetic ganglia situated in the thorax or abdomen, at a distance from the viscera they innervate; these receive preganglionic fibres that traverse the sympathetic chain, and examples include the stellate (inferior cardiac) ganglion, the coeliac ganglion and the inferior mesenteric (hypogastric) ganglion.

  7. Also included along with the sympathetic sysyem is the adrenal medulla - a pre-vertebral ganglion situated above the kidney that consists of modified post-ganglionic sympathetic cells capable of secreting the hormone, adrenaline, into the blood stream.

  8. Key Words: sympathetic and parasympathetic nerves; thoraco-lumbar and cranio-sacral outflows; autonomic ganglia; innervation of internal organs, glands and blood vessels; homeostasis


Autonomic Nerves in the Thorax

Cardiovascular System

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.

Respiratory System

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

Sympathetic Innervation

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.

Parasympathetic Innervation

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.










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