THE PARASYMPATHETIC NERVOUS SYSTEM: POINTS RELEVANT FOR THE ANESTHESIOLOGIST

🎳The anterior hypothalamus controls the parasympathetic nervous system

🎳The posterior and lateral hypothalamic areas increase blood pressure and heart rate, whereas the preoptic area decreases blood pressure and heart rate.

🎳The parasympathetic nervous system arises from neurons in the brainstem and spinal cord sacral segments (S2–S4). As the parasympathetic ganglia are located near or within their effector organs, the parasympathetic postganglionic fibres are short, and they all release acetylcholine. The distribution of parasympathetic outflow is restricted so that parasympathetic effects are more localized than sympathetic effects.

🎳Parasympathetic fibres follow the distribution of the third, seventh, ninth and tenth cranial nerves. Preganglionic fibres of the third cranial nerve arise from the oculomotor nucleus and pass through the orbit to the ciliary ganglion. Postganglionic f ibres from the ciliary ganglion supply the ciliary muscle  and  sphincter  of  the  iris  and  constrict  the pupils.

🎳Preganglionic fibres from the superior salivary nucleus of the seventh nerve form the chorda tympani and reach the submaxillary ganglion via the lingual nerve. Postganglionic fibres supply the submaxillary and sublingual salivary glands and cause salivary secretion. #TheLayMedicalMan

🎳Preganglionic fibres arising from the inferior salivary nucleus of the ninth nerve form the lesser superficial petrosal nerve and reach the otic ganglion. The postganglionic fibres are distributed to the parotid gland via the auriculotemporal nerve and also cause salivary secretion.

🎳The vagus nerve is the major part of the cranial parasympathetic outflow. The preganglionic fibres arise from the dorsal nucleus of the vagus in the medulla and terminate in the ganglia of plexuses or in the walls of visceral organs. Postganglionic fibres supply the heart and decrease cardiac excitability, contractility, conductivity and rate. Postganglionic fibres from the pulmonary plexus contract the circular muscles of the bronchi, producing bronchoconstriction. Vagal branches to the gastric plexus give rise to postganglionic fibres  to  the  stomach, liver, pancreas and spleen. Stimulation of the vagus causes increased gastric motility and secretions, with relaxation of the pyloric sphincter. The intestinal branches of the vagus  supply  the  small  and  large  intestines  down to the transverse colon and it's stimulation increases peristalsis and relaxes the ileocolic sphincter. #TheLayMedicalMan

🎳The sacral outflow of the parasympathetic system arises from the second, third and fourth sacral segments of the spinal cord, and fibres enter the hypogastric plexus to innervate the descending colon, rectum, bladder and uterus. It's stimulation contracts the muscular wall of the rectum, relaxes the internal sphincter of the anus and contracts the detrusor muscle of the bladder wall.

Ref: Principles of Physiology for the Anaesthetist , 3/e

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THE SYMPATHETIC NERVOUS SYSTEM: POINTS RELEVANT FOR THE ANESTHESIOLOGIST

🔸Posterior hypothalamus is the principal site of sympathetic nervous outflow.

🔸The sympathetic system has short preganglionic fibres and long postganglionic fibres

🔸The sympathetic nerves originate from columns of preganglionic neurons in the grey matter of the lateral horn of the spinal cord from the first thoracic segment down to the second or third lumbar segment.

🔸The preganglionic fibres leave the spinal cord through the ventral roots with the spinal nerves and then leave the spinal nerves as white rami communicantes ( myelinated B fibres ) to synapse with the postganglionic neurons in the ganglia of the sympathetic chain.

🔸The ganglia form sympathetic chains. The post-ganglionic fibres leave the ganglia as grey rami communicantes (unmyelinated C fibres) and join the spinal nerves or visceral nerves to innervate the target organ.

🔸In general, the preganglionic fibres secrete acetyl choline as neurotransmitter, while the post ganglionic ones secrete norepinephrine. The postganglionic sympathetic nerves that innervate the blood vessels of muscles, sweat glands and the hair follicles in the skin release acetylcholine instead of norepinephrine (noradrenaline).

➡️The sympathetic chains extend down the length of the vertebral column and are divided into four parts:

🔸A cervical part consisting of superior, middle and inferior galglia, supplying the head, neck and thorax. The superior cervical ganglion sends postganglionic fibres to form the internal carotid plexus. The inferior cervical or stellate ganglion is fused with the first thoracic ganglion.

🔸Branches of the thoracic part, from T 1 –T 5 supply the aortic, cardiac and pulmonary plexuses. The greater and lesser splanchnic nerves are formed from the lower seven thoracic ganglia. The lowest splanchnic nerve arises from the last thoracic ganglion and supplies the renal plexus.

🔸The coeliac plexus is derived from the lumbar sympathetic ganglia

🔸The sacral ganglia contribute to the hypogastric and pelvic plexus #TheLayMedicalMan

SYMPATHETIC SYSTEM PRODUCE:

🔸Dilatation of the pupil and retraction of the eyelid (levator palpebrae)

🔸Thoracic visceral effects of positive inotropic and chronotropic cardiac effects, pulmonary blood vessel vasoconstriction and bronchial smooth muscle relaxation.

🔸Abdominal visceral effects of increased sphincteric tone and inhibition of peristalsis, leading to relaxation of the gut and reduced motility.

🔸Pelvic visceral effects of relaxation of the bladder wall and the rectum with sphincter closure. Contraction of the smooth muscle of the seminal vesicles and prostate produces ejaculation. #TheLayMedicalMan

🔸Cutaneous effects such as piloerection, vasoconstriction and sweating.

🔸In the limbs, the arterioles to the skin constrict, whereas the skeletal muscle arterioles vasodilate.

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