PARASYMPATHETIC SYSTEM

THIS IS A PRESENTATION ON PARASYMPATHETIC NERVOUS SYSTEM.THAT COVERS THE BASIC CONCEPTS OF PSNS. USUALY DATA IS TAKN FROM PHYSIOLOGY BOOK WRITTEN BY JAYPEE'S BOOK
'ESSENTIALS OF MEDICAL PHYSIOLOGY'.

1. PARASYMPATHETIC NERVOUS SYSTEM

2. DEFINATION : Parasympathetic nervous system is a division of autonomic nervous system. Parasympathetic nervous system is sometimes referred to as the “rest and digest” system. In general,the PNS (parasympathetic nervous system) acts on the opposite way to SNS(sympathetic nervous system),reversing the effect of the fight or flight response. However it may be more correct to say that the SNS and the PNS have a complimentary relationship rather than one of opposition.

3. Neurotransmitters : Preganglionic neurotransmitter is Acetylcholine ( ach) . And postganglionic neurotransmitter is Noradrenaline . The terminal nerve endings of parasympathetic nervous system all or virtually all secrete acetylcholine. Therefore, acetylcholine is called parasympathetic transmiter . The PNS uses acetylcholine as its primary neurotransmitter, although peptides (such as cholecystokinin) may acts on the PSNS as a neurotransmitter. When stimulated, the parasympathetic nerve releases acetylcholine ( ach) at the ganglion.

4. Acetylcholine in turn acts on nicotinic receptors of post synaptic neurons.Post synaptic nerves then releases acetylcholine to stimulate the muscarinic receptors of the target organ.

5. Fibers: The fibers that secrete acetylcholine are said to be cholinergic. Those that secrete norepinephrine are said to be adrenergic, a term derived from adrenalin, which is an alternate name for epinephrine. All preganglionic neurons are cholinergic in both the sympathetic and parasympathetic nervous systems. Acetylcholine and acetylcholine like substances, when applied to the ganglia, will excite both sympathetic and parasympathetic postganglionic neurons of parasympathetic system are also cholinergic.

6. Conversely, most of the postganglionic sympathetic neurons are adrenergic .

7. The parasympathetic system, like the sympathetic system has both preganglionic and postganglionic neurons. However, except in the case of a few cranial parasympathetic nerves, the preganglionic fibers pass uninterrupted all the way to the organ that is to be controlled. PREGANGLIONIC AND POSTGANGLIONIC PARASYMPATHETIC NEURONS:

8. The postganglionic neurons are located in the wall of the organ. The preganglionic fibers synapse with these neurons, and extremely short postganglionic fibers, a fraction of a millimeter to several centimeters in length, leave the neurons to innervate the tissue of the organ. This location of parasympathetic postganglionic neurons in the visceral orange is quite different from the arrangement of the sympathetic ganglia because the cell bodies of the sympathetic postganglionic neurons are almost always located in the ganglia of the sympathetic chain or in various other discrete ganglia in the abdomen, rather in the excited organ.

9. EFFECTOR ORGANS As the previous figure demonstrates that parasympathetic fibers leave the central nervous system through cranial nerves III,VII,and X; additional parasympathetic fibers leave the lower most part of the spinal cord through the second and third sacral spinal nerves and occasionally the first and fourth sacral nerves. About 75 percent of all parasympathetic nerve fibers are in the vagus nerves ( cranial nerve X), passing to the entire thoracic and abdominal regions of the body. Therefore a physiologist speaking of the parasympathetic nervous system often thinks mainly of two vague nerves.

10. The vague nerves supply parasympathetic nerves to the hearts, lungs, esophagus, stomach, entire small intestine, proximal half of the colon, liver, gallbladder, pancreas, kidneys, and upper portions of uterers. Parasympathetic fibers in third cranial nerve go to the papillary sphincter and cillary muscle of the eye. Fibers from the seventh cranial nerve pass to the lacrimal, nasal and submandibular glands, and fibers from the ninth cranial nerve go to the parotid gland. The sacral parasympathetic fibers are in the pelvic nerves which pass through the spinal nerve sacral plexus on each side of the cord at the S2 and S3 levels.

11. These fibers then distribute to the descending colon, rectum, urinary bladder, and lower portions of ureters. Also, the sacral group of parasympathetic supplies nerve signals to the external genitalia to cause erection.

12. EFFECTS OF PARASYMPATHETIC NERVES ON EFFECTORS ORGANS EFFECTS Eye Iris Cillary muscles Contraction of circular Muscle contraction Blood vessels Usually nill Skin Sweat secretion Erection of hairs Generalized Nill

13. Heart Inhibition Salivary glands Free secretion and vasodilation Stomach Contractions Secretions Sphincters Augmentation Increase Relaxation(usually) Intestine Intestinal Movements Sphincters Secretions Augmentation Relaxation(usually) Increase

14. Gall bladder Contraction Liver Nill Spleen Nill Pancreatic Secretions Increase Bronchial muscles Contraction Bronchial Secretions Increase Kidneys Increase Ureter Contraction

15. Bladder fundus Sphincter Contraction Relaxation Sex organs (male) Erection Uterus Variable