The Parasympathetic Nervous System

It wasn't until the late 1800s that an anatomically and functionally separate division of the nervous system to the viscera was described. Since it was anatomically separate and yet supplied most of the same organs (fig. 10.4a), it was named the parasympathetic (para = alongside) nervous system. It soon became evident, however, that the mode of operation of this accessoiy system was entirely different from that of the sympathetic nervous system. In contrast to the operation of the sympathetic nervous system, "relaxation" of the parasympathetic nervous system has no meaning except in relation to each organ and each function. In contrast to the sympathetic nervous system, which causes global effects throughout the body, the parasympathetic nervous system is organ-specific. With only a few exceptions such as regulation of the heart and lungs, it does not "balance" the sympathetic nervous system. Nor does it become generally active or inactive as does the sympathetic nervous system. It presides over functions as disparate as stimulating salivary, gastric, and pancreatic secretions, stimulating peristalsis (the movement of food distally through the gastrointestinal tract), stimulating the synthesis of liver glycogen from glucose, accommodating for near vision (as for threading a needle), sexual arousal, slowing the heart rate, constricting the bronchial tree, contracting the wall of the bladder for urination, and relaxing the internal urethral and anal sphincters to facilitate elimination.

Spinal Cord Nerves Uterus

pupilla dilatici accommodation for near vision " (pupils constricted)

cervical spinal cord bronchioii dilated salivary gland \

bronchioles ' constricted heart slows stomach — and small intestine — (digestion stimulated)

spleer-contre s diger n impec i uterine contractor inhibited (non-pregi it uterus)

colon bladder empties uterine contradi s promoted (pregnant uterus sphincters contracted, impeding elimination sacral parasym pathetic plexus ovary / and uterus parasympathetic nervous system, (craniosacral outflow)

brain stem and spinal cord cranial nerves brain sympathetic nervous system, (thoracolumbar outflow)

Figure 10.4a. Parasympathetic nervous system (craniosacral outflow) outlined on the left, and sympathetic nervous system (thoracolumbar outflow) outline« on the right. (Both systems innervate both sides.) The cranial portion of the parasympathetic nervous system is included in cranial nerves 3, 7, 9, and 10, tl latter being the famous vagus nerve that innervates most of the viscera, indud ing the lungs, heart, liver, and the upper part of the digestive tract. The sacral parasympathetic plexus innervates the genitals, the hladder, and the lower par of the digestive tube. The sympathetic nervous system innervates the entire body from its source in the spinal cord between T1 and L2, which is why the sympathetic nervous system is called the thoracolumbar outflow (Dodd).

IV. KUAKATION AM) AUDITA IKiH <¡5y

IV. KUAKATION AM) AUDITA IKiH <¡5y brachial plexus (innervales upper extremity: blue on cover)

right vagus nerve (parasympathetic) (coded in green on book cover)

lumbosacral plexus (innervates lower extremity; blue on book cover)

cranial parasympathetic outflow (cranial nerves 3, 7. 9, and 10. coded in green on book cover)

coeliac (solar) plexus (red on cover)

sympathetic ganglia and chain (thoracolumbar outflow: red)

intercostal nerves (blue on cover)

Figure 10.4b. Peripheral nervous system, with components shown in four colors on the book cover (Sappey).

Parasympathetic Lumbosacral Plexus

brachial plexus (innervales upper extremity: blue on cover)

right vagus nerve (parasympathetic) (coded in green on book cover)

lumbosacral plexus (innervates lower extremity; blue on book cover)

cranial parasympathetic outflow (cranial nerves 3, 7. 9, and 10. coded in green on book cover)

coeliac (solar) plexus (red on cover)

sympathetic ganglia and chain (thoracolumbar outflow: red)

intercostal nerves (blue on cover)

sacral parasympathetic plexus and ganglia (coded in green on book cover)

Figure 10.4b. Peripheral nervous system, with components shown in four colors on the book cover (Sappey).

Although it's not in the realm of ordinary experience, there is one wa> that you would be able to experience all parasympathetic reactions at the same time. Untutored and unwary mushroom aficionados have been known to ingest the poisonous fungus Amanita muscaria, which globalh stimulates the muscarinic receptors of the parasympathetic nervous system (and which is where those receptors got their name). Don't be tempted Depending on the dose, you'll have a most disagreeable experience. Fron the top down if you don't die your pupils will constrict to pinpoints, you'll slobber and froth saliva, your heart rate and blood pressure will plumme to near death, extreme bronchiolar constriction will make it almos impossible to breathe, your digestive tract will thump, thrash, and grim like a washing machine gone berserk, and you'll suffer from cxplosiv diarrhea. Amanita muscaria is plainly not suitable as a mushroom sauc for your dinner guests, but it does provide us with the only naturall occurring illustration of a general parasympathetic reaction, and it al> makes clear the altogether different ways in which the parasympathet and sympathetic nervous systems act on internal organs.

Looking at a more typical scenario in ordinary life, if you eat a large me at noon, the portions of the parasympathetic nervous system associated wit the digestive system will preside over digestion during the course of tl afternoon, and then its effects will recede into the background. Then if y< do a session of hatha yoga followed by relaxation at 5 PM, the ma autonomic effects will first be stimulation and then "relaxation" f the sympathetic nervous system. In another possible scenario, if you were > eat a meal at 5 PM, do a vigorous practice of hatha between 6 PM and 7 1' and finally lie down to relax between 7 PM and 7:30 PM, the parasym| thetic nervous system would initiate digestion after the meal, be partia stymied around 6:15 PM by the hatha session, and kick in again to r stimulate digestion during the course of relaxation around 7:15 PM. A) 1 so it goes, day and night, year after year, without the necessity of oi conscious supervision.

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Responses

  • Jukka
    Does the parsympathetic nervous system stimulate peristalsis?
    7 years ago
  • andwise
    How the parasympathetic system relate to the brain,cranial nerves ans sacral spinal cord?
    7 years ago

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