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'.
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.
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.
10. 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.
11. 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.
12. 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.
13. 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
