Autonomic nervous system (ANS) is primarily concerned with regulation of visceral or vegetative functions of the body. So, it is also called vegetative or involuntary nervous system.
DIVISIONS OF ANS
From anatomical and
physiological point of view, ANS is divided into two divisions:
1. Sympathetic division
2. Parasympathetic
division.
SYMPATHETIC DIVISION
Sympathetic division is otherwise called thoracolumbar outflow because, the preganglionic neurons are situated in lateral gray horns of 12 thoracic and first two lumbar segments of spinal cord. Fibers arising from here are known as preganglionic fibers. Preganglionic fibers leave the spinal cord through anterior nerve root and white rami communicantes and terminate in the postganglionic neurons, which are situated in the sympathetic ganglia. Sympathetic division supplies smooth muscle fibers of all the visceral organs such as blood vessels, heart, lungs, glands, gastrointestinal organs, etc.
SYMPATHETIC GANGLIA
Ganglia of
sympathetic division are classified into three groups:
A. Paravertebral or
sympathetic chain ganglia
B. Prevertebral or
collateral ganglia
C. Terminal or peripheral
ganglia.
A. Paravertebral
or Sympathetic Chain Ganglia
Paravertebral or
sympathetic chain ganglia are arranged in a segmental fashion along the
anterolateral
surface of vertebral
column. Ganglia on either side of the spinal cord are connected with each other
by
longitudinal
fibers, to form the sympathetic chains. Both the chains extend from
skull to coccyx.
Ganglia of the sympathetic
chain (trunk) on each side are divided into four groups:
1. Cervical ganglia : 8 in
number
2. Thoracic ganglia : 12
in number
3. Lumbar ganglia : 5 in
number
4. Sacral ganglia : 5 in
number
1. Cervical ganglia
Eight cervical ganglia are
arranged in three groups:
i. Superior cervical
ganglion: It is formed by the fusion of upper four cervical ganglia. It is
the largest ganglion of ANS. It receives preganglionic
fibers from first thoracic spinalsegment (T1) via white rami.
Postganglionic fibers from this ganglion, supply the
blood vessels, glands, etc. Superior cervical ganglion also sends some fibers to heart through superior cervical
sympathetic nerve and cardiac plexus.
ii. Middle cervical
ganglion: It is formed by fifth and sixth
cervical ganglia. Preganglionic fibers
arise from
T1 segment. Postganglionic fibers from here supply the sweat
glands, thyroid
gland and parathyroid
glands. It also sends fibers to heart via middle cervical
sympathetic
nerve and cardiac plexus.
iii. Inferior cervical
ganglion: This ganglion is formed by the fusion of seventh and eighth
cervical ganglia. First
thoracic ganglion fuses with inferior cervical ganglion, forming stellate
ganglion.
It receives preganglionic fibers from T1 segment. It sends postganglionic
fibers
to heart through inferior
cervical sympathetic
Effector
organ Sympathetic division Parasympathetic division
1. Eye
Ciliary muscle Relaxation
Contraction
Pupil Dilatation
Constriction
2. Lacrimal glands Decrease
in secretion Increase in secretion
3. Salivary glands Decrease
in secretion and
vasoconstriction
Increase in secretion and vasodilatation
4. Gastrointestinal tract
Motility Inhibition
Acceleration
Secretion Decrease Increase
Sphincters Constriction
Relaxation
Smooth muscles Relaxation
Contraction
5. Gallbladder Relaxation
Contraction
6. Urinary bladder
Detrusor muscle Relaxation
Contraction
Internal sphincter
Constriction Relaxation
7. Sweat glands Increase in
secretion –
8. Heart – rate and force
Increase Decrease
9. Blood vessels
Constriction of all blood
vessels,
except those in heart and
skeletal
muscle
Dilatation
10. Bronchioles Dilatation
Constriction
nerve and cardiac plexus.
Postganglionic
fibers also form the
plexus around subclavian
artery and its branches.
2. Thoracic ganglia
There are 12 thoracic ganglia on each
side and these ganglia are evenly spaced. Thoracic ganglia receive preganglionic
fibers from the thoracic segments of spinal cord. Postganglionic
fibers from thoracic ganglia are distributed to visceral organs in the thorax and abdomen.
3. Lumbar ganglia
There are 5 lumbar ganglia.
Preganglionic fibers for these ganglia arise from first and second lumbar
spinal segments
(L1 and L2) and reach the lumbar ganglia. From here, the fibers extend
down to sacral ganglia also. Postganglionic fibers from these ganglia supply the abdominal and pelvic organs.
4. Sacral ganglia
There are 5 sacral ganglia, which
receive the preganglionic fibers from L1 and L2 segments.
Postganglionic fibers from sacral ganglia innervate the blood vessels and sweat
glands in
the lower limb.
Below the sacral level,
both the sympathetic trunks converge and fuse upon the anterior surface
of coccyx and form a terminal
swelling. This terminal swelling is known as coccygeal ganglion. Unpaired
coccygeal ganglion is also
called ganglion impar. It receives
preganglionic fibers from L1 and L2 segments. Postganglionic fibers from here are distributed to the abdominal
viscera and pelvic region.
B. Prevertebral or
Collateral Ganglia
Prevertebral ganglia are
situated in thorax, abdomen and pelvis, in relation to aorta and its branches.
Prevertebral ganglia are:
1. Celiac ganglion
2. Superior mesenteric
ganglion
3. Inferior mesenteric
ganglion.
Prevertebral
ganglia receive preganglionic fibers from T5 to L2 segments. Postganglionic
fibers from
these ganglia supply the
visceral organs of thorax, abdomen and pelvis.
C. Terminal or
Peripheral Ganglia
Terminal ganglia are
situated within or close to structures innervated by them. Heart, bronchi,
pancreas and urinary bladder are innervated by the terminal ganglia.
Sympathoadrenergic
System
Sympathoadrenergic system is a functional and phylo genetic unit that includes
sympathetic division
and
adrenal medulla. Adrenal medulla is a modified sympathetic ganglion.
Since adrenal medulla and
sympathetic division develop
from the same neural crest, their secretions and functions are almost the same.
Any increase in sympathetic activity increases the secretion of catecholamines
from adrenal medulla.
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