Autonomic System and its functions

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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