Autonomic Nervous System Notes

Autonomic Nervous System Introduction

The autonomic nervous system (ANS) is primarily concerned with the regulation of visceral or vegetative functions of the body. So, it is also called the vegetative or involuntary nervous system.

1. Divisions Of Ans: From an anatomical and physiological point of view, the autonomic nervous system is divided into two divisions:

1. Sympathetic division
2. Parasympathetic division.

Sympathetic Division

• It is otherwise called thoracolumbar outflow because the preganglionic neurons are situated in lateral gray horns of 12 thoracic and first two lumbar segments of the spinal cord. The fibers arising from here are known as preganglionic fibers.

Read And Learn More: Medical Physiology Notes

• The preganglionic fibers leave the spinal cord through the anterior nerve root and white rami communicants and terminate in the postganglionic neurons, which are situated in the sympathetic ganglia.
  Sympathetic division supplies smooth muscle fibers o- ail the visceral organs such as blood vessels, heart, lungs, glands, gastrointestinal organs, etc.

1. Sympathetic Ganglia: The ganglia of sympathetic division are classified into three groups:

1. Paravertebral or sympathetic chain ganglia
2. Prevertebral or collateral ganglia
3. Terminal or peripheral ganglia.

1. Paravertebral or Sympathetic Chain Ganglia: Paravertebral or sympathetic chain ganglia are arranged in a segmental fashion along the anterolateral surface of the vertebral column. The ganglia on either side of the spinal cord are connected with each other by longitudinal fibers to form sympathetic chains. Both chains extend from the skull to the coccyx.

The Ganglia of the sympathetic chain (trunk) on each side are divided into four groups:

1. Cervical ganglia
2. Thoracic ganglia
3. Lumbar ganglia
4. Sacral ganglia.

1. Cervical ganglia: There are 8 cervical ganglia, which are arranged in three groups.

1. Superior cervical ganglion: It is formed by the fusion of the upper four cervical ganglia. It is the largest ganglion of the autonomic nervous system. It receives preganglionic fibers from the first thoracic spinal segment (T1) via white rami. The postganglionic fibers from this ganglion supply the blood vessels, glands, etc. The superior cervical ganglion also sends some fibers to the heart through the superior cervical sympathetic nerve and cardiac plexus.
2. Middle cervical ganglion: It is formed by the 5th and 6th cervical ganglia. Preganglionic fibers arise from T₁ segment. Postganglionic fibers from here supply the sweat glands, thyroid glands, and parathyroid glands. It also sends fibers to the heart via the middle cervical sympathetic nerve and cardiac plexus.
3. Inferior cervical ganglion: This ganglion is formed by the fusion of the 7th and 8th cervical ganglia. First thoracic ganglion fuses with the inferior cervical ganglion forming the stellate ganglion. It receives preganglionic fibers from T., segment. It sends postganglionic fibers to the heart through the inferior cervical sympathetic nerve and cardiac plexus. The postganglionic fibers also form the plexus around the subclavian artery and its branches.

2. Thoracic ganglia: There are 12 thoracic ganglia on each side and these ganglia are evenly spaced. The thoracic ganglia receive preganglionic fibers from the thoracic segments of the spinal cord. The postganglionic fibers from the thoracic ganglia are distributed to visceral organs in the thorax and abdomen.

3. Lumbar ganglia: There are 5 lumbar ganglia. The preganglionic fibers for these ganglia arise from the first and second lumbar spinal segments (L₁, and L₂) and reach the lumbar ganglia. From here, the fibers extend down to the 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 pre¬ganglionic fibers from L₁ and L₂ segments. Postganglionic fibers from the sacral ganglia innervate the blood vessels and sweat glands in the lower limb.
• Below the sacral level, the two sympathetic trunks converge and fuse upon the anterior surface of the coccyx and form a terminal swelling. This terminal swelling is known as the coccygeal ganglion.
• The unpaired coccygeal ganglion is also called ganglion impar. It receives preganglionic fibers from L₁ and L₂ segments. The postganglionic fibers from here are distributed to the abdominal viscera and pelvic region.

2. Prevertebral or Collateral Ganglia: Prevertebral ganglia are situated in the thorax, abdomen, and pelvis in relation to the aorta and its branches.

• The prevertebral ganglia are:
  • Celiac ganglion
  • Superior mesenteric ganglion
  • Inferior mesenteric ganglion.

The prevertebral ganglia receive preganglionic fibers from T₅ to L₂ segments. The postganglionic fibers from these ganglia supply the visceral organs of the thorax, abdomen, and pelvis.

3. Terminal or Peripheral Ganglia: Terminal ganglia are situated within or close to struc¬tures innervated by them. The heart, bronchi, pancreas, and urinary bladder are innervated by the terminal ganglia.

• Sympathoadrenergic system: Sympathoadrenergic system is a functional and phylogenetic unit that includes sympathetic division and adrenal medulla. The adrenal medulla is a modified sympathetic ganglion. Since the 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 the adrenal medulla.

Parasympathetic Division

Parasympathetic Stimulation

• The parasympathetic division of ANS is otherwise called craniosacral outflow because the fibers of this division arise from the brain and sacral segments of the spinal cord. The cranial portion of the parasympathetic division in the brainstem innervates the blood vessels of the head and neck and many thoracoabdominal visceral organs.
• The sacral portion in sacral segments of the spinal cord innervates the smooth muscles forming the walls of viscera and the glands such as the large intestine, liver, spleen, kidneys, bladder, genitalia, etc.

1. Cranial Nerves Of Parasympathetic Division: The cranial nerves of the parasympathetic division are:

1. Oculomotor (3) nerve
2. Facial (7) nerve
3. Glossopharyngeal (9) nerve
4. Vagus (10) nerve.

The fibers of sacral outflow arise from the second to fourth sacral (S₂ to S₄) segments of the spinal cord. Thus, the preganglionic fibers of parasympathetic division arise from the neurons situated at three different levels:

Parasympathetic Stimulation

1. Tectal or midbrain level (3 cranial nerve)
2. Bulbar level or bulbar outflow (7, 9, and 10 cranial nerves)
3. Sacral outflow (sacral nerves).

The preganglionic fibers are longer and reach the postganglionic neurons, which are situated within the organs or close to the organs innervated by these nerves. Preganglionic fibers are myelinated, but the postganglionic fibers are non-myelinated.

1. Tectal or Midbrain Level: The group of cells forming the Edinger-Westphal nucleus of 3 cranial nerves gives rise to tectal fibers. The fibers from this nucleus end in the ciliary ganglion. The postganglionic fibers from here supply the sphincter pupillae and ciliary muscle.

2. Bulbar Level or Bulbar Outflow: The preganglionic fibers are the fibers of 7, 9, and 10 cranial nerves, which arise from the nuclei present in the medulla oblongata.

• Fibers of 7 cranial nen/e supply the lacrimal, nasal, submaxillary, and sublingual glands. The preganglionic fibers of this nerve end in the sphenopalatine ganglion and submaxillary ganglion. Postganglionic fibers from sphenopalatine ganglion supply lacrimal and nasal glands. The postganglionic fibers from the submaxillary ganglion supply sublingual and submaxillary glands.
• Fibers of 9 cranial nerve supply the parotid gland. The preganglionic fibers synapse with neurons of an otic ganglion. The postganglionic fibers from the otic ganglion supply the parotid gland.
• Fibers of 10 cranial nerves supply the visceral organs of the body. The preganglionic fibers terminate in the ganglia, which are situated on or near the organs. The post¬ganglionic fibers from the ganglia supply the organs. Vagus nerve supplies almost all the organs in the thorax and abdomen but not the pelvic organs.

Parasympathetic Stimulation

3. Sacral Outflow:

• The preganglionic fibers arise from anterior gray horn cells of the 2nd, 3rd, and 4th (sometimes from 1st also) sacral segments of spinal cord and form the pelvic nerve (nerve origins).
• The fibers end on the postganglionic neurons, which are situated on or near the visceral organs. The fibers from the postganglionic neurons supply descending colon, rectum, urinary bladder, internal sphincter, urethra, and accessory sex organs.
• The sacral parasympathetic fibers supply those visceral organs which are not supplied by the vagus.

Functions Of Ans

• The ANS is concerned with the regulation of functions, which are beyond voluntary control. By controlling the various vegetative functions, ANS plays an important role in maintaining the constant internal environment (homeostasis).
• Almost all the visceral organs are supplied by both sympathetic and parasympathetic divisions of ANS and, the two divisions produce antagonistic effects on each organ. When the fibers of one division supplying to an organ is sectioned or affected by a lesion, the effects of fibers from another division on the organ become more prominent.
• The actions of the sympathetic and parasympathetic fibers on various structures are given in.

Neurotransmitters Of Ans

The different nerve fibers of ANS execute the functions by releasing some neurotransmitter substances.

1. Sympathetic Fibers:

1. Preganglionic fibers: Acetylcholine (Ach)
2. Postganglionic noradrenergic fibers: Noradrenaline
3. Postganglionic cholinergic fibers: Ach

The postganglionic sympathetic cholinergic nerve fibers supply sweat glands and blood vessels in the heart and in skeletal muscle.

2. Parasympathetic Fibers:

1. Preganglionic fibers: Ach
2. Postganglionic fibers: Ach

• • Catecholamines: The synthesis and the metabolism of catecholamines are explained in.

Sympathomimetic Drugs

The drugs, which produce effects similar to the effects of stimulation of sympathetic nerve fibers, are called sympathomimetic drugs or adrenaline-like drugs. Adrenaline and noradrenaline produced in the body act only for a short duration of about 1-2 minutes. Whereas, the sympathomimetic drugs injected intravenously act for a longer period of about 30 minutes to 2 hours.

The following are the sympathomimetic drugs:

• Drugs Stimulating the Receptors Directly
  • Phenylephrine (alpha receptors)
  • Isoproterenol (beta receptors)
  • Albuterol (beta 2 receptors).
• Drugs Inducing the Release of Noradrenaline:
  • Ephedrine
  • Tyramine
  • Amphetamine.

Sympathetic Blockers

Sympathetic blockers are drugs that prevent the actions of sympathetic neurotransmitters. The sympathetic blockers act on all levels. The different actions of the blocking agents are:

1. Prevention of synthesis and storage of noradrenaline.
   Example: Reserpine
2. Prevention of release of noradrenaline.
   Example: Guanethidine
3. Blockage of alpha-adrenergic receptors.
   Examples: Phenoxybenzamine and phentolamine
4.  Blockage of beta-adrenergic receptors.
   Example: Metaprolal
5. Blockage of transmission of nerve impulse through sympathetic ganglia.
   Example: Hexamethonium.

Parasympathomimetic Drugs

The drugs, which produce effects similar to the effects of stimulation of parasympathetic nerve fibers are called parasympathomimetic drugs or Ach-like drugs. Ach produced in the body acts only for a short period whereas, the injected Ach acts for a long time. Similarly, parasympathomimetic drugs also exhibit their actions for a longer time. Following are the para-sympathomimetic drugs:

• Drugs that Act on Muscarinic Receptors: Pylocarpine and methacholine produce their effects by acting on the muscarinic receptors.
• Drugs, which Prolong the Action of Ach: The action of Ach can be prolonged by preventing its destruction. The drugs like neostigmine and physostigmine inhibit the activity of acetylcholinesterase and so the Ach is not destroyed quickly.

Parasympathetic Blockers

Parasympathetic blockers are the drugs, which prevent the actions of parasympathetic neurotransmitters. The drugs atropine, homatropine, and scopolamine inhibit the actions of Ach by blocking the muscarinic receptors.

Gadglsonic Blockers

• Ganglionic blockers are drugs that prevent the transmission of impulses from the preganglionic neurons to the postganglionic neurons. Tetraethyl ammonium ion, hexamethonium ion, and pentolinium are some of the ganglionic blockers.
• These drugs block both the sympathetic and parasympathetic ganglia. However, ganglionic blockers are commonly used to block the sympathetic ganglia rather than the parasympathetic ganglia because the sympathetic blockade overshadows the parasympathetic blockade.