Basics of Autonomic nerve system in pharmacology

Autonomic nerve system

The autonomic nervous system is also known as the visceral, vegetative or involuntary nervous system widely distributed throughout the body controlling the so called autonomic or vegetative functions.

The ANS regulates activity of structures like the smooth muscles, cardiac muscle and secretory glands other than endocrine glands.

The ANS is responsible for maintaining respiration, blood pressure, heart rate, eye function, gastrointestinal activity, urinary output and virtually all visceral functions within well defined, physiologic limits.

The ANS mediates the body’s response to emergency situations. ANS conserves energy and maintains homeostasis.

Anatomical divisions of the nervous system

The nervous system is divided into central nervous system and peripheral nervous system-

ANS – involuntary, innervates smooth muscle, cardiac muscle, glands, etc., at neuroeffector junctions.

SNS -voluntary, innervates skeletal muscle at neuromuscular junctions.

Afferent fiber

Afferent fiber is the first link in the reflex arcs of the ANS. It arise from visceral structures. this is generally non myelinated and Cell bodies are in the dorsal root ganglia of spinal nerves and sensory ganglia of cranial nerves. Afferent fibers convey information to the brain and then to the system to be acted upon by efferent.

Efferent fiber

In efferent fibers two neurons (Preganglionic and Post ganglionic fibres) + effector cells / target organs are generally involved-

1. Preganglionic fibres
   1. Exit the spinal cord and terminate in ganglia
   2. Acetylcholine (ACh) is the neurotransmitter released by the preganglionic nerves in autonomic ganglia. This connection is known as nicotinic cholinergic synapse.
2. Postganglionic fibres
   1. Exit from ganglia and innervate the effector cells /organs
   2. Neurotansmitters released at the neuroeffector junctions depending on the innervation 
      • ACh mediates muscarinic cholinergic transmission
      • Norepinephrine mediates adrenergic transmission
   3. Adrenal medulla is an exception embryologically where ganglionic transmission causes release of epinephrine ( adrenaline) into the blood 

Divisions of the ANS

Parasympathetic Nervous system

• Craniosacral outflow – Preganglionic fibres originate in the midbrain ( 3^rd (oculomotor) cranial nerve), medulla oblongata (7^th (facial), 9^th (glossopharyngeal) and 10^th (vagus) cranial nerves) and 2^nd – 4^th segments of sacral spinal cord.
• Preganglionic fibres are long and usually synapse on or within the target organ in the ciliary ganglion (3^rd cranial nerve), submandibular ganglia (7th cranial nerve) otic ganglion (9th cranial nerve), terminal ganglia in the heart, lungs, liver, spleen, GI tract, kidney (10^th cranial nerve)
• Innervation is discrete, with preganglionic to postganglionic ratio of 1:1
• Postganglionic fibres are short and innervate target organs:
  • cranial division innervates the eye, salivary glands, heart, bronchi, gastrointestinal tract
  • sacral division innervates bladder, colon, urinary and rectal sphincters and genital organ

Sympathetic nervous system

• Thoracolumbar outflow – preganglionic fibres originate in the intermediolateral columns of spinal cord from the 1^st thoracic to the 3^rd lumbar vertebrae
• Preganglionic fibres are short and usually synapse well before the target organ in either:
  • Vertebral (Paravertebral) ganglia which include the cervical ganglia, consisting of 22 pairs on either side of vertebral column
  • Prevertebral ganglia in abdomen (celiac, cranial and caudal  mesenteric)
  • Innervation is diffuse, with preganglionic to post ganglionic ratio upto 1: 20
  • Postganglionic fibres are long and innervate target organs
  • Fibres from vertebral ganglia innervate blood vessels, the eyes, salivary glands, heart, bronchi, sweat glands and hair follicles.
  • Fibres from prevertebral ganglia innervate the stomach, intestine, bladder, urinary and rectal sphincters and genital organs.
• Adrenal medulla is embryologically and functionally a sympathetic ganglion; innervated by typical sympathetic preganglionic neurons

Comparison of sympathetic and parasympathetic nervous system