Autonomic Nervous System

Autonomic Nervous System (ANS)—Introduction

Autonomic nervous system (ANS) is called autonomic as it works without the voluntary control of the individual. Its central nervous component is mainly present in the hypothalamus. Its afferent nerve fibers that take impulses from the periphery to CNS include visceral afferents, vagal afferents, and nociceptive fibers. Its efferent nerve fibers that carry impulses from the CNS to viscera make up the Parasympathetic and the Sympathetic nervous systems.

Parasympathetic system:

It consists of the cranial and the sacral outflows, together called as cranio-sacral outflow. The cranial outflow includes the cranial nerves III, VII, IX and X that carry impulses to eye, salivary glands, heart, lungs and the abdominal viscera. The sacral outflow originates from the 2nd, 3rd and 4th sacral segments of the spinal cord; and forms the pelvic nerves, which supply rectum, bladder and genital organs.

The cranial and pelvic nerves coming out from the CNS form the pre-ganglionic nerve fibers of the parasympathetic system, are quite long and terminate in the parasympathetic ganglia that lie close to the target sites. The post-ganglionic parasympathetic nerve fibers that arise from these ganglia are very short and supply discretely close by visceral organs. The organs supplied by these nerve fibers have muscarinic receptors and are listed below under the distribution of the muscarinic receptors.

Sympathetic system:

It consists of the sympathetic nerve fibers coming out from 1st thoracic to 3rd lumbar segments of the spinal cord (thoraco-lumbar outflow). These form the pre-ganglionic sympathetic nerve fibers, are very short and terminate in the sympathetic ganglia present close by in the form of sympathetic chains on both sides of the spinal cord. The post-ganglionic sympathetic nerve fibers are very long and supply wide spread target organs. The organs supplied by these nerve fibers have adrenergic receptors.

Some sympathetic ganglia that supply abdominal and pelvic viscera lie outside the sympathetic chain. Adrenal medulla is also a modified form of sympathetic ganglia. It is supplied by the pre-ganglionic fibers but does not give out the post-ganglionic fibers, rather directly secretes adrenaline (80%) and to some extent nor-adrenaline (20%) into the circulation.

[Note: Some organs are supplied by both the post-ganglionic parasympathetic and sympathetic nerve fibers. The effects of parasympathetic and sympathetic system in these organs are mostly opposite to each other and parasympathetic effects usually dominate].