Autonomic Nervous System

Human Physiology

Lesson 43 : Autonomic Nervous System, Reflex Action & Sensory Receptors

Autonomic Nervous System

The PNS is divided into two divisions called somatic nervous system and autonomic nervous system. The somatic nervous system relays impulses from CNS to skeletal muscles and function voluntary, while the autonomic nervous system transmits impulses from the CNS to the involuntary organs and smooth muscles of the body. By controlling involuntary functions, the ANS helps in regulating vital body functions such as blood circulation, respiration, digestion, excretion, metabolism and thermoregulation etc.
The autonomic nervous system is further classified into sympathetic and parasympathetic nervous systems. Presence of peripheral ganglia (nerve cell bodies) is one of the main characteristic features of the ANS. The difference between the sympathetic and parasympathetic are mentioned below.



The sympathetic nerve fibers are from thoracic and lumber spinal cord. It includes 1st to 12th thoracic vertebrae and 1 to 3 lumber vertebrae. It is thoraco-lumber in origin.

The parasympathetic nerve fibers are from brain and sacral region (cranio-sacral in origin). The cranial efferents supplies to organs of head, thorax, abdomen and includes; occulomotor or III cranial, facial or VII cranial, glossopharyngeal or IX cranial and vagus or X cranial nerves. The sacral efferents supplies to genital organ, colon, rectum and internal anal sphincter.

The pre-ganglionic neurotransmitter is acetyl choline and post-ganglionic is nor-epinephrine.

The neurotransmitter at both pre-ganglionic and post-ganglionic is acetyl choline.

These nerve fibers supplies to almost all organs of body (diffuse innervation).

These nerve fibers supplies to only visceral organs (restricted innervation).

Ganglia are located mostly away from the organ supplied.

Ganglia are located close to the organ supplied

The pre-ganglionic fiber is short and postganglionic fiber is long.

The pre-ganglionic fiber is long and postganglionic fiber is very short.

Action is excitatory in most of organs but inhibitory in gastrointestinal tract.

Action is inhibitory in most of organs but excitatory in gastrointestinal tract.

Last modified: Wednesday, 11 April 2012, 5:34 AM