Autonomic Nervous System (ANS)

• Classically called the Visceral motor system
• Innervate involuntary (smooth) muscle, modified cardiac muscle (intrinsic conducting tissue), and glands
• Visceral efferents (motor) fibers are accompanied by afferent (sensory) fibers
• Sensory fibers act in autonomic reflexes; in conducting pain impulses, and to regulate visceral function

The efferent nerve fibers and ganglia of the ANS are organized into 2 systems/divisions:

1. Sympathetic (thoraco-lumbar)
   Generally catabolic – preparing the body to “flee”
2. Parasympathetic (cranio-sacral)
   Generally anabolic – promoting normal function and conservation of energy

Conduction involves 2 neurons – pre-ganglionic and post-ganglionic

Preganglionic neuron

• cell body in brain or spinal cord
• axon is myelinated type B fiber that extends to autonomic ganglion

Postganglionic neuron

• cell body lies outside the CNS in an autonomic ganglion
• axon is unmyelinated type C fiber that terminates in a visceral effector

The anatomical distinction between the 2 divisions is based primarily on the location of the presynaptic cell bodies

Parasympathetic Nervous System:

• Reach organs via postganglionic fibers within or close to the organ
• Pre-ganglionic axons originate from Cranial Nerves 3, 7, 9 and 10
• The parasympathetic nervous system preganglionic neurons are mainly found in the brain/head and sacral regions (parasympathetic ganglia are found wherever their targets are)

Sympathetic Nervous System:

• Reach end-organs via more devious pathways down the spinal cord to clusters of sympathetic ganglia (trunk) alongside the spine
• messages are relayed to other neurons that travel largely with blood vessels to other parts of the body, and therefore do not reach avascular tissues
• The sympathetic nervous system preganglionic neurons and ganglia are found mainly in the thoracic region (T1-L2)

Development of The ANS

Sympathetic NS

• 5th week, neural crest cells in thoracic region migrate along sides of spinal cord, forming paired masses (ganglia) dorsolateral to the aorta, which connect in bilateral chain by longitudinal nerve fibers.
• Some migrate ventrally to the aorta, forming neurons in preaortic ganglia
• Other cells migrate to viscera (heart, lungs, GI) where they form plexuses near or within the organ.

Parasympathetic NS

• Arise from neurons in nuclei of the brainstem and in the sacral spinal cord.
  
  
  See: Autonomic Diseases