AUTONOMIC NERVOUS SYSTEM: INTRODUCTION.

AUTONOMIC NERVOUS SYSTEM: INTRODUCTION.

 

The cell bodies of the preganglionic sympathetic axons lie in the intermediolateral grey horn at the level T1-L2 of the spinal cord while the cell bodies of the preganglionic SACRAL parasympathetic axons lie in the intermediate grey horn at the level S2-S4 of the spinal cord.

 

The sympathetic and parasympathetic SACRAL efferents travel in the anterior root of spinal cord, the trunk and the anterior rami. From here it takes different courses.

 

SYMPATHETIC MOTOR PATHWAY

 

1. Each sympathetic efferent leave the anterior ramus and innervate in the sympathetic trunk (paravertibral trunk) at which the preganglionic axons terminate and postganglionic axons begin. The postganglionic axons again enter the same anterior ramus, from where they also go to the posterior ramus to provide both the anterior and posterior body parts. Note that the sympathetic trunks are two, on either side of the spinal cord and lie anterior to the anterior rami of spinal cord so the preganglionic axons from an anterior ramus moves anteriorly innervates the cell body of the postganglionic axon in the sympathetic trunk and then the postganglionic axon again moves posteriorly to join the same anterior ramus (Students Grays. Pg: 44 Fig: 1.45). The sympathetic efferent preganglionic axon may ascend or descend along the sympathetic trunk to innervate a postganglionic cell body at a different level. This postganglionic axon moves posteriorly from the sympathetic trunk to enter the anterior ramus of its own level. The axons also go to the posterior ramus.

2.Some sympathetic efferents after leaving the anterior ramus pass the sympathetic trunk without synapsing and synapse at the prevertibral plexus   (adjacent to the aorta and anterior to the vertebral column, hence the name PRE vertebral), the renal plexus or the adrenal medulla instead. One of the sub divisions of the prevertibral plexus is the celiac plexus. The bundles of preganglionic axons penetrate the diaphragm to synapse at the above mentioned plexus. There are three.

 

·GREATER SPLANCHNIC NERVE: that synapse at the renal plexus, celiac plexus and suprarenal medulla (the suprarenal medulla is a collection of ganglia which release norepinephrine and epinephrine).

 

·LESSER SPLANCHNIC NERVE: that synapse only at the

Renal plexus.

 

·LEAST SPLANCHNIC NERVE: that synapse only at the celiac plexus.

 

The plexus contain a connective tissue capsule, satellite cells, postganglionic cell bodies, efferent and afferent axons (both sympathetic and parasympathetic).

 

The postganglionic axons leaving the renal plexus provide the kidneys and leaving the celiac plexus provides the celiac artery.

 

Note: Each sympathetic trunk has 3 cervical ganglia, 12 thoracic ganglia, 5 lumber and 5 sacral ganglia.

 

PARASYMPATHETIC MOTOR PATHWAY

 

1.The sacral parasympathetic preganglionic efferents after leaving the anterior rami of sacral nerves S2-S4 form the pelvic splanchnic nerves that synapse at the hypogastric plexus from where postganglionic axons innervate the gastro intestinal tract.

2.The cranial parasympathetic preganglionic efferents initiate from the grey matter of the brain stem at level cranial 3, 7, 9 and 10 to form the corresponding cranial nerves. The cranial nerves synapse either at ganglions  in the head and neck or the cranial autonomic nuclei from where postganglionic axons emerge and innervate parts of the head and neck.

 

All efferent preganglioinc axons release acetylcholine.

The parasympathetic postganglionic axons also release acetylcholine but the sympathetic postganglionic mainly release norepinephrine except that to the sweat glands and blood vessels of skeletal muscles, it releases acetylcholine.

 

The parasympathetic and sympathetic postganglionic axons are also seen to release neuropeptide Y, substance P and ATP. These have an inductive effect on the primary neurotransmitters.

 

Apart from the preganglionic and postganglionc neurons, there may be interneurons called small intensely fluorescent cells-SIFs (since they contain catecholamine and shine under light). The SIFs release dopamine.