Anatomy Image Database: Autonomics and arrhythmias
Describe the autonomic pathway and the role of the autonomic system in the development of arrhythmias.
Sympathetic Pathway
Cell bodies preganglionic fibers
The cell bodies of the preganglionic sympathetic fibers are located in the first four thoracic segments of the spinal cord (intermediolateral columns)
Preganglionic fibers 1
The fibers exit the spinal cord via the ventral roots of the spinal nerves (T1-T4). (All motor fibers are contained in the ventral roots.) The spinal nerves exit via the intervertebral foramina.
Preganglionic fibers 2
The fibers next travel in the ventral rami of the T1 T4 nerves. Oops need to go to the sympathetic chain! Why?
Preganglionic fibers 3
Leave the nerve as white rami communicantes and join the sympathetic chain/trunk. What do these fibers do on reaching the chain?
Cell bodies postganglionic fibers
They happily synapse with the cell bodies in the chain ganglia. (Most synapse in the upper thoracic ganglia though some ascend up to synapse in the cervical ganglia.)
Postganglionic fibers cardiac nerves
The postganglionic fibers exit the sympathetic trunk as cardiac nerves (medial branches from these ganglia)
Postganglionic fibers cardiac plexus
These postganglionic sympathetic fibers merely pass through the cardiac plexus to innervate the heart. Sympathetic stimulation increases the heart rate and dilates the coronary arteries.
Parasympathetic Pathway
Cell bodies preganglionic fibers
These are located in the brain (vagal nuclei). Remember there are only four cranial nerves that carry parasympathetic fibers and only one coming as far down as the thorax?
Preganglionic fibers -1
Fibers in the vagus, exit the cranial cavity by passing through the jugular foramen.
Preganglionic fibers 2
The vagus containing preganglionic fibers travels vertically down the neck in the carotid sheath.
Preganglionic fibers -3
Next they continue in the cardiac branches of the vagus and reach the cardiac plexus (superficial and deep).
Cell bodies postganglionic fibers
Small parasympathetic ganglia are located in the cardiac plexus as well as the heart itself (atrial walls). It is here that the preganglionic fibers contained in the vagus synapse.
Postganglionic fibers
The postganglionic fibers are distributed to the heart. Parasympathetic stimulation slows the heart rate and constricts the coronary arteries.
Balance
Majority of the postganglionic fibers are distributed to the SA and AV node. Some fibers do supply the atrial and ventricular musculature. It is the balance between the two systems that maintains the heart rate. Too much or too little results in arrhythmia.