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) DSCN0597
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. DSCN0600
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? DSCN0629
Ventral ramus
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? spinal nerve
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.) DSCN0628
Postganglionic fibers – cardiac nerves The postganglionic fibers exit the sympathetic trunk as cardiac nerves (medial branches from these ganglia) DSCN0632
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. Cardiac Plexus
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? DSCN0016
Preganglionic fibers -1 Fibers in the vagus, exit the cranial cavity by passing through the jugular foramen. DSCN0035
Preganglionic fibers – 2 The vagus containing preganglionic fibers travels vertically down the neck in the carotid sheath. Image026
Preganglionic fibers -3 Next they continue in the cardiac branches of the vagus and reach the cardiac plexus (superficial and deep). Image033(copy)
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. Cardiac Plexus
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.