Autonomics Thorax

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Autonomics of the Thorax - Page 1 of 12Learning Modules - Medical Gross Anatomy

INTRODUCTION

AUTONOMICS OFTHE THORAX

Many organsthroughout the bodyreceive dual innervationfrom the sympatheticand parasympatheticdivisions of theautonomic nervoussystem. Within thethorax the organs ofparticular importance

are the heart, lungs andesophagus. This lessonwill describe howautonomic fibers reachtheir target organs andwhat responses can beexpected uponstimulation by eithersympathetic orparasympathetic fibers.

Copyright 2002 The University of Michigan. Unauthorized use prohibited.


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HEART -OVERVIEW

The intrinsicrhythmicity of the heartis produced by its dualinnervation withsympathetic andparasympathetic cardiacnerves. This collection ofautonomic fibers formsthe CARDIAC PLEXUS.It is located at the baseof the heart behind and

within the concavity ofthe arch of the aorta.Stimulation of thesefibers can dramaticallyalter the pace and forceof contraction of theheart.



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HEART - SYMPATHETIC

The sympathetic innervation of the heart originatesfrom the thoracic portion (T1-T4 or T5) of the spinal cordThese presynaptic fibers first travel to either the cervical(superior, middle or inferior) or the thoracic ganglia of thsympathetic chain, where they synapse. Thepostsynaptic fibers emerging from the ganglia will thentravel to the heart in small cervical or thoracic cardiacnerves (a.k.a. thoracic visceral nerves) to innervate it.Stimulation of the sympathetic nervous system within thitarget organ causes an INCREASE in heart rate andcontractility.

Why do the cardiac nerves bother to ascend into the

neck, simply to descend back down into the chest? It'sbecause the heart initially develops in the general vicinityof the neck, and then descends into the chest, drawingits nerves down with it. This is a recurrent theme inanatomy - when the adult anatomy seems illogical, lookto embryology to help explain it.



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HEART - PARASYMPATHETIC

The parasympathetic innervation of theheart is supplied by the right and left vagus(CN X) nerves which provide cervical cardiacnerves to the cardiac plexus. Additionalcardiac branches are provided by the rightand left recurrent laryngeal nerves, branchesof the vagus nerve.

Unlike the sympathetic innervation,which must first synapse within chain gangliato supply the heart with postsynaptic fibers,the parasympathetic fibers synapse at

ganglia located directly on the heart andshort postsynaptic fibers then supply thetarget organ. Parasympathetic stimulationacts to DECREASE its rate and contractility.



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LUNGS - OVERVIEW

The lungs aresupplied by both thesympathetic andparasympatheticdivisions of theautonomic nervoussystem, arranged as thePULMONARY PLEXI.These paired autonomicplexi lie on the anteriorand posterior surfacesof the roots of both

lungs. The pulmonaryplexi are directlycontinuous with thecardiac plexus at thetracheal bifurcation, andcommunicate with theesophageal plexus andautonomic fibers on theaorta. Fibers from thepulmonary plexidistribute to smoothmuscle and glands ofthe bronchi and

pulmonary bloodvessels.



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LUNGS - SYMPATHETIC

The sympathetic innervation of thelungs originates from the thoracic portion(T1-T4 or T5) of the spinal cord. Thepresynaptic fibers pass through white ramicommunicantes to reach the sympathetictrunk, where they synapse in the upperthoracic chain ganglia. The postsynapticfibers then pass via slender

into the pulmonary plexusto innervate the vasculature of the lungs,while epinephrine released from thesuprarenal cortex acts upon the bronchial

smooth muscle. Stimulation of thesympathetic nervous system acts toVASOCONSTRICT and BRONCHODILATE

thoracicvisceral nerves



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LUNGS - PARASYMPATHETIC

The parasympathetic innervationof the lung is supplied by the rightand left vagus nerves. Someparasympathetic fibers reach thepulmonary plexus from the cardiacplexus, and many additional fibersare supplied directly from the vagusnerve as it passes posterior to theroot of each lung. Just as with theheart, vagal fibers synapse directlywithin the pulmonary plexus onganglia located on bronchi and

pulmonary vessels. Parasympatheticstimulation within the plexusescauses VASODILATION andBRONCHOCONSTRICTION.



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LUNGS - ASTHMA

Bronchial asthma is a chronic disorder characterized by hyperreactive airways leading to episodic,reversible bronchoconstriction due to an increased sensitivity to irritating stimuli. Clinically patients willpresent with increased mucus secretions (mucus plugs) and show a tremendous and rapidbronchoconstriction in response to moderate stimulation. Prescription medications aimed at managingasthma are designed to mimic a sympathetic response (sympathomimetic) of bronchodilation.



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ESOPHAGUS - SYMPATHETIC

The sympathetic innervation ofthe esophagus arises from thethoracic sympathetic trunk.Presynaptic fibers will first synapsewithin the chain ganglia beforetraveling to the esophageal plexusvia smallto supply the esophageal vascularsmooth muscle. Stimulation of thesefibers will result inVASOCONSTRICTION.

thoracic visceral nerves



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ESOPHAGUS - PARASYMPATHETIC

The esophageal plexus of nerves is formed primarily by the right and left vagus nerves and supplies the lower two-thirds of this orga

In forming the plexus, the right vagus will pass primarily to the back of the esophagus to become the POSTERIOR VAGAL TRUNK as itpasses through the diaphragm, while the left vagus will pass to the front to become the ANTERIOR VAGAL TRUNK. These trunks will pathrough the esophageal hiatus of the diaphragm to supply parasympathetics to abdominal viscera. As with the cardiac and pulmonaryplexuses, presynaptic parasympathetic fibers will synapse within ganglia located in the wall of the esophagus and short postsynaptic fibewill innervate the organ. Parasympathetic stimulation of the esophagus results in the rhythmic contraction of esophageal smooth muscle,peristalsis, allowing food to pass into the stomach.



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SPLANCHNICS AND GRAY RAMI

Presynapticsympathetic fibersfrom chain gangliabetween T5-T12leave the thoracicsympathetic trunk toform the

, which willinnervate abdominalorgans. Branchesfrom chain ganglia at

T5-9 travelanteroinferiorly on thesurface of thevertebral bodies,uniting to form thegreater thoracicsplanchnic nerve,which can be easilyidentified within theposteriormediastinum. Similarbranches from T10-11 form the lesserthoracic splanchnicnerve, and T12provides the leastthoracic splanchnic,but these are moredifficult to identifywithin the thorax dueto the dome of thediaphragm. Thedetails of the thoracicsplanchnic nerves willbe covered later.

greater,lesser and leastthoracic splanchnicnerves

Remember as

well that the thoracicsympathetic trunkgives off

carrying fibers to thethoracic spinalnerves. Of course,each thoracic ventralprimary ramus(otherwise known asan intercostal nerve)is also connected to

gray ramicommunicantes


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the sympathetic trunkby a

,carrying thepresynaptic fibersthat form thesympathetic trunk.

white ramuscommunicans

Above and below - posterior chest wall, right lateral view


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SUMMARY

In summary, theautonomicinnervation ofthoracic viscera isderived from bothparasympathetic andsympathetic systems.Autonomic fibersform the cardiac,pulmonary andesophageal plexuses.

Sympatheticfibers supplyingthoracic viscera arisefrom the lateral hornof the upper thoracicspinal cord segmentsas preganglionicfibers, some of whichascend in the cervicalsympathetic trunkand synapse incervical ganglia.Postganglionic fibersthen descendthrough the neck ascardiac branches andend in the cardiacplexus of the heart.Other sympatheticfibers synapse in theupper thoracic sympathetic trunk and the postganglionic fibers travel to the thoracic viscera as smallthoracic visceral nerves ending in the cardiac, pulmonary and esophageal plexuses. The sympatheticsspeed the heart, increase its output, constrict blood vessels and dilate bronchi.

The parasympathetic supply to thoracic viscera is carried via the vagus nerve (CN X), whichdescends through the neck and into the chest. It gives rise to branches that reach the cardiac,pulmonary and esophageal plexuses. The parasympathetics slow the heart and decrease its output,

constrict bronchioles in the lungs and provide for contraction (peristalsis) of the esophagus.


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Autonomics Thorax