Integrated Systems




Learning Objectives



  1. Discuss the function of the two divisions of the ANS.



  2. Where is the origin of the sympathetic and parasympathetic divisions?



  3. Describe the pathway of preganglionic and postganglionic sympathetic fibers.



  4. Describe the pathway of preganglionic and postganglionic parasympathetic fibers.



  5. Identify sympathetic and parasympathetic ganglia.



  6. Identify the neurotransmitters and receptors found in the ANS.



  7. Identify major autonomic plexi in the thorax and abdomen.



  8. Describe the basic anatomy and function of the hypothalamus.



  9. Describe the basic anatomy and function of the limbic system.



  10. Describe the basic anatomy and function of the reticular formation.




Autonomic Nervous System (ANS)



Overview


The function of the autonomic nervous system (ANS) is to maintain homeostasis. It controls several systems such as digestion, respiration, cardiac activity, and exocrine glands among others. The ANS controls are exerted on targeted tissue and cells. The targets of the ANS are: cardiac tissue, smooth muscle, and exocrine glands. This system is made up of visceral motor (general visceral efferent [GVE]) components that operate automatically.



Organization of the ANS


The ANS is divided into three major divisions: the parasympathetic, sympathetic, and enteric nervous system. The enteric system provides the intrinsic innervation for the gastrointestinal tract and mediates reflex peristaltic activity. Due to the unique and specific function of the enteric system, which is independent of the hypothalamus, that particular division will not be included in further discussions in this chapter.




  • Both the parasympathetic and sympathetic arms of the ANS involve a two-neuron chain ().




    • The first order neuron of the chain is called a preganglionic neuron.




      • Its cell body is located within the central nervous system (CNS).



      • Its axon is myelinated.



    • The preganglionic axon terminates on the second-order neuron.




      • The second order neuron is called a postganglionic neuron.



      • Its cell body is located in a peripheral ganglion.



      • Its axon is unmyelinated.



    • The ANS carries GVE fibers that control visceral motor neurons.



  • Although the divisions are often considered as antagonistic, there are some situations where they work in concert or independently. For example, arrector pili muscles, which are associated with hair follicles in mammals, are solely innervated by sympathetic fibers. Conversely, reproduction requires both sympathetic (ejaculation) and parasympathetic (erection) activity.



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Fig. 18.1 Circuit diagram of the autonomic nervous system. The central first (presynaptic) neuron uses acetylcholine as a transmitter in both the sympathetic and parasympathetic nervous systems neuron. Acetylcholine is also used as a neurotransmitter by the second (postsynaptic) neuron in the parasympathetic nervous system. In the sympathetic nervous system, norepinephrine is used by the noradrenergic neuron. Note: The target cell membrane contains different types of receptors (= transmitter sensors) for acetylcholine and norepinephrine. Each transmitter can produce entirely different effects, depending on the type of receptor. (Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)



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Fig. 18.2 (a) The first order cell bodies of the sympathetic system are located in the intermediolateral cell column (IML) of the gray matter. (b) Organization of the gray matter, left oblique anterosuperior view. The gray matter of the spinal cord is divided into three columns (horns): (A) Anterior column (horn): Contains motor neurons (B) Lateral column (horn): Contains sympathetic or parasympathetic (visceromotor) neurons in select regions (C) Posterior column (horn): Contains sensory neurons. Afferent and efferent neurons within these columns are clustered in nuclei according to function. (Reproduced with permission from Gilroy AM, MacPherson BR. Atlas of Anatomy. Third Edition. © Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)



Sympathetic Division




  • The function of the sympathetic system is to prepare the body for increased activity. It is commonly referred to as the “fight or flight response.” Activation of this division will increase heart rate, decrease digestion, and dilate pupils ().



  • The sympathetic division of the ANS originates in the intermediolateral cell column (IML) of the spinal cord (a, b).




    • The IML is present in the lateral horn gray matter in vertebral levels T2–L1 ().




      • Due to its range of vertebral levels, the sympathetic arm is also called the thoracolumbar division of the ANS (a, b).



    • Preganglionic neurons for the head and neck are located in the IML from T1 to T4.



  • Sympathetic ganglia house the second order neurons, also known as the postganglionic neuron.




    • The thoracic sympathetic ganglia are described as paravertebral. That is, there are two interconnected “sympathetic chains/trunks” that run vertically on either side of the vertebral column ().



    • In the abdomen and pelvis, sympathetic ganglia lie anterior to the abdominal aorta and are called prevertebral ganglia.



  • The superior, middle, and inferior cervical ganglia (sympathetic ganglia) are located in the head and neck ().




    • The superior cervical ganglion is located in the retrostyloid space and lies on longus coli at approximately C2.



    • The middle cervical ganglion lies medial to the vertebral artery at vertebral level C6.



    • The inferior cervical ganglion lies at the level of C7. It often fuses with the first thoracic ganglion in which case it is referred to as the stellate ganglion.



    • Preganglionic neurons for the head and neck synapse in the cervical ganglia.



  • Sympathetic fibers are widespread in the body.




    • Preganglionic fibers exit the spinal cord via the ventral root of the spinal nerve.



    • The preganglionic fibers enter the sympathetic chain forming white communicating rami. White rami are named based on the fact that they contain preganglionic myelinated fibers. They are only present from T1 to L2/3 (a, b).



    • Some sympathetic preganglionic fibers terminate in the sympathetic chain by synapsing on second order neurons. They can synapse at the vertebral level they entered or they can ascend or descend within the sympathetic chain and synapse at a different level ().



    • Postganglionic fibers (unmyelinated) rejoin spinal nerves forming gray communicating rami. After joining the spinal nerve, the postganglionic fibers will travel to their target structure (cardiac tissue, smooth muscle, or glands). Gray rami are present the entire length of the spinal cord.



    • There are some preganglionic fibers that enter the sympathetic chain via white rami and exit without synapsing such as splanchnic nerves. Splanchnic nerves will ultimately synapse on second order neurons that are located in prevertebral ganglia (). From there, they will innervate abdominal and pelvic viscera. It should be noted that the term “splanchnic” denotes that the nerve innervates viscera. It does not indicate if it is sympathetic or parasympathetic or if it is pre- or postganglionic.



    • There are three groups of thoracic splanchnic nerves that exit the sympathetic chain at different levels ():




      • Greater splanchnics (T5–T9).



      • Lesser splanchnics (T10 and T11).



      • Least splanchnics (T12).



    • The greater, lesser, and least splanchnic nerves synapse on prevertebral ganglia in the abdomen. There is significant variability in terms of the specific target ganglia; however, the following relationships are fairly consistent:




      • Greater splanchnics will terminate primarily in the celiac and superior mesenteric ganglia. Some fibers will travel directly to the adrenal medulla.



      • The lesser splanchnics will synapse in the aorticorenal ganglia or superior mesenteric ganglion.



      • The least splanchnics will terminate in the aorticorenal ganglia.



    • Lumbar splanchnics arise from ganglia at vertebral levels L1–L4.




      • Lumbar splanchnics will pass into the inferior mesenteric ganglia or a pelvic plexus and synapse.



      • The postganglionic lumbar fibers will innervate the hindgut and pelvic viscera.



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Fig. 18.3 The sympathetic arm of the ANS is also called the thoracolumbar division due to the fact that the first order cell bodies are located in the spinal cord at the thoracic and lumbar levels. ANS, autonomic nervous system. (a: Reproduced with permission from Mattle H, Mumenthaler M, Taub E. Fundamentals of Neurology: An Illustrated Guide. 2nd Edition. © Thieme 2016.



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b: Reproduced with permission from Kahle W, Frotscher M.Color Atlas of Human Anatomy, Vol 3. 6th edition. © Thieme 2011.)



Peripheral sympathetic nervous system





























Peripheral sympathetic nervous system

Origin of Preganglionic Fibers*


Ganglion Cells


Course of Postganglionic Fibers


Target


Spinal cord


Sympathetic trunk


Follow intercostal nerves


Blood vessels and glands in chest wall


Accompany intrathoracic arteries


Visceral targets


Gather in greater and lesser splanchnic nerves


Abdomen


*The axons of preganglionic neurons exit the spinal cord via the anterior roots and synapse with postganglionic neurons in the sympathetic ganglia.


(Reproduced with permission from Gilroy AM. Atlas of Anatomy. 3rd ed. Thieme; 2017, Table 8.4.)



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Fig. 18.4 (a,b) Thoracic sympathetic ganglia are described as paravertebral because they run vertically on either side of the vertebral column. Reproduced with permission from Gilroy AM, MacPherson BR. Atlas of Anatomy. Third Edition. © Thieme 2016. Illustrations by Markus Voll and Karl Wesker.







Fig. 18.5 The superior, middle, and inferior sympathetic ganglia are located in the head and neck.







Fig. 18.6 (a) Preganglionic fibers from first order sympathetic cell bodies enter into the sympathetic chain forming white communicating rami. Postganglionic fibers rejoin spinal nerves forming gray communicating rami. (b) White rami are named based on the fact that these axons are preganglionic and therefore myelinated. (a: Reproduced with permission from Gilroy AM, MacPherson BR. Atlas of Anatomy. Third Edition. © Thieme 2016. Illustrations by Markus Voll and Karl Wesker. b: Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)







Fig. 18.7 Some preganglionic fibers terminate immediately in the sympathetic chain while others ascend or descend and synapse at a different level.



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Fig. 18.8 Some preganglionic fibers enter the sympathetic chain via white rami and exit without synapsing such as splanchnic nerves. These nerves innervate abdominal and pelvic viscera.



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Fig. 18.9 There are three groups of thoracic splanchnic nerves that exit the sympathetic chain at different levels. They are named: greater, lesser, and least splanchnics.



Synopsis of the sympathetic and parasympathetic nervous systems
















































Synopsis of the sympathetic and parasympathetic nervous systems

Organ


Sympathetic Nervous System


Parasympathetic Nervous System


Eye


Pupillary dilation


Pupillary constriction and increased curvature of the lens


Salivary glands


Decreased salivation (scant, viscous)


Increased salivation (copious, watery)


Heart


Increased heart rate


Decreased heart rate


Lungs


Decreased bronchial secretions and bronchodilation


Increased bronchial secretions and bronchodilation


Gastrointestinal tract


Decrease in secretions and motility


Increase in secretions and motility


Pancreas


Decreased exocrine secretions


Increased exocrine secretions


Male sex organs


Ejaculation


Erection


Skin


Vasoconstriction, sweating, piloerection


No effect


(Reproduced with permission from Schuenke. Atlas of Anatomy. Vol. 3, 2nd ed. Thieme; 2017, Table 10.287C.)



Parasympathetic Division




  • The function of the parasympathetic system is to conserve and restore energy. It is often called the “rest and digest response.” Activation of this division will slow heart rate, constrict pupils, increase peristalsis, and increase glandular activity ().



  • The parasympathetic division originates in the brainstem for most of the body and in the sacral area (IML S2–S4) for the hindgut and pelvic structures. The parasympathetics for the hindgut and pelvic structures are called the pelvic splanchnics ().




    • The efferent parasympathetic fibers travel in certain cranial nerves (CNs III, VII, IX, and X) and sacral spinal nerves (S2–S4). Due to its origin, it is sometimes called the craniosacral division ( ).



    • First order neurons (preganglionic) of the parasympathetic division are located in brainstem nuclei. The parasympathetic nuclei lie in close proximity to the nuclei of the cranial nerve they travel with. It should be noted that CNs III, VII, IX, and X all have a nucleus of their own motor. The preganglionic fibers from the parasympathetic nuclei travel with cranial nerves to reach their respective ganglia () (). The parasympathetic nuclei:cranial nerves associations are:




      • Edinger-Westphal nucleus: CN III



      • Superior salivatory nucleus: CN VII



      • Inferior salivatory nucleus: CN IX



      • Dorsal motor nucleus of X: CN X



    • Second order neurons are located in parasympathetic ganglia in the head or within the walls of the target viscera ().




      • Head and neck parasympathetic ganglia are:




        • Ciliary ganglion




          • Preganglionic fibers travel on CN III to ganglion.



          • Postganglionic fibers project to the orbit.



          • Targets are ciliary muscle and sphincter pupillae.



        • Pterygopalatine ganglion




          • Preganglionic fibers travel with CN VII (greater petrosal followed by nerve of the pterygoid canal).



          • Postganglionic fibers project to the nasal cavity, palate, paranasal sinuses, and lacrimal gland.



          • Targets are lacrimal gland and mucous glands.



        • Submandibular ganglion




          • Preganglionic fibers travel in CN VII (chorda tympani and lingual nerves).



          • Postganglionic fibers project to the submandibular and sublingual glands.



          • Target is salivary glands.



        • Otic ganglion




          • Preganglionic fibers travel with CN IX (tympanic branch and lesser petrosal).



          • Postganglionic fibers project to the parotid gland.



          • Target is salivary glands.


Sep 13, 2022 | Posted by in NEUROLOGY | Comments Off on Integrated Systems
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