Nervous System: ANS, SNS, and PNS

General Concepts of the Nervous System

• The nervous system comprises the central (CNS) and peripheral (PNS) divisions
• The CNS includes the brain and spinal cord
• The PNS includes 12 pairs of cranial nerves, 31 pairs of spinal nerves, and associated ganglia
• Functionally divided into somatic (voluntary actions) and autonomic (involuntary actions) nervous systems

General Concepts of the Autonomic Nervous System (ANS)

• The ANS integrates within both the central and peripheral nervous systems
• It is subject to higher control, notably by the hypothalamus
• It contains both efferent and afferent nerve fibers
• The ANS regulates involuntary structures
• The two divisions of the ANS are the sympathetic and parasympathetic nervous systems

Comparison Between Sympathetic (SNS) and Parasympathetic (PNS) Nervous Systems

• Sympathetic prepares the body for emergency, while parasympathetic conserves and restores energy
• The anatomical outflow for the SNS is T1-L2 (thoracolumbar)
• The anatomical outflow for the PNS is cranial nerves 3, 7, 9, 10 and S2-S4, originating from the craniosacral region
• Both systems are under the high control of the hypothalamus

Sympathetic Nervous System (T1-L2) Efferent Pathways

• Pathway: Lateral horn (anterior horn) to white rami communicantes to paravertebral ganglia of the sympathetic trunk
• Afferent refers to towards the CNS
• Efferent refers to away from the CNS
• Sympathetic afferent myelinated fibers bypass synapsing in sympathetic ganglia
• They travel through white rami communicantes to cell bodies at the posterior root ganglion, reaching higher centers or local reflex arcs
• SG means sympathetic ganglion

Parasympathetic Nervous System Efferent Pathways

• Connector nerve cells reside in the brainstem and sacral plexus
• Brainstem component: Cranial nerves III, VII, IX, and X
• Sacral Plexus component: Anterior horn cell (S2-S4) to sacral nerves to pelvic splanchnic nerve to hypogastric plexuses
• Excitor neurons (post-ganglionic) influence bladder, uterus, sphincters, rectum, and descending colon
• Parasympathetic afferent fibers directly connect to the target organ without synapsing
• From white rami communicantes to cell bodies of the central nervous system or posterior root ganglionic of corresponding nerve to higher center or local reflex arc

Autonomic Nervous System of the Heart and Lungs

• Heart
  • Fibers from T2-T4 (PRGF) extend to the superior, middle, inferior, and lower thoracic (PRGF)
  • This becomes sympathetic PGF to the cardiac plexus to the sinoatrial (SA) and atrioventricular (AV) nodes, cardiac muscles, and coronary arteries
  • Results in increased heart rate, force of contraction, and vasodilation of coronary arteries
  • Parasympathetic PRGF (dorsal nucleus of CN 10) via vagus nerve to postganglionic neurons in the cardiac plexus to SA and AV nodes, cardiac muscles, and coronary arteries
  • Results in decreased heart rate, force of contraction, and vasoconstriction
• Lungs
  • Fibers from T2-T5 (sympathetic PRGF) extend to sympathetic ganglia to PGF to the pulmonary plexus to blood vessels of the lungs and bronchi
  • Results in bronchodilation and vasoconstriction
  • Parasympathetic PRGF (dorsal nucleus of CN X) via vagus nerve synapse with PG neurons in the pulmonary plexus to blood vessels of the lungs and bronchi
  • Results in bronchoconstriction, vasodilatation, and increased secretion of glands
• PRGF means pre-ganglionic fibers while PGF means post-ganglionic fibers

Mediastinum

• The thoracic cavity contains the mediastinum, lungs, and pleura
• Mediastinum is the space between the lungs, from the root of the neck to the diaphragm
• Divided into superior and inferior regions at the angle of Louis (T4-T5)
• Inferior mediastinum includes anterior, middle, and posterior mediastinum
  • Anterior mediastinum is located between the sternum and pericardium
  • Middle mediastinum is located between the anterior and posterior mediastinum
  • Posterior mediastinum is located between the pericardium and vertebral column
• The superior mediastinum contains the thymus remains, brachiocephalic veins, superior vena cava, brachiocephalic artery, left common carotid artery, left subclavian artery, aortic arch, phrenic and vagus nerves, left recurrent laryngeal nerve, cardiac nerves, trachea, lymph nodes, esophagus and thoracic duct, and sympathetic trunks
• Anterior mediastinum contains sternopericardial ligaments, lymph nodes, and thymus remains
• Middle mediastinum contains the pericardium and heart, great vessel roots, phrenic nerves, trachea bifurcation, and lymph nodes
• Posterior mediastinum contains the descending thoracic aorta, esophagus, thoracic duct, azygos and hemiazygos veins, vagus nerves, splanchnic nerves, sympathetic trunks, and lymph nodes

Lymphatic System of the Trunk

• Lymphatic system composed of lymphatic tissues and vessels
• Lymphatic tissues contain lymphocytes key to host defense
• Lymphatic vessels aid the cardiovascular system by transporting fluids from interstitial spaces
• Upper trunk lymphatic drainage is through the right lymphatic and left thoracic ducts
• The right lymphatic duct drains the upper right side of the body, including the head, thorax, and upper limb (excluding structures below the diaphragm)
• The left thoracic duct drains the remainder, including below the diaphragm
• Lymph from below the diaphragm collects in the cisterna chyli, which drains into the thoracic duct
• The intestinal and lumbar trunks collect lymph from gut, liver, spleen, kidneys, pancreas, pelvic viscera, and lower limbs, draining into the cisterna chyli