17.2 The Sympathetic Division

• • Sympathetic division of the autonomic nervous system operates through a series of interconnected neurons. Efferent sympathetic neurons originate from thoracic and lumbar spinal nerves and synapse with neurons in the peripheral nervous system at a series of sympathetic ganglia. 
• Preganglionic neurons only located between T2-L2 of spinal cord
• Ganglionic neurons in three locations: sympathetic chain ganglia, collateral ganglia, adrenal medulla
• Sympathetic chain ganglia, or paravertebral ganglia, on both sides of vertebral column
• Sympathetic chain ganglia innervate effector organs of body wall, head, neck, limbs, thoracic cavity
• Collateral ganglia, prevertebral ganglia, anterior to vertebral column
• Collateral ganglia innervate effector organs in abdominopelvic cavity
• Specialized sympathetic neurons located in interior of adrenal gland, adrenal medulla, a modified sympathetic ganglion
• Adrenal medulla release neurotransmitters into blood as hormones
• Sympathetic Chain Ganglia. Each sympathetic chain ganglion has … sympathetic ganglia:

  • 3 cervical 
  • 11-12 thoracic 
  • 2-5 lumbar 
  • 4-5 sacral 
  • 1 coccygeal 
• Sympathetic Chain Ganglia. Numbers may vary because adjacent ganglia may fuse
• Sympathetic Chain Ganglia. Preganglionic sympathetic neurons only found in segments T1-L2 of spinal cord
• Sympathetic Chain Ganglia. Spinal nerves of these segments have both… 

  • White rami communicantes (preganglionic fibers) 
  • Gray rami communicantes (postganglionic fibers)
• Sympathetic Chain Ganglia. Every spinal nerve along the entire length of the spinal cord has a pair of gray rami communicantes carrying sympathetic postganglionic fibers
• Sympathetic Chain Ganglia. About 8% of the axons in each spinal nerve are sympathetic postganglionic fibers
• Collateral Ganglia. Abdominopelvic viscera receive sympathetic innervation by sympathetic preganglionic fibers that synapse in separate collateral ganglia
• Collateral Ganglia. These fibers pass through sympathetic chain without synapsing 
• Collateral Ganglia. Above forms paired splanchnic nerves, in posterior wall of abdominal cavity
• Collateral Ganglia. Anatomy of the Collateral Ganglia. Splanchnic nerves innervate three collateral ganglia
• Collateral Ganglia. Anatomy of the Collateral Ganglia. Preganglionic fibers from seven inferior thoracic segments end at celiac ganglion and superior mesenteric ganglion 
• Collateral Ganglia. Anatomy of the Collateral Ganglia. Preganglionic fibers from lumbar segments from splanchnic nerves that end at inferior mesenteric ganglion 
• Collateral Ganglia. Anatomy of the Collateral Ganglia. Sacral splanchnic nerves end in hypogastric plexus 
• Collateral Ganglia. Anatomy of the Collateral Ganglia. Celiac Ganglion. Located at base of celiac trunk
• Collateral Ganglia. Anatomy of the Collateral Ganglia. Celiac Ganglion. Innervate stomach, duodenum, liver, gallbladder, pancreas, spleen, kidney
• Collateral Ganglia. Anatomy of the Collateral Ganglia. Superior Mesenteric Ganglion. Located at base of superior mesenteric artery
• Collateral Ganglia. Anatomy of the Collateral Ganglia. Superior Mesenteric Ganglion. Innervate small intestines and initial segments of large intestine
• Collateral Ganglia. Anatomy of the Collateral Ganglia. Inferior Mesenteric Ganglion. Located at base of inferior mesenteric artery
• Collateral Ganglia. Anatomy of the Collateral Ganglia. Inferior Mesenteric Ganglion. Innervate terminal portions of large intestine, kidney, bladder, sex organs
• Adrenal Medulla. Some preganglionic fibers originating between T5-T8 pass through sympathetic chain and celiac ganglion without synapsing and proceed to adrenal medulla
• Adrenal Medulla. Synapse on modified neurons that perform endocrine function
• Adrenal Medulla. Release neurotransmitters epinephrine (E) and norepinephrine (NE)
• Adrenal Medulla. Effects resemble sympathetic postganglionic fibers, differences are… 

  • Cells not innervated by sympathetic postganglionic fibers affected by circulating levels of epinephrine/norepinephrine only if they possess receptors for these molecules 
  • Effects last longer than by direct sympathetic innervation, bc released hormones continue to diffuse out of blood for longer
• Effects of Sympathetic Stimulation. In crises, entire division response called sympathetic activation
• Effects of Sympathetic Stimulation. Affects peripheral tissues and alters CNS activity
• Effects of Sympathetic Stimulation. Sympathetic centers in hypothalamus control sympathetic activation
• Effects of Sympathetic Stimulation. When sympathetic activation occurs, we experience… 

  • Increased alertness, through stimulation of reticular activating system 
  • Energy and euphoria, associated with disregard for danger and insensitivity 
  • Increased activity in cardiovascular/respiratory centers of pons/medulla oblongata, increased heart rate and contraction strength, elevations in blood pressure 
  • Elevation in muscle tone through extrapyramidal system, tense and shiver  
  • Mobilization of energy reserves through glycogen breakdown in muscle and liver cells and release of lipids 
• Sympathetic Activation and Neurotransmitter Release. ACH released by cholinergic, preganglionic neurons during sympathetic activation always stimulates ganglionic neurons
• Sympathetic Activation and Neurotransmitter Release. Leads to postganglionic fibers release norepinephrine (NE) at neuroeffector junctions
• Sympathetic Activation and Neurotransmitter Release. Neuroeffector junctions are adrenergic, sympathetic terminals
• Sympathetic Activation and Neurotransmitter Release. Each branch resembles a string of beads and each bead or varicosity is packed with mitochondria and neurotransmitter vesicles