Anatomy(19-2)

Structure and Function of Sympathetic and Parasympathetic Divisions

• Gray rami communicantes outnumber white rami and carry sympathetic output to smooth muscle and glands
• Prevertebral ganglia, including celiac, superior mesenteric, inferior mesenteric, and aorticorenal ganglia, lie close to major abdominal arteries
• Splanchnic nerves from thoracic area synapse with postganglionic cell bodies in celiac ganglion, innervating blood vessels to stomach, spleen, liver, kidneys, and small intestine
• Lesser splanchnic nerve passes through celiac plexus to enter aorticorenal ganglion and superior mesenteric ganglion, innervating blood vessels of small intestine and proximal colon
• Lumbar splanchnic nerve terminates in inferior mesenteric ganglion, supplying blood vessels of distal colon and rectum, urinary bladder, and genital organs
• Sympathetic preganglionic neurons also extend to adrenal medullae, releasing hormones into the blood upon stimulation
• Adrenal medullae and sympathetic ganglia are derived from the same tissue, the neural crest
• Adrenal medullae release a mixture of hormones, including epinephrine and norepinephrine, intensifying responses elicited by sympathetic postganglionic neurons
• Parasympathetic preganglionic neurons are located in the nuclei of cranial nerves in the brain stem and in the lateral gray horns of the second through fourth sacral segments of the spinal cord
• Parasympathetic division is known as the craniosacral division, and its axons are referred to as the craniosacral outflow
• Cranial parasympathetic outflow consists of preganglionic axons in four cranial nerves, while sacral parasympathetic outflow consists of preganglionic axons in anterior roots of the second through fourth sacral nerves
• Cranial outflow has four pairs of ganglia and plexuses associated with the vagus nerve, with vagal axons extending to terminal ganglia in the thorax and abdomen

Enteric Division of the Autonomic Nervous System

• The enteric division is a specialized network of nerves and ganglia within the gastrointestinal tract, pancreas, and gallbladder.
• It contains around 100 million neurons, equivalent to the number in the spinal cord, and can function independently.
• The network includes sensory neurons monitoring intestinal wall tension and content composition, which relay signals to interneurons in the enteric ganglia.
• Interneurons process incoming signals and generate output signals to motor neurons, regulating motility, secretory activities, and blood supply of the digestive organs.
• Most nerve fibers innervating digestive organs arise from the myenteric plexus and the submucosal plexus within the enteric system.
• Autonomic dysreflexia is an exaggerated response of the sympathetic division that occurs in about 85% of individuals with spinal cord injury at or above the level of T6.
• Autonomic dysreflexia results from the interruption of ANS control by higher centers and causes severe vasoconstriction, elevated blood pressure, and emergency symptoms.
• Autonomic neurons are classified based on the neurotransmitter they release, with cholinergic neurons releasing acetylcholine (ACh) in the ANS.
• ACh is stored in synaptic vesicles and binds with specific cholinergic receptors, including nicotinic receptors and muscarinic receptors.
• Nicotinic receptors are present in the plasma membranes of dendrites and cell bodies of sympathetic and parasympathetic postganglionic neurons, and in the motor end plate at the neuromuscular junction.
• Muscarinic receptors are present in the plasma membranes of all effectors innervated by parasympathetic postganglionic axons, including smooth muscle, cardiac muscle, and glands.
• Cholinergic and adrenergic receptors are integral membrane proteins located in the plasma membrane of a postsynaptic neuron or an effector cell.