Organization
- Autonomic ganglia are peripheral synaptic stations where autonomic neurons communicate with one another.
- Parasympathetic ganglia are located in or near the organs they control.
- Sympathetic ganglia are located in the paravertebral chain, which runs alongside the vertebral column.
- Preganglionic neurons have their cell bodies in the central nervous system.
- Preganglionic neurons synapse in autonomic ganglia before the signal reaches most effector organs.
- Sympathetic preganglionic neurons arise from spinal cord segments T1-L3.
- Thoracolumbar outflow means sympathetic preganglionic neurons originate from thoracic and upper lumbar spinal cord segments.
- Parasympathetic preganglionic neurons arise from cranial-nerve nuclei and sacral spinal cord segments S2-S4.
- Craniosacral outflow means parasympathetic preganglionic neurons originate from cranial nuclei and sacral cord segments.
- In spinal-cord segment notation, T means thoracic, L means lumbar, and S means sacral.
- Postganglionic neurons have their cell bodies in autonomic ganglia.
- Postganglionic neurons synapse directly on effector organs.
- Effector organs for postganglionic autonomic neurons include the heart, blood vessels, and sweat glands.
- Adrenal medulla is a specialized sympathetic ganglion rather than an ordinary endocrine structure for autonomic signaling purposes.
- Preganglionic sympathetic fibers synapse directly on adrenal-medulla chromaffin cells.
- Chromaffin cells are adrenal-medulla cells that release catecholamines into the bloodstream.
- Epinephrine makes up about 80% of adrenal-medulla catecholamine secretion.
- Norepinephrine makes up about 20% of adrenal-medulla catecholamine secretion.
- Pheochromocytoma is a catecholamine-secreting tumor of the adrenal medulla.
- Vanillylmandelic acid increases in urine when catecholamine metabolism is increased, as in pheochromocytoma.
- Sympathetic preganglionic axons are short.
- Sympathetic postganglionic axons are long.
- Parasympathetic preganglionic axons are long.
- Parasympathetic postganglionic axons are short.
- Acetylcholine (ACh) is released by both sympathetic and parasympathetic preganglionic neurons in autonomic ganglia.
- Nicotinic receptors are the receptors used by acetylcholine in autonomic ganglia.
- Sympathetic postganglionic neurons usually release norepinephrine onto effector organs.
- Sweat glands are an important exception because their sympathetic postganglionic innervation releases acetylcholine.
- Parasympathetic postganglionic neurons release acetylcholine onto effector organs.
- Muscarinic receptors mediate parasympathetic postganglionic acetylcholine effects on target tissues.
- Somatic motor neurons release acetylcholine at the skeletal-muscle neuromuscular junction.
- Nicotinic neuromuscular-junction receptors mediate acetylcholine signaling from somatic motor neurons to skeletal muscle.
- Nonadrenergic noncholinergic neurons are postganglionic parasympathetic neurons that release transmitters other than norepinephrine or acetylcholine.
- Gastrointestinal nonadrenergic noncholinergic neurons may release substance P.
- Gastrointestinal nonadrenergic noncholinergic neurons may release vasoactive intestinal peptide.
- Gastrointestinal nonadrenergic noncholinergic neurons may release nitric oxide.
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