Nervous system ↗Central circuits set patterns
Hypothalamic, brainstem, spinal, limbic, sensory, and cortical inputs converge to form organ-specific autonomic output.
BRAIN & NERVES / 03
Organ control is
continuous and specific.
Sympathetic and parasympathetic pathways are not a universal gas pedal and brake. They use distinct anatomy, transmitters, receptors, baseline tone, reflexes, and target-organ patterns.
Preganglionic neurons use ACh
Both branches release acetylcholine onto nicotinic receptors in autonomic ganglia; adrenal-medulla chromaffin cells are modified sympathetic targets.
Sympathetic targets vary
Most postganglionic fibers release norepinephrine at adrenergic receptors; sweat glands are an important cholinergic exception.
Parasympathetic targets use ACh
Cranial and sacral postganglionic neurons release acetylcholine at muscarinic receptors close to or within target organs.
Heart and vessels differ
Both branches affect cardiac rate, while sympathetic pathways dominate direct systemic vascular control and local tissues retain strong autoregulation.
Gut has its own circuits
Parasympathetic and sympathetic input modifies enteric networks that already coordinate motility, secretion, sensation, and local blood flow.
Metabolism uses nerves and hormones
Liver, pancreas, adipose, adrenal, and muscle responses reflect autonomic input alongside insulin, glucagon, catecholamines, cortisol, and demand.
Reflexes close the loop
Baroreceptor, chemoreceptor, visceral, temperature, bladder, and other afferents continually update central output.