CAdrenocorticotropic hormone (ACTH) from the pituitary binds chromaffin cell receptors
DRising cortisol from the adrenal cortex directly stimulates chromaffin cell exocytosis
Chromaffin cells are functionally equivalent to modified postganglionic sympathetic neurons — they are innervated directly by preganglionic sympathetic fibers that release acetylcholine. The ACh binds nicotinic receptors on chromaffin cells, triggering catecholamine exocytosis into the bloodstream. This is why the adrenal medulla is considered neural tissue: it sits at a sympathetic synapse, just one that releases into blood rather than onto a specific organ. Option A is a common misconception — postganglionic neurons release NE at their target organs, but the medulla skips that step entirely.
Question 2 Multiple Choice
Compared to direct sympathetic nerve stimulation of the heart, epinephrine released by the adrenal medulla produces effects that are:
AFaster and more localized, because the blood carries it directly to cardiac tissue
BSlower in onset but affecting a wider range of tissues simultaneously
CIdentical in speed and scope, since both pathways involve catecholamines binding adrenergic receptors
DRestricted to alpha-receptor effects, while sympathetic nerves activate beta receptors
Direct sympathetic innervation is rapid and organ-specific — a nerve impulse reaches the heart within milliseconds. Adrenal catecholamines must travel through the bloodstream, adding a delay, but they reach every tissue with adrenergic receptors simultaneously, producing a body-wide hormonal wave. This is the key distinction between nervous system signaling (fast, localized) and endocrine signaling (slower, systemic). Chromaffin cells sacrifice speed for coverage — exactly the appropriate trade-off for coordinating a whole-body stress response.
Question 3 True / False
The adrenal medulla secretes primarily norepinephrine, with smaller amounts of epinephrine, because norepinephrine is the precursor in the catecholamine synthesis pathway.
TTrue
FFalse
Answer: False
False — this reverses both the proportion and the reasoning. The adrenal medulla secretes approximately 80% epinephrine and only 20% norepinephrine. Epinephrine IS synthesized from norepinephrine (by PNMT), but the enzyme PNMT is highly expressed in chromaffin cells because cortisol from the adrenal cortex flows directly to the medulla via a portal blood supply and induces PNMT expression. So the anatomical arrangement specifically promotes conversion of NE → E, making epinephrine the dominant product.
Question 4 True / False
The short plasma half-life of catecholamines (1–2 minutes) means that the fight-or-flight response subsides quickly once sympathetic stimulation ends, because MAO and COMT rapidly degrade epinephrine and norepinephrine.
TTrue
FFalse
Answer: True
True. Monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT) degrade catecholamines within minutes, ensuring the stress response does not persist indefinitely. This rapid clearance is physiologically important: a prolonged fight-or-flight state (elevated heart rate, vasoconstriction, glucose mobilization) would be harmful at rest. The clinical relevance is that catecholamine metabolites — metanephrines and vanillylmandelic acid (VMA) — accumulate in urine over time and are measured to diagnose pheochromocytoma, a tumor causing unregulated catecholamine secretion.
Question 5 Short Answer
Why is the adrenal medulla described as being 'at the intersection of the nervous system and the endocrine system'? What makes chromaffin cells unusual?
Think about your answer, then reveal below.
Model answer: Chromaffin cells are embryologically derived from neural crest tissue — the same lineage that produces sympathetic postganglionic neurons — and they are innervated by sympathetic preganglionic fibers via acetylcholine, placing them squarely in the autonomic nervous system circuit. But instead of releasing neurotransmitter across a synapse to one specific target, they release epinephrine and norepinephrine into the bloodstream, where these molecules function as hormones reaching all tissues simultaneously. This makes the adrenal medulla a neuroendocrine organ: it uses neural wiring to trigger hormonal output.
The key insight is that chromaffin cells are essentially postganglionic sympathetic neurons that 'forgot' to grow axons and instead secrete into blood. This explains why the adrenal medulla is the only endocrine tissue directly innervated by the sympathetic nervous system. The arrangement allows the brain to trigger a systemic hormonal surge (via the medulla) and simultaneous local sympathetic responses (via postganglionic nerves) through a single command — the hallmark of the fight-or-flight response.