Micturition Quiz

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Which receptor mainly mediates detrusor muscle contraction during the voiding phase of micturition?

A M1 muscarinic receptors on detrusor muscle
B Beta-3 adrenergic receptors on detrusor muscle
C Alpha-1 adrenergic receptors on detrusor muscle
D M3 muscarinic receptors on detrusor muscle ✓
E Nicotinic acetylcholine receptors on detrusor muscle

The answer is D: M3 muscarinic receptors on detrusor muscle

Explanation

Why D is correct

See Voiding Phase. Option D is correct. Which receptor does what in the bladder is a classic exam list, and it helps to learn it as one picture. Parasympathetic nerves in the pelvic nerve release acetylcholine onto M3 muscarinic receptors on the detrusor (the smooth muscle of the bladder wall). Muscarinic receptors are a type of acetylcholine receptor, and the M3 type makes smooth muscle contract, so the bladder pressure rises and urine is expelled. The M3 receptor is therefore the final step in voiding, and it is why drugs that block muscarinic receptors (anticholinergic drugs) can reduce detrusor contraction and cause urinary retention. The four receptor types to know are:

M3 muscarinic receptors on the detrusor cause contraction (voiding)
beta-3 adrenergic receptors on the detrusor cause relaxation (storage)
alpha-1 adrenergic receptors on the internal sphincter cause contraction (storage)
nicotinic receptors on the external sphincter cause contraction (storage, via the pudendal nerve)

Voiding receptor: Acetylcholine acts on M3 muscarinic receptors on the detrusor, which makes it contract.
Nerve: The parasympathetic fibres run in the pelvic nerve.
Drug link: Anticholinergic drugs block muscarinic receptors and can cause urinary retention.

Why the other options are incorrect

A. M1 muscarinic receptors on detrusor muscle: M1 muscarinic receptors are not the main detrusor contractile receptor. The detrusor contraction in voiding is mediated by M3 muscarinic receptors, while M1 receptors have other roles such as in the brain and gastric acid secretion.
B. Beta-3 adrenergic receptors on detrusor muscle: Beta-3 adrenergic receptors are present on the detrusor but their activation by noradrenaline relaxes the muscle during storage. They are inhibitory for detrusor contraction, so they cannot mainly mediate contraction in voiding.
C. Alpha-1 adrenergic receptors on detrusor muscle: Alpha-1 adrenergic receptors are found mainly on the internal urethral sphincter and bladder neck, where they cause contraction during storage. They are not the receptor that mediates detrusor contraction.
E. Nicotinic acetylcholine receptors on detrusor muscle: Nicotinic acetylcholine receptors are found at autonomic ganglia and on the external urethral sphincter, not on detrusor smooth muscle. Detrusor contraction uses muscarinic M3 receptors, not nicotinic receptors.

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More questions from this set. Start the quiz to answer them and see each explanation.

1
During the storage phase of micturition, which neural action helps to keep urine in the bladder?
2
Which mechanism most directly explains overflow urinary incontinence in cauda equina syndrome?

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