Sympathetic nervous system postganglionic fibres synapse on:

Question

BlackTom AI · Accepted Answer

Question: Sympathetic nervous system postganglionic fibres synapse on:

Option 1: α1 adrenoceptors in vascular smooth muscle
This is a classic and well-established target of sympathetic postganglionic fibers. Norepinephrine released from sympathetic endings binds to α1 adrenergic receptors on vascular smooth muscle, causing vasoconstriction and increased vascular resistance. The phrasing matches the typical distribution and action of α1 receptors in vascular beds.

Option 2: β1 adrenoceptors on bronchial smooth muscle
This is unlikely. In the airways, β2 adrenoceptors (not β1) mediate bronchodilation when activated by sympathetic stimuli or agonists like salbutamol. Postganglionic fibers to the bronchial smooth muscle primarily influence β2 receptors, not β1, and the option specifies β1 and bronchial smooth muscle, which misidentifies the receptor subtype.

Option 3: β2 adrenoceptors in the heart
The heart predominantly expresses β1 adrenoceptors; β2 receptors are present but not the main excitatory targets for postganglionic sympathetic fibers to the heart. Moreover, saying β2 in the heart misstates the primary receptor involvement in cardiac sympathetic responses, which are driven largely by β1 receptor activation increasing heart rate and contractility.

Option 4: β1 adrenoceptors on gastrointestinal sphincters
Sympathetic effects on the GI tract involve a mix of receptors, but the most characteristic postganglionic targets in sphincters are not specifically β1 receptors. GI sphincter tone is more commonly associated with α1-mediated contraction and various autonomic modulations. Singling out β1 receptors on GI sphincters does not align with the typical, primary receptor distribution for sympathetic postganglionic synapses in this region.

In summary, the statement in Option 1 correctly describes a primary and well-supported target of sympathetic postganglionic fibres, whereas the other options either misstate the receptor subtype (β1 vs β2) for relevant tissues or describe less representative targets.

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