Situation: A 22-year-old man with a first episode of psychosis is admitted to an acute psychiatric unit. He receives haloperidol 5 mg intramuscularly (IM) for severe agitation and is then started on oral haloperidol. The psychiatrist later changes his drug to low-dose risperidone. Why do second-generation antipsychotics usually cause fewer extrapyramidal effects than haloperidol?

First-generation drugs are strong D2 blockers, and blockade in the nigrostriatal pathway causes extrapyramidal symptoms. Second-generation drugs block both D2 and serotonin 5-HT2A receptors, which lowers extrapyramidal risk but increases metabolic effects.

Core mechanism
Haloperidol is a first-generation antipsychotic whose therapeutic effect depends on strong blockade of dopamine D2 receptors. When D2 blockade occurs in the nigrostriatal pathway, it produces the classic extrapyramidal symptoms such as acute dystonia, parkinsonism, and akathisia ^[1]. Second-generation antipsychotics such as risperidone still block D2 receptors, but they also block serotonin 5-HT2A receptors, and this dual action changes the net effect on motor pathways ^[2][3].

The reduced extrapyramidal risk of second-generation antipsychotics is explained by combined D2 and 5-HT2A receptor blockade, not by selective action on one dopamine pathway or by anticholinergic cancellation of dopamine blockade.

Proposed explanation	Why it is incorrect
They act only in the mesolimbic pathway and spare the others	No antipsychotic is anatomically selective in this way. Risperidone and other second-generation agents occupy D2 receptors in the striatum as well, especially at higher doses [1].
Their anticholinergic action cancels the dopamine blockade	Risperidone has minimal anticholinergic activity. Anticholinergic drugs can mask extrapyramidal symptoms, but that is not the mechanism by which second-generation antipsychotics lower risk [1].
They stimulate dopamine receptors instead of blocking them	All effective antipsychotics are D2 antagonists, not agonists. Stimulating D2 receptors would worsen psychosis [3][4].

Why 5-HT2A blockade matters
Serotonin and dopamine systems interact in the basal ganglia. Blocking 5-HT2A receptors on dopaminergic neurons reduces the inhibitory influence of serotonin, which in turn facilitates dopamine release in the striatum ^[2][3]. This local increase in dopamine competes with the antipsychotic at D2 receptors, so the functional D2 blockade in the nigrostriatal pathway is less complete than it would be with haloperidol alone. The result is a lower likelihood of extrapyramidal symptoms at clinically effective antipsychotic doses ^[2][3].

Key point! The protective effect is relative, not absolute. Risperidone is dose-dependent: at higher doses its D2 occupancy rises and extrapyramidal symptoms can still occur ^[1].

Clinical trade-off
The same 5-HT2A blockade that protects motor function contributes to the metabolic burden of second-generation antipsychotics, including weight gain, dyslipidemia, and glucose intolerance ^[1]. Watch out! When a patient is switched from haloperidol to risperidone, monitor both movement symptoms and metabolic parameters, because the risk profile shifts rather than disappears ^[1].

Why this matters for the exam
The question tests whether you can distinguish the receptor-level explanation from common misconceptions. The correct answer is the only option that names the actual pharmacologic difference: second-generation antipsychotics block serotonin 2A receptors as well as dopamine D2 receptors. The other options describe mechanisms that do not exist or do not apply to risperidone ^[2][3][4].