Platelet stimulation leads to a rise in Ca²⁺, which activates the fibrinogen receptor enabling aggregation (solid arrows). This pathway is accelerated by release of ADP and thromboxane A2 (TxA2) through P2Y and thromboxane receptors (dotted arrows). Clopidogrel and aspirin interfere with these pathways. The secretion of Substance P (SP) and endokinins A, B forms a third mechanism for feedback activation of platelet functions (bold arrows).