These drugs interrupt the transmission of impulses (messages) across nerve cells (neurons). In the heart these are called beta1 (cardioselective) receptors. Another type — beta2 (non-cardioselective) receptors — is sited in the airways, blood vessels, and organs such as the eye, liver and pancreas. Cardioselective beta blockers act primarily on beta, receptors, whereas noncardioselective drugs act on both varieties. (The neurotransmissions interrupted at the betareceptor sites through the body by the beta blockers are initiated in the adrenal gland: this is why these drugs are sometimes described as beta-adrenergic-blocking agents.)