Progress in understanding the central nervous system (CNS) mechanisms regulating cardiovascular function has long been linked to the neurobiology of cranial primary sensory afferents. Activation of visceral afferents with chemical substances provided seminal evidence that particular afferents even within a single organ (e.g., the heart) or sensory modality (e.g., mechanoreceptors) could have fundamentally different characteristics and evoke unique reflex outcomes. In cardiorespiratory afferent studies, early practitioners deployed a range of sometimes rather exotic exogenous compounds to probe the discharge properties of afferent nerves as well as to evoke reflex responses. These chemicals ranged from neurotransmitters, peptides, prostanoids, cytokines, phenylbiguanide, and veratridine to nicotine.1-4 Thus, the pharmacology of primary visceral afferents is intimately interwoven into the fabric of CNS processing and the physiology of autonomic reflexes.
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