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What roles do afferents play in efferent vagal autonomic regulation?

      A common feature across parasympathetic regulation of visceral organs like the heart is the presence of full reflex pathways with as few as two central neurons. Neurons of the solitary tract nucleus (NTS) receive directly synaptic terminals from primary viscerosensory afferents and these afferents belong to two broad phenotypic classes – neurons with either myelinated or unmyelinated axons. Although these sensory terminals all release glutamate as their primary transmitter, the release of glutamate is highly regulated by presynaptic mechanisms. For example, the neuropeptides vasopressin and oxytocin that are synthesized in the hypothalamus oppositely regulated glutamate release at NTS second order neurons. The segregation of viscerosensory phenotype is absolute for aortic baroreceptors and likely other viscerosensory afferents – myelinated afferents contact different neurons than do unmyelinated afferents. The ramifications of the segregation of afferent inputs to second, higher order, and projection neurons will be discussed as well as the presence of a separate pool of glutamate vesicles devoted to calcium entry associated with Transient Receptor Potential Vanilloid Type 1 receptors as a new form of tonic “afferent” presence driving brainstem circuits.
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