Functional Properties of Receptor Structures
Observations concerning the morphology and elementary physiology of visceroceptive structures began about half a century after the basic concepts of visceroception had been laid down by Cyon and Ludwig (1866), Hering and Breuer (1868), and Sherrington (1899). The reason for this delay can be traced to Langley (1922), who stated—although in a controversial manner—that the autonomic nervous system has exclusively efferent functions. At the turn of the century, even the most respected morphologists, like Ramon y. Cajal (1909), advocated this erroneous concept. The first morphological description of sensory endings in the viscera came from Dogiel (1878), but a long silence followed. Only sporadic papers emerged proposing the existence of autonomic afferent activity (Schofield, 1960). It was only in the 1930s and 1940s that evidence became clear: Adrian (1933) demonstrated different types of vagal afferent impulses originating from the heart and from the lungs. Heymans and Neil (1958) described the carotid sinus baroreceptors whose activity is contingent on the cardiac cycle. Abraham (1949, 1964) visualized these cardiac, gastrointestinal, and renal receptive structures in finely impregnated histological preparations. Similar histological analysis was undertaken by Dogiel’s followers, e.g., Larventiev (1948) and Kolosov (1956). The decisive evidence on the mixed efferent and afferent nature of the major autonomic nerves was proposed by Agostoni, Chinnock, Daly De Burgh, and Murray (1957) and by Evans and Murray (1954) who proved that afferent fibers constitute more than 80% of the vagus nerve in cats and rabbits. Of the histological classifications of visceroceptors, those of Niculescu (1958) are still valid. He distinguished three types of endings: (1) unencapsulated free nerve endings in poor or rich arborization, found especially in the large vessels and in the endocardium; (2) unencapsulated, sometimes glomerular endings, present widely in the cardiovascular, gastrointestinal, and other systems; (3) encapsulated endings of various types, like the Pacinian corpuscles of the mesentery.
KeywordsAortic Arch Carotid Body Receptor Potential Alimentary Canal Receptor Structure
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