), as well as increased arterial resistance and a reflex constriction of the venous vasculature. Conversely, baroreceptor unloading (i.e., branchial hypotension) results in the opposite cardiac response (i.e., tachycardia and increased Q. through increased vagal inhibition of the heart in response to branchial hypertension ( Sandblom and Axelsson, 2011). The efferent pathways of the reflex involve both cardiac and vascular responses, with the former leading to reduced f H and Q. The baroreflex (i.e., barostatic reflex) originates from afferent sensory nerve endings (i.e., baroreceptors) located in the branchial vasculature ( Ristori, 1970 Ristori and Dessaux, 1970 Sundin and Nilsson, 2002) and functions to buffer changes in arterial blood pressure ( Bagshaw, 1985 Jones and Milsom, 1982 Van Vliet and West, 1994). There is evidence that venous capacitance is actively controlled through various homeostatic cardiovascular reflexes in fish. fatty acids or peptides) or by lowering the pH in the gut.Įrik Sandblom, Albin Gräns, in Fish Physiology, 2017 5.4.2 Barostatic Reflexes Grove and Holmgren 1992a,b), while the impact of food composition may be studied after local injections of different breakdown products (e.g. Mechanical stimulation can be mimicked by inflating a balloon placed in the gut (e.g. The relative importance of different stimuli in triggering the reflexes in fish is so far not understood and further studies are needed. Vagal reflexes stimulate acid secretion in Atlantic cod ( Holstein and Cederberg 1980). In Atlantic cod and rainbow trout, accommodation is not affected by cutting vagal and splanchnic input, suggesting that local control mechanisms dominate ( Grove and Holmgren 1992a,b). relaxation in response to distension of either the stomach or esophagus, involves vagal pathways in mammals (see Furness 2006). Accommodation or receptive relaxation of the stomach, i.e. a stimulus sensed in the intestine will affect motility or secretion in the stomach. In vertebrates in general, the extrinsic reflexes coordinate events in different regions of the gut, e.g. In Atlantic cod intestine, electrical stimulation in one region similarly caused contractions anally and relaxation orally ( Karila and Holmgren 1995). one stimulus activates ascending (excitatory) and descending (inhibitory) pathways simultaneously ( Furness and Costa 1987). Mammalian studies have demonstrated that intrinsic reflexes controlling propagating gut motility are polarized, i.e. The corresponding genes have been isolated from several fish species (see NCBI but again, almost nothing has been published about their distribution in the fish gut. Many of the mammalian receptors are ion channels, like the TRP (transient receptor potential) family ( Holzer 2004). Their possible presence in fish has not received a lot of attention and little is also known about the types of receptors present on fish sensory nerves. Mammalian vagal afferent nerve endings have morphologically specialized characteristics like the intraganglionic laminar endings (IGLEs) that respond to mechanical stimulation ( Berthoud and Powley 1992 Zagorodnyuk et al. The stimuli may be noxious as well as physiological. These include distension (mechanical) of the gut wall, pH, composition of food, temperature, etc.
In mammals, extrinsic and intrinsic sensory neurons express various receptors responding to mechanical and/or chemical stimuli. Extrinsic sensory nerves also convey information to the brain about hunger and pain. Efferent nerves mainly innervate enteric neurons, but may also control smooth muscle in the wall, blood vessels or secretory cells. Extrinsic afferent nerves often synapse with preganglionic autonomic nerves, either in the spinal cord/brain stem or directly in sympathetic ganglia ( Fig. The extrinsic reflexes may include varying numbers of neurons.
cm, circular muscle lm, longitudinal muscle m, mucosa mep, myenteric plexus sm, submucosa. Local enteric reflexes include sensory (afferent), inter- and motor neurons. Vagal and spinal efferents innervate the enteric nervous system but also, e.g., blood vessels in the gut wall. Spinal afferents may synapse with efferent nerves in the celiac ganglion in the sympathetic chain or in the spinal cord. Vagal afferents have their nerve cell bodies in the nodose ganglia while spinal afferents have theirs in the dorsal root ganglia (DRG). Schematic overview of extrinsic and intrinsic reflexes controlling gut motility, based on mammalian models.
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