Neural reflex

Allergic Seasonal or Perennial Rhinoconjunctivitis. Histamine can cause all pathologic features of allergic rhinitis (35—37), with the exception of late-phase inflammatory reactions. Pmritus is caused by stimulation of receptors on sensory nerve endings prostaglandins (qv) may also contribute. Sneering, like pmritus, is an H -mediated neural reflex and can also be mediated by eicosanoids. Mucosal edema, which manifests as nasal... 

The regulation of the total peripheral resistance also involves the complex interactions of several mechanisms. These include baroreflexes and sympathetic nervous system activity response to neurohumoral substances and endothelial factors myogenic adjustments at the cellular level, some mediated by ion channels and events at the cellular membrane and intercellular events mediated by receptors and mechanisms for signal transduction. As examples of some of these mechanisms, there are two major neural reflex arcs (Fig. 1). Baroreflexes are derived from high-pressure barorecep-tors in the aortic arch and carotid sinus and low-pressure cardiopulmonary baroreceptors in ventricles and atria. These receptors respond to stretch (high pressure) or... 

The ocular surface and the tear-secreting glands of the eye are now known to function as an integrated unit. This unit refreshes the tear supply and clears used tears. An autonomic neural reflex loop stimulates secretion of tear fluid and proteins by the lacrimal glands. The sensitivity of the ocular surface decreases as aqueous tear production and tear clearance decreases. This results in a decrease in sensory-stimulated reflex tearing which exacerbates dry eye.29,30 Over time, wearing contact lenses also desensitizes the cornea by constant stimulation.12... 

Fig. 11.4. Model for cholinergic signalling in the intestinal mucosa, providing a possible rationale for AChE secretion by parasitic nematodes. ACh released from enteric cholinergic motor neurons stimulates chloride secretion, mucus secretion and Paneth cell exocytosis through muscarinic receptors. Secretory responses may be modulated by mast cell mediators, either directly or via the induction of neural reflex programmes. The role of muscarinic receptor-positive cells in the lamina propria of rats infected with N. brasiliensis is undetermined, as are potential mechanisms of trans-epithelial transport of the enzymes. Adapted from Cooke (1984).

Many intestinal functions are mediated through enteric neural reflexes, and delta opioid receptors appear to be present in these reflex pathways. In the rat ileum, the delta opioid receptor agonist DSLET decreases the rate of peristaltic contractions [95]. This effect may be secondary to an increased latency in the contractile response as described for DPDPE in this tissue preparation [96], DPDPE inhibits reflex contractile responses of the guinea pig intestine to volume distention by raising the stimulus threshold required to evoke contraction [97]. However, this effect is probably not mediated by conventional delta opioid receptors, as SNC80 is considerably weaker than DPDPE in inhibiting peristaltic contractions in this preparation and its effects are not attenuated by naltrindole [98]. 

Chronic dry eye is the result of an underlying cytokine and receptor-mediated inflammatory process that affects the ocular surface and lacrimal gland, leading to decreased tear production or altered tear film contents. Hormonal, anti-inflammatory, or immunomodulatory agents may be able to suppress the inflammation and normalize the neural reflex between the ocular smrfece and lacrimal glands. 

Mucosal oedema contributes to the airway narrowing present in asthma. Contraction of endothelial cells in post-capillary venules leads to the formation of gaps which allow the outflow of plasma. This occurs in response to mediators released from inflammatory cells such as H, PGs, LTs, and platelet aaivating factor (PAF) which probably act directly on endothelial cells and bradykinin which may act via neural reflexes. In addition to obstructing the airway lumen, exuded plasma may... 

Another way in which hormonal secretion is regulated involves a neuroendocrine reflex, which differs from a neural reflex in that the efferent neuron is replaced by a... 

Shibata, C., Jin, X.L., Naito, H., Matsuno, S., and Sasaki, I. (2002) Intraileal capsaicin inhibits gastrointestinal contractions via a neural reflex in conscious dogs. Gastroenterology 123 1904—1911. 

The rapid onset of the bradycardia in response to hypoxia is suggestive of a neural reflex (Fig. 2). The sensors for this reflex have been identified as the carotid chemoreceptors (27,28), which send afferent fibers in the carotid sinus nerve to a central integrating area in the brainstem (29) and whose efferent arm consists of parasympathetic cholinergic fibers to the heart carried in the vagus nerve (cranial nerve X) and a-adrenergic sympathetic fibers to resistance vessels in peripheral vascular beds (30). The absence of this bradycardic response earlier in gestation (31) is indicative of immaturity of neural reflex mechanisms. 

Whilst the initial bradycardia at the onset of an hypoxic episode can be explained in terms of neural reflex pathways, if hypoxia is maintained beyond approximately 4 or 5 min, this bradycardia begins to be slowly abolished indicating that slower-acting mechanisms may also be active (Fig. 2 and Ref 27). Moreover, carotid denervation may abolish the initial bradycardia and increase in peripheral resistance seen at the start of hypoxia (30), but there is still a late-onset rise in peripheral resistanee (27). 

Parasympathetic reflexes may also regulate mediator release and lymphocyte activity (41). Furthermore, contralateral histamine release and basophil infiltration occur following ipsilateral nasal challenge with allergen (30), an event mediated by neural reflexes. 

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