Stretch receptor

Benzonatate [104-31 ] (46) is a unique compound which appears to have both central and peripheral antitussive effects. Stmcturally it is a derivative of ji)-aminoben2oic acid and contains a long poly(ethylene glycol) side chain. The peripheral effects ate the result of local anesthetic action on the pulmonary stretch receptors. Clinical activity was first reported in 1955 (65). 

The large diameter A/l-afferent fibre enters the dorsal horn of the spinal cord through the medial division of the dorsal root. It then descends through the medial region of lamina I or II, or alternatively, curves around the medial (central) edge of the dorsal horn down to the ventral horn. On reaching deeper laminae, laminae IV and V, the AjS-fibres ascend back up into laminae III and IV where they repeatedly subdivide and form a characteristic termination pattern. The densest arborisation appears to occur in lamina III. Axons originating from specialised muscle stretch receptors have collaterals that pass ventrally to make monosynaptic connections with neurons of laminae V, VI and VII. Some also extend to laminae VIII and IX of the ventral horn where they synapse directly onto motor neurons and form the basis of monosynaptic reflexes. 

A decrease in blood volume or blood pressure may result in a decrease in the blood flow to the kidney. The kidney monitors renal blood flow by way of stretch receptors in the vessel walls. A decrease in renal blood flow stimulates the release of renin. The subsequent secretion of aldosterone causes retention of sodium and water and, therefore, an increase in blood volume and blood pressure back to normal. An increase in renal blood flow tends to cause the opposite effect. 

Because baroreceptors respond to stretch or distension of the blood vessel walls, they are also referred to as stretch receptors. A change in blood pressure will elicit the baroreceptor reflex, which involves negative feedback responses that return blood pressure to normal (see Figure 15.6). For example, an increase in blood pressure causes distension of the aorta and carotid arteries, thus stimulating the baroreceptors. As a result, the number of afferent nerve impulses transmitted to the vasomotor center increases. The vasomotor center processes this information and adjusts the activity of the autonomic nervous system accordingly. Sympathetic stimulation of vascular smooth muscle and the heart is decreased and parasympathetic stimulation of the heart is increased. As a result, venous return, CO, and TPR decrease so that MAP is decreased back toward its normal value. 

Low-pressure receptors. The low-pressure receptors are located in the walls of the atria and the pulmonary arteries. Similar to baroreceptors, low-pressure receptors are also stretch receptors however, stimulation of these receptors is caused by changes in blood volume in these low-pressure areas. An overall increase in blood volume results in an increase in venous return an increase in the blood volume in the atria and the pulmonary arteries and stimulation of the low-pressure receptors. These receptors then elicit reflexes by way of the vasomotor center that parallel those of baroreceptors. Because an increase in blood volume will initially increase MAP, sympathetic discharge decreases and parasympathetic discharge increases so that MAP decreases toward its normal value. The simultaneous activity of baroreceptors and low-pressure receptors makes the total reflex system more effective in the control of MAP. 

Pulmonary stretch receptors are responsible for initiating the Hering-Breuer reflex. These stretch receptors are located within the smooth muscle of large and small airways. They are stimulated when the tidal volume exceeds 1 1. Nerve impulses are transmitted by the vagus nerve to the medullary respiratory center and inhibit the inspiratory neurons. The primary function of these receptors and the Hering-Breuer reflex is to prevent overinflation of the lungs. 

We also have evidence of how the female assesses the size of her mating partners. She appears to do so indirectly, by gauging the size of their spermatophore, for which purpose she may use stretch receptors that female moths are known to have in the chamber (bursa) in which spermatophores are deposited (29). Male Utetheisa can be caused to produce inordinately small spermatophores if they are mated relatively recently beforehand. If such mated males are placed in competition with physically smaller males, whose spermatophores may now be the relatively larger ones, they tend to "lose out" (30). We predict from this that males, in nature, may space their matings days apart. To regain the capacity to produce full size spermatophores takes a male about a week (30). 

Besides the arterial baroreceptors, central projections from other inputs, for example cardiac mecha-no-receptors, chemo-receptors, pulmonary stretch receptors, and somatic inputs, are capable of influencing the controlling system and thereby the autonomic effectors. 

Pharmacology Benzonatate anesthetizes stretch receptors in respiratory passages, lungs, and pleura, dampening their activity, and reducing the cough reflex. It has no inhibitory effect on the respiratory center in recommended dosage. Onset of action is... 

Any sudden alteration in the mean arterial blood pressure tends to produce compensatory reflex changes in heart rate, contractility, and vascular tone, which will oppose the initial pressure change and restore the homeostatic balance. The primary sensory mechanisms that detect changes in the mean arterial blood pressure are stretch receptors (baroreceptors) in the carotid sinus and aortic arch. 

Atropine can be useful in patients with carotid sinus syncope. This condition results from excessive activity of afferent neurons whose stretch receptors are in the carotid sinus. By reflex mechanisms, this excessive afferent input to the medulla oblongata causes pronounced bradycardia, which is reversible by atropine. 

Three generally accepted mechanisms are involved in the regulation of renin secretion (Fig. 18.2). The first depends on renal afferent arterioles that act as stretch receptors or baroreceptors. Increased intravascular pressure and increased volume in the afferent arteriole inhibits the release of renin. The second mechanism is the result of changes in the amount of filtered sodium that reaches the macula densa of the distal tubule. Plasma renin activity correlates inversely with dietary sodium intake. The third renin secretory control mechanism is neurogenic and involves the dense sympathetic... 

Benzonatate (Tessalon) is related to the local anesthetic tetracaine. It anesthetizes the stretch receptors in the lungs, thereby reducing coughing. Adverse reactions include hypersensitivity, sedation, dizziness, and nausea. 

Afferent cholinergic and adrenergic autonomic fibers are similarly present in detrusor and lissosphincter units and include micturition triggers linked with stretch receptors and urinary flow receptors (Gil Ver-net, 1968 Hutch, 1972 Elbadawi, 1973, 1982, 1983 Elbadawi and Schenk, 1974 de Groat and Kawatani, 1985). Some of the afferent fibers connect to reflex circuits in the sacral cord for bladder and rectum while connecting in the sacral and thoracic cord for sexual reflex circuits. 

The renal vascular receptor functions as a stretch receptor, with decreased stretch leading to increased renin release and vice versa. The receptor is apparently located in the afferent arteriole, possibly in the juxtaglomerular cells. Stretch-induced changes in renin release are mediated by changes in Ca2+ concentration in the juxtaglomerular cells. 

Benzonatate (Tessalon Perles) [Anritussive] Uses Symptomatic relief of cough Action Anesthetizes the stretch receptors in the resp passages Dose Adults Peds. >10 y 100 mg PO dd (max 600 mg/d) Caution [C, ] Disp Caps SE Sedadon, dizziness, GI upset Interactions T CNS depression Wl antihistamines, EtOH, hypnodcs, opioids, sedadves EMS Concurrent use of antihistamines, analgesics and sedadves T level of CNS depression OD May cause restlessness, muscle tremors and Szs symptomatic and supportive... 

Although some species of cestodes (e.g. Hymenolepis diminuta, p. 236) undergo diurnal migrations within the intestine - and therefore their suckers may not always be in a state of sustained contraction - others (e.g. Echinococcus granulosus) apparently remain in the same place and appear to be able to maintain contraction. How this is achieved physiologically is not known, but it has been speculated that this may operate through a catch muscle mechanism (as in lamellibranchs) or by the involvement of special stretch receptors (796). 

The gastric phase, which occurs when food actually enters the stomach. The presence of food and the composition of gastric contents stimulate local reflexes involving intrinsic nerve plexuses and stretch receptors and initiate release of gastrin from G cells to further enhance secretion. 

Regulation of anti-diuretic hormone secretion is primarily through the plasma osmolarity. Osmolarity is sensed in the hypothalamus by neurons known as osmoreceptors, which in turn stimulate secretion from those neurons that produce anti-diuretic hormone. Secretion of antidiuretic hormone is also simulated by decreases in blood pressure and volume, conditions sensed by stretch receptors in the heart and large arteries. Changes in blood pressure and volume are not nearly as sensitive a stimulator as increased osmolarity, but are nonetheless potent in severe conditions. For example, loss of 15-20% of blood volume by haemorrhage results in a massive secretion of anti-diuretic hormone. Another potent stimulus of anti-diuretic hormone is nausea and vomiting, both of which are controlled by regions in the brain with links to the hypothalamus. 

Lin JH, Rydqvist B. The mechanotransduction of the crayfish stretch receptor neurone can be differentially activated or inactivated by local anaesthetics. Acta Physiol Scand. 1999 166 65-74. 

Carotid sinus massage activates stretch receptors external pressure is applied gently to one side at a time but never to both sides at once. Some individuals are very sensitive to the procedure and develop severe bradycardia and hypotension. 

Local anaesthetics can also be used topically in the airways to block the mucosal cough receptors (modified stretch receptors and C fibre endings) directly. Nebulised Ugnocaine, for example, reduces coughing during fibreoptic bronchoscopy and is also effective in the intractable cough that may accompany bronchial carcinoma. 

Baroreceptors are present in the carotid sinus and aortic arch and stretch receptors are situated in the left atrium (G2, H6). Distension of the left atrium causes a fall in blood ADH levels, and in experimental animals the reduction in atrial stretch which follows the deflation of a distended balloon produces a brisk rise (S3). These experimental results offer an explanation of the dilutional situation with water retention which follows the surgical release of a tight mitral stenosis in man. 

The heart contains special receptors that respond to stretching. Activation of the stretch receptors results in the release of atrial natriuretic peptide (ANF) from the heart (Reis et ai 1997). The role of stretch in provoking AMP release was shown in the laboratory by inflating a balloon within the heart (Christensen and Leistad, 1997). ANP travels through the bloodstream to the kidneys, where it reduces the rate of sodium resorption. The result is a lowering of blood pressure and the appearance more salt in the urine (Lopez et ai., 1995). ANP i a 28-amino-acid polypeptide with the following sequence (Drewett and Garbers, 1994) ... 

Whereas solid food delays stomach emptying, liquids tend to accelerate the process. Acceleration results from activation of stretch receptors in the stomach wall. When the fluid is water, activation of the inhibitory receptors is stopped. This results in rapid emptying of stomach contents into the duodenum. 

Benzonutalc reportedly possesses both peripheral and central activity in ptxxiucing its antitussive effect. It somehow hitxtks the stretch receptors thought to be respon.siblc for cough. Clinically, it is not as cITcctivc as codeine, hut it pnxiuccs far fewer side effects and has very low toxicity. It Ls available in lOO-mg capsules ("perles"). 

Stretch receptor A sensory receptor that monitors the degree of stretch, e.g. muscle spindle. 

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