Читать книгу Salivary Gland Pathology - Группа авторов - Страница 46

There is a continuous low background saliva production that is stimulated by drying of the oral and pharyngeal mucosa. A rapid increase in the resting levels occurs as a reflex in response to masticatory stimuli including the mechanoreceptors and taste fibers. Other sensory modalities such as smell are also involved. The afferent input is via the salivatory centers that are themselves influenced by the higher centers. The higher centers may be facilitory or inhibitory depending on the circumstances. The efferent secretory drive to the salivary glands passes via the parasympathetic and sympathetic pathways. There are no peripheral inhibitory mechanisms.

Cholinergic nerves (parasympathetic) often accompany ducts and branch freely around the secretory endpieces (acini). Adrenergic nerves (sympathetic) usually enter the glands along the arteries and arterioles and ramify with them. Within the glands, the nerve fibers intermingle such that cholinergic and adrenergic axons frequently lie in adjacent invaginations of a single Schwann cell (Garrett and Kidd 1993). Secretion and vasoconstriction are mediated by separate sympathetic axons whereas a single parasympathetic axon may, through serial terminals, result in vasodilatation, secretion, and constriction of myoepithelial cells.

Figure 1.14. Diagram showing the histology of the major components of the salivary glands.

Secretory endpieces are the most densely innervated structures in the salivary glands. Individual acinar cells may have both cholinergic and adrenergic nerve endings. The secretion of water and electrolytes, which accounts for the volume of saliva produced, results from a complex set of stimuli which are largely parasympathetic. The active secretion of proteins into the saliva depends upon the relative levels of both sympathetic and parasympathetic stimulation.

Although the ducts are less densely innervated than secretory acini, they do influence the composition of the saliva. Adrenal aldosterone promotes resorption of sodium and secretion of potassium into the saliva by striated ductal cells. Myoepithelial cell contraction is stimulated predominantly by adrenergic fibers although there may be an additional role for cholinergic axons.

The cholinergic parasympathetic nerves release acetylcholine that binds to M3 and to a lesser extent M1 muscarinic receptors which result in the secretion of saliva by the acinar cells in the endpieces of the duct trees. The sympathetic nerves release noradrenaline that results in the release of stored protein from both the acinar cells and the ductal cells. There is also cross talk between the calcium and cyclic AMP intracellular pathways. Additionally, other non‐adrenergic and non‐cholinergic neuropeptides released from the autonomic nerves evoke saliva secretion and parasympathetically derived vasointestinal peptide acting through endothelial cell‐derived nitric oxide. These neuropeptides play a role in the reflex vasodilatation that accompanies salivary secretion (as seen dramatically in Frey syndrome). Neuronal type, calcium activated, soluble nitric oxide within salivary cells seems to play a role in mediating salivary protein secretion in response to autonomimetics. The fluid secretion involves aquaporin 5 and the extent to which its expression on apical acinar cell membranes is upregulated by cholimimetics remains obscure (Proctor and Carpenter 2007).