• Salivary amylase

• Lingual lipase

Approximately 99.5% of saliva is water. Swallowing is facilitated by the moistening of food materials; furthermore, it serves as a solvent for molecules that stimulate the taste buds. The presence of mucus, which is thick and slippery, lubricates the mouth and the food and assists in swallowing. Lysozyme is an enzyme that lyses or kills many types of bacteria that may be ingested.

Saliva begins the process of chemical digestion with salivary amylase. This enzyme splits starch molecules into fragments. Specifically, polysac-charides, or starches, are broken down into maltose, a disaccharide consisting of two glucose molecules. Salivary amylase may account for up to 75% of starch digestion before it is denatured by gastric acid in the stomach.

A small amount of lingual lipase is also present and plays a role in the breakdown of dietary lipid. This enzyme is optimally active at an acidic pH and therefore remains active through the stomach and into the intestine.

Due to parasympathetic stimulation of the salivary glands, saliva is secreted continuously at a basal rate of approximately 0.5 ml/min. Secretion may be enhanced by two types of reflexes:

• Simple or unconditioned salivary reflex

• Acquired or conditioned salivary reflex

The simple or unconditioned salivary reflex occurs when food is present within the oral cavity and causes stimulation of chemoreceptors and pressure receptors. These receptors then transmit impulses to the salivary center in the medulla of the brainstem. Parasympathetic efferent impulses are transmitted back to the salivary glands and secretion is enhanced.

The acquired or conditioned salivary reflex is elicited in response to the thought, sight, smell, or sound of food. As demonstrated with Pavlov's dog, these stimuli result in a learned response. Another stimulus that enhances salivation is nausea. Salivary secretion is inhibited by fatigue, sleep, fear, and dehydration. Overall, 1 to 2 l of saliva may be produced per day.

Pharmacy application: effects of anticholinergic drugs on the digestive system

In addition to their therapeutic actions, many drugs have undesirable side effects that may influence the digestive system. An example of such a drug is scopolamine, one of the most effective agents used for prevention of motion sickness. This drug may be administered transdermally in a multilayered adhesive unit, or "patch" form. Its mechanism of action likely involves inhibition of muscarinic receptors in the vestibular apparatus of the inner ear. This interrupts transmission of nerve impulses from the inner ear to the emetic center in the medulla of the brainstem. As a result, vomiting in response to motion is inhibited. However, the salivary glands are also quite sensitive to the activity of muscar-inic receptor antagonists. In fact, scopolamine and other anticho-linergic agents may severely inhibit the copious, watery secretion of the salivary glands. In this case, the mouth becomes dry and swallowing and speaking may become difficult. Other anticholinergic agents may be used to: (1) reduce muscle rigidity and muscle tremor in Parkinson's disease (benztropine mesylate); (2) reduce bronchospasm and airway mucus secretion in asthma and chronic obstructive pulmonary disease (COPD) (ipratropium); and (3) reduce accumulation of secretions in the trachea and the possibility of laryngospasm prior to administration of general anesthesia. In each case, these medications also inhibit salivary secretion and cause "dry mouth."

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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