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Figure

The hormone-secreting cells of the pancreas are grouped in clusters, or islets, that are closely associated with blood vessels. Other pancreatic cells secrete digestive enzymes into ducts.

Islet of Langerhans

Islet of Langerhans

Islets Langerhans Hormones

Figure

Light micrograph of an islet of Langerhans within the pancreas (50x micrograph enlarged to 200x).

ure 13.33

Figure

Light micrograph of an islet of Langerhans within the pancreas (50x micrograph enlarged to 200x).

When glucose concentration is relatively high, as may occur following a meal, the beta cells release insulin. By promoting formation of glycogen in the liver and entrance of glucose into adipose and muscle cells, insulin helps prevent excessive rise in the blood glucose concentration (hyperglycemia). Then, when the glucose concentration falls, between meals or during the night, insulin secretion decreases (fig. 13.34).

As insulin concentration falls, less glucose enters the adipose and muscle cells, and the glucose remaining in the blood is available for cells that lack insulin receptors to use, such as nerve cells. Neurons readily tap the energy in a continuous supply of glucose to produce ATP.

Nerve cells, including those of the brain, obtain glucose by a facilitated-diffusion mechanism that is not dependent on insulin, but rather only on the blood glucose concentration. For this reason, nerve cells are particularly sensitive to changes in the blood glucose concentration. Conditions that cause such changes—excess insulin secretion, for example—are likely to affect brain functions.

At the same time that insulin concentration is decreasing, glucagon secretion is increasing. Therefore, these hormones function together to maintain a relatively constant blood glucose concentration, despite great variations in the amounts of ingested carbohydrates.

Somatostatin (similar to the hypothalamic hormone), which the delta cells release, helps regulate glucose metabolism by inhibiting secretion of glucagon and insulin. Table 13.11 summarizes the hormones of the islets of Langerhans, and Clinical Application 13.4 discusses diabetes mellitus, a derangement of the control of glucose metabolism.

O What is the name of the endocrine portion of the pancreas?

^9 What is the function of glucagon?

^9 What is the function of insulin?

□ How are the secretions of glucagon and insulin controlled?

Q Why are nerve cells particularly sensitive to changes in blood glucose concentration?

Hypoglycemia, or low blood glucose level due to excess insulin in the bloodstream, causes episodes of shakiness, weakness, and anxiety. Following a diet of frequent, small meals that are low in carbohydrates and high in protein can often control symptoms by preventing the surges of insulin that lower the blood glucose level. Hypoglycemia is most often seen when a person with diabetes injects too much insulin, but it can also reflect a tumor of the insulin-producing cells of the pancreas, or it may occur transiently following very strenuous exercise.

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