Ross & Wilson Anatomy and Physiology in Health and Illness (101 page)

•
promoting synthesis of fatty acids and storage of fat in adipose tissue (lipogenesis)

•
decreasing
glycogenolysis
(breakdown of glycogen into glucose)

•
preventing the breakdown of protein and fat, and
gluconeogenesis
(formation of new sugar from, e.g., protein).

Secretion of insulin is stimulated by increased blood glucose levels, for example after eating a meal, and to a lesser extent by parasympathetic stimulation, raised blood amino acid and fatty acid levels, and gastrointestinal hormones, e.g. gastrin, secretin and cholecystokinin. Secretion is decreased by sympathetic stimulation, glucagon, adrenaline, cortisol and somatostatin (GHRIH), which is secreted by the hypothalamus and pancreatic islets.

Glucagon

The effects of glucagon increase blood glucose levels by stimulating:

•
conversion of glycogen to glucose in the liver and skeletal muscles (glycogenolysis)

•
gluconeogenesis.

Secretion of glucagon is stimulated by a low blood glucose level and exercise, and decreased by somatostatin and insulin.

Somatostatin (GHRIH)

The effect of this hormone, also produced by the hypothalamus, is to inhibit the secretion of both insulin and glucagon in addition to inhibiting the secretion of GH from the anterior pituitary (
p. 210
).

Pineal gland

Learning outcomes

After studying this section you should be able to:

state the position of the pineal gland

outline the actions of melatonin.

The pineal gland is a small body attached to the roof of the third ventricle and is connected to it by a short stalk containing nerves, many of which terminate in the hypothalamus. The pineal gland is about 10 mm long, is reddish brown in colour and is surrounded by a capsule. The gland tends to atrophy after puberty and may become calcified in later life.

Melatonin

This is the main hormone secreted by the pineal gland. Secretion is controlled by daylight and levels fluctuate during each 24-hour period, being highest at night and lowest around midday. Secretion is also influenced by the number of daylight hours, i.e. seasonal variations. Although its functions are not fully understood, melatonin is believed to be associated with:

•
coordination of the circadian and diurnal rhythms of many tissues, possibly by influencing the hypothalamus

•
inhibition of growth and development of the sex organs before puberty, possibly by preventing synthesis or release of gonadotrophins.

Thymus gland

Learning outcomes

After studying this section you should be able to:

state the position of the thymus gland

outline the actions of thymosin.

The location and structure of the thymus gland are described on
page 132
.

Thymosin

This is the hormone secreted by the thymus gland and is involved in the development of T-lymphocytes for cell-mediated immunity (
Ch. 15
).

Local hormones

Learning outcome

After studying this section you should be able to:

outline the actions of local hormones.