Anatomy & Physiology

Anatomy & Physiology Study Guide • In skeletal muscles, epinephrine and norepinephrine trigger movement of glycogen stores and accelerate the breakdown of glucose to provide ATP, increasing muscular strength and endurance. • In adipose tissue, fat reserves are broken down into fatty acids, which are released into the bloodstream for use by other tissues during ATP production. • In the liver, glycogen molecules are broken down, resulting in glucose molecules that are released into the bloodstream, primarily for use by neural tissues, which cannot shift to fatty acid metabolism. • In the heart, the stimulation of receptors causes an increase in the force and rate of cardiac muscle contraction. The metabolic alterations that follow the release of catecholamines, such as E and NE, peak 30 seconds after suprarenal stimulation, and they persist for several minutes after. 17.6 The Pineal Gland The pineal gland is part of the epithalamus. The pineal gland contains neurons, neuroglia, and special secretory cells called pinealocytes . These cells synthesize the hormone melatonin from molecules of the neurotransmitter serotonin. Melatonin production is lowest during daylight hours and highest at night. Among the functions suggested for melatonin in humans are inhibiting reproductive functions, protecting against damage by free radicals, and setting circadian rhythms. Increased melatonin secretion in darkness has been thought to be a primary cause of seasonal affective disorder (SAD). This condition, characterized by changes in eating habits, sleeping patterns, and mood can develop during the winter in people who live at high latitudes, where sunlight is scarce or lacking. 17.7 Pancreas The pancreas lies within the abdominopelvic cavity in the loop formed between the inferior border of the stomach and the proximal portion of the small intestine. It is primarily an exocrine organ that makes digestive enzymes. The exocrine pancreas , 99 percent of the pancreatic volume, consists of clusters of gland cells, called pancreatic acini , and their attached ducts. Together, the gland and duct cells secrete large quantities of an alkaline, enzyme-rich fluid that reaches the lumen of the digestive tract through a network of secretory ducts. The endocrine pancreas is made of small groups of cells scattered among the exocrine cells. The endocrine clusters are known as pancreatic islets or the islets of Langerhans . A typical pancreas contains roughly two million pancreatic islets. The Pancreatic Islets The pancreatic islets are surrounded by an extensive, fenestrated capillary network that carries pancreatic hormones into the bloodstream. Each islet contains four types of cells. Alpha cells produce the hormone glucagon, which raises blood glucose levels by increasing the rates of glycogen breakdown and glucose release by the liver. Beta cells produce the hormone insulin. Insulin decreases blood glucose levels by increasing glucose uptake and utilization by most body cells, and by increasing glycogen synthesis in skeletal muscles and the liver. Delta cells produce a peptide hormone Achieve Page 213 of 368 ©2018