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Pancreas – Anatomy, Nerve Supply, Functions

Pancreas is an extended, accessory digestive gland that is found retroperitoneally, crossing the bodies of the L1 and L2 vertebra on the posterior abdominal wall. The pancreas lies transversely in the upper abdomen between the duodenum on the right and the spleen on the left. It is divided into the head, neck, body, and tail. The head lies on the inferior vena cava and the renal vein and is surrounded by the C loop of the duodenum. The tail of the pancreas extends up to splenic hilum. The pancreas produces an exocrine secretion (pancreatic juice from the acinar cells) which then enters the duodenum through the main and accessory pancreatic ducts and endocrine secretions (glucagon and insulin from the pancreatic islets of Langerhans) that enter the blood.

Pancreas

Structure and Function of Pancreas

The pancreas is divided into 4 parts: head, neck, body, and tail.

The head of the pancreas is the enlarged part of the gland that is surrounded by the C-shaped curve of the duodenum. On its way to the descending part of the duodenum, the bile duct lies in a groove on the posterosuperior surface of the head or is embedded in its substance.
The body of the pancreas continues from the neck passes over the aorta and L2 vertebra. The anterior surface of the body of the pancreas is covered with peritoneum. The posterior surface of the body is devoid of peritoneum and is in contact with the aorta, the superior mesenteric artery (SMA), the left suprarenal gland, left kidney, and renal vessels.
The neck of the pancreas is short. The tail of the pancreas lies anterior to the left kidney, where it is closely related to the splenic hilum and the left colic flexure. The main pancreatic duct carrying the pancreatic secretions joins with bile duct to form hepatopancreatic ampulla, which opens into the descending part of the duodenum.
The hepatopancreatic sphincter of Oddi around the hepatopancreatic ampulla is a smooth muscle sphincter that controls the flow of bile and pancreatic juice into the ampulla and inhibits reflux of duodenal substances into the ampulla.

Blood Supply of Pancreas

Arterial Supply

Branches of the splenic artery (a branch of the celiac trunk), superior mesenteric artery (SMA), and the common hepatic artery provide blood supply to the pancreas [rx][rx].

Pancreatic head – The gastroduodenal artery (a branch of the common hepatic artery) supplies the head and the uncinate process of the pancreas in the form of the pancreaticoduodenal artery (PDA). Part of the inferior portion of the head is supplied by the inferior PDA which arises from the SMA.
Body and the tail – The splenic artery and its branches supply these.

Venous Supply

Pancreatic head – The head drains into the superior mesenteric vein (SMV).
Body and the neck – The splenic vein drains these.

The SMV and splenic vein merge to form the portal vein.

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Nerves of Pancreas

The pancreas has a complex network of

Parasympathetic,
Sympathetic, and
Sensory innervations [rx]. It also has an intrinsic nerve plexus.
Sympathetic and parasympathetic fibers are dispersed to pancreatic acinar cells.
The parasympathetic fibers arise from the posterior vagal trunk and are secretomotor, but the secretions from the pancreas are predominantly mediated by cholecystokinin and secretin, which are hormones produced by the epithelial cells of the duodenum and proximal intestinal mucosa regulated by acidic compounds from the stomach.
Sympathetic innervation is via the T6-T10 thoracic splanchnic nerves and the celiac plexus.

Function of Pancreas

Pancreatic Hormones and Their Function[rx][rx][rx]

Insulin

Source – Beta cells of islets of the pancreas.
Synthesis – Insulin is a peptide hormone. The insulin mRNA is translated as a single-chain precursor called preproinsulin, and removal of its signal peptide during insertion into the endoplasmic reticulum generates proinsulin. Within the endoplasmic reticulum, proinsulin is exposed to several specific endopeptidases, which excise the C peptide (one of three domains of proinsulin), thereby generating the mature form of insulin.
Transport – insulin circulates entirely in the unbound form (T1/2 = 6 min).
Main Target cells – hepatic, muscle and adipocyte cells (i.e., cells specialized for energy storage).
Mechanism of action – Insulin binds to a specific receptor tyrosine kinase on the plasma membrane and increases its activity to phosphorylate numerous regulatory enzymes and other protein substrates.
Regulation of its secretion – Plasma glucose level is the main regulator of insulin secretion. The change in the concentration of plasma glucose that occurs in response to feeding or fasting is the main determinant of insulin secretion. Modest increases in plasma glucose level provoke a marked increase in plasma insulin concentration. Glucose is taken up by beta cells via glucose transporters (GLUT2). Increased plasma amino acid and free fatty acid concentrations induce insulin secretion as well. Glucagon is also known to be a strong insulin secretagogue.
Physiological functions – Insulin plays an important role to keep plasma glucose value within a relatively narrow range throughout the day (glucose homeostasis). Insulin’s main actions are (1) In the liver, insulin promotes glycolysis and storage of glucose as glycogen (glycogenesis), as well as the conversion of glucose to triglycerides, (2) In muscle, insulin promotes the uptake of glucose and its storage as glycogen, and (3) in adipose tissue, insulin promotes the uptake of glucose and its conversion to triglycerides for storage.

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Amylin (diabetes-associated peptide)

Source – Beta cells of islets of the pancreas. It is co-secreted with insulin in response to caloric intake (feeding state).
Target cells – Alpha cells of islets of pancreas and hypothalamus.
Physiological functions – it suppresses glucagon secretion from the alpha cells of the islets in the pancreas via paracrine interaction between beta cells and alpha cells. Amylin also slows gastric emptying which delays the absorption of glucose from the small intestine into the circulation. Also, it stimulates the satiety center of the brain to limit food consumption.

Glucagon

Source – Alpha cells of islets of the pancreas.
Synthesis – The initial gene product is the mRNA encoding preproglucagon. A peptidase removes the signal sequence of preproglucagon during the translation of the mRNA in the rough endoplasmic reticulum to yield proglucagon. Proteases in the alpha cells subsequently cleave the proglucagon into the mature glucagon molecule.
Target cells – Hepatic cells.
Mechanism of action – glucagon binds to a receptor that activates the heterotrimeric G protein Gas, which stimulates membrane-bound adenylyl cyclase. The cAMP formed by adenylyl cyclase, in turn, activates PKA, which phosphorylates numerous regulatory enzymes and other protein substrates.
Regulation of its secretion – The amino acids released by digestion of a protein meal appear to be the main determinant of glucagon secretion.
Physiological functions – Glucagon acts exclusively on the liver to antagonize insulin effects on hepatocytes. It enhances glycogenolysis and gluconeogenesis. It also promotes oxidation of fat, which can lead to the formation of ketone bodies.

Somatostatin

Source – Delta cells of the islets of the pancreas, hypothalamus and D cells of gastric glands.
Target cells – Beta cells of islets of the pancreas, somatotroph cells in the anterior pituitary gland and the G cells of the gastric glands.
Mechanism of action – Somatostatin binds to a receptor that activates the heterotrimeric inhibitory G protein, which inhibits membrane-bound adenylyl cyclase and cAMP formation.
Regulation of its secretion – Glucagon stimulates somatostatin secretion via paracrine interaction between alpha cells and delta cells of the islets of the pancreas.
Physiological functions – Somatostatin inhibits the secretion of multiple hormones, including growth hormone, insulin, glucagon, gastrin, vasoactive intestinal peptide (VIP), and thyroid-stimulating hormone.

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Ghrelin

Source – Epsilon cells of the islets of the pancreas, endocrine cells in the stomach and hypothalamus.
Target cells – Beta cells of the islets of the pancreas and somatotroph cells in the anterior pituitary gland.
Physiological functions – ghrelin inhibits the secretion of insulin from Beta cells of the islets of the pancreas via paracrine interaction between delta cells and beta cells of the islets of the pancreas. It also stimulates appetite and growth hormone secretion.

Pancreatic Polypeptide (PP)

Pancreatic polypeptide is secreted from upsilon (F) cells of the islets of the pancreas. Dietary intake of nutrients alters the secretion of the pancreatic polypeptide. Its function is not decidedly understood yet.
Paracrine Interaction Between Pancreatic Endocrine Cells
Insulin secreted by beta cells acts as a prime hormone of glucose homeostasis. Insulin and amylin inhibit glucagon secretion by alpha cells. Whereas glucagon activates insulin and somatostatin secretion, somatostatin secreted by delta cells and ghrelin by epsilon cells inhibit insulin secretion.

References

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Pancreas – Anatomy, Nerve Supply, Functions

Pancreas – Anatomy, Nerve Supply, Functions

Structure and Function of Pancreas