Neuroendocrine cells and neuropeptides
Solitary cells of the nervous system and neurosecretory nuclei of the hypothalamus synthesize specific substances—neuropeptides, of which 17 families have been identified to date. Their mechanism of action is dual in nature:
- Locally: they ensure signal transmission directly within synapses, acting as neurotransmitters.
- Systemically: they enter the systemic circulation and affect distant target organs, functioning as true hormones (e.g., ADH, oxytocin, hypothalamic statins, and liberins).
Of particular importance are specific groups of neuropeptides that exhibit pronounced analgesic activity. Opioid peptides (endorphins) bind to specific receptors, producing a powerful analgesic and sedative (calming) effect. Neurotensin is similar in action to analgin and causes a characteristic triad: analgesia, hypothermia (reduction in body temperature), and hypotension (decrease in blood pressure). In turn, delta-sleep-inducing peptide acts as an endogenous hypnotic, inducing sleep.
Hormonal regulation of the digestive system
Many gastrointestinal hormones are also found in the central nervous system. The gastric mucosa itself produces four varieties of gastrins, which stimulate gastric motility and secretion, as well as activate the release of glucagon and insulin by the pancreas. Bombesin is also synthesized there, which enhances the secretion of chloride and hydrogen ions (hydrochloric acid) and stimulates pancreatic secretion.
The duodenal mucosa produces relatively specific hormones:
- Secretin: inhibits the production of gastrins, but stimulates the release of bile and pancreatic juice.
- Cholecystokinin (pancreozymin): produced in the intestine and the epithelium of the pancreatic ducts. It causes the gallbladder and ducts to contract and also stimulates the exocrine part of the pancreas.
Nonspecific hormones include histamine and serotonin, which stimulate the secretory and motor activity of the gastrointestinal tract. Important roles are played by the antagonists somatostatin (inhibits pancreatic functions) and VIP (vasoactive intestinal peptide — stimulates them). The regulation of circadian rhythms of the gastrointestinal tract is provided by melatonin.
APUD series cells
Numerous neuroendocrine cells are grouped into the APUD series solely on the basis of their biochemical characteristics. The name of the series stands for the ability of cells to take up amine precursors (cyclic amino acids: tryptophan, histidine, tyrosine, phenylalanine) and decarboxylate them.
The main distinguishing feature of APUD series cells is the ability to simultaneously produce two types of substances:
- Specific peptide (protein) hormones.
- Biogenic amines (serotonin, catecholamines, histamine).
The synthesis of biogenic amines is not accidental; they perform vital auxiliary functions: they facilitate the release (exocytosis) of the main hormone, increase capillary permeability for rapid entry of the hormone into the blood, and make target cells more sensitive to the humoral signal.
Embryogenesis and evolution of scientific views
Endocrine cells have a diverse origin involving all germ layers and the mesenchyme:
- Neuroectoderm: hypothalamic neurons, pineal gland, C cells of the thyroid gland, adrenal medulla.
- Foregut epithelium: adenohypophysis, follicular cells of the thyroid gland, parathyroid glands.
- Intestinal endoderm: gastroenteropancreatic system.
- Coelomic epithelium (mesoderm): adrenal cortex, ovarian follicular cells.
- Mesenchyme: interstitial cells of the gonads.
The study of these cells led to a revision of the classical rule "one cell — one hormone." With the discovery of the APUD system, the postulate was refined: "one endocrinocyte — one main hormone." However, it is now known that there are exceptions to this rule. For example, in the anterior pituitary, gonadotrophs simultaneously produce FSH and LH (resulting in 6 hormones for 5 cell types). In the intermediate pituitary, a single precursor polypeptide gives rise to several hormones at once: lipotropin, endorphins, and melanocyte-stimulating hormone.
Briefly: answers to topical questions
Which specific cells of the gastroenteropancreatic system produce enteroglucagon?
Enteroglucagon is produced by L cells of the gastroenteropancreatic system.
These endocrinocytes are localized in the following sections:
- Pyloric part of the stomach
- Duodenum
- Small intestine
- Large intestine
The hormone is a functional analogue of glucagon. Its effects include increasing hepatic glycogenolysis, and it also mobilizes carbohydrate and fat stores between meals.
The triad of neurotensin effects is similar to the action of analgin: analgesia, hypothermia (temperature reduction), and hypotension (blood pressure reduction).
Class and exam questions
What is the biochemical basis of APUD series cells?
These cells are capable of taking up amino acids with a cyclic radical (amine precursors) and decarboxylating them, producing biogenic amines along with peptide hormones.
Why do APUD series endocrinocytes need biogenic amines?
Amines perform an auxiliary role: they facilitate the exocytosis of the main hormone, dilate capillaries for its transport, and increase the sensitivity of target cells.
Is the principle of "one cell — one hormone" always observed?
No, there are exceptions. For example, cells of the anterior pituitary gland (gonadotrophs) produce two hormones at once, while cells of the intermediate lobe cleave a single polypeptide into several independent hormones.
What else to study on this topic
- Features of the 17 neuropeptide families and their effect on synaptic transmission
- The mechanism of synthesis and cleavage of the precursor polypeptide in the intermediate pituitary
- The role of melatonin in regulating circadian rhythms of gastrointestinal motility
- Specific effects of bombesin of central and gastric origin
Master the entire topic in the "Histology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on the topic "Diffuse neuroendocrine system" and another 3,485 pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 137 video lessons on the subject
Other topics in the "Endocrine System" section
Free video lessons: histologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.