Structure of the endocrine system
The endocrine system is a collection of glands, tissues, and cells that synthesize hormones and secrete them directly into the blood to regulate body functions. It is organized hierarchically and includes several levels.
Central regulatory organs:
- Hypothalamus — located in the diencephalon, it serves as the link between the nervous and endocrine systems. Its neurons produce releasing hormones (releasing and inhibiting factors) transported via the portal vascular system to the pituitary gland, as well as vasopressin and oxytocin, which are transported via axons to the neurohypophysis.
- Pituitary gland — located in the sella turcica of the sphenoid bone. The anterior lobe (adenohypophysis) secretes tropic hormones: TSH, ACTH, LH, FSH, GH. The posterior lobe (neurohypophysis) does not synthesize hormones, but stores and releases oxytocin and vasopressin.
Peripheral glands: thyroid, parathyroid glands, adrenal glands, pancreas, gonads, pineal gland. Additionally, the diffuse endocrine system is distinguished — isolated cells in the mucosa of the gastrointestinal tract, lungs, and kidneys that produce local hormones (gastrin, secretin, serotonin).
Connection between the nervous and endocrine systems
The hypothalamus is simultaneously a part of the brain and the main endocrine gland: it controls the pituitary gland, and through it, the peripheral glands. Nerves can directly control glands: a classic example is the adrenal medulla, which instantly releases epinephrine in response to a nerve signal. In turn, hormones affect the central nervous system: high levels of cortisol suppress the activity of the structures that initiated its release.
Key differences between the systems:
- Nature of the signal: nervous — electrochemical and chemical (synaptic neurotransmitters); endocrine — chemical only (hormones).
- Pathway of transmission: via axons to a specific organ versus the humoral pathway via the bloodstream.
- Speed of response: milliseconds versus minutes and hours.
- Duration of effect: short-term versus prolonged (up to a day).
- Target specificity: high (via nerves) versus broad (to all cells with specific receptors).
- Function: rapid coordination of movements and reactions versus long-term regulation of metabolism, growth, and homeostasis.
Hormones: concept and biological properties
Hormone — a biologically active substance that acts as a chemical messenger. Target cell — any cell possessing a specific receptor for a given hormone. Endocrine gland — the source of the signal, secreting the hormone into the bloodstream in response to a command from the hypothalamic-pituitary system or neural influences.
Five biological properties of hormones:
- High biological activity — action at extremely low concentrations.
- High specificity — interaction exclusively with its "own" receptor via the lock-and-key principle.
- Distant action — effect on organs remote from the producing gland.
- Alteration of the rate of enzymatic reactions without participating in them as an enzyme or cofactor.
- Short duration of action — rapid inactivation in the body.
Classifications of hormones
By chemical nature:
- Protein-peptide: peptides (vasopressin, oxytocin), simple proteins (insulin), complex glycoprotein proteins (TSH, FSH).
- Steroids — cholesterol derivatives: cortisol, aldosterone, sex hormones.
- Amino acid derivatives: catecholamines (epinephrine, norepinephrine), iodothyronines (thyroxine, triiodothyronine).
By glandular affiliation: hormones of the hypothalamus, pituitary gland (anterior and intermediate lobes), pineal gland, adrenal cortex and medulla, pancreas, gastrointestinal tract, thyroid and parathyroid glands, gonads, and placenta.
By biological effects: regulators of protein, carbohydrate, lipid, and energy metabolism (insulin, glucagon, cortisol, thyroxine, GH); water-salt metabolism (aldosterone, vasopressin); calcium-phosphorus metabolism (parathyroid hormone, calcitonin); reproductive function (estradiol, progesterone, testosterone, FSH, LH); endocrine gland function (releasing factors, inhibiting factors, tropic hormones); tissue hormones (cytokines, histamine, secretin, gastrin).
Pathways of hormone action
- Endocrine (classical) — secretion into the blood and action on distant targets. Example: epinephrine from the adrenal glands acts on the heart.
- Paracrine — local effect on neighboring cells without entering the bloodstream (prostaglandins during inflammation).
- Autocrine — the cell responds to its own signaling molecule; typical for immune regulation and growth, including tumor cells.
- Juxtacrine — signal transmission via direct contact of neighboring cell membranes; important in embryogenesis.
- Neurocrine (neuroendocrine) — neurons secrete neurohormones not into a synapse, but into the blood (hypothalamic releasing hormones).
Mechanisms of hormone action
The hormone effect is triggered upon binding to a receptor. Receptors are divided into two groups based on localization.
Membrane receptors bind protein-peptide hormones, catecholamines, and prostaglandins. The signal is transmitted across the membrane involving G proteins and the formation of secondary messengers — cAMP or cGMP. Protein kinases and tyrosine kinase are activated, triggering protein synthesis and a physiological response. Example: the insulin receptor consists of an α-subunit (extracellular) and a β-subunit (cytoplasmic); insulin binding activates glucose transport into the cell, glycogen and protein synthesis, and lipogenesis.
Intracellular (cytoplasmic and nuclear) receptors bind steroid and thyroid hormones, vitamin D₃, and retinoids. The hormone-receptor complex interacts with DNA, activating or repressing the transcription of specific genes. The resulting mRNA is translated on ribosomes into specific proteins and enzymes. Example: cortisol activates genes for gluconeogenesis and hepatic glycogen synthesis, enhances protein catabolism, and suppresses immunity.
Principles of hormonal regulation
Most hormones are regulated by the feedback principle — a closed loop ensuring secretion stability.
Negative feedback — the predominant mechanism. Hormone A stimulates the secretion of hormone B, while hormone B inhibits the further release of hormone A. Classic example: ACTH of the adenohypophysis stimulates cortisol synthesis in the adrenal cortex, and elevated cortisol levels inhibit further ACTH secretion.
Positive feedback — hormone B additionally enhances the production of hormone A. Example: during the menstrual cycle, rising concentrations of LH stimulate estradiol secretion; upon reaching a threshold level, estradiol triggers theovulatory LH surge, initiating ovulation. After reaching the maximum, the LH level declines, reflecting the self-limiting nature of such systems.
Key points
- The endocrine system is hierarchical: hypothalamus → pituitary gland → peripheral glands + diffuse endocrine system.
- The hypothalamus controls the adenohypophysis via releasing hormones and the vascular portal system; the neurohypophysis merely stores vasopressin and oxytocin.
- Endocrine regulation is slow, prolonged, and widely targeted, unlike rapid and pinpoint neural regulation.
- By chemical nature, hormones are divided into protein-peptide, steroid, and amino acid derivatives.
- Endocrine, paracrine, autocrine, juxtacrine, and neurocrine pathways of action are distinguished.
- Membrane receptors operate via G proteins and secondary messengers (cAMP, cGMP); intracellular receptors operate via the regulation of gene transcription.
- Negative feedback (ACTH–cortisol axis) maintains homeostasis; positive feedback (LH–estradiol) ensures ovulation.
- Properties of hormones: high activity, specificity, distance action, regulation of enzymatic reactions, short duration of action.