Five levels of regulation of the reproductive system
The functioning of the system is ensured by the interaction of five levels, where higher-lying structures control lower-lying ones:
- Cerebral cortex and extrahypothalamic structures (limbic system, amygdala, hippocampus). This is the highest center that integrates external and internal signals. This is why severe stress can cause amenorrhea or pseudocyesis syndrome.
- Hypothalamus. Its hypophysiotropic area consists of neurosecretory nuclei (arcuate, ventromedial, paraventricular, etc.). The cells here are unique: they generate nerve impulses and simultaneously secrete hormones—releasing hormones (liberins) (stimulate the pituitary) and inhibiting hormones (statins) (suppress them).
- Pituitary gland. The anterior lobe (adenohypophysis) secretes gonadotropic hormones (FSH and LH) as well as prolactin under the influence of the hypothalamus.
- Ovaries. They perform generative (egg maturation) and hormonal (synthesis of estrogens, progesterone, androgens) functions. Out of 2 million primordial follicles present at birth, 200,000 to 400,000 remain by menarche (first menstruation). The remaining 99% undergo atresia (regression).
- Target organs. Tissues that respond to fluctuations in sex steroids (primarily the endometrium of the uterus).
Communication mechanisms and neurotransmitters
The system operates on the principle of negative feedback: an increase in the concentration of ovarian hormones in the blood suppresses the secretion of the corresponding trophic hormones of the pituitary and releasing hormones of the hypothalamus.
Information is transmitted to the central nervous system via specific steroid receptors, as well as with the help of neurotransmitters and neuropeptides:
- Stimulate the release of gonadotropin-releasing hormone (GnRH): norepinephrine, acetylcholine, GABA.
- Inhibit the production of GnRH: dopamine, serotonin, melatonin, and endogenous opioid peptides (endorhpins, enkephalins).
The connection between the hypothalamus and the pituitary gland is provided by the portal blood system. It allows hormones to circulate in both directions. The main regulator here is GnRH (gonadoliberin). It is secreted in a pulsatile, circannual [Note: Keep as circannual/circhoral based on context, here circhoral] mode. The frequency and amplitude of these impulses determine whether the pituitary gland will secrete FSH or LH.
The role of the pituitary gland and cycle phases
Changes in the ovaries and endometrium occur synchronously and are divided into two main phases:
- Follicular (1st phase). FSH (follicle-stimulating hormone) dominates. It triggers follicular growth, causes proliferation of granulosa cells, and increases the activity of aromatases (enzymes that convert androgens into estrogens). During this time, the functional layer of the endometrium regenerates and thickens in the uterus. The phase ends with ovulation.
- Luteal (2nd phase). LH (luteinizing hormone) dominates. It drives ovulation itself and stimulates the formation of the corpus luteum at the site of the ruptured follicle. The corpus luteum begins actively synthesizing progesterone. Secretory transformation occurs in the endometrium—preparing for embryo implantation.
If fertilization does not occur, the functional layer of the endometrium is shed—menstruation (desquamation) begins.
Prolactin and neurohypophysis
A special place in regulation is occupied by prolactin. Unlike gonadotropins, it is primarily under the inhibitory (statin) influence of the hypothalamus—its release is blocked by dopamine. Prolactin stimulates mammary gland growth, regulates lactation, and controls progesterone secretion by the corpus luteum. In pathology (hyperprolactinemia), follicular maturation is disrupted, which leads to a lack of ovulation.
The posterior pituitary (neurohypophysis) is not a true endocrine gland and does not synthesize hormones. It serves as a depot for oxytocin and vasopressin (ADH), which are produced in the hypothalamus and travel down the axons of large neurons.
To easily remember the regulatory hierarchy, use the acronym C-H-P-O-T: Cortex — Hypothalamus — Pituitary — Ovaries — Targets.
Class and exam questions
What is the circchoral pattern of secretion?
This is a pulsatile release of gonadotropin-releasing hormone (GnRH) by the hypothalamus at a frequency of approximately one pulse every 1–3 hours (from the word circhoral — around the hour).
How does severe stress lead to delayed menstruation?
Stress activates the cerebral cortex and the production of endogenous opioid peptides (e.g., endorphins). They bind to receptors and suppress GnRH synthesis in the hypothalamus, halting the entire downstream regulatory cascade.
What is the difference between the functions of FSH and LH?
FSH is responsible for follicular growth, egg maturation, and estrogen synthesis in the first phase of the cycle. LH ensures ovulation and stimulates progesterone production by the corpus luteum in the second phase.
What else to study on this topic
- Mechanisms of follicular atresia and the formation of the ovarian reserve
- The role of prolactin in the reproductive system and causes of hyperprolactinemia
- Synthesis and functions of neurohypophyseal hormones (oxytocin and vasopressin)
- Detailed analysis of the thyroid and adrenal regulatory axes
- The effect of neuropeptide Y and galanin on the menstrual cycle
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