14.1 Hormonal Control

14.1 Hormonal Control

A hormone is a chemical messenger secreted into the blood. It changes cells that carry the matching receptor, including cells far from the gland that released it. Negative feedback is the usual pattern: the response opposes the change that started it, so the variable returns toward a set point. Positive feedback amplifies a change and runs only until some other event stops it. Sex steroids use negative feedback for most of reproductive life. The surge of luteinizing hormone before ovulation is the exception, not the everyday rule.

The hypothalamus controls the anterior pituitary

The hypothalamus connects the nervous system to the endocrine system. It secretes releasing hormones into a short portal circulation that reaches the anterior pituitary, also called the adenohypophysis. Those releasing hormones control how much hormone the anterior lobe secretes. The anterior lobe is glandular tissue, and its cells synthesize the hormones they release. The hypothalamus also sends a few inhibitory signals. Either way, the anterior pituitary takes its instructions from the hypothalamus. It is not a set of independent glands.

Six hormones and their targets

The releasing hormones match those axes. Thyrotropin-releasing hormone promotes TSH. Corticotropin-releasing hormone promotes ACTH. Gonadotropin-releasing hormone promotes both FSH and LH. Growth-hormone-releasing hormone promotes GH. Dopamine holds prolactin down, and somatostatin holds GH down. Match by target when a stem gives few names. The adrenal cortex means ACTH. The thyroid gland means TSH. The ovarian follicle means FSH. Milk production means prolactin. Milk ejection is a different hormone, oxytocin, released from the posterior lobe.

Negative feedback closes most of these loops. Thyroid hormone restrains TSH. Cortisol restrains ACTH. Estrogen, progesterone, and testosterone usually restrain gonadotropin-releasing hormone, FSH, and LH. Assume that direction unless the stem describes the midcycle switch from negative feedback to positive feedback.

The posterior lobe releases hormones it did not make

The posterior pituitary, or neurohypophysis, is a downgrowth of the brain, not a second factory for its two famous hormones. Antidiuretic hormone (ADH, also called vasopressin) and oxytocin are synthesized in hypothalamic neurons. They travel down axons and are stored in nerve endings inside the posterior lobe. A nerve impulse releases them into the blood. The posterior lobe does not synthesize ADH or oxytocin.

ADH acts on kidney collecting ducts and increases water reabsorption, so urine becomes more concentrated when the body must save water. Oxytocin stimulates uterine smooth muscle in labor and causes milk ejection during nursing. Prolactin, from the anterior lobe, promotes making milk. Oxytocin only moves milk that is already made. Labor can amplify by positive feedback, but that loop is separate from the LH surge.

Thyroid hormone, calcium, and the adrenal gland

Thyroid hormone raises basal metabolic rate, the energy the body uses at rest. It also supports normal growth and development of the nervous system. TSH is the anterior signal that drives the thyroid. When thyroid hormone rises, negative feedback lowers TSH. When thyroid hormone is low, that brake lifts and TSH rises. The direction of the pituitary response is the clue that the loop is working.

Calcitonin, from parafollicular cells of the thyroid, lowers blood calcium by opposing bone breakdown and favoring calcium deposition in bone. Parathyroid hormone, from the parathyroid glands, raises blood calcium. It increases calcium release from bone, increases calcium reabsorption in the kidney, and promotes activation of vitamin D so the intestine absorbs more calcium. Calcitonin does not raise blood calcium.

The adrenal gland is two endocrine tissues in one organ. The adrenal cortex secretes steroid hormones. Cortisol supports the longer stress response, raises blood glucose, and damps inflammation. ACTH drives cortisol, and cortisol feeds back on the hypothalamus and the anterior pituitary. Aldosterone causes the kidney to retain sodium and excrete potassium, which supports blood volume and blood pressure. The adrenal medulla secretes epinephrine for the rapid fight-or-flight response: a faster heart, wider airways, and quick mobilization of fuel. Cortex means cortisol and aldosterone. Medulla means epinephrine.

The pancreatic pair, pointed back to homeostasis

As in glucose homeostasis, insulin lowers blood glucose and glucagon raises it.

FSH, LH, and the ovarian cycle

Gonadotropin-releasing hormone from the hypothalamus causes the anterior pituitary to secrete FSH and LH. In the ovarian cycle, FSH supports growth of an ovarian follicle. The growing follicle secretes estrogen. Through the early and middle parts of the cycle, estrogen mostly exerts negative feedback, and the gonadotropins stay moderate. When estrogen has been high for long enough, the hypothalamus and pituitary switch to positive feedback. The pituitary releases an LH surge. Ovulation follows the surge: the mature follicle ruptures and releases the secondary oocyte.

The ruptured follicle becomes the corpus luteum, which secretes progesterone and maintains the uterine lining. If no pregnancy occurs, the corpus luteum fades about two weeks after ovulation, progesterone falls, and menstruation begins. If an embryo is present, the early embryo secretes human chorionic gonadotropin (hCG). hCG maintains the corpus luteum through early pregnancy. It is an embryonic signal, not an anterior pituitary hormone, and it is not what starts the LH surge.

Testosterone from Leydig cells

In the testis, LH stimulates Leydig cells between the seminiferous tubules to secrete testosterone. Testosterone supports sperm production and male secondary sex characteristics, including facial hair, a deeper voice, and increased muscle mass. FSH acts on Sertoli cells, which support developing sperm together with testosterone. FSH does not secrete testosterone.