Glands at a Glance: A Horse's Endocrine System
Think of the equine body as an enlightened corporal corporation. Under the direction of president hypothalamus, CEO pituitary gland processes messages to and from department managers, known collectively as endocrine glands, and circulates them bodywide.
These interoffice memos are the hormones. Every single cell has at least one "in" basket--a receptor--and maybe several, to receive hormonal memos specifically related to its duties. Those messages of no significance to the individual cell's function just float on by unread.
This corporation really cares about its workers' well-being, along with their output. Feedback from the cells is as important as the hypothalamus directives telling them what to do. Individual cells grow old and wear out or are lost to disease or trauma, and these are replaced in an orderly process. But the constant goal is survival of the organism, which is assured only when all of life's basic needs are maintained within certain parameters, a state called homeostasis.
Of course, the brain is ultimately in charge of this hypothetical corporation. As head of the nervous system, it sends out electronic commands that are far quicker than the endocrine system's chemical memos. But the brain couldn't do the job alone. Without the endocrine system's attention to well-being of the rank and file, the brain would quickly die, along with its insufficiently nourished, hydrated or defended underlings.
The brain and the endocrine system are in constant communication with each other and with individual cells. The two communications systems integrate through the hypothalamus, which translates electronic information supplied by the senses into the chemical orders that lead to cellular responses. For example, when your horse's skin registers extremely cold temperatures, the nervous system responds immediately in two ways: It causes the horse's hair to stand up for better heat retention, and it makes the horse shiver as an emergency means of producing heat. Meanwhile, the hypothalamus monitors this sensory input; if the cold persists over many days, it acts via the pituitary gland to produce more lasting warmth. The pituitary notifies the thyroid gland to increase the body's rate of fuel consumption--to turn up the thermostat, so to speak--which it does by releasing more of its energy-enabling hormone.
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At any given time, 30 to 40 different hormones are in distribution in your horse's blood stream, orchestrating energy usage, water and mineral balance, heart rate and blood pressure, growth and sexual development, reproduction and stress reactions. Endocrine memos arrive in two formats--as hormones derived from proteins, which bind to receptor cells' outer membranes, and as hormones derived from solid steroid alcohols, which enter into the cells and bind to internal receptors.
These messages usually instruct the cell to "do" or "do more," as in do produce your protein, do excrete your substance, do contract or dilate--whatever its function may be. Reduction or absence of the hormone means "slow down" or "don't." So long as the hormone is present on the receptor, the switch stays on. When feedback informs the boss that the level of production is adequate or excessive, the order is scaled back or withdrawn, and the output remains stable or dwindles.
Some functions, however, require such fine-tuning that pairs of hormones may act on the same cells in antagonistic fashion. For instance, a hormone from the thyroid gland enables calcium to leave the blood stream and go into storage in bones, while a hormone from the parathyroid glands orders the same mineral out of storage and into circulation. These opposing hormonal actions permit a more rapid response to a circulatory excess of deficiency of this critical mineral than a simple on/off mechanism controlled by a single gland.
Read on to discover how the astonishing endocrine glands - the smallest of all the organs, squirting out minute quantities of specialized chemicals--manage your horse's life-sustaining processes second by second, day by day, season by season, over a lifetime. It's a marvel of corporate communication.
The Pituitary Gland
Aliases: the "hypophysis" (from the Greek for "to grow underneath"); the "master gland."
Location: within the skull, in its own niche in the floor of the braincase, next to the brain stem and underlying the hypothalamus.
Appearance: a flattened, oblong structure embedded in connective tissue; in horses, about the size of a prune (pea size in people).
Structure: a two-part gland attached by special blood vessels and nerves to the hypothalamus.
- The neurohypophysis derives from the nervous system and serves as storage for two direct-acting hormones manufactured by cells in the hypothalamus.
- The adenohypophysis develops from cells in the roof of the mouth and is itself divided into three sections. Some hormones produced here act directly on target cells, but most pituitary hormones serve to stimulate other glands to release their hormones.
Hormones and actions: control most of the vial bodily functions.
- circulation and kidney function - Vasopressin, also called antidiuretic hormone (ADH), increases blood pressure by signaling the kidneys to retain water and by constricting the small arteries when blood pressure drops. The hypothalamus produces ADH.
- metabolism and growth - Thyroid-stimulating hormone (TSH) triggers hormone synthesis and secretion in the thyroid gland; adenocorticotropic hormone (ACTH) regulates steroid production in the adrenal glands; growth hormone (GH) orchestrates protein synthesis, carbohydrate metabolism and related processes throughout the body as a foal grows to maturity.
- reproduction - Follicle-stimulating hormone (FSH), luteinizing hormone (LH) and others stimulate the ovaries and testes for their roles in conception and pregnancy; prolactin supports milk production in the udder. Oxytocin, produced by the hypothalamus, triggers the smooth-muscle contractions of the uterus and mammary glands involved in birth, expulsion of the placenta and milk "let down."
Diseases: Equine Cushing's syndrome (also known as pituitary pars intermedia dysfunction) occurs when feedback signals from other glands begin to fail, causing the pituitary gland to stimulate overproduction of the steroid hormone cortisol by the adrenal glands. This, in turn, triggers over-activity of various other endocrine metabolic disturbances produce obesity, muscle weakness, behavioral changes and, most visibly, abnormally long or curly haircoat and failure to shed.
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