After production of these 2 hormones in the cell bodies of the neurosecretory cells, oxytocin and ADH are packaged into secretory vesicles, which move by fast axonal transport to the axon terminals in the posterior pituitary, where they are stored until nerve impulses trigger exocytosis and release of the hormone.
Oxytocin
During foal delivery, oxytocin enhances contraction of smooth muscle cells in the wall of the uterus; and after delivery, it stimulates milk ejection from mammary glands in response to mechanical stimulus of a suckling.
In males, the function of oxytocin and in non-pregnant females is unknown.
The control of OT is a positive feedback system.
Antidiuretic Hormone (ADH)
This is a substance that decreases urine production by causing the kidneys to return more water to the blood, thus decreasing urine volume. Alcohol consumption causes frequent urination because it inhibits ADH. ADH also decreases water loss through sweating and causes constriction of arterioles – increasing blood pressure. ADH’s other name Vasopressin reflects this action. The amount of ADH secreted is dependant on blood osmotic pressure and blood volume.
Thyroid Gland
Anatomy: The thyroid gland is a butterfly-shaped gland located just inferior to the larynx on either side of the trachea. The gland contains 2 lobes connected by an isthmus. It is dark red brown in colour. The size of the thyroid depends on the activity of the gland but roughly the size of a plum. Secreting Cells: Thyroid Follicles make up most of the thyroid gland. The wall of each follicle consists of cells called Folliclular Cells . A few cells called Parafollicular Cells lie between the follicles.
Hormones: The follicular cells produce 2 hormones:
Thyroxin (T4)
Triiodothyronine (T3) .
The parafollicular cells produce the hormone Calcitonin.
Target: T3 & T4 target all body cells, while calcitonin targets bone tissue.
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