Note: Numbers within parentheses indicate the number of amino acids in the protein or peptide s. John W. This content is distributed under a Creative Commons Attribution 3. Kinds of Hormones There are two major classes of hormone: 1 proteins, peptides, and modified amino acids and 2 steroids. Proteins, peptides and modified amino acids These hydrophilic and mostly large hormone molecules bind to receptors on the surface of "target" cells; that is, cells able to respond to the presence of the hormone.
The receptors are transmembrane proteins that pass through the plasma membrane 7 times, with their N-terminal exposed at the exterior of the cell and their C-terminal projecting into the cytoplasm. Binding of the hormone to the receptor activates a G protein associated with the cytoplasmic C-terminal This initiates the production of a "second messenger".
The cell begins to produce the appropriate gene products in response to the hormonal signal it had received at its surface.
Steroid Hormones Steroid hormones , being hydrophobic molecules, diffuse freely into all cells. Figure Hormone Regulation The levels of hormones circulating in the blood are tightly controlled by three homeostatic mechanisms: When one hormone stimulates the production of a second, the second suppresses the production of the first. Example: The follicle stimulating hormone FSH stimulates the release of estrogens from the ovarian follicle. A high level of estrogen, in turn, suppresses the further production of FSH.
Antagonistic pairs of hormones. Example: Insulin causes the level of blood sugar glucose to drop when it has risen. Glucagon causes it to rise when it has fallen. Hormone secretion is increased or decreased by the same substance whose level is decreased or increased by the hormone. So less blood returns to the heart - decreased venous return - new input signal - resulting in decreased cardiac output new output signal.
So you see that in both cases the output signal is in the same direction as the input signal. Serious bleeding - This cardiac output mechanism in section c, working by itself, can result in shock after serious injury. If you sustain a major injury and are seriously bleeding, with each beat of the heart, less blood returns to the heart input signal because some of the blood actually leaves your body. The heart responds to reduced return of blood by pumping out less blood the output signal.
This leads to even less blood returning to the heart new input signal and the result is even less blood pumped out of the heart new output signal.
Eventually so little blood leaves the heart that it is not enough to maintain consciousness and the body lapses into a coma. Endocrine glands in the head and their secretions 1. Location - this pea-sized gland is nestled in the sella turcica of the sphenoid bone and is connected to the hypothalamus by a stalk called the infundibulum.
The pituitary consists of 2 parts: the adenohypophysis anterior pituitary , and the neurohypophysis posterior pituitary. The infundibulum conducts a portal vein from the hypothalamus to the adenohypophysis which permits hypothalamic influence on the adenohypophysis through hypothalamic hormones see below released into the blood. The infundibulum conducts axons from the hypothalamus to the neurohypophysis which permits hormones made in the hypothalamus to travel down the axons axon flow to the neurohypophysis to be stored there and secreted when required see below.
Function 1 There are 6 adenohypophyseal hormones hormones from the anterior pituitary : a GH growth hormone, somatotropin - directly stimulates mitosis and protein synthesis in most body cells, especially bone, cartilage and muscle leading to growth in height in children, and thickening of bone and hypertrophy of skeletal muscles in exercising adults.
It also stimulates fat breakdown from adipose tissue. GH also stimulates the liver to release somatomedins that do the same thing as GH only with a longer half-life. The major stimulus for the release of GH into the blood is vigorous exercise.
Since baseline levels of the hormone drop with age, to prevent muscle atrophy and fat accumulation with age, one must exercise more. The typical American high carbohydrate fast-food diet suppresses GH secretion. This stimulates fat accumulation and muscular atrophy - the typical flabby-muscled, overweight, middle-aged adult on a diet, only between meals.
Higher protein diets supply amino acids which act as a stimulus for GH secretion particularly the amino acid arginine. In general, the glucocorticoids help the body resist and overcome stress. Stimulates the secretion of testosterone from the testes. The hypothalamus sends this signal when the body is dehydrated. The action of ADH is to stimulate water retention from fluid flowing in the kidneys. The body is trying to conserve its water content.
The result is urine with minimal amounts of water - a concentrated urine often deeply colored and with an odor. Hypothalamus a. Location - the hypothalamus, part of the diencephalon, extends from the optic chiasma to the mammillary bodies and forms part of the walls of the 3rd ventricle. It is connected to the pituitary by the infundibulum.
Function - The hypothalamus sends 7 hormones by blood to the anterior pituitary, and 2 hormones by axonal flow to the posterior pituitary. It is clear that the CNS regulates many of the endocrine secretions, working through the hypothalamus. These hormones reach the anterior pituitary through the hypothalmic-hypophyseal portal system through the blood. These hormones travel to the posterior pituitary through axon flow and are released from the pituitary upon nerve signal from neurons of the hypothalamus the same neurons that send the hormones down their axons.
Location - the pineal is easily observed in the sheep's brain by gently depressing the brainstem and cerebellum and observing the region inferior to the occipital lobes: the pineal is seen as a medial, pea-sized bulge just superior to the corpora quadrigemina. In humans, the pineal, once proposed to be the residence of the human soul, is just below the posterior portion of the corpus callosum, on the roof of the third ventricle, and not easily seen.
Function - The gland produces serotonin during the day and melatonin during the night. It is thought to be related to human biorhythms such as helping to determine the onset of puberty and also helping to regulate human cycles of sleep and wakefulness. It is largest in children and becomes smaller and more fibrous or even calcified in adults. Definitive, detailed knowledge of the gland's actions is still lacking.
Endocrine glands in the neck and their secretions 1. Location - this largest of endocrine glands consists of two large lobes connected by an isthmus and is located just above the soft spot in your neck just anterior to the suprasternal notch. It is wrapped around the anterior and lateral portions of the trachea. Function - The thyroid's secretion of hormones is regulated by TSH from the pituitary gland.
Since the pituitary secretion of TSH is regulated by TRH from the hypothalamus, physiologists speak of a hypothalamic-pituitary-thyroid axis. Three major hormones are secreted by this gland: T 3 , T 4 , and calcitonin. These hormones increase the production of adrenal gland and pituitary hormones, raise blood pressure, respiratory rate and body heat production, stimulate bone teeth and nail growth in adults and during fetal development, and promotes fat breakdown for energy.
As T 4 is broken down, TSH secretion increases. This is a negative feedback mechanism. During pregnancy and lactation, the reduction of blood calcium as it passes from the maternal blood into the fetus or the milk, results in parathyroid gland activity see below that increases blood calcium levels.
Whenever calcium levels of the blood rise, calcitonin is then secreted by the thyroid: it activates osteoblasts resulting in an overall reduction of blood calcium levels as some calcium is deposited in the woman's bones as well as entering the milk and the fetus.
This hormone is also important in controlling the blood calcium concentration of infants and children whose blood calcium levels are generally higher than those of adults due to the bone growth and remodeling.
Calcitonin has been used therapeutically to help individuals with osteoporosis. Parathyroid glands a. Location - Although their number and location can vary, most people have 4 yellowish, rice-grain sized parathyroid glands located on the posterior surface of the thyroid gland.
Function - They secrete the hormone, parathormone , when the blood calcium level drops. Parathormone has the following effects, all of which attempt to raise the blood calcium level back to the normal range a negative feedback mechanism : 1 Stimulates osteoclast activity which releases calcium into the blood from osseus tissue. Calcitriol then increases calcium, phosphate and magnesium absorption from digested food in the intestine.
Endocrine glands in the thoracic cavity and their secretions 1. Location - The thymus is easily observed in the mediastinum of a pig or cat as a large cap of tissue covering the anterior surface of the heart cranial portion of the heart.
In humans the thymus is large in infants, gets larger during childhood and covers the superior and anterior portion of the heart. It is much smaller in adults and is located superior to the heart and medial to the upper superior lung lobes. It becomes very small and fibrous in old age.
If the receptors are damaged, the hormone will not work. Type II diabetes results when the insulin receptors no longer bind the hormone insulin. The symptoms are the same as if there were insufficient amount of the hormone released. Where phosphorylation is involved, the phosphates are removed by specific phosphotases. Whereas hydrophilic signaling molecules travel freely through the bloodstream, but require membrane proteins to influence their target cells.
Hydrophobic signaling molecules can pass through membrane, but require water soluble carrier proteins to travel through the blood stream. There are a number of different hydrophobic signaling molecules: the include the steroid hormones and the retinoids.
Steroids are derived form cholesterol, while retinoids are derived from vitamin A. Both steroids and retinoids regulate gene expression in a similar manner. The pass through the plasma membrane, and interact with intracellular receptor proteins. They are allosteric effectors of these receptors. The activated receptor complex binds to specific DNA sequences and acts to regulate the transcription of specific genes. NO signaling. Another type of hydrophobic signaling pathway is that characterized by the nitrogen oxide or NO pathway.
NO is a gas and can diffuse through the plasma membrane. NO is generated from arginine. The best understood site of NO signaling is in blood vessels.
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