Chemical Signals in Animals

2 communication systems in body. 1Nervous system, 2endocrine system. Endocrine system - glands and tissues that secrete hormones, chemical messengers produced by cells to act on other cells.

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Some hormones act on nearby cells (paracrine signals) others on same cells that made them (autocrine signal). Signals in endocrine system take longer to reach destination (carried by blood), longer lasting than nervous impulses.

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Endocrine glands secrete hormones directly into bloodstream. Exocrine secretions do not contain hormones; released through ducts into body compartment. Example - pancreas in digestion.

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Hormones classified into 2 groups: steroid hormones, peptide hormones. Both must bind to protein receptor on target cell, peptide hormones must bind to receptors on cell surface since they cannot cross membrane. Causes signal to be transmitted to inside of cell.

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Signal can activate 2nd messengers, which amplify signal and alter cell activities - called signal transduction cascade because process amplified as it continues down path.

Steroid hormones usually smaller and can pass through membrane. Most come from cholesterol and usually enter nucleus of cell, altering protein synthesis (transcription). Steroid hormones not stored, unlike protein hormones - need to be regulated in order to be secreted.

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Endocrine glands Regulation of production and secretion of hormones done through feedback loop. Some hormones regulate release of other hormones.

1Hypothalamus and pituitary – found in forebrain, located above pituitary gland. Pituitary has 2 parts: anterior pituitary, posterior pituitary. Posterior - vasopressin (antidiuretic hormone or ADH) - acts on kidney to conserve water; oxytocin - aids in childbirth.

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Hormones made in hypothalamus, pass through posterior pituitary - secreted. Anterior pituitary regulated by hypothalamus through portal blood circulation - carries blood directly from hypothalamus to pituitary.

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Hypothalamus stimulated - releasing factors into portal blood circulation carried to pituitary - cause release of hormone from anterior pituitary. Growth hormone promotes growth in body tissues.

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Thyroid stimulating hormone (TSH) stimulates thyroid to secrete thyroxin. Adrenocorticotrophic hormone (ACTH) stimulates adrenal cortex to secrete corticosteroids in response to stress.

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Prolactin - responsible for milk production. Follicle-stimulating hormone (FSH) promotes maturation of seminiferous tubules in males, ovaries in females. Luteinizing hormone (LH) - promotes testes to secrete testosterone in males; causes ovulation of egg in females.

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Endorphins act on central nervous system to block pain signaling (like opiates). 2Thyroid gland – thyroxine accelerates metabolism. Person deficient can develop goiter, lethargy, obesity. Hyperthyroidism causes profuse sweating, weight loss, increased BMR.

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Thyroxine secretion stimulated by hypothalamus in response to environment (like cold), acts on thyroid gland. Thyroid gland also produces calcitonin - regulates calcium concentration in blood.

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Calcitonin increases stimulates bone formation, decreases bone destruction. Calcitonin opposed by parathyroid hormone. 3Parathyroid glands – located on backside of thyroid gland, secretes parathyroid hormone - regulates calcium, phosphate balance between blood, other tissue.

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Increased parathyroid hormone increase bone resorption, elevates plasma calcium. Decreased calcium in blood causes secretion of parathyroid hormone which increases activity of osteoclasts (bone breaking cells) - remodels bones to release calcium.

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4Pancreas – performs both exocrine, endocrine functions. Endocrine function occurs in islets of Langerhans which contain alpha and beta cells that secrete glucagon and insulin. Insulin stimulates muscles and other cells to release glucose from blood.

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Causes muscles and liver to convert glucose to glycogen (storage form of glucose). Glucagon responds to low levels of blood glucose - stimulates breakdown of glycogen to glucose. Diabetics cannot control levels of insulin.

5Adrenal glands – found on top of kidneys, consist of adrenal cortex on exterior + adrenal medulla on inside of gland. Medulla responsible for epinephrine and norepinephrine in response to stress.

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Epinephrine responsible for adrenaline release as well as causing blood to shunt away from skin, digestive organs, kidneys; increases blood flow to heart, brain, skeletal muscle. Also increases metabolic activity.

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Cortex responsible for secretion of corticosteroids - regulated by nervous system in response to stress, 3 types. AGlucocorticoids help to raise blood glucose levels. High doses help in inflammation response.

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BMineral corticoids - promote reabsorption of Na+, excretion of K by kidneys. CSex hormones – androgens responsible in part for female sex drive.

6Ovaries, Testes - testes produce testosterone in males, estrogen and progesterone in females. 7th week of development, information on Y chromosome (if present) starts to produce testosterone. Absent - female hormones are produced.

ATestes – releases androgens (like testosterone) responsible for production of sperm and secondary male sex characteristics. BOvaries – secrete estrogen and progesterone.

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1Estrogen responsible for stimulating lining of uterus to grow and secondary sex characteristics of females. 2Progesterone responsible for promoting lining of uterus to grow.

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7Pineal gland - small mass of tissue near center mammalian brain. Secretes melatonin - aids in reproduction and cycle of light and dark.

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