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# Lecture 25: The Endocrine System ## Types of signaling ### 1. Neural * Electrical signal travels along a neuron and releases a chemical signal. * Fast and specific. ### 2. Endocrine * Chemical signal (hormone) secreted into body fluids (usually blood). * Slower and general. ### 3. Ne...

# Lecture 25: The Endocrine System ## Types of signaling ### 1. Neural * Electrical signal travels along a neuron and releases a chemical signal. * Fast and specific. ### 2. Endocrine * Chemical signal (hormone) secreted into body fluids (usually blood). * Slower and general. ### 3. Neuroendocrine * Combination of the 2. ## Hormones * Hormones mediate responses to environmental stimuli and regulate growth, development, and reproduction. * Hormones bind to target cells that have hormone receptors. * Hormone binding triggers signal transduction. ### Types of Hormones 1. **Peptide/Protein:** * Water-soluble * Cannot pass plasma membrane * Bind receptor on cell surface $\rightarrow$ signal transduction $\rightarrow$ cellular response * Example: Insulin 2. **Steroid:** * Lipid-soluble * Can pass plasma membrane * Bind receptor in cytoplasm or nucleus $\rightarrow$ change in gene expression * Example: Testosterone ## Control Pathways ### 1. Simple endocrine pathway * Stimulus $\rightarrow$ endocrine cell $\rightarrow$ hormone $\rightarrow$ blood vessel $\rightarrow$ target cells $\rightarrow$ response ### 2. Simple neuroendocrine pathway * Stimulus $\rightarrow$ sensory neuron $\rightarrow$ hypothalamus/other neuroendocrine cell $\rightarrow$ hormone $\rightarrow$ blood vessel $\rightarrow$ target cells $\rightarrow$ response ## Feedback regulation loops ### 1. Negative feedback * Response reduces initial stimulus * Example: Blood glucose ### 2. Positive feedback * Response reinforces stimulus, leading to an even greater response * Example: Childbirth ## The hypothalamus and pituitary * The hypothalamus integrates endocrine and nervous system signals * The pituitary is located at the base of the hypothalamus ## Posterior Pituitary * Extension of the hypothalamus * Neuroendocrine cells of the hypothalamus secrete hormones into the posterior pituitary where it is stored and released * Example: Antidiuretic hormone (ADH) ## Anterior Pituitary * Hypothalamus secretes releasing and inhibiting hormones into portal vessels connecting the hypothalamus and anterior pituitary * Each hypothalamic hormone regulates secretion of a particular anterior pituitary hormone ## Examples of Anterior Pituitary Hormones | Hormone | Target | Primary effects | | :----------------------- | :------------------------ | :----------------------------------------------------------------------------------------------------------------------------- | | Growth Hormone (GH) | Liver, bone and other | Stimulates growth through production of growth factors; regulates carbohydrate and lipid metabolism | | Prolactin (PRL) | Mammary glands | Stimulates milk production and secretion | | Follicle stimulating | Testes or ovaries | In testes, stimulates sustentacular cells; in ovaries, stimulates growth of follicles | | hormone (FSH) | | | | Luteinizing hormone (LH) | Testes or ovaries | In testes, stimulates Leydig cells to produce testosterone; in ovaries, stimulates ovulation and formation of the corpus luteum | | Thyroid-stimulating | Thyroid | Stimulate thyroid to produce thyroid hormones | | hormone (TSH) | | | | Adrenocorticotropic | Adrenal cortex | Stimulates adrenal cortex to produce glucocorticoids | | hormone (ACTH) | | | | Melanocyte-stimulating | Melanocytes in skin | Stimulates melanocytes | | hormone (MSH) | | | | Endorphin | Brain pain receptors | Pain relieving effect | ## Thyroid Hormone * **T3 and T4** * Stimulate metabolism and influence development and maturation * Hyperthyroidism: high body temperature, weight loss, irritability * Hypothyroidism: weight gain, lethargy, cold intolerance * **Calcitonin** * Lowers blood calcium levels * Inhibits removal of calcium from bone, stimulates calcium uptake by bones, and promotes excretion of calcium by kidneys ## Parathyroid Hormone (PTH) * Increases blood calcium levels * Releases calcium from bone and stimulates calcium uptake in kidneys * Indirectly increases calcium uptake in the intestines ## Adrenal Glands ### 1. Adrenal medulla * Secretes epinephrine and norepinephrine * “Fight or flight” response * Increase heart rate, blood pressure and breathing rate, increase metabolic rate, and change blood flow patterns ### 2. Adrenal cortex * Secrete corticosteroids (steroid hormones) * **Glucocorticoids** (e.g. cortisol) * Influence glucose metabolism and the immune system * Inhibit the immune system * **Mineralocorticoids** (e.g. aldosterone) * Affect salt and water balance ## Gonadal Sex Hormones * **Androgens** (e.g. testosterone) * Stimulate development and maintenance of male reproductive system * Responsible for male secondary sex characteristics * **Estrogens** (e.g. estradiol) * Stimulate development and maintenance of female reproductive system * Responsible for female secondary sex characteristics * **Progestins** (e.g. progesterone) * Involved in preparing and maintaining the uterus