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# Lecture 24: The Endocrine System ## The Endocrine System ### What is it? - A collection of glands that secrete hormones into the blood which travel to target cells. - Endocrine signals are transmitted more slowly than neuronal signals but have longer lasting effects. - The hypothalamus bridges...

# Lecture 24: The Endocrine System ## The Endocrine System ### What is it? - A collection of glands that secrete hormones into the blood which travel to target cells. - Endocrine signals are transmitted more slowly than neuronal signals but have longer lasting effects. - The hypothalamus bridges the endocrine and nervous systems. ### Classes of Hormones 1. **Peptides and proteins:** e.g. insulin 2. **Amino acid derivatives:** e.g. epinephrine 3. **Steroids:** e.g. testosterone ### Mechanisms of Hormone Action 1. **Receptor in plasma membrane** - Peptide and amino acid hormones - Signal transduction - Changes in protein activity, gene expression 2. **Receptor in cytoplasm or nucleus** - Steroid hormones - Alter gene expression ### Major Endocrine Glands 1. **Hypothalamus**: Brain region that controls the endocrine system, connected to the pituitary. 2. **Pituitary gland**: At the base of the hypothalamus, has anterior and posterior parts. - **Posterior**: Extension of the hypothalamus. - **Anterior**: Endocrine gland controlled by the hypothalamus. 3. **Thyroid gland**: In the neck, produces hormones that regulate metabolism. 4. **Parathyroid glands**: In the neck, regulate calcium levels in the blood. 5. **Adrenal glands**: On top of the kidneys, involved in stress response and osmoregulation. 6. **Pancreas**: In the abdomen, regulates blood sugar levels. 7. **Pineal gland**: In the brain, produces melatonin which regulates sleep. 8. **Ovaries (female)** and **Testes (male)**: Produce sex hormones. ### The Hypothalamus and Pituitary - The hypothalamus is physically connected to the pituitary gland. - **Posterior pituitary**: Hormones are produced in the hypothalamus and stored in the posterior pituitary until they are released into the blood. - **Antidiuretic hormone (ADH)**: Promotes water retention in the kidneys. - **Oxytocin**: Stimulates uterine contractions during birth and milk ejection. - **Anterior pituitary**: The hypothalamus releases hormones that control the release of hormones from the anterior pituitary. - **Tropic hormones**: Hormones that regulate other endocrine glands. ### Examples of Tropic Hormones 1. **Thyroid-stimulating hormone (TSH)**: Stimulates the thyroid gland to release thyroid hormones. 2. **Adrenocorticotropic hormone (ACTH)**: Stimulates the adrenal cortex to release hormones. 3. **Follicle-stimulating hormone (FSH)** and **Luteinizing hormone (LH)**: Stimulate the ovaries and testes to produce sex hormones. ### Non-Tropic Hormones - Hormones that directly stimulate target cells. 1. **Prolactin**: Stimulates milk production. 2. **Melanocyte-stimulating hormone (MSH)**: Affects skin pigmentation. 3. **Growth hormone (GH)**: Stimulates growth and metabolism. ## The Thyroid Gland ### Thyroid Hormones - **Triiodothyronine ($T_3$)** and **Thyroxine ($T_4$)**: Stimulate metabolism. - Lack of iodine leads to hypothyroidism, which can cause a goiter. - Hyperthyroidism can cause weight loss, high blood pressure, and irritability. ### Regulation of Thyroid Hormone Levels - Hypothalamus releases **Thyrotropin-releasing hormone (TRH)**, which stimulates the anterior pituitary to release TSH. TSH then stimulates the thyroid gland to release $T_3$ and $T_4$. $T_3$ and $T_4$ inhibit the release of TRH and TSH. ## The Adrenal Glands ### Adrenal Medulla - Secretes **epinephrine** (adrenaline) and **norepinephrine** (noradrenaline). - These hormones mediate the "fight or flight" response. - Increase heart rate, blood pressure, and blood flow to muscles. ### Adrenal Cortex - Secretes corticosteroids. - **Glucocorticoids** (e.g. cortisol): Increase blood sugar levels. - **Mineralocorticoids** (e.g. aldosterone): Promote sodium and water retention in the kidneys. ## The Pancreas ### Endocrine Function - **Islets of Langerhans**: Clusters of endocrine cells in the pancreas. - **Alpha cells**: Produce **glucagon**, which increases blood sugar levels. - **Beta cells**: Produce **insulin**, which decreases blood sugar levels. ### Regulation of Blood Sugar - After a meal, blood sugar levels rise, which stimulates the release of insulin. Insulin promotes the uptake of glucose by cells, which lowers blood sugar levels. - When blood sugar levels are low, glucagon is released. Glucagon stimulates the breakdown of glycogen in the liver, which raises blood sugar levels. ### Diabetes Mellitus - Caused by a deficiency of insulin or a decreased response to insulin in target tissues. - Type 1 diabetes: Autoimmune destruction of beta cells. - Type 2 diabetes: Reduced responsiveness to insulin. ## Sex Hormones ### Testes - **Androgens** (e.g. testosterone): Promote development of male secondary sex characteristics and spermatogenesis. ### Ovaries - **Estrogens** (e.g. estradiol): Promote development of female secondary sex characteristics and regulate the menstrual cycle. - **Progesterone**: Prepares the uterus for implantation and maintains pregnancy. ### Pineal Gland - Secretes **melatonin**, which regulates sleep cycles. - Melatonin is released at night and inhibited during the day. ## Hormone Disruptors - Chemicals that interfere with the endocrine system. - Can mimic or block the action of hormones. - Examples: pesticides, plastics ## The diagram shows the human endocrine system Parts include: 1. **Pineal Gland** 2. **Pituitary Gland** 3. **Thyroid Gland** 4. **Parathyroid Glands** 5. **Adrenal Glands** 6. **Pancreas** 7. **Ovaries** 8. **Testes** Diagrams are displayed on both male and female identifying figures.