29.1 Hormones A hormone is a chemical signal that is produced in one part of the body and that acts at a distant site Three advantages to using chemical signals 1. Can spread to all tissues via the blood 2. Can persist much longer than electric signals 3. Many can act as hormones Different hormones can target different tissues

Hormones are produced by endocrine glands These glands do not have ducts Hormones are secreted directly into the blood Hormones are under the control of the neuroendocrine system The hypothalamus is the main switchboard Issues commands to the nearby pituitary gland Pituitary sends chemical signals to the various hormone-producing glands in body

Fig. 29.1 Major glands of the human endocrine system

The path of communication of a hormone signal 1. Issuing the command Hormones are effective because they are recognized by specific receptors These are on or in target cells The path of communication of a hormone signal 1. Issuing the command By the hypothalamus 2. Transporting the signal By the blood 3. Hitting the target Binding to the receptor 4. Having an effect A change in cell activity

Fig. 29.2 How hormonal communication works

29.2 How Hormones Target Cells Steroid hormones Manufactured from cholesterol Lipid-soluble and thus can across the lipid bilayer of the cell membrane Bind to receptors within the nucleus usually The hormone-receptor complex binds to DNA causing changes in gene activity Anabolic steroids Synthetic compounds resembling testosterone Cause muscle cells to produce more protein

Fig. 29.3 How steroid hormones work

Short peptides or full-sized proteins Fig. 29.4 Peptide hormones Short peptides or full-sized proteins Bind to receptors embedded in the cell membrane Induce a change in cytoplasmic end of the receptor protein Triggers events within the cytoplasm

Trigger a cascade of enzymic activation within cell The change in cellular activity is facilitated by second messengers such as cAMP Trigger a cascade of enzymic activation within cell Fig. 29.5

29.3 The Hypothalamus and the Pituitary The pituitary gland is located in a bony recess in the brain below the hypothalamus It produces nine major hormones It is actually two glands Posterior lobe Anterior lobe

The Posterior Pituitary Stores and releases two short peptide hormones Vasopressin (antidiuretic hormone, ADH) Regulates water conservation by the kidney Oxytocin Initiates uterine contractions during childbirth Causes milk letdown Both synthesized within hypothalamus neurons Transported down nerve axons to the posterior lobe

Fig. 29.6 Posterior pituitary

The Anterior Pituitary A complete gland that produces seven major hormones 1. Thyroid-stimulating hormone (TSH) 2. Luteinizing hormone (LH) 3. Follicle-stimulating hormone (FSH) 4. Adrenocorticotropic hormone (ACTH) 5. Growth hormone (GH) 6. Prolactin 7. Melanocyte-stimulating hormone (MSH)

Fig. 29.7 The role of the pituitary

The hypothalamus controls the anterior pituitary Secretes releasing and inhibiting hormones into capillaries Hypothalamo-hypophyseal portal system carries the hormones to the pituitary Fig. 29.8

Hypothalamus and anterior pituitary are themselves controlled by negative feedback inhibition Regulated by the hormones whose secretion they stimulate! Fig. 29.9 Thyrotropin-releasing hormone Gonadotropin-releasing hormone Corticotropin-releasing hormone

29.4 The Pancreas The pancreas gland is located behind the stomach Secretes a number of digestive enzymes into the small intestine through a narrow tube Islets of Langerhans govern blood glucose levels Insulin Secreted by beta cells Storage hormone that promotes accumulation of glycogen in liver and triglycerides in fat cells Glucagon Secreted by alpha cells Cause release of stored glucose and fat breakdown

Fig. 29.10

Diabetes mellitus Affected individuals cannot take up glucose from the blood Type I ~ 5-10% of diabetics Autoimmune disorder in which immune systems attacks the Islets of Langerhans Develops before age 20 (juvenile-onset diabetes) Treated by daily injections of insulin Type II ~ 90-95% of diabetics Caused by abnormally low number of insulin receptors Develops after age 40 (adult-onset diabetes) Typically in obese individuals Treated by diet and exercise

29.5 The Thyroid, Parathyroid and Adrenal Glands The thyroid gland lies just below the Adam’s apple in front of the neck Its two most important hormones are Thyroxine Increases metabolic rate and promotes growth Contains iodine Lack of iodine in diet causes goiters Calcitonin Stimulates calcium deposition in the bone

Fig. 29.11 The thyroid gland secretes thyroxine Goiter

The parathyroid glands are four small glands attached to the thyroid Produce parathyroid hormone (PTH) One of only two hormones essential for survival Acts as a fail-safe mechanism ensuring that calcium levels never fall too low Calcium ions are key to muscle contractions When levels are low, PTH stimulates osteoclast cells to break down bone matrix This raises calcium levels in the blood PTH also acts on kidneys to reabsorb calcium ions from the urine

Fig. 29.12 Maintenance of proper calcium levels in the blood

The adrenal glands are two glands, one located just above each kidney Each composed of two parts Medulla – Inner core Adrenaline and norepinephrine Released in times of stress to stimulate rapid deployment of body fuel Cortex – Outer shell Cortisol – Stimulates carbohydrate metabolism and reduces inflammation Aldosterone – Stimulates the kidney to uptake sodium and other ions from the urine This is the other hormone that is absolutely essential for survival

29.6 A Host of Other Hormones Steroid sex hormones Estrogen, progesterone and testosterone Produced by gonads to regulate sexual development Atrial natriuretic hormone Secreted by the right atrium of the heart Stimulate kidneys to excrete salts and water in the urine Erythropoietin Secreted by the kidney Stimulates the bone marrow to produce RBC

Molting and Metamorphosis in Insects Molting is triggered by the molting hormone, also called ecdysone The production of ecdysone is itself controlled by the brain hormone These two hormones are balanced by a third hormone, the juvenile hormone If present in high levels, it inhibits the formation of the pupa and adult forms

Fig. 29.13 The hormonal control of metamorphosis in the silkworm moth, Bombyx mori