Chapter 46: Chemical Signals in Animals

46.1 Cell-to-Cell Signaling: An Overview

• Autocrine signals affect the same cell that releases them.
  • Perhaps the best-studied autocrine signals are cytokines.
  • ==Most cytokines amplify the response of a cell to a stimulus.==
• Paracrine signals diffuse locally and act on target cells near the source cell.
• Like some cytokines, the cell-cell signals named insulin and glucagon cross categories.
• These molecules are produced by two distinct populations of cells within the regions of the pancreas called the islets of Langerhans.
• The cells that produce endocrine signals may be organized into discrete organs called glands or may be interspersed among the cells of other organs-as are the islets of Langerhans in the pancreas.
• Even though they are released from neurons, neuroendocrine (“ nerve-inside-separated”) signals share a key attribute with endocrine signals.
  • They act on distant cells. For this reason, they are called neurohormones.
• Antidiuretic hormone (ADH; also called vasopressin) is a particularly well-studied neuroendocrine signal.
  • ADH is produced by neurons that have their cell bodies in a brain region called the hypothalamus.
• All three types of signaling pathways- endocrine, neuroendocrine, and neuroendocrine-to-endocrine-are regulated by negative feedback.
  • ==In negative feedback, the product of a process inhibits its production.==
• Positive feedback, in contrast, occurs when the product of a process stimulates its production, resulting in greater and greater production of that product.
• Organs that secrete a hormone into the bloodstream are called endocrine glands.
• Exocrine glands, in contrast to endocrine glands, deliver their secretions through outlets called ducts into a space other than the circulatory system.

46.2 How Do Hormones Act on Target Cells?

• Chief among these are that they can have strong effects even at very low concentrations, and they exert these effects by binding to proteins called hormone receptors in target tissues.
• The hormone epinephrine, also known as adrenaline, is synthesized in the medulla of the adrenal glands from the amino acid tyrosine.
• The hormone cortisol is synthesized in the cortex of the adrenal glands from the steroid cholesterol.
• Estrogens are steroid hormones at direct the development of female secondary sex characteristics in many animal species.
• In humans and other mammals, the most important estrogen is the molecule estradiol.
• Follow-up work confirmed at steroid hormone-receptor complexes binds to specific sites in DNA called hormone-response elements.
• By purifying components of the liver cell extracts and testing them one by one, researchers eventually found the key ingredient in the activation of phosphorylase: a molecule called cyclic adenosine monophosphate, or cyclic AMP ( cAMP).

46.3 What Do Hormones Do?

• The process of changing from an immature, aquatic larva to a sexually mature, terrestrial frog, toad, or salamander is an example of metamorphosis.
• In response to a signal from the brain， the pituitary gland secretes thyroid-stimulating hormone (TSH).
• If juvenile hormone (JH) is present at a high concentration in the larva, surges of the hormone ecdysone induce the growth of the larva via molting.
• The increase in day length or increasing photoperiod-during spring is particularly important in stimulating the release of sex hormones in seasonally reproducing mammals, lizards, and birds.
• In mammals, photoreceptors in the retinas of the eyes send signals to the pineal gland via a pathway leading through the brain and spinal cord.
  • The pineal gland secretes the hormone melatonin, which relays photoperiodic information to the hypothalamus, a brain region at initiates a series of signals directing the production of sex hormones.
  • ==The pineal gland also regulates sleep-wake cycles.==
• The short-term reaction, called the fight-or-flight response, is triggered by the sympathetic nervous system.
• Insulin normally stimulates adipocytes and resting muscle cells to remove glucose from the bloodstream.
• When the sodium ion concentration in body fluids is low, aldosterone is released from the adrenal cortex.
  • ==Because aldosterone increases the reabsorption of sodium ions in the distal tubules of the kidney, it plays a key role in homeostasis with respect to electrolyte concentrations and the overall volume of body fluids.==
  • Adrenal hormones with this effect are called mineralocorticoids.
• Erythropoietin (EPO) is a crucial component in the homeostatic system that maintains the blood oxygen level.

46.4 How Is The Production of Hormones Regulated?

• This molecule soon came to be called adrenocorticotropic hormone, or ACTH.
• After years of effort, a different team of researchers succeeded in purifying a peptide of just 41 amino acids long-called corticotropin-releasing hormone (CRH).
• The hypothalamic-pituitary axis forms two anatomically distinct systems because the pituitary gland has two segments： the anterior pituitary and the posterior pituitary.
• . Both types of hypothalamic neurons synthesize and release neurohormones and are therefore called neurosecretory cells.
• Oxytocin stimulates uterine contractions during birthing and subsequent milk release in female mammals.
• In addition, some sympathetic nerves release the related molecule norepinephrine directly onto target cells.
• Epinephrine and norepinephrine, which differ from one another only by the presence of an additional methyl group on epinephrine, are members of the family of molecules called catecholamines.