Chemical Signals in Animals


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Coordination between organs in animals requires some form of communication. Two overlapping systems of internal communication in animals are the nervous system and the endocrine system.

Because the nervous and endocrine systems are structurally, chemically, and funtionally related, they often are integrated into regulatory processes necessary for homeostasis.

Each system has its particular strengths.

The nervous system has evolved for responding to rapid changes in the environment, and for precise control over specific tissues.

Slow changes involving growth and development do not require speedy responses. The endocrine system is made-to-order for slow steady communication relying on simple diffusion or the bulk flow of the circulatory system to deliver its chemical messengers.

This lecture will present an overview of the endocrine system describing how it coordinates homeostasis and regulates growth, development, and reproduction through over 50 different chemical signals called hormones.


General Classes of Chemical Signals

Hormone molecules travel in blood stream to reach the target cells. One of two forms of long distance signaling.

Specialized cells of the nervous system also synthesize and secrete hormones. A second form of long distance signaling

Synaptic signaling is a form of local chemical signaling. Here a nerve cell releases neurotransmitter molecules into a narrow space -- a synapse -- between it and its single target cell.

Paracrine signaling involves a secreting cell which affects nearby target cells by discharging local regulators into the interstitial fluid. White blood cells often signal other white cells this way.

Chart above adapted from Campbell Biology (4th edition).

I. Hormones

  • An animal hormone is a chemical messenger that is:
  • secreted into body fluids (usually the blood)
  • produced by specialized cells (either endocrine or nerve cells)
  • effective in minute amounts
  • detected only by "target" cells possessing special protein receptors embedded in their plasma membranes
  • often regulated by a second antagonistic hormone

     

    Endocrine cells are grouped together into secretory organs called endocrine glands. These glands lack ducts and are not to be confused with exocrine glands which generally secrete materials out of the body such as sweat, mucus, and digestive enzymes.

    Hormones come in 2 chemical varieties

    1. Steroid hormones:
      • are chemically derived from cholesterol
      • enter the cell and in combination with a receptor directly interact with the cell's DNA
    2. Amino acid derived hormones
      • may be modified amino acids, short peptide chains, or proteins
      • interact with membrane bound receptor molecules

    Each hormone has a specific shape which is only recognized by the target cell's receptors.

    To prevent runaway positive feedback most hormones have an antagonistic mate which can produce just the opposite effect. The balance offered by antagonistic twins helps to establish set-points required by homeostasis.


II. Pheromones

Pheromones are chemical signals that behave just like hormones except their target tissue is in another individual of the same species.

Pheromones can be classified according to their function including:

  1. mate attractants
  2. territorial markers
  3. alarm signals

These small volatile molecules are readily dispersed throughout the environment and can be detected by the target organism in parts per trillion or less.


III. Local Regulators

Any chemical messanger that affects target cells within the same tissue or very close to the point of secretion functions in local regulation.

Examples include:

  1. Neurotransmitters - from neuron to neuron, neuron to muscle, or neuron to other target cell. Synaptic signaling is the most direct form of chemcial communication.
  2. Paracrine signaling occurs when a cell secretes regulatory substances into the surrounding interstitial fluid, affecting only nearby target cells. Regulatory substanced used by paracrine signals include:
    • defense signals of the immune system
    • growth factors
    • prostaglandins


Hormonal Control via Signal Transduction Pathways

Nonsteroid hormones are first messengers which temporarily bond to a membrane-surface receptor of a target cell.

The hormone receptor complex results in the activation of a second messenger within the cytoplasm of the target cell. This step is called "signal transduction" because the second messenger is able to multiply the effect of the initial contact by many fold. Second messenger are generally small molecules like Ca2+ or cyclic AMP which can be released in great numbers from storage vacuoles.

Hundreds of second messenger molecules produce rapid changes in the target cells but may also promote activation of one or more genes which produce long-term changes to the cell.

The rapid effects of a second messenger results from the activation of enzymes already existing in the cell. Eventually the genes are called upon to produce additional enzymes or other gene products designed to impliment needed changes to the cell's metabolism or structure.


Antagonistic Hormone Pairs

Insulin and glucagon regulate blood glucose with a given set point.
Insulin is produced by cells in the pancreas when glucose levels rise above the set point. Insulin increases the used of glucose by muscle cells or it storage in the liver as glycogen which causes glucose concentrations to drop.

Glucagon is produced by another group of cells in the pancreas when glucose levels drop below the set point. Target cells in the liver are stimulated to break down the glycogen which reverses the drop in glucose concentration.

See Hormones of the Pancreas.

 

PTH and Calcitonin regulate the release and storage of calcium ions.

PTH is made by the Parathyroid gland when the concentration of Ca2+ ions drops below the set point. PTH instructs the kidneys to reabsorb calcuim, bone cells, osteoclasts, to release Ca2+ and epithelium of the intestine to increase its uptake.

Calcitonin production is increased by the thyroid if the level of Ca2+ rises above the set point. The effects of calcitonin are just the opposite of PTH.


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