Coordinating Mechanisms. In these last three lectures we will discuss the coordinating mechanisms used by organisms to integrate the stimuli received from their internal and external environment via receptor cells and organs also called sensory cells and sensory organs. All responses to external stimuli are carried out by effector cells or organs which are either muscles or glands. These effector cells and organs respond to the instructions of the coordinating mechanisms. The chain of events is: environmental stimulus receptor cells coordinating mechanism effector cells. An example would be: food (stimulus)eye (receptor cells)CNS (central nervous system, the coordinating mechanism)salivary glands (effector cells).

There are two distinct coordinating mechanisms found in all animals. The most ancient of the coordinating mechanisms depends on the release and circulation of chemical messengers, the hormones. In fact, chemical communication is found the organisms of all kingdoms including Monera. Hormones which are released into the air instead of a fluid are called pheromones. Insects use pheromones in attracting mates. Even human produce airborne hormones. An example is the mechanism which causes human females who live together to have menstrual cycles in synchrony.

A second mechanism, the nervous system, is far superior in speed and selectivity. It depends on a specialized system of nerve cells, or neurons, which receive and give instructions by means of electrical impulses directed over specific pathways. As we will soon see, even the receptor cells and neurons rely on chemical messengers to stimulate the next cell in the chain.

Hormones and the Endocrine System. Hormones are produced by endocrine glands. In humans and many other animals, hormones are either peptide hormones (modified amino acids, peptides, small proteins) or steroid and steroid-like hormones. The endocrine glands are ductless glands and secrete hormones directly into the blood vessels nearby. The hormones are then carried throughout the body and act upon target cells and tissues. The target cells have protein receptors which recognize the hormone and bind it. This interaction of the hormone and its specific receptor occurs either in the cell membrane (amino acid, peptide, or small protein hormone) or in the cell interior (steroid hormones). In either case, this combination begins a chain of cellular reactions which result in the release of a chemical by the receptor cells which stimulate the next cell, a sensory neuron.

In humans there is an X linked disorder caused by the absence of the androgen (male hormone) receptor. Fetuses with the androgen insensitivity syndrome, begin life as 46, XY, form normal testes and produce testosterone (and other androgens). However, since they have a mutation in their androgen receptor gene, they do not have the intracellular receptor for these steroid sex hormones, their target cells will not be responsive. Thus, they will fail to develop the normal internal and external "male plumbing." The default sex is female in this situation.

As a small digression we will describe another kind of gland, the exocrine gland. These glands are effector organs and are stimulated by neurons or by hormones. Exocrine glands, unlike endocrine glands, are ducted glands and carry their product to the site of action via a tube called a duct. They do not release their products into the circulatory system. Examples of exocrine glands are salivary glands, glands that produce digestive enzymes, sweat glands, and that part of the gonads that produce gametes.

In humans we recognize several endocrine glands and cells. The pituitary gland is located at the base of the brain and is often called the master gland because it controls the secretions of other endocrine glands as well as other body tissues. It has a posterior lobe called the hypothalamus which contains some interesting neurosecretory cells. The other endocrine glands of vertebrates include the pineal, thyroid, parathyroid (which sit atop the thyroid), the thymus, pancreatic islets, adrenal gland, gonads, and the endocrine cells of the stomach, small intestine, liver and heart.

We learned in an earlier lecture that the function of the thyroid is one of the newborn screens done in most states. I have personally seen a young boy who was hypothyroid and was removed from the hospital before he was given the newborn screening tests. He was taken to Mexico by his parents and received no tests there nor was he diagnosed until he was older. By then he had irreversible brain damage. The condition is referred to as "cretinism." Hypo or hyperthyroidism in adults can be treated and does not damage the brain since the brain has completed its development. Iodine is needed to form the active form of thyroxin and most salt sold in the US is iodized for this reason.

The pancreas and the gonads (both ovaries and testes) are examples of organs with have both endocrine and exocrine functions. The endocrine function of the pancreas is the production of the hormones, insulin, glucagon and somatostatin which are picked up by surrounding capillaries and carried throughout the body. The exocrine function of the pancreas is the production of digestive enzymes which travel down a duct or tube from the pancreas into the small intestine. (It would not do for the digestive enzymes to be released into the bloodstream!!) The endocrine function of the gonads is the production of the sex hormones (estrogens and androgens) which are released into the nearby capillaries and circulated throughout the body, affecting only their target tissues. The exocrine function of the gonads is the production of the gametes which travel via special tubes or ducts to the exterior (sperm) or interior (eggs) site of use.

In vertebrates, the endocrine glands are themselves usually under the control of the nervous system. An exception is that the acid of the stomach directly stimulates the endocrine cells of the stomach which produce secretin.

Receptor cells and organs. Receptor cells and organs are also known as sensory cells and organs. Their function is to receive information from the external and internal environment and to convey that information to the nervous system. One way to classify sensory receptors is to consider them as biological transducers......meaning they take one form of energy and convert it to another. One can divide them according to their sensitivity to various forms of energy. Mechanoreceptors respond to pressure or tension. These include touch and hearing. The ear has the ability to process sound waves and to detect acceleration and orientation of the body. Chemoreceptors respond to chemical stimuli such as the molecules which give taste and smell. The chemicals are dissolved in fluids and for aquatic animals, taste and smell are the same. The thermoreceptors respond to temperature changes. The photoreceptors respond to light and include our eyes which are image forming. Pain receptors, unlike the other receptors, is often a "raw" nerve ending and does not utilize a specialized receptor cell or organ.

Whether a receptor is a part of a neuron or in close contact with one, all convert the energy they receive into the electrical energy of the nerve impulse by depolarizing the connecting nerve cell's membrane. They do this by producing a chemical transmitter as a consequence of receiving the external stimulus. This chemical transmitter then depolarizes the next cell in the chain which is a sensory neuron belonging to the peripheral nervous system. We will discuss this in the next lecture.

Receptor cells in any of these categories, show much variability in structural appearance, location and associations with other cells. They can be single cells, a few cells or an entire organ. As an example, mechanoreceptors detect blood pressure, sound, position or muscle stretch. And these cells may be in special organs or dispersed singly.

Receptors sensitive to light energy are a common in many organisms. Even Cnidaria have photoreceptor cells. Many organisms can detect light and may either go toward it or away depending on their life style. A photosynthetic protist or moneran will go toward light while a flatworm or other organism such as a "mealy bug" will move away since light usually means heat and drying out. But image forming eyes are found only among vertebrates, mollusks, and arthropods. The cephalopod (e.g., squid, octopuses) eye is an example of convergent evolution. They also have an image forming eye rather like ours but which is derived differently embryologically. The actual photoreceptor cells of our eye are in the retina at the back of the eye. The rest of the eye is for image formation.
 

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