Anatomy and Physiology: The Structure of the Muscles and Muscle Cells
The Structure of the Muscles and Muscle Cells
You might have noticed a running trend in this so far: The body is more complex than it seems. It is oh so easy to assume that problems within the body have a single cause, that structures and systems have a single function. With a little time, however, it becomes apparent that nothing about the body is that simple. Even a seemingly simple system such as the muscular system has multiple functions.
Amid this complexity there is, nonetheless, common ground. The same processes of cell transport used elsewhere, particularly in the nervous system, are used here as well. The muscular system is one of my favorites because a number of small details (integrating the concepts of active transport, facilitated diffusion, exocytosis, oxygen and energy usage, and so on, into a coherent whole) suddenly make sense when you apply them to the big picture, which is when and how muscles contract.
True to form, muscles weren't content doing one thing. Muscle tissue actually has four functions in the body:
- Motion. This refers not only to getting from place to place, but also to movements within the body, such as peristalsis (see The Digestive System, or even developing ?goose bymps?!
- Stability and posture. If you could cause all of your muscles to relax you would collapse in a heap! It is surprising how many muscles relate to posture.
- Controlling organ volume. Hollow organs, such as the gallbladder, need to release their contents from time to time, and they need muscles to do it.
- Generating body heat, or thermogenesis. (Break it down: thermo = heat and genesis = creating, so thermogenesis = creation of heat!) The breakdown of glucose produces 36 ATP (with O2 present), and releases 62 percent of the energy as heat! Most body heat is produced through the contraction of muscles.
All these glorious functions are related to four rather cool characteristics of muscle tissue: excitability, contractility, extensibility, and elasticity. Excitability, or (as I prefer to call it) irritability, refers to the ability of muscles to be stimulated by nerves. Contractility makes sense, because, after all, muscles contract. Extensibility is a little more difficult, because muscles don't lengthen on their own, but the fact that they can do so without causing damage is crucial. Last is the idea of elasticity, which means that, after any shortening or lengthening, the muscle will return to its original shape.
Excerpted from The Complete Idiot's Guide to Anatomy and Physiology 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.