Biomedical Engineering Reference
In-Depth Information
A motor unit consists of a motor neuron and the muscle fibers that it innervates. All
the muscle fibers in a single motor unit contract at the same time, whereas muscle fibers
in the same muscle but belonging to different motor units may contract at different times.
When a contracted muscle relaxes, it returns to its original (resting) length if another
contracting muscle moves it or if it is acted upon by gravity. During relaxation, ATP is
expended to move calcium back to the cisternae. The active sites that were needed for
cross-bridge formation become covered so actinandmyosincannolongerinteract.When
the cross-bridges disappear, the muscle returns to its resting length—that is, it relaxes.
The human body contains two different types of skeletal muscle fibers: fast and slow.
Fast fibers can contract in 10 ms or less following stimulation and make up most of the
skeletal muscle fibers in the body. They are large in diameter and contain densely packed
myofibrils, large glycogen reserves (used to produce ATP), and relatively few mitochon-
dria. These fibers produce powerful contractions that use up massive amounts of ATP
and fatigue (can no longer contract in spite of continued neural stimulation) rapidly.
Slow fibers take about three times as long to contract as fast fibers. They can continue
to contract for extended periods of time because they contain (1) a more extensive net-
work of capillaries, so they can receive more oxygen; (2) a special oxygen-binding mole-
cule called myoglobin; and (3) more mitochondria, which can produce more ATP than
fast fibers. Muscles contain different amounts of slow and fast fibers. Those that are domi-
nated by fast fibers (e.g., chicken breast muscles) appear white, while those that are domi-
nated by slow fibers (e.g., chicken legs) appear red. Most human muscles appear pink
because they contain a mixture of both. Genes determine the percentage of fast and slow
fibers in each muscle, but the ability of fast muscle fibers to resist fatigue can be increased
through athletic training.
3.5 HOMEOSTASIS
Organ systems work together to maintain a constant internal environment within the
body. Homeostasis is the process by which physical and chemical conditions within the inter-
nal environment of the body are maintained within tolerable ranges even when the external
environment changes. Body temperature, blood pressure, and breathing and heart rates are
some of the functions that are controlled by homeostatic mechanisms that involve several
organ systems working together.
Extracellular fluid—the fluid that surrounds and bathes the body's cells—plays an
important role in maintaining homeostasis. It circulates throughout the body and carries
materials to and from the cells. It also provides a mechanism for maintaining optimal
temperature and pressure levels, the proper balance between acids and bases, and concen-
trations of oxygen, carbon dioxide, water, nutrients, and many of the chemicals that are
found in the blood.
Three components—sensory receptors, integrators, and effectors—interact to maintain
homeostasis (Figure 3.39). Sensory receptors, which may be cells or cell parts, detect
stimuli—that is, changes to their environment—and send information about the stimuli
to integrators. Integrators are control points that pull together information from one or
more sensory receptors. Integrators then elicit a response from effectors. The brain is an
Search WWH ::




Custom Search