All muscle tissues are specialized for contraction, possessing the ability to shorten and therefore create a contractile force. Substantial metabolism is necessary to produce energy for these contractions and a useful by-product of chemical reactions is heat. Therefore, muscles contribute to thermoregulation, the maintenance of body temperature. Functions of muscles in our bodies include:
support and movement
propulsion of blood through vessels
movement of food or body secretions through tracts
thermoregulation
Muscle cells possess other attributes besides contractility. All muscles are excitable, able to respond to stimuli, an important capability also common to nervous tissues. Muscles are extensible in that they can be stretched and still maintain contractile ability. As we will see, some muscles are better at this than others. Finally, muscle cells are associated with elastic connective tissues. These connective tissue elements enable muscles to contract or stretch and then return to their original length, an attribute called elasticity.
Muscle types vary in their appearance histologically but one common attribute applies to all varieties, the cells of muscles are long and narrow. For this reason, the term fiber is common descriptive term for muscle cells. Muscle cells, therefore are also muscle fibers or more specifically myofibers. There are three types of myofibers:
skeletal
cardiac
smooth
Skeletal and cardiac muscles are classified as striated types while smooth is a non-striated type. The term "striated" describes the repeating dark and light bands visible in longitudinal views of cardiac and skeletal muscle types. These bands are important in the identification of skeletal and cardiac muscle in longitudinal sections. You will learn later these striations are the result of highly organized arrangements of contractile proteins within muscles. Smooth muscle also has contractile proteins but these are arranged randomly.
This is a high magnification view of a striated muscle type! Remember, cardiac and skeletal are striated types!
Muscle types are also described as voluntary or involuntary. These terms imply some muscles are controlled voluntarily while others are not. Think about this for a moment! You can control the muscles moving your fingers but you cannot consciously control contractions of muscles in your heart or stomach. Intuitively, you already know cardiac and smooth muscles are involuntary types. Skeletal, the muscle type attaching to bones can be consciously controlled and therefore is a voluntary type. The three muscle types we will view are then:
Skeletal-voluntary muscles attached to skeletal elements and cartilage.
Cardiac-involuntary muscles making up the myocardium of the heart.
Smooth-involuntary muscles in the walls of hollow organs, blood vessels, and all other body locales where muscles perform work(i.e. think of the eye with its pupillary dilation or constriction).
Some muscles have the ability to contract independently without stimulation by the nervous system. Smooth muscle and cardiac muscle possess this ability, they are subsequently called autorhythmic types!
Connective Tissue Components of Muscles
All muscles are invested with connective tissue elements. Delicate fibers wrap around individual muscle cells as endomysium. Heavier connective tissue with more collagen surrounds groups of muscle fibers as perimysium, dividing each muscle into fascicles. Dense irregular c.t. surrounding entire muscles is epimysium. This outer connective tissue covering is the anchor for attachment of surrounding fascia, tendons, ligaments, or aponeuroses (broad, flat tendons). Collagen fibers of these tissues intermingle to form tight connections between muscles and other body structures. Without connective tissues, forces generated by muscles during contraction would not be transferred to tissues where specific functions are performed. Keeping this point in mind reminds us that all slides containing muscle tissue will also possess connective tissue components. Watch for these and remember vascular elements pass through these connective tissues as they deliver blood to and from the muscles. Now, take a look at some sections of muscle to see fascicles with some of these connective tissue components!
Now, here is a section of cardiac muscle where the fascicles are visible but connective tissue components are not!
Finally, in this view of skeletal muscle, some fascicles are visible as are striations in some longitudinal myofibers.
Think of fascicles as being a key feature to look for in the identification of muscle tissues in cross-sections. In longitudinal sections fascicles may not be as obvious and other criteria such as striations are more important to identify muscles.
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