Muscle cells are highly specialized for contractions. Such contractions may result in the movement of the whole body or a portion of it, if the muscles are attached to a movable part of the skeleton. If the muscle is located in the wall of a hollow organ, its contractions may cause the contents of the organ to move, e.g. peristaltic movement of material through the digestive tract.
Several specific terms are used exclusively for muscle tissue. For example, muscle cells are called fibres; their cytoplasm is termed sarcoplasm; and their cell membrane is referred to as sarcolemma.
Three types of muscle tissue are distinguished on the basis of structural, functional and locational differences:
a) skeletal or striated,
a) Skeletal (Striated) Muscle (Slide #18)
Skeletal muscles form the "flesh"; sometimes referred to as the "red meat" of an animal's body. They are attached to, and result in, the movement of the bones of the skeleton. For example, the biceps brachii and pectoralis are skeletal muscles. As the contraction of the skeletal muscles is under conscious control, they are also called voluntary muscles.
A typical skeletal muscle cell is a highly modified, giant, multi-nucleate cell (fibre). Each fibre is cylindrical in shape with blunt, rounded ends. The flattened nuclei are located mainly at the periphery of the cell, just inside the sarcolemma. The "cross-striped" (or striated) appearance of light and dark banding results from the arrangement of myofibrils, small protein contractile units embedded in the sarcoplasm (Figure 14).
Examine slide #18 of a section of the tongue. Attempt to locate an area on the slide showing a longitudinal view of parallel skeletal muscle fibres (Note: this is often difficult as the tongue contains interlacing bundles of skeletal muscle cells oriented at various angles.) Note the position of the nuclei and the prominent, regular cross-striations (Figure 14). It is nearly impossible to judge the length of the individual cells (or to see the "rounded ends") in this type of section.
Figure 14A: Schematic representation of skeletal muscle.
Figure 14B: Microscopic view of skeletal muscle.
b) Smooth Muscle (Slides #92 and #19)
Smooth muscle is abundant throughout the internal organs of the body especially in regions such as the digestive tract. As its contraction is not under conscious nervous control, it is referred to as involuntary muscle.
Smooth muscle fibres are spindle-shaped structures with a prominent centrally located nucleus (Figure 15). In comparison with skeletal muscle fibres, they are much shorter in length and they do not exhibit striations. The cells occur as individual fibres within organs or as groups of fibres closely interlaced in sheets or bands.
Re-examine slide #92 showing the cross-section of the small intestine. Try to locate the inner circular and outer longitudinal bands of smooth muscle tissue. The cell membranes of the individual muscle fibres are difficult to discern in this type of section. But, the lack of cross-striations is usually apparent and so is the central location of the nucleus (especially in the cells of the outer longitudinal layer).
Also, examine slide #19 showing isolated smooth muscle cells. Note the characteristic spindle cell shape, the absence of cross-striations and the prominent nucleus.
Figure 15A: Schematic representation of smooth muscle.
Figure 15B: Microscopic view of smooth muscle.
c) Cardiac Muscle (Slide #12)
Cardiac muscle is a highly specialized tissue restricted to the wall of the heart. It is also an involuntary type of muscle, as its contraction is not consciously controlled.
Unlike smooth or striated fibres, cardiac fibres tend to form long chains of cells which branch and intertwine. This arrangement results in the peculiar "wringing" action of the heart. The junction of one cell with another in a particular chain is known as an intercalated disc and appears as a heavy dark line running across the fibre.
Each cell has a somewhat cylindrical shape with one centrally-located, oval nucleus. Cross-striations are apparent but they are not as regular nor as prominent as those of skeletal muscle (Figure 16).
Examine Slide #12 of a section of the heart. Note the shape of cells, the intercalated discs and the, the nucleus, and the cross-striations.
Figure 16A: Schematic representation of cardiac muscle.
Figure 16B: Microscopic view of cardiac muscle.
Cardiac muscle showing intercalated disks.
Heart muscle showing branching fibers.