Muscle Tissue Types

Sections

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Skeletal muscle

• Skeletal muscle tissue comprises long, parallel cells, which are also called muscle fibers.
• Multinucleated
• Sarcomeres, which are the contractile units of skeletal muscle, lend a striated, or striped, appearance
  • The dark stripes are called A-bands
  • Lighter stripes are called I-bands
  • Z-discs lie in the middle of the I-bands, and M-lines lie within the A-bands
  • A single sarcomere comprises the space between two adjacent z-lines. This construction allows the thick and thin filaments to shorten the sarcomeres and contract the skeletal muscles.
• Are located throughout the body, and attaches to bones (or other connective tissues) to provide body movement.
• Unlike cardiac and smooth muscle, skeletal muscle tissue is under conscious neural control, though it also participates in reflex arcs

Cardiac muscle tissue

• Branching cells with 1-2 centrally located nuclei.
• Sarcomeres lend striations.
• Cardiac muscle cells are connected at intercalated discs, which can appear as light-staining "breaks" or dark-staining lines between cardiac muscle cells.
• Intercalated discs comprise interdigitating processes that hold adjacent cells together via complex junctions, including desmosomes and gap junctions.
  • Gap junctions serve as "electrical synapses" that spread contraction signals from cell to cell.
• Cardiac muscle is in the heart, where it contracts involuntarily to provide a rhythmic heartbeat.
  • The pacemaker sets the heart rate, but can be overridden by neural controls.

Smooth Muscle Tissue

• Comprises spindle-, aka, fusiform, shaped elongated cells, with a single central nucleus.
• Although they comprise contractile elements, the filaments are not arranged in sarcomeres; therefore, there are no striations.
• Intermediate filaments attached to dense bodies form a mesh-like cytoskeleton, which resists tension during contraction
  • Thin filaments also attach to the dense bodies; during contraction, the cell shortens and takes on a cork-screw appearance.
• Smooth muscle tissue is located in the walls of hollow organs, such as urinary and digestive tracts, and in arterial walls.
• Involuntary smooth muscle contraction produces peristalsis, a wave-like pattern of movement that propels materials through the body channels (for example, foods through the digestive tract).
• Contraction of vascular smooth muscle regulates blood flow.
• Involuntary regulation of smooth muscle occurs via autonomic nervous stimulation, hormonal triggers, and local stimuli, including stretch.

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• Here we will compare and contrast some key histological features of skeletal, smooth, and cardiac muscle tissue.

• To begin, let's draw skeletal muscle and label some features of a histological sample.

• First, show that skeletal muscle tissue comprises long, parallel cells, which are also called muscle fibers.

• Indicate that they are multinucleated, and, that

• Sarcomeres, which are the contractile units of skeletal muscle, lend a striated, or striped, appearance;
  • The dark stripes are called A-bands;
  • Lighter stripes are called I-bands.

• To remember this, notice that the word dark contains an A, which is the name of the A-bands, while the word light contains an I, which is the name of the I-bands.

• In our diagram, show that z-discs lie in the middle of the I-bands, and M-lines lie within the A-bands;

• Then, show that a single sarcomere comprises the space between two adjacent z-lines.
  • Elsewhere, we'll learn how this construction allows the thick and thin filaments to shorten the sarcomeres and contract the skeletal muscles.

• Write that skeletal muscle is located throughout the body, and attaches to bones (or other connective tissues) to provide body movement.

• Unlike cardiac and smooth muscle, skeletal muscle tissue is under conscious neural control, though it also participates in reflex arcs.

Next, we'll look at cardiac muscle tissue, which comprises chains of branching cells;

• Draw a single "link" of this chain, and show that cardiac muscle cells have 1-2 centrally located nuclei.

• As in skeletal muscle, sarcomeres lend striations.

• However, unlike skeletal muscle, cardiac muscle cells are connected at intercalated discs, which can appear as light-staining "breaks" or dark-staining lines between cardiac muscle cells.

• Intercalated discs comprise interdigitating processes that hold adjacent cells together via complex junctions, including desmosomes and gap junctions;
  • Gap junctions serve as "electrical synapses" that spread contraction signals from cell to cell.

• As you've probably guessed, cardiac muscle is in the heart, where it contracts involuntarily to provide a rhythmic heartbeat.
  • The pacemaker sets the heart rate, but can be overridden by neural controls.

• Finally, show that smooth muscle tissue comprises spindle-, aka, fusiform, shaped elongated cells, with a single central nucleus.

• Although they comprise contractile elements, the filaments are not arranged in sarcomeres; therefore, there are no striations.

• Indicate that intermediate filaments attached to dense bodies form a mesh-like cytoskeleton, which resists tension during contraction;
  • Though not shown here, thin filaments also attach to the dense bodies; during contraction, the cell shortens and takes on a cork-screw appearance.

• Smooth muscle tissue is located in the walls of hollow organs, such as urinary and digestive tracts, and in arterial walls.

• Involuntary smooth muscle contraction produces peristalsis, a wave-like pattern of movement that propels materials through the body channels (for example, foods through the digestive tract).

• Contraction of vascular smooth muscle regulates blood flow.

• Involuntary regulation of smooth muscle occurs via autonomic nervous stimulation, hormonal triggers, and local stimuli, including stretch.

Images:

Histology (Mark Braun, MD, & Indiana University: http://medsci.indiana.edu/c602web/602/c602web/toc.htm; http://www.indiana.edu/~anat215/virtualscope2/start.htm)