Muscle Tissue

Muscle is a very specialized tissue that has both the ability to contract and the ability to conduct electrical impulses. Muscles are are classified both functionally as either voluntary or involuntary and structurally as either striated or smooth. From this, there emerges three types of muscles: smooth involuntary (smooth) muscle, striated voluntary (skeletal) muscle and striated involuntary (cardiac) muscle. The names in the brackets are the common names given to the particular classification of muscle.

Smooth Involuntary (Smooth) Muscle Tissue

The slide shown on the left is a section of the small intestine showing smooth muscle in the lower right and dense irregular connective tissue in the upper left. Note how the smooth muscle forms neat, parallel lines, while the dense irregular connective tissue is more wavy and less organized. The slide on the right is a close up the smooth muscle seen on the left. Smooth muscle features long, narrow "spindle" shaped cells with a single central, somewhat elongated, nucleus. These cells are arranged parallel to one another in situ and do not show any striations microscopically. This type of muscle is called "involuntary" because it acts to contract and relax without conscious thought (i.e. you can't will your intestines to contract, they just do). Smooth muscle is found in the walls of the digestive tract, uterus, bladder, blood vessels and other internal organs.

Striated Voluntary (Skeletal) Muscle Tissue
Skeletal muscle fibres

This slide above shows skeletal muscle prepared in such a way that the individual fibres (1) have been teased apart and isolated. The fibres of skeletal muscle should not be confused with the fibres of connective tissue. Connective tissue fibres are extracellular elements, whereas skeletal muscle fibres describe the individual skeletal muscle cells. Each cell is called a fibre because it is thin and very long, making it look like a thread, or fibre of clothing, when isolated. Note that a single fibre (cell) can span the entire length of the muscle. Microscopically, each cell is cylindrical, unbranched and contains many nuclei (2 - this is also called multinucleate). The nuclei are arranged around the periphery of the cell just beneath the cell membrane. The characteristic striations seen are a result of the orderly arrangement of actin and myosin filaments within the muscle cell (seen well on the slide above). In the body, skeletal muscle cells are arranged into bundles (called fascicles) surrounded by thin layers of connective. These fascicles are again arranged into larger bundles, which form the particular muscular organ (i.e. the biceps). The connective tissue surrounding these fascicles extends as dense regular connective tissue to anchor the muscle to the bone in the form of a tendon.

Striated Involuntary (Cardiac) Muscle Tissue
Cardiac muscle

Cardiac muscle is unique in that it shows some features of skeletal muscle and some features of smooth muscle. As the name implies, cardiac muscle is the muscle that makes up the wall of the heart. Cardiac muscle is similar to skeletal muscle in that it is striated, and similar to smooth muscle in that the nuclei are centrally located. It differs from both skeletal muscle and smooth muscle in that its cells branch and are joined to one another via intercalated discs.

Cardiac Muscle Cell
Nuclei

Intercalated Discs

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This slide above shows skeletal muscle prepared in such a way that the individual fibres (1) have been teased apart and isolated. The fibres of skeletal muscle should not be confused with the fibres of connective tissue. Connective tissue fibres are extracellular elements, whereas skeletal muscle fibres describe the individual skeletal muscle cells. Each cell is called a fibre because it is thin and very long, making it look like a thread, or fibre of clothing, when isolated. Note that a single fibre (cell) can span the entire length of the muscle. Microscopically, each cell is cylindrical, unbranched and contains many nuclei (2 - this is also called multinucleate). The nuclei are arranged around the periphery of the cell just beneath the cell membrane. The characteristic striations seen are a result of the orderly arrangement of actin and myosin filaments within the muscle cell (seen well on the slide above). In the body, skeletal muscle cells are arranged into bundles (called fascicles) surrounded by thin layers of connective. These fascicles are again arranged into larger bundles, which form the particular muscular organ (i.e. the biceps). The connective tissue surrounding these fascicles extends as dense regular connective tissue to anchor the muscle to the bone in the form of a tendon.