Muscular Histology General Information

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Muscular tissues are one of the four tissues of the human body (the other being epithelial, connective, and nervous).  Muscular tissue is different than the other 3 tissues in that muscular tissue can contract, exerting force.  

Muscle Fibers

Each of the three types of muscular tissue has a specific cell shape.  Skeletal muscle’s cell shape is the easiest to visualize as a cylinder. Smooth muscle fibers have a spindle shape, which can be compared to the shape of a flame.  Cardiac muscle fibers have branched cylinders similar to the wood of a tree with branches.

Try this for yourself.  Collect paper towels or toilet paper rolls.  Gather them all together in a bundle with a rubber band or some string.  Make the two cuts mentioned above and view the cut faces.

Striations

The striations of skeletal and cardiac muscle are created by the alternating pattern of the proteinsLarge molecules made of amino acids with various functions in the body. actin and myosin.  Smooth muscle contains actin and myosin however they are arranged in a way that does not yield striations.  These striations are visible at high magnifications, such as the pictures below.  

Recall that nucleiClusters of neurons in the CNS responsible for processing information. are squished and flattened against the sarcolemma (cell membrane) in order to make more room for myofibrils, the contractile proteinsProteins (actin and myosin) responsible for muscle contraction..  The orientation of the nuclei will be in the same direction as the sarcolemma, which looks like white or lightly colored lines.  The striations run perpendicular to the sarcolemma.  The picture on the left is cardiac muscle tissue with the branching cylinders oriented horizontally, much like a fallen tree.  In the picture of skeletal muscle below, the faint pink lines of the sarcolemma run vertically with a slight lean to the right.  The striations therefore run horizontally across the picture but are very faint and difficult to see.  Try increasing the magnification of your screen to see the striations.

Vascularization and Innervation

Muscular tissue, unlike epithelial tissues, is highly vascularizedTo invade a tissue with blood capillaries. with blood vessels and innervated with nervous tissue.  Skeletal muscle is innervated by nerve fibers that allow you to have voluntary control of contractions in your skeletal muscle. Smooth muscle and cardiac muscle are involuntarily controlled by your nervous systemThe organ system that controls body functions using electrical and chemical signals.. 

Terminology

Muscle fiber = muscle cell

Because each muscle cell is the product of the fusing of myoblasts, they are called muscle fibers instead of muscle cells.  This fusing is also the result of the mane nuclei in one muscle fiber.

Sarcolemma = cell membrane

The prefix sarco- is a common prefix for muscle-associated structures.  Myo- and myso- are also common.  The suffix -lemma means “sheet” and refers to how the cell membrane covers a muscle fiber like a sheet.  

MyofibrilsCylindrical structures within muscle fibers that contain myofilaments. = contractile proteins

These cylinder-shaped proteins are often mistaken for the cylinder-shaped muscle fibers of skeletal muscle.  

Sarcomere = small