Muscle is one of the four primary tissue types of the body (along with epithelial, nervous, and connective tissues), and the body contains three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle (Figure 10.1.1). All three muscle tissues have some properties in common; they all exhibit a quality called excitability as their plasma membranes can change their electrical states (from polarized to depolarized) and send an electrical wave called an action potential along the entire length of the membrane. While the nervous system can influence the excitability of cardiac and smooth muscle to some degree, skeletal muscle completely depends on signaling from the nervous system to work properly. On the other hand, both cardiac muscle and smooth muscle can respond to other stimuli, such as hormones and local stimuli.

A unique property common to all three types of muscle is contractility, which is the ability of the cells to shorten and generate force.  While muscle tissue can shorten with contractions, it also displays extensibility or the ability to stretch and extend beyond the resting length of the cells.  After being stretched, the elasticity of muscle allows it to recoil back to its original length.

The muscles all begin the mechanical process of contracting (shortening) when a protein called actin is pulled by a protein called myosin, and differences in the microscopic organization of these contractile proteins exist among the three muscle types.  In both skeletal and cardiac muscle, the actin and myosin proteins are arranged very regularly in the cytoplasm of individual muscle cells, which creates an alternating light and dark striped pattern called striations. The striations are visible with a light microscope under high magnification (see Figure 10.1.1).  Smooth muscle (named for it’s lack of striations), does not produce this striped pattern because the contractile proteins are not arranged in such regular fashion.

Skeletal muscle cells (also called muscle fibers) are unique in that they are multinucleated with the nuclei located on the periphery of the cell under the cell plasma membrane (also called sarcolemma in muscle).  During early development, embryonic myoblasts, each with its own nucleus, fuse with hundreds of other myoblasts to form long multinucleated skeletal muscle fibers. Cardiac muscle cells each generally have one nucleus centrally located in the cell, but the cells are physically and electrically connected to each other so that the contraction signals spread through cells and the entire heart contracts as one unit.  Smooth muscle cells contain a single nucleus and can exist in electrically linked units contracting together as a single-unit or as multi-unit smooth muscle where cells are not electrically linked.

Muscle Functions

The best-known feature of skeletal muscle is its ability to contract and cause movement. Skeletal muscles act not only to produce movement but also to stop movement, such as resisting gravity to maintain posture. Small, constant adjustments of the skeletal muscles are needed to hold a body upright or balanced in any position. Muscles also prevent excess movement of the bones and joints, maintaining skeletal stability and preventing skeletal structure damage or deformation.  Skeletal muscles are located throughout the body at the openings of internal tracts to control the movement of various substances. These muscles allow functions, such as swallowing, urination, and defecation, to be under voluntary control. Skeletal muscles also protect internal organs (particularly abdominal and pelvic organs) by acting as an external barrier or shield to external trauma and by supporting the weight of the organs.

Skeletal muscles contribute to the maintenance of homeostasis in the body by generating heat. Muscle contraction requires energy, and when ATP is broken down, heat is produced. This heat is very noticeable during exercise, when sustained muscle movement causes body temperature to rise, and in cases of extreme cold, when shivering produces random skeletal muscle contractions to generate heat.

Cardiac muscle is only found in the heart and functions to generate force and build pressure gradients to drive blood flow throughout the body. Smooth muscle in the walls of arteries is a critical component that regulates blood pressure and blood flow through the circulatory system.  Smooth muscle in the skin, visceral organs, and internal passageways is also essential for moving materials through the body. Neither cardiac nor smooth muscle connect to bone and therefore they cannot produce the gross movements we associate with skeletal muscle.

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