Muscle Tissues

Types of Tissues

The human body is primarily made up of four major types of tissues: epithelial, connective, muscle, and nervous. Epithelial tissues cover body surfaces, cover and line internal organs, and make up the glands. Connective tissues are widely distributed throughout the body, filling internal spaces, and func-tion to bind, support, and protect body structures. Muscle tissues are specialized for contraction and include the skeletal muscles of the body, the heart, and the muscular walls of hollow organs. Skeletal muscles are attached to bones and are used for movement of the body. Nervous tissues carry information from one part of the body to another via electrical impulses. They are found in the brain, spinal cord, and nerves (TABLE 5-1).

The human body is primarily made up of four major types of tissues: 

 1. epithelial, 

 2. connective

 3. muscle, and 

 4. nervous.

Muscle Tissues

Muscle tissues can contract by shortening their elongated muscle fibers. This action moves body parts. Myofilaments in muscle cells are responsible for the muscles’ ability to move or contract. The three types of muscle tissue are skeletal muscle tissue, smooth mus-cle tissue, and cardiac muscle tissue.

Skeletal Muscle Tissue

Skeletal muscle tissue is known as voluntary muscle tissue because it is found in muscles controlled by con-scious effort. It attaches to bones and is composed of long thread-like cells that have light and dark markings called striations (FIGURE 5 -9). These multinucleated muscle cells contract when stimulated by nerve cells. The term “multinucleated” describes the several hun-dred nuclei distributed inside the muscles’ plasma membranes. Skeletal muscle tissue moves the head, trunk, and limbs, allowing all voluntary movements in these body areas. Skeletal muscle cells are also called muscle fibers. They are long and cylindrical, con-taining many peripherally located nuclei.

The cells of skeletal muscle tissue may be one foot or more in length. The fibers cannot divide. New fibers are produced when satellite cells divide. These are stem cells that remain in adult skeletal muscle tissue. There-fore, skeletal muscle tissue can partially repair itself after injury. Skeletal muscle is also known as striated voluntary muscle. It actually contains all four types of body tissue, not just muscle tissue. Adjacent fibers are bound by collagen and elastic fibers that blend into an attached tension or aponeurosis, which con-ducts the force of contraction. Movement is, therefore, produced when skeletal muscles contract and pull on attached bones.

Smooth Muscle Tissue

Smooth muscle tissue is composed of elongated, spindle-shaped cells in muscles not under voluntary control. The cells have tapered ends and a single, oval nucleus. Smooth muscle fibers are shorter than striated fibers, having only one nucleus per spindle-shaped fiber. They are also called nonstriated involuntary muscles or unstriated muscles (FIGURE 5-10). Smooth muscle cells have actin and myosin filaments that are different from skeletal or cardiac muscle. They may contract on their own. Gap junctions exist between adjacent cells, which coordinate the contractions of individual cells.

Smooth muscle cells can divide; therefore, they regenerate after being injured. Smooth muscle tissue composes hollow internal organ walls (such as the intestines, stomach, urinary bladder, blood vessels, and uterus). Smooth muscle cannot, in most cases, be ­controlled by conscious effort. This type of tissue moves food through the digestive tract, empties the urinary bladder, and constricts blood vessels. It accom-plishes these tasks by either contracting or relaxing.

Cardiac Muscle Tissue

Cardiac muscle tissue is also called the myocardium, the thick middle layer of the heart wall. The contractile tissue of the myocardium is composed of fibers, with the characteristic, prominent cross-striations of mus-cular tissue. These striations branch frequently and are interconnected, forming a network. Myocardial muscle­ contains less connective tissue than skeletal muscle and is usually uninucleate (having one nucleus, located centrally). However, some cardiocytes­ have up to five nuclei. Cardiac muscle is involuntary and makes up most of the heart. It relies on pacemaker cells for regular contraction.

Cardiac muscle cells, known as cardiocytes, are branched and fit together tightly at junctions known as intercalated discs. In these locations, membranes are joined by desmosomes, gap junctions, and proteo-glycans. As ions move through the gap junctions, they help synchronize cardiac muscle cell contractions. The desmosomes and proteoglycans lock the cells together as they contract. The cells do not rely on nerve activ-ity to begin a contraction; specialized pacemaker cells establish regular contraction rates instead. The nervous system has the ability to alter the rate of pacemaker cell activity. However, it does not provide any volun-tary control over individual cardiac muscle cells. As a result, cardiac muscle is called striated involuntary muscle. Cardiac muscle tissue is not able to signifi-cantly repair itself. After an injury to the heart, some muscle cells are able to divide. However, repairs are not complete and some heart function will usually be lost.

1. What type of muscle cells is striated?

2. Identify examples of voluntary and involuntary muscles.

3. What type of muscle cells is branched and has intercalated discs?