- Identify the three types of muscle tissue
- Compare and contrast the functions of each muscle tissue type
- Explain how muscle tissue can enable motion
Muscle Tissue: Key Properties and Classifications
Muscle tissue is generally characterized by properties that allow movement. A critical property is that muscles are excitable and are able to respond to a variety of stimuli. They are contractile, meaning they can shorten and generate a pulling force. When attached between two movable objects, in other words, bones, contractions of the muscles cause the bones to move.
Some muscle movement is voluntary, which means it is under conscious control. For example, a person decides to open a book and read a chapter on Psychology. Other movements are involuntary, meaning they are not typically under conscious control, such as the contraction of your pupil in bright light or the rhythmic contraction of your heart muscles.
Muscle tissue used for voluntary and involuntary movement can be classified into three main types according to structure and function: Skeletal, Cardiac, and Smooth. Table 1 below illustrates the distinctions between these three muscle types.
Comparison of Structural and Functional Properties of Muscle Types
|Skeletal||Long cylindrical fiber, striated, many peripherally located nuclei||Voluntary movement, produces heat, protects organs||Attached to bones and around entrance points to body (e.g., mouth, anus)|
|Cardiac||Short, branched, striated, single central nucleus||Contracts to pump blood||Heart|
|Smooth||Short, spindle-shaped, no evident striation, single nucleus in each fiber||Involuntary movement, moves food, involuntary control of respiration, moves secretions, regulates flow of blood in arteries by contraction||Walls of major organs and passageways|
Skeletal muscle is attached to bones and its contraction makes possible locomotion (i.e. walking), facial expressions, maintaining posture, and other voluntary movements of the body. Skeletal muscles also generate heat as a byproduct of their contraction and thus participate in thermal regulation. Shivering is an involuntary contraction of skeletal muscles in response to perceived lower than normal body temperature.
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. Joints can become misaligned or dislocated entirely by pulling on the associated bones; muscles work to keep joints stable.
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 muscle tissue is arranged in bundles surrounded by connective tissue. Under the light microscope, muscle cells appear striated with many nuclei squeezed along the membranes. The striation is due to the regular alternation of the contractile proteins actin and myosin, along with the structural proteins that couple the contractile proteins to connective tissues. The cells are multinucleated as a result of the fusion of many precursor cells to form each long muscle fiber.
Cardiac muscle forms the contractile walls of the heart. The cells of cardiac muscle, known as cardiomyocytes, also appear striated under the microscope. Unlike skeletal muscle fibers, cardiomyocytes are single cells typically with a single centrally located nucleus. A principal characteristic of cardiomyocytes is that they contract on their own intrinsic rhythms without any external stimulation. Cardiomyocytes attach to one another with specialized cell junctions called intercalated discs. Intercalated discs have both anchoring junctions and gap junctions. Attached cells form long, branching cardiac muscle fibers that are, essentially, a mechanical and electrochemical syncytium allowing the cells to synchronize their actions. The cardiac muscle pumps blood through the body and is under involuntary control. The attachment junctions hold adjacent cells together across the dynamic pressure changes of the cardiac cycle.
Smooth muscle tissue contraction is responsible for involuntary movements in the internal organs. It forms the contractile component of the digestive, urinary, and reproductive systems as well as the airways and arteries. Each cell is spindle shaped with a single nucleus and no visible striations (Figure 4.18).