Medical brain notes 41

  Muscle Tissue and Physiology of Muscle Contraction All body functions involving movement require muscle activity. Muscle activity is required for skeletal movement, heart contraction, food moving through the gut, urination, and breathing, among many oth-ers. Certain muscles, even when not producing move-ment, remain contracted to maintain posture and op-pose the effects of gravity. To perform these varied activities, three types of muscle are present in the body:  skeletal, cardiac,  and  smooth.  The skeletal muscles constitute about 40% to 50% of body weight. They are attached to the skeleton and are responsible for skeletal movement and stabilizing body position. Cardiac muscle is lo-cated in the heart; smooth muscle is present in the gut, around the bronchi in the lungs, urinary tract, reproductive organs, and blood vessels. Cardiac mus-cle helps move blood throughout the body, and smooth muscle helps move fluid and food matter. Smooth muscle also helps re...

  STRUCTURE OF SKELETAL MUSCLE Macroscopic Structure of Muscle Skeletal muscle, as we know it (e.g., biceps brachii), is a collection of muscle cells, nerves, connective tis-sue, and blood vessels. Each muscle  cell  is known as a  muscle fiber.  Muscle fibers are cylindrical, arranged parallel to each other, and run through the entire length of the muscle. The fibers are held in place by connective tissue, which surround individ-ual fibers, bundles of muscle fibers and, finally, the entire muscle. It is the connective tissue that attaches muscle to the periosteum of bones and conveys the force generated by the muscle to the bone, across one or more joints. Other than the connective tissue sur-rounding individual fibers, bundles of fibers, and the entire muscle, connective tissue ( fascia ) separates muscle from the skin ( superficial fascia  or  subcuta-neous layer ) and holds groups of muscles with simi-lar functions together ( deep fascia ). The con...

  THE MYOFILAMENT: THE SPECIALIZED PROTEINS OF MYOFIBRILS Each myofibril is made up of  myofilaments,  which are regular arrangements of protein filaments (Figure 4.3). Myofilaments, unlike the myofibrils, do not run the entire length of the muscle fiber, but are arranged in smaller sections called  sarcomeres.  The sarcom-ere is the functional unit (the smallest structure(s) of an organ that can perform the function) of the mus-cle, and it is the activity at the level of the sarcomere that causes muscle to contract. Myofilaments consist of two types of protein,  actin  and  myosin.  Because of size, actin is known as the  thinfilament,  and myosin is known as the  thick filament. The thick and thin filaments are arranged in a specific manner to facilitate muscle contraction. The filaments are arranged parallel, with bundles of thick filaments alternating with bundles of thin. When the muscle is viewed under the microscope, the...

  SLIDING FILAMENT MECHANISM   The sliding filament mechanism explains the process of muscle contraction at the molecular level. This process is initiated by impulses from the nerve that innervates the muscle fiber. Muscle-Nerve Communication Skeletal muscle only contracts when stimulated by the communicating nerve. Each muscle fiber is in contact with a nerve ending. The cell body of the nerve fiber (a single neuron) is located in the spinal cord, brainstem, or brain, according to where the skeletal muscle is located and to where it originated in the embryonic stage. The axons of these neurons  extend from the cell bodies to individual muscles.   For example, when we say that the ulna nerve sup-plies the muscles of the thumb, we are indicating the bundles of axons of motor neurons that go together as the ulna nerve before they split to supply the indi-vidual muscle fibers of muscles that move the thumb. The axons branch when they reach the muscles they supply, and e...

  CHARACTERISTICS OF WHOLE MUSCLE CONTRACTION Although the contractile mechanism is the same, muscle characteristics are modified in many ways to enable the body to adjust force and speed of contraction and direction and range of motion. At the microscopic level, variations in contraction duration exist between muscles. Macroscopically, variations in fascicle arrangement and size of motor units are present. Together, structural and functional variations allow us to execute both crude and intricate, movements. The ultimate force transmitted to bone across joints depends, to a large extent, on the way muscle fibers are organized (e.g., the direction of fiber arrangement, where the muscle originates and inserts—closer or further from the joint—and in which direction they pull). Of course, the range of motion in a joint also de-pends on the  articular surface,  the  pliability of structures around the joint,  such as ligaments and tendons,and the  relaxation of...