![]() Muscles enable animals and man to perform very important physiological functions, such as movement of the body or its individual parts, blood circulation, respiration, passing of chyme through the digestive organs, maintenance of vascular tonus, and excretion. The body tissue of the skeletal and visceral musculatures. The major physiological properties of these muscles are their intrinsic ability to contract spontaneously and their dual regulation by the autonomic nerves of the sympathetic and parasympathetic systems. Smooth muscle is usually arranged in sheets or layers, commonly oriented in different directions. Those in the walls of various ducts and tubes in the body act to move the contents to their destinations, as in the biliary system, ureters, and reproductive tubes. Smooth muscle fibers in the skin regulate heat loss from the body. Smooth muscle forms the major contractile elements of the viscera, especially those of the respiratory and digestive tracts, and the blood vessels. These fibers contract relatively slowly and have the ability to maintain contraction for a long time. The size of such fibers varies greatly, from a few micrometers up to 0.02 in. These muscles consist of elongated fusiform cells which contain a central oval nucleus. Smooth muscle, also designated visceral and sometimes involuntary, is the simplest type. This permits muscles to contract quickly when stimulated without having to overcome the inertia of total relaxation. During resting states, some of the fibers in the musculature are maintained in a state of partial contraction, known as muscle tone. The point of contact between nerve and muscle is the neuromuscular junction, where the chemical substance acetylcholine is secreted, initiating the changes that cause the muscle to contract. A muscle fiber is stimulated to contract by electrical impulses from the nervous system. ![]() The energy required for muscle contraction comes from the breakdown of adenosine triphosphate (ATP), a substance that is present in the cells and is formed during cellular respiration. Actin and myosin interact before muscle contraction, forming the contractile material actomyosin. Each muscle fiber contains several hundred to several thousand tightly packed strands called myofibrils that consist of alternating filaments of the protein substances actin and myosin. This type of muscle is composed of numerous cylindrically shaped bundles of cells, each enclosed in a sheath called the sarcolemma. Perhaps because its action is most varied, striated muscle has been studied most extensively. Striated muscle is usually considered phasic, while cardiac and smooth muscle are thought to be tonic. The means by which all types of muscles contract is thought to be generally the same, although muscles are classified as phasic, or fast twitch, and tonic, or slow twitch, to differentiate between the various lengths of time a muscle may require to move in response to stimulation. Skeletal muscles are attached (with some exceptions, such as the muscles of the tongue and pharynx) to the skeleton by means of tendons, usually in pairs that pull in opposite directions, e.g., the biceps (flexor) and triceps (extensor) that move the forearm at the elbow. The contractions of cardiac muscle are stimulated by a special clump of muscle tissue located on the heart (the pacemaker), although the rate of contractions is subject to regulation by the autonomic nervous system. It is found only in the heart, where it forms that organ's thick walls. Cardiac muscle is striated like skeletal muscle but, like smooth muscle, is controlled involuntarily. Its speed of contraction is slower than that of striated muscle, but it can remain contracted longer. Smooth muscle lines the blood vessels, hair follicles, urinary tract, digestive tract, and genital tract. Smooth muscle fibers are spindle-shaped, not striated, and generally are arranged in dense sheets. Smooth muscle, which lines most of the hollow organs of the body, is not under voluntary control, but is regulated by the autonomic nervous system. It is called striated because it appears striped, in alternating bands of light and dark, when viewed under a microscope, and animals have conscious control over most of their striate muscles. Striated, or skeletal, muscle forms the bulk of the body's muscle tissue and gives the body its general shape. Muscle tissue in the higher animals is classified as striated, smooth, or cardiac, according to its structure and function. Muscle, the contractile tissue that effects the movement of and within the body.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |