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The Haversian Arrangement and Skeletal Muscle Fibers

Bones are the amazing structural units that provide us with support, protection, movement, mineral storage, and blood cell formation. Bone consists of living cells and a predominate, non-living matrix that is calcified. Every bone in the skeleton has an external dense, smooth layer of compact bone. Internal to the compact bone is spongy bone. The Haversian (osteon) arrangement consists of the lamellae and Haversian canals, and is the basic unit of structure in compact bone. It consists of intricate passageways for blood vessels, lymphatic vessels, and nerves. Compact bone is arranged in concentric cylindrical layers parallel to the long axis of the bone and main compression stresses called lamellae. These lamellae are part of an osteon, which resembles the rings on a cross section of a tree and can be compared to tiny weight-bearing pillars. Between the lamellae are small spaces called lacunae, which contain osteocytes. Osteocytes are mature bone cells that no longer secrete bone matrix materials. Radiating in all directions from the lacunae are minute canals called canaliculi, which are filled with extracellular fluid. Inside the canaliculi are slender fingerlike processes of osteocytes. The canaliculi connect lacunae with one another and, eventually with the Haversian canal within the center of the concentric lamellar layers. The Haversian canal contains capillaries and lymph vessels. Covering the bone, except at joints, is a fibrous membrane called the periosteum. H. Blood vessels, lymphatic vessels, and nerves from the periosteum penetrate the compact bone through Volkmann's canals. The blood vessels of these canals connect with the blood vessels and nerves of the medullary cavity and those of the Haversian canals.

Muscle tissue provides us with movement, stabilization of joints, posture and body position, regulation of organ volume, and thermogenesis (creation of heat). There are 3 types of muscle fibers: skeletal muscle tissue, cardiac muscle tissue of the heart, and smooth muscle tissue. Muscle cells are long and narrow and are usually referred to as muscle fibers. The muscle fiber's cell membrane is called the sarcolemma and surrounds the muscle fiber's sarcoplasm, the cytoplasm of the muscle fiber. At high magnification, the sarcoplasm appears to be made up of little threads that are referred to as myofibrils. The myofibrils are surrounded by fluid filled structures called sarcoplasmic reticulum, which stores calcium ions when in a relaxed state. Myofibrils extend lengthwise within the muscle fiber and show alternating light and dark bands called striations. They contain even smaller structures called myofilaments. There are thin myofilaments called actin and thick myofilaments called myosin. The dark striations of a myofibril are called A Bands and the light striations of a myofibril are called I Bands. The light area that runs through the middle of an A Band is known as the H Zone. A dark stripe that runs through the middle of an I Band is known as the Z Line. The section of a myofibril that extends from one Z Line to the next Z Line is referred to as a sarcomere. Sarcomeres are the basic contractible unit of skeletal muscle. T tubules are tunnel-like folds of the sarcolemma and can conduct impulses to the deepest regions of muscle cells. They penetrate the muscle fiber at right angles to the sarcoplasmic reticulum and the myofilaments. T tubules open to the outside of the muscle fiber. On either side of a T tubule are sacs of sarcoplasmic reticulum. A T tubule and the two sacs of sarcoplasmic reticula on either side of the T tubule are referred to as a triad. Actin and myosin are myofilaments, which make up myofibrils which are surrounded by a sarcoplasmic reticulum and bundles of these myofibrils make up a muscle fiber cell that is surrounded by the endomesyum. Bundles of muscle fibers make up fascicles, which are surrounded by the perimysium. Groups of fascicles make a muscle, which is covered by the epimysium.

The Haversian system and skeletal muscle are similar in several ways. Both are intricately layered and packed in bundles. They both are long, cylindrical structures and are designed to be very strong and stable. Blood and nerves are abundant in these two different structures. Skeletal muscles and the Haversian system in long bones grant us mobility and form.

The differences, however, far outweigh the similarities. The Haversian system has an obvious center structure (the central canal), while muscle tissues are just bundles of myofilaments with no specific center structure. Haversian systems are perpendicularly intersected often by Volkmannís canals, yet muscle fibers continuously run parallel to each other throughout the length of the muscle. The Haversian system is designed to optimize stability, strength, and resistance to pressure while supplying the necessary nerves and blood vessels, but flexible muscle fibers specialize in contracting and extending quite a lot. The Haversian arrangement is surrounded by a single membrane that covers the outside of the compact bone and is called a periosteum. Muscle fibers have many membranes used to contain bundles of myofilaments, myofibrils, or muscle fiber. The collagen fibers in the Haversian system twist in the opposite direction of the layer beneath it, but muscle tissue is always parallel and constant. In the Haversian system blood vessels and nerves are the central point in the middle of a central canal, but are found on the outside corners of bundles of muscle fibers called fascicles. One of the main differences between the Haversian system and muscle fibers is that when damaged, the Haversian system can heal quickly and very effectively, while muscle fibers can be permanently lost.

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