Musculo-skeletal Anatomy

Musculo-skeletal Anatomy

1. Cartilage (or gristle as it is also called) is a hard but flexible, compressible and elastic tissue. It is found principally at joints and between bones. Cartilage consists of specialised cells called chondrocytes embedded in a matrix which are generally present in groups of 2-4 cells. Each cell lies in a fluid space called lacuna. Cartilage does not contain blood vessels but is covered by a membrane perichondrium from which it derives blood supply. Nutrients and oxygen must move by diffusion through the matrix in order to reach the chondrocytes.

2. There are 3 main varieties of cartilage:

   • 1.Hyaline -This consists of collagen fibres embedded in a clear, glassy, tough ground substance called matrix. It is firm and elastic and is found covering the ends of the long bones as articular cartilage, in the nose, larynx, trachea and bronchial tubes where it keeps open the orifice. The cells of hyaline cartilage are arranged in small groups, set in a tough matrix.
     • 2. Fibrous cartilage – Here the matrix contains large number of fibres. On the basis of type of fibres present, it is of 2 types. White fibro-cartilage (fibro cartilage) Here the small amount of matrix of cartilage is packed with large number of bundles of thick white fibres, and hence it the toughest, strongest and less flexible It has no perichondrium. It is found in intervertebral disc as shock absorber
       • 3. Yellow fibro-cartilage (elastic cartilage) Here the matrix is packed with yellow or elastic fibres which run in all directions to form a network. It appears yellow and opaque. The presence of the yellow fibres makes it very flexible. It is found in the epiglottis, pinna of the ear, and at the tip of the nose.

3. Compact bone is found on the outside of most bones. It is the hard outer layer that gives bones their smooth, white appearance. This type of bone has a much greater mass than the same amount of spongy bone, which is found in places such as joints and the ends of bones.

   • Bone cells are embedded in a firm, calcified matrix. About 30% of the matrix is composed of organic material, whist 70% is inorganic bone salts.
     • The organic material consists mostly of (the protein) collagen. The chief inorganic constituent of bone is needle-like crystals of hydroxyapatite, Ca10(PO4)6(OH)2, a form of calcium phosphate.
       • Organic (collagen) Help to resist bone fracture Inorganic (hydroxyapatite) Help prevent bone compression

4. Bones begin life as cartilage. As the young baby develops in the mother's womb, the skeletal system becomes organised into a framework of cartilage. After birth, hardening of all parts of the skeleton begins. This process continues throughout life until, in old age, very little cartilage is left and the bones are old and brittle. The process of hardening cartilage into bone is called ossification.

   • Ossification is achieved by bone-forming cells called osteoblasts (osteo- means "bone" in Greek). The old osteoblasts produce bone tissue and also secrete the enzyme phosphatase which allows calcium salts to be deposited in the newly formed bone tissue. This makes the tissue hard.
     • Osteoclasts are responsible for breaking down the bone matrix. Minerals such as calcium move into the bloodstream for use by the body. As resorption occurs, new bone cells form, which is a function of osteoblasts.

5. Rickets is a condition that affects bone development in children. It causes the bones to become soft and weak, which can lead to bone deformities. Rickets in adults is known as osteomalacia.

   • Brittle bone disease (osteogenesis imperfecta) is a rare genetic disorder that causes bones to fracture easily.
     • Like brittle bone disease, osteoporosis is a condition that weakens bones, making them fragile and more likely to break.

6. Thick filaments are made from many myosin molecules. Each myosin molecule has a tail and a head which projects from the surface of the thick filament.

   • Thin filaments consist of two strands of globular actin molecules which twist round each other. The protein tropomyosin is wrapped around the actin fibre and the protein troponin is found at positions along the actin/tropomyosin fibre

7. 1. A nervous impulse causes calcium ions Ca2+ to be released from the sarcoplasmic reticulum (a system of membrane-bound tubules that surrounds muscle fibrils). 2. Ca2+ binds to troponin and changes the shape of the troponin molecule. 3. This causes tropomyosin to change position exposing the myosin binding sites on the actin. 4. Myosin heads can then form cross bridges with the myosin binding sites on the actin. 5. The myosin head bends pulling the actin past the myosin. This is called the ‘power stroke’. 6. ATP at the end of the myosin is hydrolysed into ADP and Pi which are released. 7. The cross bridge is broken when ATP attaches to the myosin head which returns to its original position. 8. More ATP is hydrolysed to ADP and Pi and a cross-bridge forms with the thin filament further along. 9. The process continues until Ca2+ is actively pumped back into the sarcoplasmic reticulum.

8. The functions of the skeleton The skeleton performs the following functions: 1. Support: It is the framework of the body. It supports the softer tissues and provides points of attachment for most muscles. 2. Assisting in movement: Skeletal muscles are attached to bones. Therefore, when the associated muscles contract, they cause the bones to move. 3. Protection: It protects the soft organs, particularly the brain (the cranial bones), spinal cord (the vertebrae), heart and lungs (the ribcage). 4. Production of blood cells. Blood cells are produced by cell division in the bone marrow of certain bones. Red blood cells are produced in red bone marrow by a process called erythropoiesis. This process is stimulated by a hormone called erythropoietin which is released when there is a decrease in blood oxygen levels. 5. Storage of minerals: Bone tissues store several minerals including calcium and phosphorous. When required, bone releases the minerals into the bloodstream.

9. The action of a muscle pulling on a bone often works like a type of simple machine called a lever. A lever is a rigid rod (usually a length of bone) able to rotate about a fixed point known as a fulcrum (formed by the joint). There are three types of lever but all levers have three parts: 1. The Fulcrum: the point of movement or pivot, generally at the centre of a joint. 2. A Load: the body's weight or some external object. 3. An Effort: the contraction of the muscle to move the load. Types of Levers in the Body Levers are classified according to the positions of the fulcrum, effort and load.

   • Muscles which have opposing actions are described as antagonistic. When a muscle such as the biceps contracts, its antagonist, the triceps should relax.