Compendium Topic Two

Skeletal and Muscular System

I.  Skeletal System

    Function and Anatomy

    Bone Growth, Remodeling and Repair

    Bones of the Axial Skeleton

    Bones of the Appendicular Skeleton

    Articulations

II.  Muscular System

    Types and Function

     Skeletal muscle Fiber Contraction

     Whole Muscle Contraction

     Muscular Disorder

      Homeostasis

I.  SKELETAL SYSTEM

The skeletal system provides the shape and form for our bodies. It is also important in supporting, protecting, facilitating movement, producing blood for the body, and storing minerals and fat. Bone, cartilage, and fibrous tissue make up this framework.    

Function and Anatomy

It's 206 bones form the basic frame from which soft tissue and organs are attached. As we stand the skeleton supports the body from the legs up. Support comes from the legs bones, to the pelvic girdle, and through the abdominal cavity. The organs in the body are protected by the skeleton. The spinal cord is protected by the vertebrae, the brain by the skull, and the heart and lungs by the rib cage and sternum. Blood cells are produced in the bone marrow. Bones sore minerals such as calcium and phosphorous, and fat is stored in the yellow marrow. 

The long bone is encased by periosteum a connective tissue layer containing blood vessels, lymphatic vessels, and nerves. The length of the bone, the main portion called the diaphysis has a large medullary cavity filled with yellow marrow. The widened areas at the end of the bone are epiphysis, spongy bone containing red bone marrow. It is covered by hyaline cartilage. This is where red bone marrow is made. Along it's length long bone is composed of compact bone. It is made of organized tubular units known as osteocytes. Osteocytes exchange nutrients and wastes with the blood vessels.

Softer than bone cartilage is gel like and contains no blood vessels and nerves. There are three types. Hyaline is found at the ends of long bones, in the nose, ribs, larynx and trachea, and is firm and flexible. In the knees and vertebrae fibrocartilage provides support. Elastic cartilage is the most flexible located in the ear flaps.

The ligaments that connect bone to bone are made of fibrous connective tissue and are also called articulations.

Bone Growth, Remodeling, and Repair

The growth of bone starts at about six weeks and can last till 25. However it can continue to change due to factors of external stress. It's change in shape, size and strength is called remodeling. Different cells are involved in growth, remodeling, and repair. Osteoblasts are bone forming cells and osteocytes are mature bone cells derived from osteoblasts. Osteoclasts break down bone assisting in depositing calcium and phosphate to the blood.

There are two forms of ossification, formation of bone.  In Intramembraneous ossification bones develop between sheets of fibroooous connective tissue. Endochondral ossification replaces cartilage with bone. This is how most of the bones are formed. The stages of ossification change cartilage to bone, then spongy bone is created by osteoblasts further creating the medullary cavity. As the bone lengthens so does the diameter. Once the epiphyseal plates close growth stops.

Specific  hormones are important to the growth of bone. Vitamin D is converted to a hormone in the intestinal tract and is important to the absorption of calcium and growth hormone is essential for overall growth.

The functioning of bone remodeling also promotes homeostasis. The renewal of bone lets the body regulate the amount of calcium in the blood. This is also accomplished by specific hormones. Remodeling happens because osteoblasts form bone and osteoclasts break down bone.

Bone repair is similar to bone growth except in it's first stage. There are four stages. Hematoma, the place of injury where blood clots. Fibrocartilaginous callus then fills the space which eventually becomes bony callus. And the last stage remodeling occurs when new compact bone is built.

Bones of the Axial Skeleton

The axial skeleton lie in the mid- line of the body. These include the skull, hyoid bone, vertebral column, and the rib cage. The skull is made of eight cranial and fourteen facial bones. Cranial bones protect the brain and they are easy to remember because  the major bones bones share the same names as the lobes of the brain. The frontal, parietal, occipital and foramen magnum where the spinal cord joins the base of the skull. Below the parietal bone lies the temporal bone and the sphenoid bone spans the cranium behind the eye sockets. The facial bones consist of the two jaw bones, the mandable and maxillae, the zygomatic bones of the cheekbone, and the nasal bones. 

Within the larynx sits the hyoid bone. It' s purpose is to secure the tongue and is involved in the muscle that allow swallowing.

The vertebral column protects and houses the spinal column. It consists of 33 vertebrae named according to their location and has 4 curvatures to provide resilience and strength. Fibrocartilage can be found between the disks. The cartilage cushions the spine to reduce friction caused by movement.

The rib cage or thoracic cage consists of 12 thoracic vertebrae and 12 pairs of ribs. The upper seven pairs connect to the sternum. The cartilage connecting the ribs to the sternum expand and contract as the lungs breathe.   

      

 Bones of the Appendicular Skeleton

The appendicular skeleton are the appendages that attach to the axial skeleton. It consists of 126 bones functioning to produce movement. The pectoral girdle and upper limb produce flexibility and the lower pelvic girdle and lower limbs stabilize the body and are the strongest bones in the body. For each part there is a left side and a right side. 

  

 Articulations

Where bones meet it forms a joint. there are three types. Fibrous joints cannot move such as in the cranium. Cartilaginous joints in the ribs move slightly and synovial joints are filled with synovial fluid which enables it to move freely. Sacs called bursae  cushion bone and muscle movement. 

II.  MUSCULAR SYSTEM

The human body contains more than 650 muscles which attached to the skeleton which provides us with movement. Each muscle fiber has several nuclei. Skeletal muscle can make up 40% of an adults body weight.

Types and Function

All muscles are able to contract. There are three types of muscle. Smooth muscle is involuntary and has striations and are found in the walls of organs and veins. The heart walls contain cardiac muscle that is striated. Intercalated disks in the membrane have gaps to enable the heart to contract and relax. Skeletal muscle attach to the skeleton made of bundles of muscle fibers and are voluntary. They function to support posture and provide movement. Contraction of the muscle produces heat caused by ATP breakdown, and the pressure created by contraction keep the blood and lymph moving. The muscles also protect the organs as well. The contraction of these muscles at death is called rigor mortis. Whole muscles are made up of bundles of muscle fibers called fascicles. These muscle fibers are surrounded by connective tissue that extends beyond the muscle creating tendons. Skeletal muscles operate in pairs and the nervous system stimulates the group of muscles to provide the right balance of movement.. Thus one muscle that is performing the most work is called the prime mover and the supporting muscles are called the synergists. An example of this is the biceps brachii and the triceps brachii.   

Skeletal muscle Fiber Contraction

The striated features of skeletal muscle  represent the  arrangement of myofilaments in the muscle fiber. There are special  names for the cell components in muscle fiber. The plasma membrane is called the sarcolemma, the cytoplasma is sarcoplasma, and the endoplasmic reticulm is the sarcoplasmic reticulum. A unique feature to the the cell is it's T-tubules that dip into the cell. The expanded areas of the reticulum are calcium storage sites important to muscle contraction. The sarcoplasmic reticulum contains hundreds to thousands of myofibrils, and interestingly contains glycogen and myoglobin. The units of myofibrils make up sacromeres that contain myosinand actin. The filaments act in different ways. Thick filaments are made of myosin and contain myosin heads. The thin filaments are made up of intertwined strands of actin and troponin and tropomyosin. Muscle contraction is created when when the sacromere shortens due to the sliding effect of the actin and myosin filaments. 

Nerve impulses carried by motor neurons can stimulate several muscle fibers because of it's branched  ends. Between the axon terminal and a muscle fiber there is a gap called the synaptic cleft. When the impulse arrives at the axon terminal neurotransmitters are released and diffuses binding to receptors in the sarcolemma. The impulse is then spread over the sarcolemma, down the t-tubules to the sarcoplasmic reticulum. As calcium is released it binds to troponin creating myosin binding sites. ATP is then split  to ADP and P as the myosin head attaches to the actin filament forming a cross bridge. The release of ADP and P creates the pulling action and when ATP again binds to myosin the cycle is repeated. 

Whole Muscle Contraction

Whole muscle contraction is dependent on motor units. It includes the components of nerve fiber with all the muscle fibers it is effecting. Because of this there is a all-or-none law. From the impulse they either contract or they do not. Infrequent impulses cause muscle twitches. Sumation  is increased muscle contraction and tetnus is the maximal sustained motor unit contraction. Intensifying nervous stimulation activates more muscle units. In order to sustain contraction some motor units contract maximally while others are relaxed.

Muscles  have four energy sources. They are triglycerides, fatty acids, glucose, and glycogen. Glycogen and triglycerides are stored i the blood. Muscles require ATP for contraction. There are three ways that muscles to get ATP. One way is by formationof ATP by the creatine phosphate pathway. The second is by fermentation and the third is by cellular respiration. The creatine phosphate pathway is the easiest  and quickest way but is limited in it's storage and available for short term high intensity activities. Fermentationis also fast acting but creates lactate buldup. It creates two ATP from the breakdown of glucose. For lower intesity activities cellular respiration doesn't immediately supply ATP. It is slower because it must be carried through the blood. This process also burns fatty acids and glucose.    

Muscular Disorder

Muscular disorders can be mild or serious. Common conditions include include spasms, cramps, convulsions, and sprains and strains. Inflammation in the joint is known as tendinitis. Conditions that require medical attention are fibromyalgia, muscular dystrophy, myasthenia gravis, and amyotrophic lateral sclerosis.  

Homeostasis

The muscular skeletal system plays an important part in the homeostasis. It's many functions enable the body to move, regenerate, store nutrients, protect the body, maintain body temperature, and aid in the movement of fluid. 

References:

http://www.mnsu.edu/emuseum/biology/humananatomy/skeletal/skeletalsystem.html

http://en.wikipedia.org/wiki/Muscle