Human Skeletal System: Functions, Regions, Structures, And Joints

Skeletal System Functions

Discuss about the Human Skeletal System.

1. Skeletal System Functions

Provides support for the body organs forming other body systems such as the digestive system, endocrine system, muscles among others according to Kisała & Pluskiewicz (2015).

The skeletal system also protects different body organs. For instance, the ribs protect delicate lungs, the heart and other organs

The skeletal system further enables the movement of organs and the whole body for humans (Nagaraja, 2013). It is through the help of the bones and the muscles that an individual is able to effectively make movements from one place to another.

Skeletal system in relation to muscle attachment and locomotion

Skeletal bones are usually held to each other by the ligaments. However, tendons attach these bones to the muscles. In this regard, the contraction in the muscles makes the bones to move.

2. Regions of the Spine

The spine consists of four regions.

The cervical spine-This region includes the spinal bones within the neck region (Senoglu et al, 2010). It comprises of 7 vertebras which support the skull and further ensures that the brain stem is protected.

The thoracic spine- This consists of 12 vertebra starting below the last cervical vertebrae and forms the thorax (Senoglu et al, 2010). These bones are larger when compared to the cervical vertebrae

The Lumbar spinal region-consists of only 5 vertebrae beneath the last thoracic vertebrae, with larger structural elements than rest of the spinal vertebrae

Sacral spine-This region is found behind the pelvis and it is made up of 5 fused bones Senoglu et al, 2010). The fusion of the bones forms the sacrum, a triangular bone structure behind the pelvis.

Anatomical Bones

1. Skull
2. Mandible
3. Hyoid bone
4. Cervical vertebrae
5. Clavicle
6. Sternum
7. False Ribs
8. True Ribs
9. Scapula
10. Humerus
11. Radius
12. Ulna
13. Carpals
14. Metacarpals
15. Phalanges

4. Long Bones include;

1. Humerus
2. Femur
3. Radius
4. Fibula
5. Tibia

5. Structure of Long bones

A long bone typically comprises of the body known as the diaphysis and two epiphyses at the terminal ends. The medullary cavity of a long bone is a container for the bone marrow and is an enclosure filled with the yellow bone marrow (Nagaraja, 2013). Other components of a typical long bone are discussed below.

Haversian canal-Long bones have the Haversian canal which is a central osteon canal that runs length-wisely enclosing the nerves and the blood vessels.  

Periosteum- Long bones also have the periosteum, a fibrous membrane that is rich in blood vessels and functions as an envelope to the bone except where there exist articular surfaces. The periosteum contributes to the thickness of a long bone.

Skeletal system in relation to muscle attachment and locomotion

The Volkmann’s canals- The Volkmann’s canals on long bones transverses the compact bone canals and connect each Haversian canal to each other, to the medullary cavity and to the long bone periosteum.

Osteon-The Osteon is the elementary long bone compact structure that is cylindrical (Senoglu et al, 2010). It consists of between 4 and 20 concentric plates of bones and it surrounds the Haversian canal on a long bone.

Concentric lamellae-These are the bony layers on the osteon that consist of collagen fibers that are concentrically arranged around the long bone Haversian canal (Senoglu et al, 2010). The concentric lamellae usually form as the bones grow.

Articular cartilage- The articular cartilage comprises of smooth resistant and elastic tissues that cover the terminal parts of a long bone. They are found particularly at the parts where the bone articulates with other bones and enables movement and while absorbing shocks in case of accidents.

The spongy bone- It comprises of compartments that are bony set apart from each other by the cavities that are full of blood vessels, the nerves and further, the bone marrow (Nagaraja, 2013).  The spongy bone is the structure that makes long bones to have a characteristic lightness.

Blood vessel –Long bones have a channel within them where circulation of blood occurs and thus depositing required nutrients and crucial mineral salts.
The compact bone-Is the dense tissue of the bone that comprises of osteons that resist both pressure and shocks on the bones ((Morrison & Hill, 2011). The tissue also protects the bone’s spongy tissue while forming the bone diaphysis in long bones.

Bone Marrow-The bone marrow- is the soft tissue that is found in the cavities of bones cavities and produces the blood cells.

6. The Structure of a Synovial Joint

Synovial joints are the most movable joints in all mammalian bodies as they comprise of articulations that enable free movement.  The joint is made of continuous surfaces of bones which are covered with the articular cartilages (Thomsen et al, 2012). They are connected by the ligaments that are en-lined by the synovial membrane. Usually, a synovial joint can fully or partially be divided by a synovial meniscus and/or by articular disks. The periphery however, is continuous and consists of a fibrous capsule. The free surfaces of the joint are covered by the synovial membrane.

Just like other joints a synovial joint achieves movement at the contact point of its articulating bones. A major structural difference between synovial joints and fibrous joints is that synovial joints have capsules surrounding bone articulating surfaces and the lubricating synovial fluid within these capsules (Rogers et al, 2009). The synovial joints consist of a synovial cavity between the joining bones and filled with the synovial fluid. Its articular capsule that is continuous with the bone periosteum of the articulating bones is fibrous (Thomsen et al, 2012). The capsule surrounds the diarthrosis to unite the joint articulating bones. There are two layers of the articular capsule. The first layer is the outer fibrous membrane which usually contains ligaments. Secondly, the capsule has the inner synovial membrane which produces and releases the synovial fluid that works as a lubricant at the joint, absorbing shock and nourishing the joint.

Regions of the Spine

Further, the synovial joint bones are covered by hyaline cartilage layer which lines the ends of the joint bone epiphyses with a slippery and smooth surface that ensure that they are not bound together. The articular cartilage absorbs shock and reduces friction when there is movement at the joint.

Joint Classification of Structures and Ranges of Movement for the Following Joints

1. Hinge

The hinge joint is classified as part of synovial joints found at the ankle, the knee and at the elbow. Usually, a hinge joint is formed between two or even more bones particularly where these bones are only able to move by flexing or extending along only one axis (Hewitt & Stringer, 2008). Therefore, the range of movement for a hinge joint is limited only one axis. The hinge joint functions by enabling the bending and the straightening of the limbs.

1. Ball and Socket

The ball and socket joint consists of one ball-shaped end of a bone which can fit into a cavity that is cup-shaped to accommodate it (Hewitt & Stringer, 2008). The ball and socket joint allows the movement of the bones in all directions unlike in hinge joints. It is found for instance where the femur articulates with the glenoid fossa.

• Pivot

The pivot joint consists of a joint where one bone usually has to rotate within a collar, which is formed by another bone (Nagaraja, 2013). For instance, there exists a pivot joint between the skeletal atlas and axis which are found at the cervical neck region. This joint allows an individual to have their heads turn to either side.

1. Gliding

A gliding joint is also classified as the plane joint and forms part of the synovial joints too. Gliding joints are found between the bones which meet at both flat and almost flat surface (Nagaraja, 2013). The gliding joins allows an individual’s bones to glide past each other in whatever direction but along the joint plane. A gliding joint can enable free movement of bones up and down, diagonally and to the left and the right.

7. Differences between bone and cartilage

Bones and cartilages are different both structurally and in terms of their functions.  Bones are hard tissues which from the body’s skeletal structure. In comparison, cartilages are not as hard, and have no rigidity as that in the bones (Nagaraja, 2013). Cartilages are found in the ear, the nose, and the joints. Cartilages in the joints cover the ends of joint bones and therefore act as shock absorbers in order to prevent the bones from creating friction when they rub against each other.

Anatomical Bones

Physical Structural difference- Bones consist of both living cells and dead cells that are embedded in the matrix.  The compact bone is the outer layer which contains some spaces within it. The porous spongy tissue in the inner part of the bone contains both the bone marrow and the blood vessels (Senoglu et al, 2010).  Cartilages on the other hand consist of the chondrocyte cells that produce the extracellular matrix which comprises of proteoglycan, elastin, and collagen fibers. The different cartilage types have varying proportions of these structures. Unlike the bones, the cartilage has no blood vessels.

Cellular Structure Difference-The bone consists of osteoblasts that produce osteocytes. The osteocytes are mature cells in the bone, while osteoclasts are large bone cells which breakdown the bone tissues during repair, growth and remodeling (Kisała & Pluskiewicz, 2015). The bone also has lining of cells which regulate calcium levels and phosphate ion movement into and out of a bone. The cartilages consist of chondrocytes that are produced by the precursor cells called chondroblasts. The chondroblasts secrete elastin and collagen fiber dense matrix both of which form an embedment for mature chondrocyte cells.

8. The Gross and Microscopic Structure a striated Muscle

Gross Anatomy of the Muscle

Grossly, muscles have both blood and nerve supply which enables neural control and an adequate delivery of nutrients together with removal of wastes. Definitely, a muscle consists of numerous muscle cells, connective tissues, nerve fibers and blood vessels (Kawashima &Sasaki, 2011). Muscles consist of sheaths of connective tissues which are found at the different structural muscle levels in every muscle. The endomysium according to Rossi et al (2008) for instance, is the sheath that surrounds every muscle fiber while the perimysium is the sheath which surrounds different groups of the muscle fibers. Further, the epimysium sheath surrounds all muscles wholly.

Microscopic Anatomy of a Skeletal Muscle Fiber

Microscopically, muscles consist of fibers which are both cylindrical and long cells that consisting of numerous nuclei under the cellular sarcolemma. The muscles have myofibrils which forms the largest component of the cellular volume. Myofibrils form the main contractile components of the cells in a muscle (Jaggi et al, 2006).  They have repeating units referred to as sarcomeres that are the myofibril contractile unit, with overlapping myofilaments that are connected to the muscle Z discs at the two ends of the sarcomere.

The myofilaments which form myofibrils are both thick and/or thin in structure. The thick myofilaments are the myosin while the thin ones are actin respectively. The muscular Z disc of the muscle is a component that is mainly made up of actinin, a protein alpha which connects to other similar discs on the myofibrils that are adjacent through the desmin-made intermediate filaments (Jaggi et al, 2006). The muscle has an elastic filament known as titin which anchors thick filaments on to the Z discs. The titin filament also runs through the thick filaments to the muscle M-line. Components such as the dystrophin link the thin filaments in a muscle to the cell sarcolemma, while proteins such as Nebulin, myomesin and vimentin bind together the filaments and/or the sarcomeres (Kisała & Pluskiewicz, 2015). Muscular striations are as a result of dark A-band repeating series which polarize the visible light, and the I-bands which are light and do not polarize the visible light. A-bands in a muscle are muscular parts where both thin and thick filaments overlap. The I-bands on the other hand are found along the muscular Z lines where the thin filaments are mainly present

Structure of Long bones

9. The sliding filament theory (How Muscles Contract)

The sliding filament theory stipulates that muscle contraction mechanism is mainly based on the sliding of muscle proteins past each other and thus leading to movement. The theory indicates that the region where sliding filament contraction process occurs is in the sarcomere (Morrison & Hill, 2011). The contraction occurs when the myosin myofilaments are ratcheted over the actin myofilaments and thus contracting the muscle cell sarcomere (Telley & Denoth, 2007). The sarcomere consists of major regions referred to as the H and the I-band which compress and then expand facilitating the movement.

However, the theory states that the myofilaments neither expand nor contract themselves (Mungal et al, 2015). While the amount of force and the movement generated individual level of a sarcomere can be, small, the multiplication of this effect due to the numerous sarcomeres in a given myofibril makes the force and the movement more significant. Myofibrils form myocytes, which form the muscles and they contain the numerous sarcomere that generate the significant force to enable movement.

10. Stages of Bone Development from Fertilization to Adulthood

At fertilization(Fetal Stage)

The process of bone formation and development at the fetal stage happens in two different processes. These include the intramembranous ossification process and/or the endochondral ossification process.

Intramembranous ossification process-This process occurs majorly when the flat skull bones form. It also involves the formation of the mandible, the maxilla, and the clavicles in a fetus (Snow, Keiver, 2007). At this stage, the bones form from connective tissues like the mesenchyme tissue and not from cartilages. First, the ossification center develops and is followed by the calcification of the bone tissue. The trabeculae form in within the fetus after which the periosteum develops forming the bone structure.

Endochondral ossification- This process starts with points of primary ossification centers in the cartilages. These centers usually appear at the time the fetus is developing. However, some short bones start the primary ossification process after birth (Snow, Keiver, 2007). The primary ossification centers lead to the formation of the long-bone diaphyses, short-bone formation, among other parts found on irregular bones.

1. Secondary Ossification

The process of secondary ossification in humans happens once the baby is born. This process involves the formation of long-bone epiphyses and the irregular/flat-bone extremities. The diaphyses and two long-bone epiphyses are usually separated by the cartilage zone that grows at this stage known as the epiphyseal plate (Senoglu et al, 2010). At skeletal maturity when the child is between the age of 18 and 25 years old, the rest of the cartilage gets replaced by bone tissue. This involves the fusing of the diaphysis and the two epiphyses together forming the epiphyseal closure.

The Structure of a Synovial Joint

Remodeling is a process that is also referred to as bone turnover and involves the resorption and replacement of bones with some little changes in their shapes. This process takes place throughout one’s life and goes on beyond the initial stages of bone development. The coupled osteoblasts and osteoclasts through paracrine cell signaling process form the bone remodeling unit (Snow, Keiver, 2007).  According to studies, an approximate of 10% of an adult’s skeletal mass is remodeled yearly. The remodeling period for bones comprise of the duration for the resorption, osteoclastic reversal, and the formation periods of both growth and the development of bones. This period is the average of the total duration that a single bone remodeling cycle occurs on the bone surface. Remodeling mainly meant for calcium homeostatic regulation and in the repairing of bones that have been micro-damaged due to stress (Plochoki et al, 2016). Remodeling also helps in shaping and sculpting the skeleton during the bone growth process. Repetitive stress activities like weight-bearing exercise and/or the healing of bones leads to the thickening of the bone at points, which involve maximum stress.

References

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