Compendium Review: Skeletal System and Muscular System

Skeletal System

Table of Contents:

Overview of Skeletal System
Bone Growth, Remodeling, and Repair
Bones of the Axial System
Bones of the Appendicular Skeleton
Articulations

Overview of the Skeletal System

The skeletal system consists of 206 bones in and adult, along with the cartilage and fibrous connective tissue which occurs in the ligaments in the joints. The skeleton supports the body, protects soft body parts, produces blood cells, stores minerals and fat, and along with muscle, permits flexible body movement.


http://biology.clc.uc.edu/graphics/bio105/skeleton.jpg

The shaft, or main portion of a long bone, is called the diaphysis. The diaphysis has a large medullary cavity, whose walls are composed of compact bone, lined with endosteum, and filled with yellow bone marrow. The expanded region at the end of a long bone is the epipysis and is composed largely of spongy bone that contains red bone marrow. The epiphyses are coated with articular cartilage. Except for the articular cartilage, a long bone is covered with periosteum (a layer of fibrous connective tissue.)


http://www.web-books.com/eLibrary/Medicine/Physiology/Skeletal/long_bone.jpg

Compact bone is highly organized and composed of tubular units called osteons. Bone cells called osteocytes lie in lacunae which are tiny chambers arranged in concentric circles around a central canal. Spongy bone has a much less organized appearance and contains thin plates called trabeculae separated by unequal spaces. Spongy bone is lighter than compact bone, but designed for strength. The spaces of spongy bone are filled with red marrow.


http://www.web-books.com/eLibrary/Medicine/Physiology/Skeletal/compact_spongy_bone.jpg

Cartilage is not as strong as bone, but is more flexible because the gel-like matrix contains collagenous and elastic fibers. The cells, chondrocytes, are irregularly grouped. There are 3 types of cartilage: hyaline, fibrocartilage, and elastic cartilage.


http://washington.uwc.edu/about/faculty/schaefer_w/TISSUES/hyaline_cartilage1.jpg

Fibrous connective tissue contains rows of cells called fibroblasts separated by bundles of collagenous fibers. This tissue makes up the ligaments that connect bone to bone and the tendons that connect muscles to a bone joint.


http://www.bweems.com/gctenmp1.jpg

Bone Growth, Remodeling and Repair

Bones are composed of living tissues as exemplified by their ability to grow, remodel and repair. Several types of cells are involved in bone growth:

• Osteoblasts: bone forming cells that secrete the organic matrix of bone
• Osteocytes: mature bone derived from osteoblasts
• Osteoclasts: bone absorbing cells that break down bone and assist in depositing calcium and phosphate in blood

The term ossification refers to the formation of bone. The bones of the skeleton form during embryonic development in 2 distinctive ways:

• Intramembranous ossification: bones develop between sheets of fibrous connective tissue and cells derived from connevtive tissue cells become osteoblasts, which form spongy bone
  
• Endochondrial ossification: bones replace the cartilaginous models of the bone; gradually the cartilage is replaced by calcified bone

When the epiphyseal plates close, bone length can no longer occur. Growth hormone (GH) stimulates growth of the epiphyseal plates, as well as bone growth in general. Too much or too little GH produces gigantism or dwarfism, respectively.


http://www.gigantism.com/robert-wadlow.jpg

As much as 18% of bone is recycles each year by osteoclasts. Bone recycling allows the body to regulate the amount of calcium in the blood. 2 hormones in particular are involved in regulating the blood calcium level: parathyroid hormone (PTH) and calcitonin. Bone remodeling also accounts why bones can respond to stress. If you engage in an activity that calls upon the use of a particular bone, it will enlarge in diameter at the region most affected by the activity.


http://www.nature.com/ncprheum/journal/v2/n10/images/ncprheum0298-f1.jpg

Repair of a bone is required after it breaks or fractures. Fracture repair takes place over a span of several months in a series of 4 steps:

• Hematoma
• Fibrocartilaginous callus
• Bony callus
• Remodeling

The naming of a fracture tells you what kind of break occurred:


http://images.medicinenet.com/images/illustrations/typical_fractures.jpg

Bones of the Axial Skeleton

The 206 bones of the skeleton are classified according to whether they occur in the axial skeleton or the appendicular skeleton. The axial skeleton lies in the midline of the body and consists of the skull, hyoid bone, vertebral column, and the rib cage.

The skull is formed by the cranium and the facial bones. The cranium protects the brain and in adults is composed of 8 bones fitted tightly together. In newborns, certain cranial bones are not completely formed and are joined by membranous regions called fontanels.


http://www.aafp.org/afp/20030615/2547_f1.gif

The most prominent facial bones are the mandible, the maxillae, the zygomatic bones, and the nasal bones. The mandible (lower jaw) is the only movable portion of the skull and its action permits chewing. The lips and cheeks have a core of skeletal muscle. The zygomatic bones are the cheekbones prominences and the nasal bones. Other bones are part of the nasal septum which divides the interior of the nose into 2 cavities.


http://training.seer.cancer.gov/module_anatomy/popup_illustrations/images/illu_facial_bones.jpg

The hyoid bone is not part of the skull, however it part of the axial skeleton. It is the only bone in the body that does not articulate with another bone. It is attached to the temporal bones by by muscles and ligaments and to the larynx by a membrane. The hyoid bone anchors the tongue.


http://www.yorku.ca/earmstro/journey/images/larynx.gif

The vertebral column consists of 33 vertebrae. Normally, the vertebral column has 4 curvatures that provide more resilience and strength for an upright position. The spinal cord passes through the vertebral canal and gives off spinal nerves at the intervertebral foramina.


http://www.univie.ac.at/cga/courses/be524/spinal/spine2-BB.jpg

The rib cage is also called the thoracic cage and is composed of the thoracic vertebrae, the ribs and their associated cartilages, and the sternum. A rib is a flattened bone that originates at the thoracic vertebrae and proceeds toward the anterior thoracic wall. There are 12 pairs of ribs. The sternum lies in the midline of the body and helps protect the heart and lungs. The sternum is composed of 3 bones: the manubrium, the body, and the xiphoid process.


http://www.daviddarling.info/images/rib-cage.jpg

Bones of the Appendicular Skeleton


http://academic.kellogg.cc.mi.us/herbrandsonc/bio201_McKinley/f8-1a_appendicular_skel_c.jpg

Articulations

Bones are joined at the joints, which are classified as fibrous, cartilaginous, or synovial. Some joint are fibrous and immovable. Cartilaginous joints are connected by hyaline cartilage.

A synovial joint is a joint having a cavity filled with synovial fluid. Synovial joints allow for the movements of flexion, extension, adduction, abduction, rotation, circumduction, inversion and eversion.


http://www.palaeos.com/Vertebrates/Lists/Glossary/Images/SynovialJoint.jpg

Muscular System

Table of Contents:

Overview of Muscular System
Skeletal Muscle Fiber Contraction
Whole Muscle Contraction
Muscular Disorders
Homeostasis

Overview of Muscular System

Humans have 3 types of muscle tissue: smooth, cardiac, and skeletal. The cells of these tissues are called muscle fibers. Smooth muscle fibers are spindle shaped cells, each with a single nucleus. The cells are arranged in parallel lines forming sheets. Striations are seen in cardiac and skeletal, but not in smooth. Smooth muscle is located in the walls of hollow internal organs and it causes the walls to contract. It does not fatigue easily.


http://www.arunnersplace.co.uk/images/smooth.jpg

Cardiac muscle forms the heart wall. Its fibers are generally uninucleated, striated, tubular, and branched, which allows the fibers to interlock at intercalated disks.


http://webanatomy.net/histology/cardiac/cardiac_muscle.jpg

Skeletal muscle fibers are tubular, multinucleated, and striated. Skeletal muscles have numerous functions:

• Skeletal muscles support the body
• Skeletal muscles make bones move
• Skeletal muscles help maintain a constant body temperature
• Skeletal muscles contraction assists movement in cardiovascular and lymphatic vessels
• Skeletal muscles help protect internal organs and stabilize joints

Skeletal muscles are well organized. A whole muscle contains bundles of skeletal muscle fibers called fascicles. Fascia is a type of connective tissue that extends beyond the muscle and becomes its tendon. Tendons quite often extend past a joint before anchoring a muscle to bone.
Skeletal muscles usually function in groups and the nervous system stimulates the appropriate group of muscles.


http://people.uleth.ca/~little/PE2110/funcanatomy/2front%20muscle.jpg


http://people.uleth.ca/~little/PE2110/funcanatomy/2back%20muscle.jpg

Skeletal Muscle Fiber Contraction

A muscle fiber is a cell containing the usual cellular composition, but special names have been assigned to some of these components. The plasma membrane is called the sarcolemma;the cytoplasm is the sarcoplasm;and the endoplasmic reticulum is the sarcoplasmic reticulum.

http://www.sport-fitness-advisor.com/images/muscle_anatomy.jpg

A muscle fiber divides into even smaller parts. Within each fiber are strands of myofibrils. These long cylindrical structures appear striped due to strands of tiny myofilaments. Myofilaments have two types of protein: actin (thin myofilaments) and myosin (thick myofilaments).


http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/an/m5/s5/assets/images/anm5s5_1.jpg

Muscle fibers are stimulated to contract by motor neurons whose axons are in nerves. The axon of one motor neuron can stimulate from a few to several muscle fibers of a muscle because each axon has several branches. The entire region is called a neuromuscular junction.


http://www.colorado.edu/intphys/Class/IPHY3430-200/image/figure7m.jpg

Axon terminals contain synaptic vesicles that are filled with the neurotransmitter acetylcholine (ACh). When nerve impulses traveling down a motor neuron, arrive at an axon terminal, the synaptic vesicles release ACh into the synaptic cleft. ACh quickly diffuses across the cleft and binds to receptors on the sarcolemma. 2 other protens are associated with an actin filament: tropomyosin and troponin.


http://www.octc.kctcs.edu/GCaplan/anat/images/Image331.gif


Whole Muscle Contraction

A motor unit is comprised of the nerve fiber and all the muscle fibers it innervates. When a motor unit is stimulated by infrequent electrical impulses, a single contraction occurs that lasts only a fraction of a second. The response is a muscle twitch which is divided into 3 stages: latent, contraction, and relaxation. Muscles use various fuel sources for energy and they have various ways of producing ATP during muscle contraction.


http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Creatine.gif


Muscle cells store limited amounts of ATP, but they have 3 ways to acquire more ATP: formation of ATP by the creatine phosphate (CP) pathway, fermentation, and cellular respiration. Some muscle fibers metabolize both aerobically and anaerobically. Fast twitch fibers tend to rely on the CP pathway and fermentation which are anaerobic, slow twitch fibers prefer cellular respiration, which is aerobic.

Muscular Disorders

Muscular disorders are divided into those that are commonly seen and those that are more serious. Spasms are sudden involuntary muscular contractions most often accompanied by pain. Multiple spasms of the skeletal muscles are called convulsions. A strain is caused by the stretching or tearing of a muscle.

Tendinitis is an inflammation of a tendon. Bursitis is inflammation of a bursa, a slippery surface where muscle and tendons glide over. NSAIDs are the most common OTC medications for treating arthritis, bursitis, and tendinitis.

Myalgia and Fibromyalgia refer to achy muscles. Myalgia without a traumatic history is usually from a viral infection.


http://simplefibrocure.com/fibrographics.jpg

Muscular dystrophy is a broad term applied to a group of disorders that are characterized by a progressive degeneration and weakening of the muscles. As muscle fibers die, fat and connective tissue take their place.


http://www.dinf.ne.jp/doc/english/global/david/dwe002/dwe002g/dwe00212g01.gif

Myasthenia gravis is an autoimmune disorder characterized by weakness that affects the muscles of the eyelids, face, neck, and extremities. The immune system produces antibodies that destroy ACh which leads to the impairment of muscle contraction.


http://www.myasthenia.org.au/assets/images/eyes2.jpg

Amyotrophic Lateral Sclerosis (ALS) is better known as Lou Gehrig's disease. ALS sufferers experience gradual loss of the ability to walk, talk, chew, and swallow. Mental ability is not affected. There is no cure.


http://graphics.jsonline.com/graphics/news/img/aug07/als_080307_big.gif

Homeostasis

The muscular and skeletal systems work together to achieve homeostasis. Both systems protect body parts and the soft internal organs. They work together to produce movement. The skeletal system (in conjunction with the endocrine system) performs tasks that are vital for calcium homeostasis and cellular metabolism. The bones contain red bone marrow, which is the site of blood production. White blood cells originate in the red bone marrow and they defend the body against pathogens. Muscles also help maintain body temperature.

References:

Mader, S. (2008). Skeletal System and Muscular System. Human Biology, 10th Ed. (p 207-246)

Muscular System (2008.) www.kumc.edu/instruction/medicine/anatomy/histoweb/muscular/muscular.htm

Skeletal System (2008). www.mnsu.edu/emuseum/biology/humananatomy/skeletal/skeletalsystem.html