Compendium Review: Movement

Picture from School House Rock's "Not So Dry Bones."

http://www.school-house-rock.com/Bone.html

I Introduction


II Skeletal System

• Function

III Anatomy of Bones

• Bone Growth

IV Muscular System

V Muscle Fibers

• Muscle Contraction


• Energy
  

VI Conclusion

I Introduction

The skeletal and muscle systems work together to give us movement. Skeletal muscle via impulses from the nervous system enable us to move the various parts of our body. In the next few paragraphs I will be discussing the bone, muscles, and skeleton and how they work. I will start with the bones and end with the muscles.

Graphic pointing out the axial and appendicular skeleton found at www.aclasta.co.nz/osteoporosis/index.htm

II Skeletal System

There are 206 bones in the human skeleton. The skeletal system serves several important purposes. First, the skeleton serves as a frame and support for the human body. The design of our skeleton promotes flexibility and enables us to perform different articulations and complex movements. Second, certain bones like those of the skull, rib cage, and vertebral column help protect important body parts for example the brain and lungs. Our bones also produce blood cells and store minerals and fat.

The skeletal system is composed of the axial skeleton and the appendicular skeleton. The axial skeleton consists of the skull the vertebral column, and the ribcage. The skull includes the cranium and the facial bones. The cranium itself (in adults) is composed of 8 bones. The vertebral column includes 33 vertebrae that protect the spinal cord. There are intervertebral disks between vertebrae that serve as a kind of padding to keep vertebrae from grinding against each other. Twelve ribs on each side of the body originate from the thoracic vertebrae. The seven upper ribs connect to the sternum which is also called the breastplate, this bone helps protect the heart and lungs.

An illustration of the skull from www.georgehernandez.com/.../Health/Anatomy.asp

The appendicular skeleton is made up of the bones of the limbs and the pectoral and pelvic girdles. Pectoral means shoulder and pelvic refers to the hip, the arms and legs are attached respectively to the pectoral and pelvic girdles. Because of ball and socket joints our arms and legs are able to perform a wide range of motion. Synovial joints, having cavities filled with fluid to lubricate the joint allow free movement. Synovial joints (AKA hinge joints) allow movement mostly in one direction. Ligaments which connect bone to bone help to make joints stronger. The longest bone in the body is the femur which is the thighbone. The hands and feet are composed of many bones to promote flexibility.

Illustration showing synovial joint movements from Human Biology page 223, Sylvia S. Mader.

III Anatomy of Bones

Long bones have a main portion called the diaphysis. Inside of the diaphysis is the medullary cavity which is filled with yellow bone marrow that stores fat. The ends of long bones are called epiphyses and are coated with a layer of hyaline cartilage. Except for this cartilage at the ends long bones are covered by fibrous connective tissue called periosteum which contains blood vessels.

Compact Bone

Compact bone is composed of osteons and is highly organized. Bone cells called osteocytes are arranged in tiny compartments called lacunae around a central canal. The lacunae are connected to each other by tiny canals called canaliculi. Osteocytes exchange nutrients and wastes with blood vessels.

Spongy Bone

Unlike compact bone, spongy bone is disorganized and contains lots of thin plates separated by unequal spaces. In many cases, the spaces of spongy bone are filled with red bone marrow. Red bone marrow produces all types of blood cells.

Cartilage

Cartilage is more flexible than bone. It is composed of cells called chondrocytes. There are also no nerves or blood vessels in cartilage making it slow to heal.

Fibrous Connective Tissue

Ligaments are made of fibrous connective tissue. Ligaments connect bone to bone at the joints.

Bone Growth

The skeleton of a human embryo begins forming at 6 weeks. Bone growth can continue through age 25. Bones can also heal through bone repair and respond to stress through remodeling.There are different cells involved in bone growth. Osteoblasts are bone forming cells, osteocytes are maintain the structure of bone, and osteoclasts break down bone.

The formation of bone is called ossification. During embryonic development bones are formed in two ways, intramembranous ossification and endochondral ossification.

Intramembranous Ossification: Flat bones like those in the skull are formed in this way. In this form of growth, bones develop between sheets of connective tissue. Osteoblasts are responsible for secreting the living matrix of bone made up of mucopolysaccharides and collagen fibrils.

Endochondral Ossification: This is the way most of our bones are formed. in this process cartilaginous models are replaced by bone. Bone formation spreads from the center to the ends. It starts with the cartilage model, which is made out of hyaline cartilage and shaped like the future bone. Osteoblasts secrete a living bone matrix which calcifies. the result is known as the bone collar. Osteoblasts in the interior begin to lay down spongy bone, this area is called the primary ossification center. Osteoclasts absorb the spongy bone of the diaphysis and the resulting cavern like space is called the medullary cavity. Secondary ossification centers are formed in the epiphyses shortly after birth. Between the primary and secondary ossification centers is the epiphyseal growth plate. The bones will continue to grow as long as this plate is present. There are four layers to this plate. The top layer is the resting zone, then comes the proliferating zone, the degenerating zone and the ossification zone where bone is forming. The growth plates in the arms and legs typically close in women around age 18 and in men around age 20.

This chart shows endochondral ossification of a long bone. Human Biology, page 210, Sylvia S. Mader

This graphic shows bone growth and the epiphyseal growth plate, Human Biology, page 211. Sylvia S. Mader

Bone remodeling helps keep bones strong. remember that bone is constantly broken down by osteoclasts and reformed by osteoblasts. This kind of "recycling" allows our bodies to regulate the amount of calcium we have in our blood. remodeling also helps our bones respond to stress. Exercise and strength training can stimulate the work of oesteoblasts, making our bones stronger. Osteoporosis is a condition of low bone density. Calcium is removed from bones quicker than it is replaced. This can lead to fractures.

Bones can repair themselves when a fracture occurs. First a hematoma (a mass of clotted blood) is formed and fills the space between the broken bones. Tissue repair begins when a fibrocartilaginous callus fills the space and stays there for about two to three weeks. Spongy bone is produced by osteoblasts and begins to join the bones together. Remodeling is the last step, new compact bone is formed at the periphery and a medullary cavity is formed. Fractures can be complete (broken all the way through the bone) or incomplete.

IV Muscular System

Muscle are made to contract and cause some part of the body to move. As we discussed in a past unit, there are three types of muscle, Smooth muscle, cardiac muscle, and skeletal muscle. All of these muscle tissues are made up of muscle fibers but they also have different characteristics. For this compendium I am focusing on skeletal muscle.

The skeletal muscles have many important functions. Our muscles help support us and keep us upright against the force of gravity. They also make our bones move. Muscle contractions are also responsible for movements like facial expressions and breathing. Muscle contraction causes ATP breakdown which results in heat. This helps our body maintain its temperature. Skeletal muscle contraction also plays a role in keeping are blood and lymph moving. Organs and joints are also protected by skeletal muscle by support and padding. I think it is safe to say looking at all these functions that the skeletal muscle system plays a role in homeostasis.

A whole muscle is composed of bundles skeletal muscle fibers known as fascicles. Muscles have a covering of connective tissue called fascia which becomes tendon. Muscles usually work in groups and have an origin, on a stationary bone and an insertion on a moving bone. The single muscle that does most of the work is called a prime mover. Other muscles, called synergists are often working with the prime mover to make the action more effective. A muscle that works opposite a prime mover is called the antagonist. Muscles are named for characteristics like size, shape, location, direction of muscle fibers, attachment, number of attachments, and action.

V Muscle Fibers

Muscle fibers are cells and have the normal cellular componets, except some of these compomponents have different names. The cell membrane is known as the sarcolemma; the cytoplasm is called the sarcoplasm, and the endoplasmic reticulum is known as the sarcoplasmic reticulum.A unique feature of the muscle fiber is the T system. T tubules in the sarcolemma penetrate the cell with larger parts of the sarcoplasmic reticulum. It is here that calcium is stored, and calcium is necessary for muscle contraction. Myofibrils are encased inside the sarcoplasmic reticulum. Myofibrils are contractile portions of muscle fibers. Myofibrils run the length of muscle fibers. Inside of muscle fibers are dark colored striations made up of myofilaments within myofibrils called sacromeres. A sacromere stretches between two dark lines known as Z lines. Within sacromeres are two kinds of protein myofilaments. One is thick and called myosin, the thin myofilament is actin. An I band is light colored and contains actin filaments attached to a Z line. The dark parts of the A band have overlapping actin and myosin filaments. The H zone contains only myosin filaments.

This graphic taken from Human Biology page 233, shows skeletal muscle structure and function. Sylvia S. Mader

Muscle Contraction

Motor neurons stimulate muscle fibers to contract. I talked about nerve impulses in my last compendium. In the case of a muscle contraction the neurotransmitter is acetylcholine (ACh). ACh is released into the synaptic cleft when a nerve impulse arrives at the axon terminal. Once released, ACh diffuses across the synaptic cleft and binds to receptors in the sarcolemma. Next, impulses generated by the sarcolemma spread down T tubules to the sarcoplasmic reticulum.The sarcoplasmic reticulum releases Ca2+ which causes sacromere contraction.

This graphic illustrates a neuromuscular junction. You can see ACh in the lower right corner diffusing across the synaptic cleft. Human Biology, Page 234, Sylvia S. Mader.

When a muscle fiber contracts, the sacromeres (inside myofibrils) shorten. This causes actin filaments to slide past the myosin. The I band shortens, the Z line moves inward, and the H zone almost disappears. This is known as the sliding filament model.

There are two other proteins involved with the actin filament. Tropomyosin winds around the actin filament and troponin is found in intervals along the threads. When released, CA2+ combines with troponin, causing tropomyosin to change position. Now myosin binding sites are exposed and myosin and actin can now bind.

This illustration shows how calcium, and myosin work in muscle contraction. Human Biology, page 235, Slvia S. Mader.

A motor unit is made up of a nerve fiber and all the muscle fibers it stimulates. If one muscle fiber in a motor unit is stimulated, all its counterparts in that unit are as well. The number of muscle fibers within a motor unit varies. Some are very large, like those controlling the gastrocnemius, and some are much smaller like in the case of of the muscles moving the eyes.

Energy

Fuel for excercise can come from stored muscle or blood. Glycogen and fat are stored in muscles and the amount used depends on how hard and how long a person is working their muscles. Energy is also taken from blood glucose and plasma fatty acids. These sources of energy are provided by the blood. Muscle cells can get more ATP by formation of ATP in the creatine phosphate pathway, through fermentation, or through cellular respiration. Normal aerobic exercise relies on cellular respiration.

VI Conclusion

I guess the main thing I got out of this topic is that movement is a very complicated process. We don't often stop and think about the amount of energy and activity that is involved in simple muscle movements. The multitude of funtions of the skeletal and muscular systems was aslo very interesting to me. The roles these systems have in homeostasis was not something I had considered before reading these chapters.

Works Cited:

Human Biology, Sylvia S. Mader

Movement PowerPoint, Professor Frolich

www.georgehernandez.com/.../Health/Anatomy.asp

www.aclasta.co.nz/osteoporosis/index.htm

www.school-house-rock.com (not really a work cited, more for fun)