Lesson 12

In this lesson, we will learn how the muscles and skeleton function to allow movement in animals. Take a moment to review the learning objectives for this lesson. We will cover each one in turn. We will begin this lesson with the skeleton. There are three types of skeletons found in animals. A hydrostatic skeleton incorporates a fluid-filled central cavity surrounded by muscles. The muscles work against the fluid. And exoskeleton consists of a rigid outer covering that protects internal organs and provides an attachment site for muscles. The endoskeleton found in vertebrates and echinoderms consists of rigid internal elements that form a framework and offer attachment points for muscles. The hydrostatic skeleton of earthworms uses two sets of muscles working against the fluid-filled body cavity. Contraction of circular muscles, pressurizes the fluid. Contraction of longitudinal muscles causes bristle like external structures called chaetae, to protrude and contact the ground to hold the body in place. Waves of circular muscle contraction down the body, followed by waves of longitudinal muscle contraction allow for movement. Arthropods such as insects and crustaceans have an exoskeleton made of the polysaccharide chitin. Exoskeleton is not as strong as a bony skeleton. And as we will see an upcoming lessons, the respiratory system sets a limit on the body size. An exoskeleton completely encases the muscles and internal organs and therefore limits growth of the animal. Because of this, the exoskeleton must be periodically shed and a new larger one regrown. During this period of molt, the animal is vulnerable to predation. The endoskeleton is a rigid internal skeleton that forms the body's framework and offers a surface for muscle attachment. Endoskeletons of echinoderms such as sea urchins and starfish, are composed of calcite or calcium carbonate. Vertebrate endoskeletons are composed of bone and cartilage and dense fibrous connective tissue. Some large animals like sharks and rays, have an endoskeleton composed primarily of cartilage. Most vertebrates bone is the primary skeletal components. Bone is stronger but less flexible than cartilage. Both bone and cartilage are living tissues. We will now take a closer look at bone and cartilage. Cells that make up bone and cartilage are derived from embryonic connective tissue called mesenchyme. Undifferentiated mesenchyme cells can give rise to three types of cells involved in bone development. Fibroblasts are cells that produce collagen. Chondroblasts produced cartilage, and eventually become mature chondrocytes and osteoblasts, which form bone and become osteocytes. The osteoclasts shown in this figure are not derived from mesenchymal cells. They develop from a type of white blood cell and are involved in bone removal or dissolving. Cartilage is a specialized connective tissue. Cartilage is designed to withstand compression and tension. It is tough but flexible and is found in many joints between bones. Like all connective tissues. It has cells surrounded by a matrix. The cells are derived from chondroblasts. Chondroblasts secrete extracellular matrix and often becomes stuck inside cocoon like spaces called lacunae and are then called chondrocytes are cartilage cells. The matrix secreted by the chondroblasts is composed of a glycoprotein rich ground substance and collagen fibers. Oxygen and nutrients reach the cells by diffusion from surrounding blood vessels. There are no blood vessels within cartilage itself. Because cartilage is a vascular and aging chondroblasts lose their ability to divide. Cartilage heals slowly, as many people with sports injuries can attest to. Bone is also a specialized connective tissue and is composed of both organic and inorganic components. The matrix in this case is made of collagen fibers and polysaccharides which add to the flexibility of bone and its ability to stretch and twist. The majority of bone consists of the inorganic hydroxyapatite or mineral salts. The osteoblasts secrete an enzyme which causes calcium phosphate to form the crystalline hydroxyapatite, which accounts for the hardness of bone. Some of the osteoblasts become trapped in the matrix and differentiate into osteocytes. Osteocytes reside in lacunae which have little canals extending out called canaliculi. Starburst like extensions of the osteocytes can interact with neighboring cells through the colliculi, providing a means of intercellular communication. Most mammals have vascular bones with internal blood vessels. The Halvorson system is the internal organization of vascular bone. Halvorson lamella are layers of bone laid down around narrow channels called and canals. The canals run parallel to the length of the bone and contain blood vessels and sometimes nerve fibers. Note the difference in bone type between the spongy bone in the epiphysis of the bone, the medullary bone inside the shaft, and the compact bone found in the perimeter of the shaft. Each type of bone is specialized for its location. Pause the lecture and review the lesson objectives covered so far. Bone tissue develops from fibrous membranes and cartilage, and there are two types of development, intramembranous development and endochondral development. Intramembranous development occurs between membranes and is typical of flat bones, such as the bones in the skull and the sternum. This figure illustrates endochondral bone development. Bones begin as cartilaginous models. Calcified bone replaces the outer covering and then replaces the internal cartilage. Vertebrae, ribs and long bones of the limbs go through endochondral development. Growth of endochondral bones occurs at the widened ends of the bone called the epiphysis. Inside the epiphysis, our growth plates made of cartilage. The cartilage contains young cells that undergo mitosis. These cells thicken as they age and eventually die. The thickening of the cells lengthens the bone. The cartilage is simultaneously calcified from the shaft side. Growth in length stops in late adolescence, but growth in width can continue throughout life. Bones are continually changing. At any particular time, osteoblasts are depositing new bone while osteoclasts are resorbing established bone. Together, these two processes are called bone remodeling. Bone remodeling is stimulated by mechanical stresses and regulatory hormones, which will be discussed in future lectures. In the image above, we will see that a bone will remodel as additional stresses are placed on it. Part B of the figure shows an exaggerated image of a bone bending due to compressive load. The osteoblasts are signaled to produce more matrix and the newly thickened bone resists bending as shown in D. Once the bone is thick enough to resist bending, bone deposit stops. Some individuals suffer from osteoporosis, which is when bone resorption outpaces bone deposit. Bones become so fragile that normal activity can break them. In addition to a diet high in calcium and vitamin D. Another treatment for osteoporosis is increased physical exercise, including light weightlifting. Based on this diagram, can you explain the rationale for that treatment? There are many types of joints in the body. The diagram above depicts some of the joints that allow movement. All of these joints are powered by muscles. Ball and socket joints allow the greatest amount of movement and are located in the hip and shoulder. Hinge joints allow only movement forward or backward and are found in the knee, elbow, and within the fingers. Gliding joints allow sliding of one surface to another. And it can be found between vertebrae and the bones in the hand, among others. The joints in the jaw or an example of a combination joint that allows both rotation and side-to-side movement. All of the joints described produce motion powered by muscles. Muscles Power Movement at joints when they contract, muscles may attach directly to bone or tendons may attach the muscle to the bone. One attachment of the muscle, the origin remains stable while the other end, the insertion is attached to the portion of the bone that moves. Typically muscles are arranged so that movement produced by contraction of one muscle can be reversed by contraction of another muscle, which is often called the antagonist muscle. This figure illustrates the antagonistic movement produced by the contraction of the hamstring muscles, which caused the lower leg to move backward, and the contraction of the quadriceps which pulls the lower leg forward. Pause the lecture and review the lesson objectives covered so far. You should remember from earlier lectures that there are three kinds of muscles in the body. Smooth, skeletal and cardiac. Smooth muscle is normally found in the internal organs, is in voluntarily controlled and tends to contract and relax very slowly. The cardiac muscle cells are found only in the walls of the heart, are involuntarily controlled. And the network of fibers are connected by intercalated discs, which allow the fibers to contract as a unit. Furthermore, as we will see in future lessons, the cardiac muscles are self exciting and rhythmic, which means the heart directs its own beating. The last type of muscle is skeletal muscle, and we will focus on that type for the remainder of the lesson. Vertebrate skeletal muscle power is voluntary skeletal movements. So let's now look at the structure of skeletal muscle. Each skeletal muscle consists of bundles of long skeletal muscle fibers or muscle cells. Notice that muscle cells are also multinucleated. The individual muscle fibers contain a bundle of four to 20 myofibrils, which are composed of thick and thin myofilaments. It is the myofibrils that account for the contraction of skeletal muscles. And we will spend the next few minutes looking at the interaction of the thick and thin myofilaments. When viewed under a microscope. The myofibrils have alternating light and dark bands. The dark bands are produced by stacked thick filaments. These dark bands are called the A bands. The light bands are formed by the thin filaments alone. These are the I bands. Each I-band is divided in half by the Z-line, which is a disc of protein. This structure repeats itself from Z-line to Z-line and is called a sarcomere. The sarcomere is the smallest unit of muscle contraction. Note the gray colored zone within the A band called The H band, where only the thick filaments are visible. Now compare the top figure to the lower failure. To see how a sarcomere shortens during muscle contraction, you will notice that there is greater overlap of the thick and thin filaments in the contracted muscle. And the I and H bands have narrowed significantly because the sarcomeres repeat down the length of the myofibrils. When all sarcomeres contract, the myofibrils shortens considerably. The following animation demonstrates the functioning of sarcomeres via the sliding filament mechanism. In a relaxed muscle, actin and myosin myofilaments lie side-by-side and the H zones and I-band are at maximum width. During contraction, the actin and myosin myofilaments interact. The actins are pulled toward the center of each myosin myofilament. As a result, the sarcomeres shorten. In the fully contracted muscle, the ends of the actin myofilaments overlap. The H zones disappear and the I band becomes very narrow. Myosin proteins are motor proteins that are able to convert the chemical energy in ATP into mechanical energy. This figure of the cross-bridge cycle illustrates the series of events that occur during this conversion. In figure A, the hydrolysis of ATP to ADP and Pi causes a conformational change that moves the myosin head into an energized or ready state. The energized head can form a cross-bridge with the actin filaments shown in B. During the power stroke, ADP and Pi or released. And the myosin head shifts into a low-energy position that pulls the attached myosin filament in one direction. Following the power stroke. ATP can bind to the myosin head, which weakens the link between myosin and actin. And the cross-bridge breaks. ATP hydrolysis returns myosin to the energized conformation and the cycle can begin again. The cycle continues as long as the muscle is stimulated to contract. Note that it takes energy to break the cross-bridge between actin and myosin. Living cell always has enough ATP to allow the myosin head to detach. However, dead cells do not produce ATP. Thus the cross-bridges do not break. This is what causes the muscle stiffness following death called rigor mortis. Rigor mortis begins approximately 3 h after death and reaches peak rigidity around 12 hours. Once muscle proteins start breaking down, rigor mortis gradually decreases. The cross-bridge cycle outlined in the previous slide occurs during a muscle contraction. But muscles also exist in a relaxed state when they are not stimulated to contract. In a relaxed muscle, the myosin heads are in the energized confirmation, but they do not form cross bridges with actin because the attachment site is blocked by the protein tropomyosin, which is the filamentous protein shown in green. Attached to tropomyosin is a globular protein called troponin, which is shown in blue. When a muscle is stimulated to contract, the concentration of calcium ions in the muscle fiber cytoplasm increases. Calcium ions bind to troponin, which changes its shape and causes the attached tropomyosin filaments to move, thus revealing actin binding site. At the end of a contraction, calcium ions must be pumped out of the cells so that the muscle can return to its relaxed state. Muscles need a steady supply of calcium for contraction and store it in a modified endoplasmic reticulum called the sarcoplasmic reticulum. In this figure we see the neuromuscular junction between a motor neuron and a myofibril of a muscle. The calcium storing sarcoplasmic reticulum is shown in purple. When a motor neuron releases a neurotransmitter into the synapse at the neuromuscular junction and the muscle cell is depolarized. The depolarization is conducted down the cell membrane and down the transverse tubules. Transverse tubules are invaginations of the cell membrane which carry the depolarization deep into the muscle cell, to the sarcoplasmic reticulum, which then releases calcium ions. You should note that at the neuromuscular junctions, motor neurons release the neurotransmitter acetylcholine. Pause the lecture and review the lesson objectives covered so far. The somatic motor neurons that stimulate muscle cells have branched axons, which allow them to make synapses with a number of muscle fibers. In humans, each muscle cell has only a single synapse with a branch of one axon. The set of muscle fibers innervated by a single branch axon is called a motor unit. All muscle fibers in a motor unit contract simultaneously when stimulated. As illustrated in this figure, precise muscle actions like toe tapping have small motor units and larger muscle movements like running require additional motor units. The more motor units activated, the stronger the contraction. The graph above demonstrates the relationship between amplitude of a muscle contraction or tension and time. The response of a motor unit to a single action potential is called a twitch. Muscle fibers contract and then quickly relax. Rarely does this occur naturally, but it will result when muscles are stimulated with a single electric shock. Most natural muscle contractions are smooth and very intention. When multiple stimuli are applied to a muscle before it can relax, the subsequent twitches will be stronger than the first. This is called summation. If the frequency of stimulation is increased, there is no relaxation between twitches and the contraction is smooth and sustained. As in normal muscle contraction. This is called incomplete tetanus and performs as a sustained but shaky muscle contraction. If stimulation frequency continues to increase, maximum tension can be achieved. No relaxation occurs and a smooth sustained contraction plateaus. This is called complete tetanus and rarely occurs. Instances of complete tetanus are when people encounter emergency situations and get a dose of superhuman strength. It should be noted that tetanus should not be confused with the bacterial disease that causes involuntary muscle contractions. Skeletal muscle fibers are divided on the basis of their contraction speed into slow-twitch and fast-twitch fibers. Fast-twitch fibers provide rapid generation of power and some can respire anaerobically using a large storage of glycogen. Thus, they require fewer capillaries, have fewer mitochondria and less myoglobin, which is a pigment that improves delivery of oxygen to muscle fibers. Fast-twitch fibers often appear white, although they contract rapidly, they also tire quickly. Slow twitch muscle fibers rely on aerobic respiration for sustained action, and therefore require more capillaries, more mitochondria, and more myoglobin. Because of the additional capillaries and myoglobin, these fibers are darkly colored. Everyone's muscles contain a mixture of these types of fibers. The percentage of each is determined by body part, but also genetics and can be modified by exercise. For example, marathon runners have a high percentage of slow twitch fibers in their legs, while sprinters have a higher percentage of fast twitch fibers. The graph above demonstrates the contraction of a fast-twitch muscle, slow twitch muscle, and a muscle with the percentage of both types. Pause the lecture and review the lesson objectives covered so far. This completes this lesson. In the next lesson, we'll take a look at the digestive system.