Vertebrates have an endoskeleton (versus the exoskeleton of insects and the hydroskeleton of other invertebrates; e.g. earthworms, squid). Serves as the framework against which the muscles work. Comprised of bone made from protein (collagen) fibers and crystals of calcium phosphate. The two principal components of the skeletal system are:
The axial skeleton supports the main axis of the body, including the cranium, the vertebral column, and the rib cage. The cranial plates are not completely fused until after birth (usually about 18 months). "Soft spots" are called the fontanels. The thirty three vertebrae are stacked on cartilaginous discs. When these discs slip or are ruptured, the encased nerves are irritated or damaged. This often takes the form of shooting pain in the leg, involving the sciatic nerve.
The appendicular skeleton supports the limbs of the body. The long bones are made of compact bone (for support) and spongy bone (the site of red blood cell production). The compact bone contains structures called Haversian canals which allow oxygen and nutrients to be delivered to the cells (osteoblasts) that make the bone. As a result, bone is very active metabolically, heals relatively quickly, and is constantly being remodeled (with uptake of bone by osteoclasts) to meet demands. Tendons and ligaments are composed of collagen fibers, secreted by fibroblasts. Because of the way the fibroblasts are isolated, tendons and ligaments are much less metabolically active than bone and heal slowly. They attach the muscles to appendicular bones. Joints in the skeletal system connect the bones and they can either be movable or immovable. Arthritis involves inflammation of cartilage in the joints, and the degeneration of that cartilage.
II. Muscles
Muscles generate force only by contraction (shortening). Even muscles that appear to generate force by extension in fact do so by contraction, often involving antagonistic pairs of muscles that exert force in opposite directions, relative to the plane of the body. Skeletal muscle propels the skeletal system and is made up of giant cells called muscle fibers. Smooth muscle propels food through the digestive tract, and provides tension in the bladder, uterus, mammary glands, and arteries. Cardiac muscle, found only in the heart, contracts spontaneously. All three types of muscle rely on the same biochemical events for contraction.III. Exercise PhysiologyIn skeletal muscle, a nerve impulse travel to the muscle, causing the release of calcium ions into the muscle fiber. The release of calcium ions setsin motion the sequence of events resulting in muscle contraction. In the sliding filament hypothesis for muscle function, calcium attaches to a protein called troponin attached to a larger protein filament called actin. When calcium attaches to troponin, this allows a crossbridge to form between the actin filament and an adjacent filament called myosin. The crossbridge, which requires an ATP to form, has the effect of sliding the filaments over one another, like a ratchet-type movement. The crossbridge can only be broken if: 1) Calcium is removed from the system and 2) A new ATP molecule is supplied. Calcium is pumped out of the system to a special type of internal membrane system, the sarcoplasmic reticulum (SR). The faster the muscle, the more SR must be present to handle rapid calcium turnover. Energy is required to pump the calcium away from the filaments. In rigor mortis, no ATP is supplied and crossbridges remain in place. During electrical shocks, calcium removal is impeded, and muscles contract continuously and involuntarily. Cramps result from ionic imbalances in the blood and body fluids, causing an involuntary release of calcium.
Fiber type of muscles - red (or oxidative) fibers versus white (glycolytic) fibers. Any one muscle is made up of a combinationof both types of cells. Although muscles do change in response to training, fiber type appears to be largely determined by genetics. Red fibers are smaller, slower, less powerful, more richly invested with mitochondria, capillaries, and myoglobin (a special oxygen storage pigment). In fact, "red" fibers appear red because of the myoglobin. Because they rely on aerobic metabolism (burning either glycogen or fats) they have high endurance.
White fibers are big, powerful, fast, with few mitochondria, capillaries, and no myoglobin. They are packed with contractile proteins (myosin and actin), and have extensive SR. Reliance on anaerobic glycolysis causes buildup of lactic acid and rapid fatigue.
Performance correlates of fiber type - variation among individuals predisposes athletes to success in specific events. World class cross-country skiers and marathoners have 75-80% red fibers, wrestlers, sprinters have predominantly white. Animals also vary even more dramatically in their fibertype. Ectotherms (animals that cannot generate sufficient metabolism to elevate body temperature) have mostly glycolytic fibers, whereas endothermic (birds, mammals, some sharks, tuna, pythons, insects, even flowers) vertebrates have predominantly red fibers.
Causes of fatigue Creatine phosphate depletion, for short bursts of intense exercise (< 10 sec)
Lactic acid buildup, for exercise lasting 2-15 min
Glycogen depletion ("hitting the wall") for exercise lasting 20 min to several hours
Neuromuscular dysfunction (for very long term exercise over many hours or days) Effects of training
There are two basic types of training-- power training and endurance training. Fiber type does not change with either of these. Power training makes muscles more glycolytic, increasing fiber size (but not number) and speed.In endurance training, muscles become more oxidative, the heart is larger,and the blood volume increases. This means that well-trained enduranc athletes have low resting heart rates and blood pressure. Increases in blood volume help to dissipate heat loads during exercise. Training also increases the blood concentrations of HDL's, reducing the chance of atherosclerosis.