Skeletal Muscle Contraction Day 1 Introduction Muscle fibers

Skeletal Muscle Contraction (Day 1) Introduction Muscle fibers and their associated connective tissues are flexible, but can tear if overstretched. This type of injury, common in athletes, is called muscle strain. The seriousness of the injury depends on the degree of damage the tissues sustain. If the strain is mild, only a few muscle fibers are injured, the fascia remains intact, and loss of function is minimal. In a severe strain, however, many muscle fibers as well as fascia tear and muscle function may be completely lost. Such a severe strain is painful and produces discoloration and swelling. 1. What is a muscle strain? 2. What activities generally lead to muscle strain? 3. Who typically gets muscle strains? 4. What do you think could be done to limit muscle strains?

Skeletal Muscle Contraction (Day 1) Explicit Instruction Neuromuscular Junction ◦ The site where the motor neuron and muscle fiber meet is the neuromuscular junction. ◦ The muscle fiber membrane forms a motor end plate in which the sarcolemma is tightly folded and where nuclei and mitochondria are abundant. ◦ The cytoplasm of the motor neuron contains numerous mitochondria and synaptic vesicles storing neurotransmitters. Motor Units ◦ A motor neuron and the muscle fibers it controls make up a motor unit; when stimulated to do so, the muscle fibers of the motor unit contract all at once.

Skeletal Muscle Contraction (Day 1) Explicit Instruction Muscle contraction involves several components that result in the shortening of sarcomeres, and the pulling of the muscle against its attachments. Role of Myosin and Actin ◦ Myosin consists of two twisted strands with globular crossbridges projected outward along the strands. ◦ Actin is a globular protein with myosin binding sites; tropomyosin and troponin are two proteins associated with the surface of the actin filaments.

Skeletal Muscle Contraction (Day 1) Explicit Instruction Role of Myosin and Actin (continued) ◦ According to the sliding filament theory of muscle contraction, the myosin cross-bridge attaches to the binding site on the actin filament and bends, pulling on the actin filament; it then releases and attaches to the next binding site on the actin, pulling again. ◦ Energy from the conversion of ATP to ADP is provided to the cross-bridges from the enzyme ATPase, causing them to be in a “cocked” position.

Skeletal Muscle Contraction (Day 1) Explicit Instruction Stimulus for Contraction ◦ The motor neuron must release the neurotransmitter acetylcholine from its synaptic cleft in order to initiate a muscle contraction. ◦ Protein receptors in the motor end plate detect the neurotransmitters, and a muscle impulse spreads over the surface of the sarcolemma and into the T tubules, where it reaches the sarcoplasmic reticulum. ◦ Upon receipt of the muscle impulse, the sarcoplasmic reticulum releases its stored calcium to the sarcoplasm of the muscle fiber.

Skeletal Muscle Contraction (Day 1) Explicit Instruction Stimulus for Contraction (continued) ◦ The high concentration of calcium in the sarcoplasm interacts with the troponin and tropomyosin molecules, which move aside, exposing the myosin binding sites on the actin filaments. ◦ Myosin cross-bridges now bind and pull on the actin filaments, causing the sarcomeres to shorten. ◦ After the nervous impulse has been received, acetyl cholinesterase rapidly decomposes the acetylcholine. ◦ Then, calcium is returned to the sarcoplasmic reticulum, and the linkages between myosin and actin are broken.

Skeletal Muscle Contraction (Day 1) Guided & Independent Practice SLIDING FILAMENT MODEL GP Draw a diagram of the sliding filament model and explain what happens during contraction. Be sure to include in your drawing: Actin A band Myosin ATP Tropomyosin ADP + P Troponin Cross-Bridges Sarcomere Ca++ Z line THE PROCESS OF CONTRACTION IP 1. Explain how a motor nerve impulse can trigger a muscle contraction. 2. Explain how the filaments of a myofibril interact during a muscle contraction.

Skeletal Muscle Contraction (Day 2) Introduction The bacterium Clostridium botulinum produces a 1. poison, called botulinum toxin, that can prevent 2. the release of acetylcholine from motor neuron axons at neuromuscular junctions, causing botulism, a very serious form of food poisoning. Most cases of botulism are cause by eating home 3. -processed food that has not been heated enough to kill the bacteria in it or to inactivate the toxin. Botulinum toxin blocks stimulation of muscle fibers, paralyzing muscles, including those responsible for breathing. Without prompt medical treatment, the fatality rate for botulism is high. Very small amounts of botulism toxin (“botox”) are injected into facial skin to temporarily smooth wrinkles by preventing local muscles from contracting. If done to excess, botox can cause the face to appear frozen. What is botulinum toxin? What effect does botulinum toxin have on the body? How is botulinum toxin used in the medical world?

Skeletal Muscle Contraction (Day 2) Explicit Instruction Energy Sources for Contraction ◦ Energy for contraction comes from molecules of ATP. This chemical is in limited supply and so must often be regenerated. ◦ Creatine phosphate, which stores excess energy released by the mitochondria, is present to regenerate ATP from ADP and phosphate. ◦ Whenever the supply of ATP is sufficient, creatine phosphokinase promotes the synthesis of creatine phosphate. ◦ As ATP decomposes, the energy from creatine phosphate can be transferred to ADP molecules, converting them back to ATP.

Skeletal Muscle Contraction (Day 2) Explicit Instruction Oxygen Supply and Cellular Respiration ◦ The early phase of cellular respiration yields few molecules of ATP, so muscle has a high requirement for oxygen, which enables the complete breakdown of glucose in the mitochondria. ◦ Hemoglobin in red blood cells carries oxygen to muscle. ◦ The pigment myoglobin stores oxygen in muscle tissue.

Skeletal Muscle Contraction (Day 2) Explicit Instruction Oxygen Debt ◦ During rest or moderate activity, there is enough oxygen to support aerobic respiration. ◦ Oxygen deficiency may develop during strenuous exercise, and lactic acid accumulates as an end product of anaerobic respiration. ◦ Lactic acid diffuses out of the muscle cells and is carried in the bloodstream to the liver. ◦ Oxygen debt refers to the amount of oxygen that liver cells require to convert the accumulated lactic acid into glucose, plus the amount that muscle cells need to resynthesize ATP and creatine phosphate to their original concentrations. ◦ Repaying oxygen debt may take several hours.

Skeletal Muscle Contraction (Day 2) Explicit Instruction Muscle Fatigue ◦ When a muscle loses its ability to contract during strenuous exercise, it is referred to as fatigue. ◦ Muscle fatigue usually arises from the accumulation of lactic acid in the muscle. ◦ A lowered p. H as a result of accumulation of lactic acid prevents the muscle from contracting. ◦ A muscle cramp occurs due to a lack of ATP required to return calcium ions back to the sarcoplasmic reticulum so muscle fibers can relax.

Skeletal Muscle Contraction (Day 2) Explicit Instruction Heat Production ◦ Concentration of skeletal muscle represents an important source of heat for the body. ◦ Much of the energy produced through the reactions of cellular respiration is lost as heat (another source of heat for the body).

Skeletal Muscle Contraction (Day 2) Guided Practice STEROIDS AND ATHLETICS GP Read “Steroids and Athletes – An Unhealthy Combination” on page 186 in your textbook. 1. Who is Ben Johnson? 2. What evidence was there to support that he used performance enhancing drugs? 3. What are the negative effects of using steroids? 4. Where did steroid abuse begin and why?

Skeletal Muscle Contraction (Day 2) Independent Practice MUSCLE CONTRACTION IP 1. Which biochemical provide the energy to regenerate ATP? 2. What are the sources of oxygen for aerobic respiration? 3. How are lactic acid, oxygen debt, and muscle fatigue related? 4. What is the relationship between cellular respiration and heat production?