1. A Few Muscular Disorders http://www.hvrsd.org

2. 2 Much of the text material is from, “Principles of Anatomy and Physiology, 14th edition” by Gerald J. Tortora and Bryan Derrickson (2014). I don’t claim authorship. Other sources are noted when they are used. Mappings of the lecture slides to the 12th and 13th editions are provided in the supplements.

3. 3 Muscular Atrophy Muscular atrophy is a condition that involves wasting away of skeletal muscle. In this condition, the individual fibers decrease in size due to the loss of myofibrils. Chapter 10, page 295

4. 4 Disuse Atrophy Disuse atrophy can occur when skeletal muscles are not used, such as in bedridden individuals or in the zero-gravity environment of long- duration spaceflight. The condition results from far less action potentials to skeletal muscle fibers. Disuse atrophy is often reversible. Chapter 10, page 295

5. 5 Denervation Atrophy Denervation atrophy results if the nerve supply to a muscle is cut or disrupted. The muscle shrinks to about one-quarter of its original size in 6 to 24 months. The muscle fibers are irreversibly replaced by fibrous connective tis- sues. Chapter 10, page 295

6. 6 Muscular Hypertrophy Muscle hypertrophy results from increased diameter of muscle fibers due to increased production of myofibrils, mitochondria, sarcoplasmic reticulum, and other organelles. It can occur from forceful, repetitive muscular activity such as strength training. Hypertrophied muscles produce more forceful contractions due to the increased number of myofibrils. Chapter 10, page 295

7. 7 Exercise-Induced Muscle Damage Skeletal muscles may become sore within 12 to 48 hours of intense exercise. Stiffness, tenderness, and swelling may result—recovery often occurs quickly. Chapter 10, page 323

8. 8 Exercise-Induced Muscle Damage (continued) Electron micrographs of muscle tissue from athletes after intense exercise sometimes show exercise-induced muscle damage. The damage includes torn sarcolemmas, damaged myofibrils, and disrupted Z discs. Increases concentrations of some types of proteins—including myo- globin and some enzymes usually confined to muscle fibers—may be detected in the blood. Damaged skeletal muscle fibers can undergo repair, which involves the replacement of torn sarcolemmas and synthesis of new muscle proteins. Chapter 10, page 323

9. 9 Rigor Mortis Rigor mortis is not a muscle disorder, but it is included in this lecture module. Cell membranes, including for skeletal muscle tissue, become leaky soon after the death of an organism. Ca 2+ leaks from the sarcoplasmic reticulum and into the sarcoplasm, enabling myosin heads to bind to the myosin binding sites on actin to form crossbridges. ATP synthesis stops soon after breathing stops, and therefore myosin in the crossbridges cannot detach from actin. Chapter 10, page 305

10. 10 Rigor Mortis (continued) The skeletal muscles become rigid since they can neither contract nor stretch. Rigor mortis begins about 3 to 4 hours after death, and lasts about 24 hours, when enzymes from the lysosomes in the muscle fibers break- down the crossbridges between myosin and actin in the thick and thin filaments. Chapter 10, page 305

11. 11 Botulinum Toxin Botulinum toxin from the bacterium, Clostridium botulinum blocks the release of ACh from the synaptic vesicles—therefore, muscle contrac- tion cannot occur. The bacterium, which can grow in improperly canned foods, is one of the most lethal chemicals known. Breathing stops due to paralysis of respiratory muscles of the external intercostals and diaphragm. Chapter 10, page 307

12. 12 Botulinum Toxin (continued) Injections of the medicine, Botox ® derived from the botulinum toxin can help patients who have: - Crossed eyes (strabismus) - Uncontrolled blinking (blepharospasm) - Spasms of the vocal cords Botox ® is also used to relax the skeletal muscles that cause facial wrinkles. It is also used to alleviate chronic back pain due to muscle spasms in the lumbar region. Chapter 10, page 307

13. 13 Curare Curare, a plant derivative, causes paralysis by binding to and block- ing the ACh receptors in the motor end plates of the neuromuscular junctions. The result is a slowing of the removal of ACh from the postsynaptic region. Skeletal muscles remain in a prolonged stage of contraction (known as tetanus). The diaphragm and external intercostal muscles are no longer able to control inhalation and breathing stops. Chapter 10, page 307

14. 14 Electromyography Electromyography (EMG) is a technique for measuring electrical activity (muscle action potentials) in skeletal muscles. Electrodes are attached to the skin—the electrical potentials are ampli- fied and displayed on an oscilloscope or computer screen. The amount of electrical activity increases with increased forcefulness of the muscle contractions. Chapter 10, page 307

15. 15 Electromyography (continued) EMG can be used to help determine if muscle weakness or paralysis is due to a malfunction of either the muscle or the nerve fibers that supply the muscle. The technique can be used in diagnosing some muscle disorders such as muscular dystrophy. Chapter 10, page 307

16. 16 Myasthenia Gravis Chapter 10, page 323 Myasthenia gravis is an autoimmune disease that causes damage to the neuromuscular junctions, which leads to progressive weakening of the skeletal muscles. The immune system produces antibodies that bind to and block many ACh receptors in the motor end plates. The number of available receptors to initiate a muscle action potential is therefore diminished. Autoimmune disease = an illness that occurs when the body tissues are attacked by its own immune system. (http://www.medterms.com)

17. 17 Myasthenia Gravis (continued) Myasthenia gravis condition occurs in about one in 10,000 people. It is more common in women, with a typical onset between ages 20 and 40. The onset in men is somewhat later, typically between ages 50 and 60. Anticholinesterase drugs are the first-line treatment for inhibiting the enzyme (AChE) that breaks-down ACh. Steroid drugs may be used to reduce antibody levels responsible for the autoimmune reaction. Chapter 10, page 323

18. 18 Muscular Dystrophy Muscular dystrophy is a collection of inherited genetic diseases that can cause progressive degeneration of skeletal muscle fibers. A common type, Duchenne Muscular Dystrophy (DMD), is the result of a recessive gene on the X chromosome of the 23rd chromosomal pair (sex chromosomes). DMD affects boys almost exclusively since males usually have only one X chromosome. Women would need to have the recessive gene on both X chromo- somes, a much rarer event. Chapter 10, page 323

19. 19 Muscular Dystrophy (continued) The incidence of DMD is about 1 in 3,500 newborn males. The rate can be much higher in some tight-knit communities such as in the Amish of southeastern Pennsylvania and other localities where genetic exchange has been restricted. The disorder often becomes apparent between the ages of 2 and 5, when parents notice their child has difficulty in performing physical activities. By age 12, most boys with DMD are unable to walk, and have great difficulty in breathing. Chapter 10, page 323

20. 20 Muscular Dystrophy (continued) The gene that codes for the structural protein dystrophin is mutated in DMD so that little or none of the protein is present in the sarcolemma. The sarcolemma can therefore tear easily during muscle contractions, causing the muscle fibers to rupture and die. Chapter 10, page 323