1. CHAPTER 46 THE MUSCULAR- SKELETAL SYSTEM AND LOCOMOTION
Prepared by
Brenda Leady, University of Toledo
Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. Skeleton
Structure or structures that serve one or more functions related to support, protection, and locomotion
3 types
Hydrostatic
Exoskeleton
Endoskeleton

3. Hydrostatic skeleton Combination of muscles and water
Water is nearly incompressible
Hydrostatic pressure can be used to move the body
Cnidarians – body and tentacles can elongate or shorten
Earthworms – move forward by passing a wave of muscular contractions along the length of the body
Circular muscles squeeze and elongate while longitudinal muscles shorten and widen

5. Exoskeleton External skeleton surrounding and protecting body
Vary in complexity, thickness, and durability
Arthropods – made of chitin, segmented for movement, must be shed to grow

7. Endoskeleton Internal structures
Do not protect body surface, only internal organs and other structures
Sponges, echinoderms, and vertebrates
Minerals give firmness

9. Vertebrate skeleton 2 parts
Axial – main longitudinal axis
Appendicular – limb bones and girdles
Joint – formed where 2 or more bones come together
Pivot joints
Hinge joints
Ball-and-socket joint

11. Bone Living, dynamic tissue
Organic components – cells that form and break down bone and collagen (for strength and flexibility)
Mineral components – Ca2+, PO4- and other ions

12. Skeletal muscle structure
Muscle is a grouping of cells (muscle fibers) bound together by connective tissue
Tendons link bones to skeletal muscle
Skeletal muscle fibers increase in size during growth but no new fibers are formed

13. Myofibrils Striated muscle named for striped pattern
One unit is a sarcomere
Thick filaments made of myosin
Thin filaments are made of actin, troponin, and tropomyosin

14. A band – wide band of myosin
H zone – narrow region in center of A band, space between the 2 sets of thin filaments
M line – in center of H zone, proteins that link central regions of adjacent thick filaments
Z line – 2 sets of thin filaments anchored to network of proteins at this point
2 successive Z lines make a sarcomere
I band – contains portions of thin filaments that do not overlap thick filaments
Cross-bridges extend from surface of myosin toward thin filaments

16. Sliding filament mechanism
Sarcomeres shorten as thin filaments slide past stationary thick filaments
Myosin cross-bridges attach to thin filament and force thin filament toward center of sarcomere
Cross-bridge repeats motion as long as stimulation to contract continues

18. Actin
Molecules form 2 intertwined helical chains associated with troponin and tropomyosin
Each actin molecule contains a binding site for myosin

19. Myosin
6 protein subunits combine to form a protein with 2 heads and a long tail
Tail lies along axis of thick filament
2 heads form cross-bridges
Each head contains bonding site for actin and ATP
Myosin proteins at 2 ends of thick filament oriented in opposite directions so thin filaments brought toward center of sarcomere

21. Did an Ancient Mutation in Myosin Play a Role in the Development of the Human Brain?
Myosins are among the most ancient of eukaryotic proteins
Also among the most highly conserved
MYH16 expressed in jaw muscles of 8 nonhuman primates but is a nonfunctional mutant in 100% of people tested
Mutation occurred 2.4 mya
Same time Homo appeared
Smaller, less muscular jaws may have let the skull and brain grow larger
Recently, analyses showed mutation may have arisen much earlier

23. Cross-bridge cycle
Sequence of events between time when cross-bridge binds to a thin filament and when it is set to repeat the process
4 steps
Cross-bridge binds to actin
Cross-bridge moves and filament slides past each other – power stroke
ATP binds to myosin, causing cross-bridge to detach
Hydrolysis of ATP re-energizes the cross-bridge

25. Regulation of contraction
Tropomyosin
Rod-shaped molecule composed of 2 intertwined proteins
Arranged along length of actin thin filament
In absence of calcium, cover myosin-binding sites

26. Troponin Smaller protein bound to both tropomyosin and actin
Binds Ca2+ and drags tropomyosin off of myosin-binding site and contraction begins
Removal of Ca2+ reverses the process and contraction stops

28. Excitation-contraction coupling
Skeletal muscle cells are capable of generating and propagating action potentials
Causes rise in cytosolic Ca2+ released from sarcoplasmic reticulum
Transverse or T-tubules are invaginations of plasma membrane that conduct the action potential from the outer surface to inner regions
Triggers contraction
Ion pumps will return calcium to the sarcoplasmic reticulum, troponin and tropomyosin slide back in place, and contraction stops

30. Neuromuscular junction
Junction of motor neuron’s axon and muscle fiber
Axon divides into terminals containing vesicles of acetylcholine
Region of muscle fiber under axon terminal is folded into junctional folds to increase surface area
ACh receptor is ligand-gated ion channel
Na+ flows into muscle cell leading to depolarization and an action potential

32. Skeletal muscle function
Different muscle fibers contain forms of myosin that differ in the maximal rates at which they can hydrolyze ATP
Fast fibers contain myosin with high ATPase activity
Slow fibers have myosin with a lower ATPase activity
Maximal force produced by each is the same, only speed varies

33. Oxidative fibers Glycolytic fibers
Contain numerous mitochondria and have a high capacity for oxidative phosphorylation
Depends on blood flow to deliver oxygen and nutrients for ATP production
Contain large amounts of myoglobin as an intracellular reservoir of oxygen
Glycolytic fibers
Few mitochondria but a high concentration of glycolytic enzymes and large stores of glycogen
Contain little myoglobin – makes them pale or white

34. 3 major types Slow-oxidative fibers Fast-oxidative fibers
Low rates of myosin ATP hydrolysis but can make large amounts of ATP, used for prolonged, regular activity
Fast-oxidative fibers
High myosin activity and can make large amounts of ATP, particularly suited for rapid actions
Fast-glycolytic fibers
High myosin activity but cannot make as much ATP, best suited for rapid, intense actions but fatigues quickly

35. Exercise
Increased amounts of exercise can produce an increase in the size of muscle fibers and their capacity for ATP production
Increase in muscle size due to increase in size of individual fibers
Atrophy is the reduction in size of a muscle
Happens if neurons at neuromuscular junction becomes nonfunctional

36. Evans and Colleagues Activated a Gene to Produce “Marathon Mice”
Discovered one way in which ratios of oxidative and glycolytic fibers change in skeletal muscle
Activation of PPAR-delta results in expression of genes that enable cells to more efficiently burn fat
Started as research in obesity prevention
Created transgenic mice expressing PPAR-delta in skeletal muscle
Transgenics gained less weight and had dramatic shift to slow-oxidative muscle fibers
Switch in fiber type does not require exercise per se

38. Contracting muscle exerts a force on bones through its connecting tendons
Contracting muscle exerts only a pulling force
Muscles that bend a limb are flexors
Muscles that straighten a limb are extensors
Groups of muscles with oppositely directed motions at a joint are antagonists

39. Muscles, bones, and joints arranged in lever systems
Lever system amplifies the velocity of muscle shortening
Short, relatively slow movements of a muscle produce faster movements of the hand

40. Animal locomotion Water Greatest challenge is the density of water
Resistance increases exponentially as speed increases
Streamlined bodies reduce drag and make swimming more efficient
Energetic advantage is that swimmers do not need to provide lift to overcome gravity
Swimming similar among many vertebrates
Invertebrates use means other than swimming – squid propulsion, move passively on current, crawl on rocks

42. Land – walking and running
Energetically costliest means of locomotion
Gravity must be overcome at each step
Accelerating and decelerating with each step is more important
Most animals limit ground contact to reduce friction
Except mollusks on mucus and snakes undulating

45. Air – flying Evolved on 4 occasions
Pterosaurs, insects, birds, and mammals (bats)
Numerous advantages – escape, scan large areas, inhabit inaccessible areas
Mechanics require overcoming gravity and air resistance
Resistance reduced by streamlined bodies
Lift and thrust in vertebrates provided by pectoral and back muscles

46. Bird and bat wings are modifications of the forelimbs
Bat wings more maneuverable with fingers at the end of forelimb/wings but can’t glide for long
Birds have less control but large birds can glide for long periods of time

47. Impact on public health
Rickets
Improper mineral deposition
Usually due to inadequate dietary calcium intake or inadequate absorption of calcium from the small intestine
Prevented or treated with vitamin D

48. Osteoporosis Result of prolonged disuse of muscles
Force produced by active skeletal muscle contractions helps maintain bone mass
May also result from hormonal imbalances like decline in estrogen at menopause
Can be minimized with adequate calcium intake, weight-bearing exercise, and adequate dietary vitamin D

49. Myasthenia gravis – MS
Characterized by skeletal muscle fatigue and weakness
Affects 10,000 – 30,000 Americans
Body’s immune system produces antibodies that bind to and inactivate ACH receptors on skeletal muscles – autoimmune disease
Treatments range from enzyme inhibitors that allow Ach to remain longer to immune system suppression to plasmapheresis to remove antibodies from the blood

50. Muscular dystrophy
Group of diseases affecting 1 in 3500 American males (less common in females)
Associated with progressive degeneration of skeletal and cardiac muscles ultimately leading to death
Most common form is Duchenne muscular dystrophy – sex-linked recessive disorder