47.0 Introduction

1. Mobility of Animals
1. Most Animals Move From Place to Place
1. Plants and fungi move by growing
2. Only animals explore environment via locomotion fig 47.1

2. Animals Use Contraction of Muscles to Move

47.1 The vertebrate skeleton is composed of bone

1. Types of Skeletons
1. How Animal Locomotion Is Accomplished
1. Results from force of muscles acting on rigid skeleton
2. Three types of animal skeletons
1. Hydraulic skeletons
2. Exoskeletons
3. Endoskeletons

3. Hydraulic skeletons
1. Found in soft bodied invertebrates
2. Fluid-filled cavity surrounded by muscle fibers
3. Pressure of fluid raised when muscles contract
4. Example: Earthworms
1. Wave of contraction of circular muscles compresses body
2. Fluid pressure pushes body forward
3. Longitudinal muscle contractions pulls rest of body forward

4. Exoskeletons
1. Surround body as a rigid, hard case
2. Found in arthropods, made of chitin
3. Limit size due to weight of large exoskeleton

5. Endoskeletons
1. Found in vertebrates and echinoderms
2. Muscles attached to rigid internal skeleton
3. Soft, flexible exterior stretches to accommodate movement of skeleton
4. Vertebrate skeleton composed of bone
1. Bone is a living tissue capable of growth
2. Can self-repair and remodel in response to stress

2. The Human Skeleton fig 47.2
1. The axial skeleton
1. 80 bones
2. Form axis of body, protect organs in head, neck and chest

2. The appendicular skeleton
1. 126 bones attached to axial skeleton
2. Include bones of limbs, pectoral and pelvic girdles

3. Functions of skeletal systems
1. Support and protect body
2. Serve as levers for forces of muscle contractions
3. Blood cells form in bone marrow
4. Matrix of bone serves as reservoir of calcium and phosphate ions

2. The Structure of Bone
1. Special Form of Connective Tissue
1. Organic extracellular matrix of collagen fibers
2. Impregnated with hydroxyapatite (calcium phosphate crystals)
3. Composition of crystals with collagen is unique
1. Hydroxyapatite is strong and rigid but brittle
2. Collagen is flexible but weak
3. Collagen spreads stress over many crystals
4. Makes bone more resistant to fractures

2. Formation of Bone fig 47.3
1. A bone is living, dynamic tissue
1. New bone formed by osteoblast cells
2. Secrete collagen fibers that are subsequently calcified
3. After calcification cells called osteocytes, trapped within lacunae of bone
4. Osteoclasts dissolve bone, help remodel bone when physical stress

2. Microstructure of bone: A Haversian system
1. Lamellae: Concentric layers of bone surrounding Haversian canals
2. Haversian canals interconnect, carry blood vessels and nerve cells
3. Blood flow allows osteocytes to remain alive when embedded in calcified matrix

3. Two types of bone formation
1. Flat bones like skull
1. Osteoblasts located in web of dense connective tissue
2. Produce bone within that tissue

2. Long bones
1. Cartilage skeleton initial template for bone formation
2. Bone formed as cartilage degenerates
3. Cartilage remains at joint surface and growth plates
4. Growth in height occurs at growth plates
5. Plates ultimately calcified, growth stops

4. Long bones composed of two elements
1. Ends and interiors are open lattice of spongy bone tissue
1. Spaces contain marrow
2. Most blood cells formed in bone marrow

2. Surrounded by concentric layers of compact bone tissue
1. Bone is much denser
2. Gives bone strength to withstand mechanical stress

47.2 Skeletal muscles contract to produce movements at joints

1. Types of Joints
1. Bones Interact at Joints or Articulations
1. Skeletal movements produced by contraction and shortening of muscles
2. Tendons attach skeletal muscles to bones

2. Three Kinds of Joints fig 47.4
1. Immovable joints
1. Include sutures
2. Example: Cranial bones
3. Open areas of dense connective tissue in fetus as skull is not fully formed
1. Allows for passage of head through birth canal
2. Bone later replaces most connective tissue

2. Slightly moveable joints
1. Bones bridged by cartilage
2. Example: Vertebral bones in spine
1. Pads of cartilage are intervertebral disks
2. Cushion and allow flexibility

3. Also called cartilaginous joints

3. Freely moveable joints
1. Called synovial joints
2. Articulated end located within synovial capsule with lubricating fluid
3. Ends of bone capped with cartilage
4. Synovial capsule strengthened by ligaments
5. Bones move in direction dictated by structure of joint
1. Arm-shoulder joint has ball-and-socket structure
2. Elbow joint has hinge-like movement

2. Actions of Skeletal Muscles
1. Skeletal Muscles Produce Movement of Skeleton
1. Muscles attach to bones
1. Are usually attached to two different bones
2. May be attached to another structure like skin

2. Connection of muscle to bone called tendon
1. Attachment at origin remains relatively stationary during contraction
2. Insertion end of muscle is attached to bone that moves

2. Muscles May Work in Groups
1. Synergists produce same action at joint
2. Antagonists produce opposing actions
3. Example: Lower leg muscles fig 47.5
1. Quadriceps group extends knee joint
2. Quadriceps muscles are synergists to each other
3. Hamstring muscles contract cause flexion at knee
4. Quadriceps and hamstrings are antagonists
5. Muscles that antagonize are relaxed when opposing set is contracted

3. Muscle Fiber Twitches
1. Twitch: Single brief contraction
1. Muscle fiber stimulated by single impulse on motor neuron
2. Fiber contracts rapidly and relaxes
3. Increasing stimulus voltage increases strength of twitch to a maximum
4. Strength of muscle contraction is graded, varied

2. Summation
1. Second twitch adds to first
2. Time for relaxation between twitches gets shorter
3. Strength of contraction increases

3. Tetanus
1. No visible relaxation between twitches
2. Produces smooth, sustained contraction

4. Isometric and Isotonic Contractions
1. Muscles must generate force greater than opposing forces preventing movement
2. Example: Lifting a weight
1. Force of muscle movement greater than force of gravity on weight
2. Muscle and all fibers shorten in length

3. Isotonic contraction
1. Muscle shortens
2. Constant force of contraction throughout shortening process
3. Means "same strength"

4. Isometric contraction
1. Means "same length"
2. Opposing forces too great, too few fibers activated

47.3 Sliding of the myofilaments produces muscle contraction

1. The Sliding Filament Mechanism of Contraction
1. Microscopic Anatomy of Skeletal Muscle fig 47.7
1. Each muscle contains numerous muscle fibers
1. Each fiber encloses bundle of 4-20 myofibrils
2. Myofibril composed of thin and thick myofilaments
1. Have cross-striations that produce alternating light-dark appearance
2. Result from organization of myofilaments

2. Light and dark banding results from thin and thick myofilaments
1. Thick filaments stack together to form dark A bands
2. Thin filaments alone form light I bands

3. Structure of a sarcomere fig 47.8
1. I band divided in half by Z line, disc of protein
2. Thin filaments anchored to protein disks of Z line
3. Myofibril structure repeats from Z line to Z line
4. Repeating structure called a sarcomere, smallest subunit of muscle contraction

4. Thin and thick filaments overlap
1. Overlap not complete in resting muscle
2. Center, lighter portion called H band
3. Portion on either side contains interdigitating filaments

5. Appearance changes when muscle contracts
1. Muscle contracts and shortens because myofibrils contract and shorten
2. Myofilaments do not shorten
3. This filaments slide deeper into A band fig 47.9
4. H bands become narrower, ultimately disappear
5. I bands also become narrower as A bands come closer together

6. Process called sliding filament mechanism of contraction

2. Molecular Aspects of Muscle Contraction
1. Cross-bridges extend from thick to thin filaments
1. Thick filament molecular structure
1. Myosin proteins each with a protruding head fig 47.10
2. Heads form cross-bridges

2. Thin filament molecular structure
1. Globular actin proteins twisted into double helix fig 47.11
2. Molecular appearance of sarcomere fig 47.12a

2. Muscle contraction associated with cleaving ATP to ADP + Pi
1. At rest myosin heads function as ATPase enzymes
2. Hydrolysis activates myosin heads
3. "Cocks" heads so they can bind to actin and form cross-bridges

3. Cross-bridge formation causes conformational change
1. Pulls thin filament toward center of sarcomere fig 47.12b
2. Called the power stroke
3. Binding another ATP detaches myosin head from actin
1. Lack of ATP in dead animal causes myosin to remain bound to actin
2. Causes stiffened condition called rigor mortis

4. Cleaving that molecule activates myosin head again
5. Myosin head is slightly closer to the Z line at the next cycle

2. The Role of Ca⁺⁺ in Contraction
1. Involves Regulatory Proteins Troponin and Tropomyosin
1. In relaxed muscle myosin heads are "cocked", unable to bind to actin
2. Attachment site physically blocked by tropomyosin
1. Tropomyosin lies against thin filament
2. Cross-bridges can't form, filaments can't slide

3. Troponin moves tropomyosin out of the way
1. Troponin binds to tropomyosin
2. Form complex regulated by calcium ion concentration in muscle cell cytoplasm
3. In relaxed muscle fig 47.14
1. Ca⁺⁺ in cytoplasm is low
2. Tropomyosin blocks myosin heads from binding to actin prevents contraction

4. Increase levels of Ca⁺⁺
1. Ca⁺⁺ binds to troponin
2. Ca⁺⁺-troponin- tropomyosin complex pulled from myosin-binding sites on actin
3. Cross-bridges can form, power strokes occur, contraction produced

4. Muscles store Ca⁺⁺ in sarcoplasmic reticulum (SR) fig 47.15
1. Modified endoplasmic reticulum
2. Stimulation of muscle fiber causes release of Ca⁺⁺ from SR
3. Ca⁺⁺ diffuses into myofibrils
4. Ca⁺⁺ binds to troponin, causes contraction

5. Muscle contraction regulated by nerve activity
6. Nerves influence distribution of Ca⁺⁺ in muscle fiber

3. Nerves Stimulate Contraction
1. Initiation of Skeletal Muscle Contraction
1. Muscle contraction stimulated by nervous system
2. In skeletal muscle associated with somatic motor neurons
1. Axon of one neuron synapses with a number of muscle fibers
2. One axon can stimulate several fibers
3. Fiber has a single synapse with a branch of an axon

3. Events associated with muscle contraction
1. Motor neuron releases neurotransmitter chemical
1. Acetylcholine (Ach) released by somatic motor neurons
2. Excites muscle fiber, stimulates it to produce own electrical impulses

2. Impulses carried along membrane of muscle fiber
1. Also carried along infoldings called transverse (T) tubules
2. Tubules extend deep into muscle fiber

3. Impulses along T tubules stimulate release of Ca⁺⁺ from sarcoplasmic reticulum
1. Calcium ions released into cytoplasm
2. Binds to troponin
3. Causes troponin-tropomyosin complex to shift position on thin filaments
4. Stimulates contraction

4. When impulses stop, nerve no longer releases Ach
1. Impulses in muscle fiber cease
2. No impulse in T tubules, Ca⁺⁺ returned to SR by active transport
3. Troponin not bound to Ca⁺⁺
4. Tropomyosin returns to inhibitory position
5. Muscle relaxes

5. Process called excitation-contraction coupling
1. Neurons produce electrical excitation of muscle fiber
2. Electrical excitation indirectly produces myofilament sliding and contraction
3. Coupled to contraction through action of Ca⁺⁺

2. Motor Units and Recruitment
1. Motor unit: Set of muscle fibers controlled by one neuron fig 47.16
2. Motor neuron produces impulses, all fibers in motor unit contract together
3. Allows for fine gradation of strength of muscle contraction
1. Motor unit with few fibers requires lowest level of activation
2. Results in small contractile force
3. For greater force more motor units are activated

4. Motor units occur in variety of sizes in most muscles
1. Weakest contractions, activation of few small units
2. Stronger contraction may activate more small units
3. Initial increments are small, more units brought on

5. Recruitment
1. Use of increased number and sizes of motor units
2. Causes greater contraction of muscle

4. Types of Muscle Fibers
1. Slow-Twitch versus Fast-Twitch Muscle Fibers fig 47.17
1. Fast-twitch fibers classed as type II fibers
1. Muscles that move eyes have high proportion of fast-twitch fibers
2. Reach maximum tension in 7.3 msec

2. Slow-twitch fibers classed as type I fibers
1. Soleus muscle in leg has high proportion of slow-twitch fibers
2. Requires 100 msec to reach maximum tension

3. Some muscles must be able to sustain contraction for long time without fatigue
1. Resistance to fatigue characteristic of slow-twitch fibers
2. Have high capacity for aerobic respiration
1. Rich capillary supply
2. Numerous mitochondria
3. Numerous aerobic respiratory enzymes
4. High concentration of myoglobin pigment, improves delivery of oxygen
5. Also called red fibers due to myoglobin

4. Comparison to fast-twitch fibers
1. Fewer capillaries and mitochondria
2. Less myoglobin, called white fibers
3. Adapted to respire anaerobically
1. Large stores of glycogen
2. High concentrations of glycolytic enzymes

4. Provide rapid generation of power
5. Grow thicker and stronger with weight training

5. Human muscles also contain type intermediate to I and II
1. Are fast-twitch with high oxidative capacity
2. Are more resistant to fatigue
3. Proportion increased by endurance training

2. Muscle Metabolism During Rest and Exercise
1. Resting muscles utilize aerobic respiration of fatty acids
2. Exercising muscles also use muscle glycogen and blood glucose as energy sources
3. Large amounts of ATP required in muscle contraction
1. Needed for movement of cross-bridges
2. Required to pump Ca⁺⁺ into sarcoplasmic reticulum for relaxation

4. Rapid production of ATP associated with creatine phosphate
1. Combine ADP with phosphate from creatine phosphate
2. Previously formed by adding creatine to phosphate from ATP in respiration

5. Skeletal muscles initial respire anaerobically in heavy exercise
1. Time required to increase oxygen supply to muscles
2. Continues with aerobic respiration if exercise is moderate

6. Aerobic capacity
1. Maximal oxygen uptake, maximum oxygen consumption by aerobic respiration
2. Affects intensity of exercise for a given person

7. Lactate threshold also affects exercise intensity
1. Percentage of maximal oxygen uptake with rise in blood lactate levels
2. Associated with anaerobic exercise

3. Muscle Fatigue
1. Use-dependent decrease in ability to generate force
2. Mainly occurs from operating under anaerobic conditions
1. High activity causes buildup of lactic acid
2. Also depletes stores of glycogen in muscle
3. Energy production then comes from fat

3. Athletes can perform more exercise before muscle fatigue sets in
4. Endurance training does not increase muscle size
1. Muscle size increase dependent on high intensity resistance
2. Exemplified by weight training
3. Increases thickness of fast-twitch fibers
4. Muscles increase via hypertrophy not by increase in numbers by cell division

4. The Oxygen Debt
1. Oxygen consumption remains high at end of strenuous exercise
2. Extra oxygen consumed refer to as oxygen debt
1. Oxygen removed from hemoglobin and myoglobin
2. Oxygen required by tissues warmed during exercise
3. Oxygen associated with metabolism of lactic acid

47.4 Cardiac and Smooth Muscles Are Involuntary

1. Comparing Cardiac and Smooth Muscles
1. Similarities of Cardiac and Smooth Muscle
1. Found within internal organs
2. Generally not under conscious control

2. Differences Between Cardiac and Smooth Muscle
1. Cardiac muscle is striated, contracted via sliding filaments
2. Smooth muscle is not striated

3. Cardiac Muscle
1. Composed of striated fibers, orientation different than skeletal fibers
1. Composed of shorter branched cells with individual nucleus
2. Cells interconnect at intercalated disks fig 47.18
3. Fused membranes pierced by gap junctions
1. Permit diffusion of ions
2. Spread electrical excitation from one cell to next

4. Mass of cells form single, functioning unit called myocardium

2. Structure critical to heart muscle function
1. Contraction initiated at one location called pacemaker
2. Not initiated by impulses in motor neurons
3. Impulses spread from pacemaker throughout myocardium via gap junctions

3. Heart has two myocardia
1. One receives blood from body
2. Other ejects blood to body
3. Cells in each chamber of heart stimulated as single unit
1. Cardiac muscles cannot summate or show tetanus
2. Would interfere with cycle necessary for pumping

4. Smooth Muscle
1. Surrounds hollow internal organs like stomach, intestines, bladder, uterus
2. Surrounds blood vessels (except capillaries)
3. Long, spindle-shaped cells with individual nucleus
1. Individual myofibrils of actin and myosin not organized into sarcomeres
2. Parallel arrangements of thick and thin filaments cross diagonally
3. Thick filaments attached to dense bodies or plasma membrane
4. Have 10-15 thin filaments per thick filament
5. Striated muscle fibers have 3 thin filaments per thick filament

4. Smooth muscle cells do not have sarcoplasmic reticulum
1. Ca⁺⁺comes from extracellular fluid
2. Ca⁺⁺ binds to calmodulin in extracellular fluid
3. Complex activates certain enzyme
4. Enzyme phosphorylates myosin heads, permits formation of cross-bridges

5. Strength of contraction increases with amount of Ca⁺⁺ that enters cytoplasm
1. Drugs can block entry of Ca⁺⁺ into cells, causing vascular smooth muscles to relax
2. Blood vessels dilate, reduces work heart must do to pump blood through them

6. Some smooth muscles contract only when stimulated by nervous system
1. Example: Muscles lining walls of blood vessels, in iris of eye

7. Other smooth muscle like gut lining can contract spontaneously
1. Contain special cells that produce electrical impulses
2. Spread impulses to adjacent cells through gap junctions
3. Leads to slow, steady contraction of tissue

8. Smooth muscle can contract even when greatly stretched
1. Skeletal and cardiac muscle can't contract if too stretched
1. Thick and thin filaments must interdigitate
2. Otherwise cross-bridges can't form

2. Example: Uterus
3. Internal organs are frequently stretched, must still be able to contract