Structure and Function of Exercising Muscle Chapter 1 Structure and Function of Exercising Muscle

Chapter 1 Overview Functional anatomy of skeletal muscle Muscle fiber Myofibril Muscle fiber contraction Skeletal muscle and exercise Muscle fiber types Muscle fiber (motor unit) recruitment Orderly recruitment of muscle fibers (size principle) Fiber type and athletic success

Three Types of Muscle Tissue Smooth muscle: involuntary, hollow organs Cardiac muscle: involuntary, heart Skeletal muscle: voluntary, skeleton

Figure 1.1

Skeletal Muscle Anatomy Entire muscle Surrounded by epimysium Consists of many bundles (fasciculi) Fasciculi Surrounded by perimysium Consists of individual muscle cells (muscle fibers) Muscle fiber Surrounded by endomysium Consists of myofibrils divided into sarcomeres

Figure 1.2

Structure of Muscle Fibers Plasmalemma (cell membrane) Fuses with tendon Conducts action potential Maintains pH, transports nutrients Satellite cells Muscle growth, development Response to injury, immobilization, training

Structure of Muscle Fibers Sarcoplasm Cytoplasm of muscle cell Unique features: glycogen storage, myoglobin Transverse tubules (T-tubules) Extensions of plasmalemma Carry action potential deep into muscle fiber Sarcoplasmic reticulum (SR): Ca2+ storage

Figure 1.3

Myofibrils and Sarcomeres Muscle  fasciculi  muscle fiber  myofibril Hundreds to thousands per muscle fiber Sarcomeres Basic contractile element of skeletal muscle End to end for full myofibril length

Figure 1.4

Sarcomeres Distinctive striped appearance (striations) A-bands: dark stripes I-bands: light stripes H-zone: middle of A-band M-line: middle of H-zone Common boundary structure: Z-disk

Sarcomere: Protein Filaments Used for muscle contraction Actin (thin filaments) Show up lighter under microscope I-band contains only actin filaments Myosin (thick filaments) Show up darker under microscope A-band contains both actin and myosin filaments H-zone contains only myosin filaments

Myosin (Thick Filaments) Two intertwined filaments with globular heads Globular heads Protrude 360° from thick filament axis Will interact with actin filaments for contraction Stabilized by titin

Actin (Thin Filaments) Actually composed of three proteins Actin: contains myosin-binding site Tropomyosin: covers active site at rest Troponin: anchored to actin, moves tropomyosin Anchored at Z-disk Equally spaced out by titin

Figure 1.5

Motor Units • a-Motor neurons innervate muscle fibers Motor unit Single a-motor neuron + all fibers it innervates More operating motor units = more contractile force Neuromuscular junction Site of communication between neuron and muscle Consists of synapse between a-motor neuron and muscle fiber

Figure 1.6

Skeletal Muscle Contraction (Excitation-Contraction Coupling) 1. Action potential (AP) starts in brain 2. AP arrives at axon terminal, releases acetylcholine (ACh) 3. ACh crosses synapse, binds to ACh receptors on plasmalemma 4. AP travels down plasmalemma, T-tubules 5. Triggers Ca2+ release from sarcoplasmic reticulum (SR) 6. Ca2+ enables actin-myosin contraction

Figure 1.7

Role of Ca2+ in Muscle Contraction AP arrives at SR from T-tubule SR sensitive to electrical charge Causes mass release of Ca2+ into sarcoplasm Ca2+ binds to troponin on thin filament At rest, tropomyosin covers myosin-binding site, blocking actin-myosin attraction Troponin-Ca2+ complex moves tropomyosin Myosin binds to actin, contraction can occur

Sliding Filament Theory: How Muscles Create Movement Process of actin-myosin contraction Relaxed state No actin-myosin interaction at binding site Myofilaments overlap a little Contracted state Myosin head pulls actin toward sarcomere center (power stroke) Filaments slide past each other Sarcomeres, myofibrils, muscle fiber all shorten

Figure 1.8

Sliding Filament Theory: How Muscles Create Movement After power stroke ends Myosin detaches from active site Myosin head rotates back to original position Myosin attaches to another active site farther down Process continues until Z-disk reaches myosin filaments or AP stops, Ca2+ gets pumped back into SR

Figure 1.9

Energy for Muscle Contraction Adenosine triphosphate (ATP) Binds to myosin head ATPase on myosin head ATP  ADP + Pi + energy Necessary for muscle contraction

Muscle Relaxation AP ends, electrical stimulation of SR stops Ca2+ pumped back into SR Stored until next AP arrives Requires ATP Without Ca2+, troponin and tropomyosin return to resting conformation Covers myosin-binding site Prevents actin-myosin cross-bridging

Muscle Fiber Types Type I Type II ~50% of fibers in an average muscle Peak tension in 110 ms (slow twitch) Type II Peak tension in 50 ms (fast twitch) Type IIa (~25% of fibers in an average muscle) Type IIx (~25% of fibers in an average muscle)

Figure 1.10

Type I Versus Type II Differentiation Speed of myosin ATPase varies Fast myosin ATPase = fast contraction cycling Slower myosin ATPase = slower contraction cycling Muscle biopsy Small (10-100 g) piece of muscle removed Frozen, sliced, examined under microscope Gel electrophoresis Type I versus II fibers have different types of myosin Separates different types of myosin by size

Muscle Biopsy

Figure 1.11

Type I Versus Type II Sarcoplasmic reticulum Motor units Type II fibers have a more highly developed SR Faster Ca2+ release, 3 to 5 times faster Vo Motor units Type I motor unit: smaller neuron, <300 fibers Type II motor unit: larger neuron, >300 fibers

Type I Versus Type II: Peak Power Peak power: type IIx > type IIa > type I Effects of different SR, motor units, etc. Single muscle fiber recording Regardless of fiber type, all muscle fibers reach peak power at ~20% peak force

Single Muscle Fiber Peak Power

Table 1.1

Distribution of Fiber Types: Type I:Type II Ratios Each person has different ratios Arm and leg ratios are similar in one person Endurance athlete: type I predominates Power athlete: type II predominates Soleus: type I in everyone

Type I Fibers During Exercise High aerobic endurance Can maintain exercise for prolonged periods Require oxygen for ATP production Low-intensity aerobic exercise, daily activities Efficiently produce ATP from fat, carbohydrate

Type II Fibers During Exercise Type II fibers in general Poor aerobic endurance, fatigue quickly Produce ATP anaerobically Type IIa More force, faster fatigue than type I Short, high-intensity endurance events (1,600 m run) Type IIx Seldom used for everyday activities Short, explosive sprints (100 m)

Table 1.2

Fiber Type Determinants Genetic factors Determine which a-motor neurons innervate fibers Fibers differentiate based on a-motor neuron Training factors Endurance versus strength training, detraining Can induce small (10%) change in fiber type Aging: muscles lose type II motor units

Muscle Fiber Recruitment Also called motor unit recruitment Method for altering force production Less force production: fewer or smaller motor units More force production: more or larger motor units Type I motor units smaller than type II Recruitment order: type I, type IIa, type IIx

Orderly Recruitment and the Size Principle Recruit minimum number of motor units needed Smallest (type I) motor units recruited first Midsized (type IIa) motor units recruited next Largest (type IIx) motor units recruited last Recruited in same order each time Size principle: order of recruitment of motor units directly related to size of a-motor neuron

Fiber Type and Athletic Success Endurance athletes—type I predominates Sprinters—type II predominates Fiber type not sole predictor of success Cardiovascular function Motivation Training habits Muscle size

Types of Muscle Contraction Static (isometric) contraction Muscle produces force but does not change length Joint angle does not change Myosin cross-bridges form and recycle, no sliding Dynamic contraction Muscle produces force and changes length Joint movement produced

Dynamic Contraction Subtypes Concentric contraction Muscle shortens while producing force Most familiar type of contraction Sarcomere shortens, filaments slide toward center Eccentric contraction Muscle lengthens while producing force Cross-bridges form but sarcomere lengthens Example: lowering heavy weight

Generation of Force Motor unit recruitment Type II motor units = more force Type I motor units = less force Fewer small fibers versus more large fibers Frequency of stimulation (rate coding) Twitch Summation Tetanus

Generation of Force Length-tension relationship Optimal sarcomere length = optimal overlap Too short or too stretched = little or no force develops Speed-force relationship Concentric: maximal force development decreases at higher speeds Eccentric: maximal force development increases at higher speeds

Figure 1.12

Figure 1.13

Figure 1.14