Structure and Function Chapter 8 Skeletal Muscle Structure and Function

Skeletal Muscle Human body contains over 400 skeletal muscles 40-50% of total body weight Functions of skeletal muscle Force production for locomotion and breathing Force production for postural support Heat production during cold stress

Connective Tissue Covering of Muscle

Structure of Skeletal Muscle: Microstructure Sarcolemma Muscle cell membrane Myofibrils Threadlike strands within muscle fibers Actin (thin filament) Troponin Tropomyosin Myosin (thick filament) Myosin head Crossbridge location

Microstructure of Skeletal Muscle

Structure of Skeletal Muscle: The Sarcomere Functional unit Z-line – “anchor” points for actin filaments Sarcomere is Z-line to Z-line Within the sarcoplasm Transverse tubules Communication into the cell Sarcoplasmic reticulum Storage sites for calcium Terminal cisternae

Within the Sarcoplasm

The Neuromuscular Junction Site where motor neuron meets the muscle fiber Separated by gap called the neuromuscular cleft Motor end plate Pocket formed around motor neuron by sarcolemma Acetylcholine is released from the motor neuron Causes an end-plate potential Depolarization of muscle fiber

Illustration of the Neuromuscular Junction

Muscular Contraction The sliding filament model Muscle shortening is due to movement of the actin filament over the myosin filament Reduces the distance between Z-lines

The Sliding Filament Model of Muscle Contraction

Cross-Bridge Formation in Muscle Contraction

Energy for Muscle Contraction ATP is required for muscle contraction Myosin ATPase breaks down ATP as fiber contracts

Excitation-Contraction Coupling Depolarization of motor end plate (excitation) is coupled to muscular contraction Nerve impulse travels along sarcolemma and down T-tubules to cause a release of Ca++ from SR Ca++ binds to troponin and causes position change in tropomyosin, exposing active sites on actin Permits strong binding state between actin and myosin and contraction occurs ATP is hydrolyzed and energy goes to myosin head which releases from actin

Steps Leading to Muscular Contraction

CD ROM

Properties of Muscle Fibers Biochemical properties Oxidative capacity Type of ATPase on myosin head Contractile properties Maximal force production Speed of contraction / speed of stimulation Muscle fiber efficiency

Individual Fiber Types Slow fibers Type I fibers Slow-twitch fibers Slow-oxidative fibers Fastest fibers Type IIb(x) fibers Fast-twitch fibers Fast-glycolytic fibers Fast fibers Type IIa fibers Intermediate fibers Fast-oxidative glycolytic fibers

Muscle Fiber Types (x)

Comparison of Maximal Shortening Velocities Between Fiber Types

Histochemical Staining of Fiber Type Type IIb (x) Type IIa Type I

Fiber Types and Performance Successful Power athletes Sprinters, jumpers, strikers Possess high percentage of fast fibers Successful Endurance athletes Distance runners, cyclists, swimmers Have high percentage of slow fibers Others Skill sport athletes and probably non-athletes Have about 50% slow and 50% fast fibers

How Is Fiber Type Distributed In a Population? 50% Each 100% ST 100% FT

How do I get more FT muscle fibers? Grow more? Change my ST to FT? Pick the right parents?

Alteration of Fiber Type by Training Endurance and resistance training Cannot make FT into ST or ST into FT Can make a shift in IIa…. toward more oxidative properties toward more glycolytic properties

So how do I gain strength?

Protein Arrangement and Strength Increase sarcomeres (myofibrils) in parallel more cross bridges to pull requires more room in diameter requires more fluid hypertrophy

Protein Arrangement and Strength Increase in sarcomeres in series? Only with growth in height / limb length Sarcomeres would crowd each other Would result in decreased force production

Protein Arrangement and Strength Hyperplasia? more cells stem cells – for injury repair

Protein Arrangement and Strength Fiber arrangement Fusiform penniform

Protein Arrangement and Strength More sarcomeres in parallel, more tension produced More cross bridges generating tension

Age-Related Changes in Skeletal Muscle Aging is associated with a loss of muscle mass (sarcopoenia) independent of training status associated with less testosterone Regular exercise / training can improve strength and endurance Cannot completely eliminate the age-related loss in muscle mass

Types of Muscle Contraction Isometric Muscle exerts force without changing length Pulling against immovable object Stabilization Postural muscles

Types of Muscle Contraction Isotonic (dynamic) Concentric Muscle shortens during force production Eccentric Muscle produces force but length increases

Types of Muscle Contraction Isokinetic (dynamic) Constant speed of contraction Machine assisted

Isotonic and Isometric Contractions

Force and Speed of Contraction Regulation in Muscle Types and number of motor units utilized More motor units = greater force “Recruitment” More FT motor units = even greater force “Contribution”

Histochemical Staining of Fiber Type Type IIb Type IIa Type I

Force Regulation in Muscle Initial muscle length “Ideal” length for force generation Maximum vs. minimum cross bridging

Length-Tension Relationship in Skeletal Muscle

Force Regulation in Muscle Nature of the motor units’ neural stimulation Frequency of stimulation Simple twitch, summation, and tetanus

Illustration of a Simple Twitch

Simple Twitch, Summation, and Tetanus

Relationship Between Stimulus Frequency and Force Generation

Determinants of Contraction Velocity - Single Fiber Nerve conduction velocity Myofibrillar ATPase activity

Determinants of Contraction Velocity - Whole Muscle Same factors as for single fiber Nerve conduction velocity ATPase activity Number of FT fibers recruited

Force-Velocity Relationship At any absolute force the speed of movement is greater in muscle with higher percent of fast-twitch fibers The fastest fibers can contribute while the slow cannot The maximum force is greatest at the lowest velocity of shortening True for both slow and fast-twitch fibers All can contribute at slow velocities

Force-Velocity Relationship Force (Newtons) FT ST Velocity (deg./sec)

Force-Power Relationship At any given velocity of movement the power generated is greater in a muscle with a higher percent of fast-twitch fibers The peak power increases with velocity up to movement speed of 200-300 degrees•second-1 Force decreases with increasing movement speed beyond this velocity

Force-Power Relationship

Is there a safety mechanism?

Receptors in Muscle Muscle spindle Detect dynamic and static changes in muscle length Stretch reflex Stretch on muscle causes reflex contraction

Muscle Spindle

Receptors in Muscle Golgi tendon organ (GTO) Monitor tension developed in muscle Prevents damage during excessive force generation Stimulation results in reflex relaxation of muscle

Golgi Tendon Organ

Questions?

End

Force-Velocity Relationship

Structure of Skeletal Muscle: Connective Tissue Covering Epimysium Surrounds entire muscle Perimysium Surrounds bundles of muscle fibers Fascicles Endomysium Surrounds individual muscle fibers

Speed of Muscle Contraction and Relaxation Muscle twitch Contraction as the result of a single stimulus Latent period Lasting only ~5 ms Contraction Tension is developed 40 ms Relaxation 50 ms

Illustration of a Simple Twitch

Illustration of the Steps of Excitation-Contraction Coupling

Training-Induced Changes in Muscle Fiber Type