Skeletal Muscle

Structure and Function

Chapter 8

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• Storage sites for calcium

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

Fast fibers Slow fibers

Characteristic Type IIb Type IIa Type I

Number of mitochondria Low High/moderate High

Resistance to fatigue Low High/moderate High

Predominant energy system Anaerobic Combination Aerobic

ATPase activity Highest High Low

Vmax (speed of shortening) Highest Intermediate Low

Efficiency Low Moderate High

Specific tension High High Moderate

(x)

Comparison of Maximal Shortening Velocities Between Fiber Types

(x)

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?

100% FT100% ST

50% Each

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

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

Protein Arrangement and Strength

Hyperplasia?– more cells– stem cells – for injury repair

Protein Arrangement and Strength

Fiber arrangement– Fusiform

– penniform

More sarcomeres in parallel, more tension produced

More cross bridges generating tension

Protein Arrangement and Strength

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 RelationshipF

orce

(N

ewto

ns)

Velocity (deg./sec)

FT

ST

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