Chapter 10, Part 2:Muscular Tissue

Muscle Metabolism

Production of ATP in Muscle Fibers A huge amount of ATP is needed to:▪ Power the contraction cycle▪ Pump Ca++ into the SR

The ATP inside muscle fibers will power contraction for only a few seconds

ATP must be produced by the muscle fiber after reserves are used up

Muscle fibers have three ways to produce ATP▪ 1) From creatine phosphate▪ 2) By anaerobic cellular respiration▪ 3) By aerobic cellular respiration

Muscle Metabolism

Muscle Metabolism

Creatine Phosphate Excess ATP is used to synthesize

creatine phosphate▪ Energy-rich molecule

Creatine phosphate transfers its high energy phosphate group to ADP regenerating new ATP

Creatine phosphate and ATP provide enough energy for contraction for about 15 seconds

Muscle Metabolism

Anaerobic Respiration Series of ATP producing reactions that do not

require oxygen Glucose is used to generate ATP when the supply of

creatine phosphate is depleted Glucose is derived from the blood and from

glycogen stored in muscle fibers Glycolysis breaks down glucose into molecules of

pyruvic acid and produces two molecules of ATP If sufficient oxygen is present, pyruvic acid formed

by glycolysis enters aerobic respiration pathways producing a large amount of ATP

If oxygen levels are low, anaerobic reactions convert pyruvic acid to lactic acid which is carried away by the blood

Anaerobic respiration can provide enough energy for about 30 to 40 seconds of muscle activity

Muscle Metabolism

Aerobic Respiration Activity that lasts longer than half a minute depends on

aerobic respiration Pyruvic acid entering the mitochondria is completely

oxidized generating▪ ATP▪ carbon dioxide▪ Water▪ Heat

Each molecule of glucose yields about 36 molecules of ATP

Muscle tissue has two sources of oxygen▪ 1) Oxygen from hemoglobin in the blood▪ 2) Oxygen released by myoglobin in the muscle cell

Myoglobin and hemoglobin are oxygen-binding proteins Aerobic respiration supplies ATP for prolonged activity Aerobic respiration provides more than 90% of the

needed ATP in activities lasting more than 10 minutes

Muscle Metabolism

Muscle Fatigue Inability of muscle to maintain force of

contraction after prolonged activity Factors that contribute to muscle fatigue Inadequate release of calcium ions from

the SR Depletion of creatine phosphate Insufficient oxygen Depletion of glycogen and other

nutrients Buildup of lactic acid and ADP Failure of the motor neuron to release

enough acetylcholine

Muscle Metabolism

Oxygen Consumption After Exercise After exercise, heavy breathing

continues and oxygen consumption remains above the resting level

Oxygen debt▪ The added oxygen that is taken into the body

after exercise This added oxygen is used to restore

muscle cells to the resting level in three ways▪ 1) to convert lactic acid into glycogen▪ 2) to synthesize creatine phosphate and ATP▪ 3) to replace the oxygen removed from

myoglobin

Control of Muscle Tension The tension or force of muscle cell

contraction varies

Maximum Tension (force) is dependent on The rate at which nerve impulses arrive The amount of stretch before contraction The nutrient and oxygen availability The size of the motor unit

Control of Muscle Tension Motor Units

Consists of a motor neuron and the muscle fibers it stimulates

The axon of a motor neuron branches out forming neuromuscular junctions with different muscle fibers

A motor neuron makes contact with about 150 muscle fibers

Control of precise movements consist of many small motor units▪ Muscles that control voice production have 2 - 3 muscle

fibers per motor unit▪ Muscles controlling eye movements have 10 - 20 muscle

fibers per motor unit▪ Muscles in the arm and the leg have 2000 - 3000 muscle

fibers per motor unit The total strength of a contraction depends on the size of

the motor units and the number that are activated

Control of Muscle Tension

Control of Muscle TensionTwitch Contraction

The brief contraction of the muscle fibers in a motor unit in response to an action potential

Twitches last from 20 to 200 msec Latent period (2 msec)▪ A brief delay between the stimulus and

muscular contraction▪ The action potential sweeps over the

sarcolemma and Ca++ is released from the SR Contraction period (10–100 msec)▪ Ca++ binds to troponin▪ Myosin-binding sites on actin are exposed▪ Cross-bridges form

Control of Muscle Tension

Relaxation period (10–100 msec)▪ Ca++ is transported into the SR▪ Myosin-binding sites are covered by

tropomyosin▪ Myosin heads detach from actin▪ Muscle fibers that move the eyes have contraction

periods lasting 10 msec▪ Muscle fibers that move the legs have contraction

periods lasting 100 msec Refractory period▪ When a muscle fiber contracts, it temporarily

cannot respond to another action potential▪ Skeletal muscle has a refractory period of 5

milliseconds▪ Cardiac muscle has a refractory period of 300

milliseconds

Control of Muscle Tension

Control of Muscle Tension

Control of Muscle TensionMuscle Tone

A small amount of tension in the muscle due to weak contractions of motor units

Small groups of motor units are alternatively active and inactive in a constantly shifting pattern to sustain muscle tone

Muscle tone keeps skeletal muscles firm Keep the head from slumping forward on

the chest

Control of Muscle TensionTypes of Contractions

Isotonic contraction▪ The tension developed remains constant

while the muscle changes its length▪ Used for body movements and for moving

objects▪ Picking a book up off a table

Isometric contraction▪ The tension generated is not enough for the

object to be moved and the muscle does not change its length▪ Holding a book steady using an outstretched

arm

Control of Muscle Tension

Types of Skeletal Muscle Fibers Muscle fibers vary in their content of

myoglobin Red muscle fibers▪ Have a high myoglobin content▪ Appear darker (dark meat in chicken legs and

thighs)▪ Contain more mitochondria▪ Supplied by more blood capillaries

White muscle fibers▪ Have a low content of myoglobin▪ Appear lighter (white meat in chicken

breasts)

Types of Skeletal Muscle Fibers Muscle fibers contract at different speeds,

and vary in how quickly they fatigue Muscle fibers are classified into three main

types 1) Slow oxidative fibers 2) Fast oxidative-glycolytic fibers 3) Fast glycolytic fibers

Types of Skeletal Muscle Fibers Slow Oxidative Fibers (SO fibers)

Smallest in diameter Least powerful type of muscle fibers Appear dark red (more myoglobin) Generate ATP mainly by aerobic cellular

respiration Have a slow speed of contraction▪ Twitch contractions last from 100 to 200 msec

Very resistant to fatigue Capable of prolonged, sustained contractions

for many hours Adapted for maintaining posture and for

aerobic, endurance-type activities such as running a marathon

Types of Skeletal Muscle Fibers Fast Oxidative–Glycolytic Fibers (FOG

fibers) Intermediate in diameter between the other

two types of fibers Contain large amounts of myoglobin and many

blood capillaries Have a dark red appearance Generate considerable ATP by aerobic cellular

respiration Moderately high resistance to fatigue Generate some ATP by anaerobic glycolysis Speed of contraction faster▪ Twitch contractions last less than 100 msec

Contribute to activities such as walking and sprinting

Types of Skeletal Muscle Fibers Fast Glycolytic Fibers (FG fibers)

Largest in diameter Generate the most powerful contractions Have low myoglobin content Relatively few blood capillaries Few mitochondria Appear white in color Generate ATP mainly by glycolysis Fibers contract strongly and quickly Fatigue quickly Adapted for intense anaerobic movements of

short duration▪ Weight lifting or throwing a ball

Types of Skeletal Muscle Fibers

Types of Skeletal Muscle FibersDistribution and Recruitment of

Different Types of Fibers Most muscles are a mixture of all three

types of muscle fibers Proportions vary, depending on the

action of the muscle, the person ’s training regimen, and genetic factors▪ Postural muscles of the neck, back, and legs

have a high proportion of SO fibers▪ Muscles of the shoulders and arms have a

high proportion of FG fibers▪ Leg muscles have large numbers of both SO

and FOG fibers

Exercise and Skeletal Muscle Tissue

Ratios of fast glycolytic and slow oxidative fibers are genetically determined Individuals with a higher proportion of

FG fibers▪ Excel in intense activity (weight lifting,

sprinting) Individuals with higher percentages of

SO fibers▪ Excel in endurance activities (long-distance

running)

Exercise and Skeletal Muscle Tissue

Various types of exercises can induce changes in muscle fibers Aerobic exercise transforms some FG

fibers into FOG fibers▪ Endurance exercises do not increase muscle

mass Exercises that require short bursts of

strength produce an increase in the size of FG fibers▪ Muscle enlargement (hypertrophy) due to

increased synthesis of thick and thin filaments

Cardiac Muscle Tissue

Principal tissue in the heart wall Intercalated discs connect the ends of cardiac muscle

fibers to one another▪ Allow muscle action potentials to spread from one cardiac

muscle fiber to another Cardiac muscle tissue contracts when stimulated by its

own autorhythmic muscle fibers▪ Continuous, rhythmic activity is a major physiological

difference between cardiac and skeletal muscle tissue Contractions lasts longer than a skeletal muscle twitch Have the same arrangement of actin and myosin as

skeletal muscle fibers Mitochondria are large and numerous Depends on aerobic respiration to generate ATP▪ Requires a constant supply of oxygen▪ Able to use lactic acid produced by skeletal muscle fibers to

make ATP

Smooth Muscle Tissue

Usually activated involuntarily Action potentials are spread through the

fibers by gap junctions Fibers are stimulated by certain

neurotransmitter, hormone, or autorhythmic signals

Found in the▪ Walls of arteries and veins▪ Walls of hollow organs▪ Walls of airways to the lungs▪ Muscles that attach to hair follicles▪ Muscles that adjust pupil diameter▪ Muscles that adjust focus of the lens in the eye

Smooth Muscle Tissue

Smooth Muscle Tissue

Microscopic Anatomy of Smooth Muscle Contains both thick filaments and thin

filaments▪ Not arranged in orderly sarcomeres

No regular pattern of overlap thus not striated

Contain only a small amount of stored Ca++

Filaments attach to dense bodies and stretch from one dense body to another

Dense bodies▪ Function in the same way as Z discs▪ During contraction the filaments pull on the dense

bodies causing a shortening of the muscle fiber

Smooth Muscle Tissue

Physiology of Smooth Muscle Contraction lasts longer than skeletal muscle

contraction Contractions are initiated by Ca++ flow primarily

from the interstitial fluid Ca++ move slowly out of the muscle fiber

delaying relaxation Able to sustain long-term muscle tone▪ Prolonged presence of Ca++ in the cell provides for a

state of continued partial contraction▪ Important in the:▪ Gastrointestinal tract where a steady pressure is

maintained on the contents of the tract▪ In the walls of blood vessels which maintain a steady

pressure on blood

Smooth Muscle Tissue

Physiology of Smooth Muscle Most smooth muscle fibers contract or

relax in response to:▪ Action potentials from the autonomic nervous

system▪ Pupil constriction due to increased light energy

▪ In response to stretching▪ Food in digestive tract stretches intestinal walls

initiating peristalsis▪ Hormones▪ Epinephrine causes relaxation of smooth muscle in

the air-ways and in some blood vessel walls▪ Changes in pH, oxygen and carbon dioxide

levels

Smooth Muscle Tissue

Aging and Muscular Tissue Aging

Brings a progressive loss of skeletal muscle mass

A decrease in maximal strength A slowing of muscle reflexes A loss of flexibility

With aging, the relative number of slow oxidative fibers appears to increase

Aerobic activities and strength training can slow the decline in muscular performance

End of Chapter 10, Part 2