Bio 201 Chapter 10, Part 2 Lecture

Chapter 10, Part 2: Muscular Tissue

Muscle Metabolism  Production

Fibers

of ATP in Muscle

 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

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

Control of Muscle Tension  The

tension or force of muscle cell contraction varies

 Maximum

Tension (force) is dependent on    

The The The The

rate at which nerve impulses arrive amount of stretch before contraction nutrient and oxygen availability 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 Tension  Twitch 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 Tension  Muscle 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 Tension  Types 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 Fibers  Distribution

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

Muscle

Anatomy of Smooth

 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

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