Muscle and Muscle Tissue

BIO 231 - Anatomy and Physiology I

Types of Muscle

• Skeletal

– Striated and

– Contracts more rapidly

• Cardiac

– Heart muscle, striated & involuntary

– Contracts at a rate set by pacemaker - also neural override

• Smooth

– Visceral, nonstriated &

– Contractions are slow and sustained

Muscle Function

• Producing movement

• Maintaining posture

• Stabilizing joints

• Generating heat

– Muscle = of body mass

Connective Tissue Wrappings of Skeletal Muscle

Muscle Action

• Generally when a muscle contracts - one bone stationary the other moves

• Origin of muscle - less movable bony attachment

• Insertion - the more movable bony attachment

• Muscle always pull - never

• Agonists

• Antagonists

Skeletal Muscle Cell

• Elongate and multinucleate

• Show cross striations

• Skeletal muscle fiber - basic unit of histological organization

• Myofibrils - bundles of contractile

• Sarcolemma - cell membrane

• Sarcosomes - mitochondria

• Sarcoplasmic reticulum - stores

• T system - connects with sarcolemmal

SR and T Tubules

Microanatomy of Skeletal Muscle

Structure of Skeletal Muscle

• Sarcomere - z line to z line

• Thick filaments -11nm

• Run the length of the A band (dark bands)

• Thin filaments - attach to z lines

• Run through the I bands (light bands) into the A bands

• H zone - middle of A band

• M band or line - m protein

EM of Skeletal Muscle

Thin Filaments

• Actin

– g actin - globular

– f actin - polymer of g actin

• Troponin (TN) - complex of molecules

– TNT - bind complex to tropomyosin

– TNI - inhibits ATPase activity of actomyosin

– TNC binds Ca⁺⁺

• Tropomyosin (TM) - 40 nm long

– Two strands of TM per thin filament

Structure of Thin Filaments

Thick Filaments

• Extraordinarily specific

• Begins with the end to end association of tails of myosin

• About 200 myosin molecule/thick filament

• Globular heads (cross bridgee) - oriented in opposite directions on each half filament

• Successive cross bridges are rotated by 60° - helical arrangement

Structure of Thick Filaments

Thick and Thin Filaments

Structural Events of Contraction

• Large Scale

• Change in the banding pattern of sarcomere

• Small scale

• Shortening of

• Explained by Sliding Filament Model

How do Filaments Slide?

• Formation of cross bridges - linking of thick and thin filaments

• Structural events

• Cross bridges change orientation

• Orientation change not synchronous

Cross Bridge Cycle

1. Cross bridge attachment - myosin heads attach to on actin

2. Power stroke - as the myosin head binds, it pivots changing its high-energy configuration to a low-energy shape - which pulls the thin filament

• ADP & Pi are released from myosin head

•Cross Bridge Cycle

• 3. Cross bridge detachment - New ATP molecule binds to myosin head - breaks loose from

• 4. Cocking of the myosin head - hydrolysis of ATP to ADP + Pi returns the myosin head to its high-energy or “cocked” position

Cross Bridge Cycle

Role of Ca⁺⁺ in Contraction

Regulation of Contraction

• Muscle contraction requires - AP in a motor neuron

• The motor neuron releases acetylcholine (ACh) at the neuromuscular junction

• Post-synaptically the muscle has a motor end plate - with ACh

• An AP then propagates over the surface of the muscle and cause Ca++ to be released within the muscle cytoplasm

• Ca⁺⁺ triggers contraction

Excitation Contraction Coupling

• Latent period

• External AP ® internal chemical signal

• AP travels down

• This brings about the release of Ca⁺⁺

• Ca⁺⁺ above 10-6 to 10-7 ® contraction occurs

Excitation Contraction Coupling

Relaxation of Skeletal Muscle

• Pump Ca⁺⁺ back into SR

• 2 moles of Ca⁺⁺ sequestered/ mole of ATP hydrolyzed

• Ca⁺⁺ pumps have higher affinity for free Ca⁺⁺ than does

Relaxation of Skeletal Muscle

• Crossbridge cycling stops

• Calsequestrin in lumen of SR - weakly binds Ca⁺⁺ & lowers effective conc of Ca⁺⁺ so pump doesn’t work as hard

Studying Skeletal Muscle

• Resting - unstimulated state

• Active - state

• Isotonic recording - muscle attached at one end & the other end is free to lift a load

• Isometric recording - muscle is firmly attached at both ends - only small changes in length can occur

Wave (Temporal) Summation

Muscle Metabolism

• Stored ATP - enough for 4 to 6 seconds only a few twitches

• Direct Phosphorylation of ADP

– Creatine phosphate

– Creatine - P + ADP ® ATP +

– Buffer while other ATP mechanisms are being turned on

Muscle Metabolism

• Anaerobic Glycolysis

– Very rapid - readily meets ATP demands of rapidly contracting muscle

– 2 moles of ATP / mole of glucose

– 3 moles of ATP / mole of gluc from

Muscle Metabolism

• Oxidative phosphorylation

– From fatty acids mostly - primary source for muscles that are frequently active (slow muscle)

– 25 moles of ATP/ mole of glucose

– Can operate continuously when circulation is adequate

– process - not fast enough for rapidly contracting muscle

Oxygen Debt

• Vigorous exercise - muscle chemistry changes

• For a muscle to recovery to resting state

– O₂ reserves must be replenished, acid must be converted to pyruvate & glycogen stores replaced

• Called an oxygen debt

• 100 yd dash in 12 seconds - requires 6L of O₂ for totally aerobic respiration - VO₂ max in 12 seconds is 1.2 L

– Thus you have incurred a 4.8 O₂ debt - requires breathing

– O2 needed to metabolize lactate