Chapter 10 Muscle Fibers

Functions of Muscle

Body movement
Maintinance of posture and body position

Potection and support

Storage and movement of materials

heat production

Types of muscles ( where found too and differences between them)

Skeletal- Striated , long, multi nucleated, all voluntary muscles

Cardiac- Striated, short, usually one nucleus, heart only

smooth- not striated, tapered at ends Hollow organs and blood vessels

Properties of muscle Cells

▪ Excitability – ability to respond to stimuli

▪ Conductivity – ability to conduct electrical changes

across entire plasma membrane

▪ Contractility – contractile proteins in cells draw closer

together

▪ Extensibility – ability to be stretched without rupturing

▪ Elasticity – ability to return to original length after

stretching

Skeletal Muscles are made of: 

Muscle fibers (myocytes)

Blood vessels

Nerves

Connective tissues

Connective tissues of Skeletal Muscles Function

(1) anchor, (2) separate, (3) organize, and (4) electrically

insulate

Connective Tissues Found on Skeletal Muscles

• Epimysium

• Perimysium

• Endomysium

Tendon

• Aponeurosis

Epimysium 

layer of CT surrounding entire muscle

Perimysium

Layer of CT surrounding fascicles (bundle of nerves or muscles)

Endomysium

Layer of CT surrounding each muscle fiber

Tendon

Cord like strand of dense regular CT that attaches muscles to bones

Aponeurosis 

Wide flat white sheet of CT that attaches neighboring muscles to bone or other muscles 

Skeletal Muscle Fiber general shape

Skeletal Muscle Fiber with Triad general outline

Myofilaments shape (image) 

Sarcomere image

Sarcolemma

the plasma membrane; the fine transparent tubular sheath which envelops

the myofibers

Sarcoplasm

the cytoplasm; contains the organelles

Transverse Tubules (T-Tubules)

inward extensions of the sarcolemma; surrounds myofibril

organelles and forms a tunnel-like network through which extracellular fluid flows

Myofibril

long cylindrical bundle of proteins specialized for contraction (myofilaments);

most abundant organelle

Sarcoplasmic Reticulum

modified web-like smooth ER; stores Ca2+ ions; contains end sacs

called terminal cisternae

Myofilaments

two contractile protein filaments that make up myofibrils; known as thin and

thick filaments

Sarcomere

structural unit of muscle contraction; composed of alternating thin and thick

filaments

Motor Unit image 

Motor Unit

Motor Unit = the motor neuron plus all the muscle fibers it controls.

A motor neuron is a cell from the nervous system that transmits a signal from the brain or spinal cord to tell

the muscle fiber to contract.

Neuromuscular Junction (NMJ)

Where the motor neuron and muscle fiber meet.

Parts:

▪ Axon terminal: end of the motor neuron; contains

vesicles filled with Acetylcholine (ACh)

▪ Synaptic cleft: small space b/n motor neuron and

muscle cell

▪ Motor end plate: folded surface of the muscle cell

directly under the synaptic cleft

Skeletal Muscle Fiber Contraction 

Myosin and actin want each other

(can’t get together because of

tropomyosin and troponin).

Troponin wants Ca++ but Ca++ is

unavailable…

Phase 1 Muscle contraction

Excitation

1. Action potential arrives at the axon terminal and triggers CA2+ channels in the terminal to open.

2. CA2 entry triggers exocytosis of synaptic vesicles

3. Synaptic vesicles release ACh into the synaptic cleft

4. ACh binds to ligand gated ion channels in the motor end plate

5. ion channels open and Na enters the muscle fiber

6. Entry of Na depolarizes the sarcolemma locally, producing Na end-plate potential

Phase 2

Excitation-Contraction Coupling 

1. End plate potential stimulates an action potential 

2. Action potential is propagated down the T tubule 

   1. T-Tubule depolarization leads to the opening in calcium channels in the sarcoplasmic reticulum and CA enters the cytosol 

Phase 3 of muscle contraction

Contraction

1. Calcium binds to troponin

2. Tropomyosin moves and the active site of actin are exposed

3. Calcium binds to troponin, causing a conformational change in

tropomyosin to expose the myosin binding site on actin.

4. The myosin heads can bind to actin, forming a cross-bridge

5. Begin siding filament mechanism of contraction

Sliding filament mechanism of contraction

Sarcomeres are contracted to the A band, bringing z discs closer together

Crossbridge Cycling 

▪ ATP bound to myosin is hydrolyzed

causing myosin head to assume

“cocked” position

▪ Myosin head binds to actin forming

cross-bridge

▪ ADP is released resulting in power stroke

▪ New ATP molecule binds to myosin

head, separating the cross-bridge

▪ Cycle starts over

Muscle Relaxation

1. Motor neuron signal ceases

▪ ACh release stops

▪ Acetylcholine esterase degrades remaining ACh in synaptic cleft

▪ Membranes return to resting membrane potential and ion channels close

2. Calcium is actively pumped back into SR

▪ Troponin and tropomyosin return to original conformations

▪ Myosin binding sites are covered

Bacteria which interfere with the muscular contraction control mechanism 

Tetanus: 

▪ Releases a toxin that interferes with the ability of the CNS to

inhibit unwanted contractions

▪ Results in spastic paralysis → “Lock Jaw”

Botulism:

▪ Releases a toxin that blocks ACh release from synaptic knob

▪ Results in flaccid paralysis of affected muscle

ATP importance in muscular contractions

Required to:

▪ Power Na+/K+ pumps that maintain ion gradients

▪ Release myosin heads

▪ Pump calcium back into SR for muscle relaxation

Sources of ATP:

▪ Creatine phosphate metabolism

▪ Anaerobic cellular respiration (glycolysis)

▪ Aerobic cellular respiration

Creatine Phosphate Metabolism

Immediate ATP supply: creatine phosphate can immediately regenerate enough

ATP for about 10 seconds of maximum muscle activity

Anaerobic Cellular Respiration (Glycolysis)

Short-Term ATP Supply: provides energy for muscle contraction once immediate

sources are depleted; can supply ATP for 30-40 seconds of sustained contraction.

Aerobic Cellular Respiration

Predominant energy source, Long term ATP supply:

allows for longer-lasting muscle contractions (more ATP than glycolysis)

▪ Muscles store small amount of excess oxygen in myoglobin to allow for more efficient muscle contractions and less fatigue

Classes of skeletal muscle 

Type 1/ Slow

Type 2/ Fast 

Based on Myosin ATPase activity (speed of power stroke) 

Type 1 Fibers

Slow oxidative (SO)

▪ Slow, less powerful contractions, but

efficient (Fatigue resistant)

▪ Low myosin ATPase activity

▪ Rely on aerobic respiration

▪ Numerous mitochondria and myoglobin molecules

▪ Well-developed blood supply → “dark muscle”

Type 2 fiber types

Fast oxidative (FO)

Fast Glycolytic (FG)

Fast Oxidative (FO)

▪ Intermediate strength, speed, and fatigability

▪ Moderate blood supply and myoglobin content

▪ “dark muscle”

Fast Glycolytic (FG)

▪ Strongest strength, speed, and fatigability

▪ High glycogen content, little myoglobin and blood supply

▪ “white muscle”

Muscle Tension

The force generated when skeletal muscle is stimulated

Maximum force of contraction can be improved by:

Fast glycolytic fibers

Large motor units

Greater stimulus frequency 

Muscles at resting length 

Muscle twitch

a single, brief contraction followed by relaxation in response to a single stimulus

Threshold 

min. voltage needed to generate twitch

Latent Period

lag time between stimulus and twitch

Contraction period 

repetitive power strokes resulting in increased tension in muscle, start of muscle tension to the peak of muscle tension

Relaxation period

Release of cross bridges due to decreasing calcium levels. Where the peak tensions starts to drop to the end of relaxation

Effect of motor unit recruitment on tension 

An increase in voltage causes a greater number of motor units to contract,m increasing tension 

Effect of stimulus frequency on muscle tension

Tetany

Continues smooth contraction of muscle (no relaxation)

Length of sarcomere determines

▪ Amount of overlap between thick and thin filaments

▪ Number of pivoting cross-bridges

Muscle fatigue

Occurs when muscles can no longer perform a required activity

Muscle fatigue is caused by

▪ Depletion of metabolic reserves (creatine phosphate, glycogen, glucose)

▪ Decreased calcium or neurotransmitter at NMJ

▪ Decreased availability of oxygen (oxygen debt) to muscle fibers (increased

demand during exercise)

Major differences of cardiac muscle from skeletal muscle 

▪ Shorter, branched cells

▪ 1-2 nuclei in center of cell

▪ Abundant myoglobin and mitochondria

▪ Intercalated discs link cells together (permit

heart to contract as a coordinated unit)

▪ Stimulated by autorhythmic pacemaker

▪ Modulated by autonomic nervous system

Myoglobin and Hemoglobin

Bind oxygen to blood

Smooth Muscle Differences to skeletal muscle

▪ No sarcomeres or intercalated discs (no striations)

▪ Myosin and actin filaments arranged differently

▪ Fatigue-resistant

▪ Modulated by autonomic nervous system

Primary muscle cells

Myocytes

Sarcoplasmic reticulum

SR, plasma membrane of muscle sarcomeres

Sarcolemma

outermost membrane covering each individual muscle fiber

Sarcomere

structural unit of myofibril (one segment of myofibril)

myofilaments

each individual filament inside of a myofibril

Myofibril

Elongated contractile thread made of myofilaments

Organization of muscle fiber

Myofilaments make myofibrils

sections of myofibrils are called sarcomeres

Myofibrils are covered by the sarcoplasmic reticulum

Bundles of myofibrils make one muscle fiber covered by sarcolemma

T-tubule

Invagination of sarcolemma that transports electrical signals into cell

Terminal Cisternae

Enlarged regions of SR that store and release calcium

Triad

Combination of T-tubules and terminal cistern that release calcium into cells cytoplasm when the muscle is stimulated

Thick myofilament

Thick and made from myosin heads

Thin myofilament

More resembles beads on a string and has tropomyosin, troponin, and myosin binding sites

Sarcomere structure

Thin filaments bundle around thick filaments to form the median line. Z disk is the outside edge where thin filaments end

Synaptic cleft

small space between motor neuron and muscle cell

Axon terminal

end of the motor neuron, contains vesicles filled with acetylcholine

motor end plate

folded surface of the muscle cell directly under the synaptic cleft

Myosin binds to

Actin

Troponin binds to

Ca2+

Thick and thin filaments contract muscle in a 

ratcheting motion where myosin heads bind to actin and pull it forward, called crossbridging

Power stroke

muscle contraction where myosin binds to actin and pulls

z disc

Structural end of a sarcomere. Marks the end of a sarcomere and the beginning of another.

myosin heads bind to

Actin