Chapter 8: Skeletal Muscle

Skeletal muscle consists of..

Nerve tissue

Blood

Connective tissue

Individual muscles are separated by ___

Fascia

What are the functions of skeletal muscles?

-Force production for locomotion and breathing

-Force production for postural support

-Heat production during cold stress

Flexors __ joint angle

Extensors __ joint angle

Decrease

Increase

Surrounds the entire muscle..

Surrounds bundles of muscle fibers/fascicles..

Surrounds individual muscle fiber...

Just below endomysium..

Muscle cell memebrane...

Epymisium

Perimysium

Endomysium

Basement membrane

Sarcolemma

what are the undifferentiated cells that play a key role in muscle growth and repair?

Satellite cells

*activation, proliferation, regeneration

**In response to resistance training they become activated and divide

Satellite cells are able to differentiate and/or fuse to augment existing muscle fibers or to form muscles fibers creating...

increased myonuclei which should theoretically enhance protein synthesis and lead to muscle growth

The achieve maximum muscle growth an increase in __ is required.

Myonuclei

Contractile proteins of a muscle?

Actin (thin filament)

Myosin (thick filament)

*these are myofibrils

Z line of a sarcomere...

defines boundaries of sarcomere and anchors thin, actin filaments

A band of a sarcomere...

contains all of the thick myosin filaments plus any overlapping think actin filaments

I band of a sarcomere...

contains only thin actin filaments

H zone of sarcomere...

contains thick filaments of myosin only

M line of a sarcomere..

Links the myosin filaments

Titan is the protein that ...

stabilizes muscle fibers while connecting z lines

Surrounds each myofibril and is the location of Ca2+ release required to trigger muscle contraction..

Sarcoplasmic reticulum

Plays a role in Ca2+ storage and release during muscle contraction and is part of the sarcoplasmic reticulum..

Terminal cisternae

Extend from the sarcolemma into muscle fiber and pass completely through the fiber to SR..

transverse tubules

*following a neural depolarization of muscle fiber, t-tubules are also depolarized, releasing Ca from terminal cisternae

Each skeletal muscle fiber is connected to a __ that extend from spinal cord.

Nerve fiber called a motor neuron

What makes up a motor unit?

The motor neuron and all the muscle fibers it innervates

Steps leading up to muscle contraction at neuromuscular junction...

1. Nerve impulse at neuromuscular junction causes ACh to release and diffuse across synaptic cleft

2. The binding of ACh to receptors opens the Na+ channels on sarcolemma allowing Na+ to diffuse inward

3. Na+ diffusion depolarizes the t-tubules and creates an end-plate potential (EPP)

4. EPP is always large enough to exceed threshold and signals to begin contraction

Neuromuscular junction represents a potential site for ___. This is why __ training results in adaptations to NMJ.

Fatigue

Endurance and strength

*can increase size of NMJ and number of synaptic vesicles on post-synaptic membrane

the myosin head requires __ to dissociate, cock back into position and repeat.

ATP

The process of the myosin head pulling the actin filaments over it is called..

power stroke

*reduction of distance between Z lines of sarcomere

**leads to shortening of muscle fiber and called 'excitation-contraction coupling'

ATP is required for muscle contraction and __ located on the myosin head breaks it down into ADP and Pi

ATPase

*release of energy serves to energize myosin cross-bridges, which in turn pull actin over myosin leading to shortening of sarcomere (~1% of resting length)

Phrase describing events leading to depolarization ( excitation) and ultimately shortening and force production (contraction) of the muscle...

Excitation-Contraction cycling

What is the 3 step excitation process?

1. Depolarization of motor neuron (located in spine)

2. Action potential causes ACh release into synaptic cleft

3. ACh binds to receptors on motor end plate, leading to EPP, leading to depolarization of muscle cell

**depolarization continued down t-tubules causing Ca2+ to be released from SR

__ lies in a groove between the double rows of actin while __ is attached directly to it. These regulate muscle contraction by...

Tropomyosin; Troponin

Controlling interaction of actin and myosin

**in a relaxed muscle tropomyosin blocks active site on actin where myosin cross-bridge attaches

Contraction process steps after the Ca2+ is released...

1. Ca2+ binds to troponin, leading to a position change of tropomyosin so the active binding sites of actin are uncovered

2. Cross-bridge binding initiates release of stored energy in myosin head causing a power stroke

3. Available ATP provides energy to break cross bridge allowing another cross-bridge to form on another active site

The contraction cycle can be repeated as long as..

Ca2+ is available to bind to troponin

Process of stopping the excitation-contraction coupling?

-Absence of nerve impulses at NMJ so Ca2+ gets pumped back into SR

-Removing the Ca2+ from troponin causes tropomyosin to move back and cover actin binding sites

Severe and/or prolonged exercise results in..

decline in muscle's ability to generate force and power

*reduce force generation and reduce shortening velocity

Causes of exercise-induced muscle fatigue are complex and may involve factors within both...

Central nervous system (CNS) and active muscles (peripheral fatigue)

**exact causes of muscle fatigue remain under investigation

Other contributing factors that affect muscle fatigue...

-fitness level

-nutritional status

-fiber type composition

-intensity and duration of exercise

During high intensity exercise fatigue can be related to..

-Decreased Ca2+ from SR

-Increase in accumulation of metabolites (lactate ions, hydrogen ions that bind to Ca2+ binding sites on tropomin)

-Phosphate ions (Pi) and Free radicals (modify myosin cross-bridges)

Decreased __ reduces muscle force production.

Myofilament sensitivity to Ca2+

Fatigue associated with moderate intensity exercise likely not due to..but instead a build up of..

build up of H+ or Pi

Free radicals impact the myosin cross-bridges

___ also likely contributing factor to decrease in muscle force like 'hitting a wall'

Depletion of muscle glycogen

-in moderate intensity exercise

Muscle cramps are spasmodic ___ and are often associated with prolonged high intensity exercise.

Involuntary muscle contractions

Most exercise associated cramps are not caused by ___ but rather likely due to...

An electrolyte or dehydration balance

Hyperactive motor neurons in spinal cord

Rigorous exercise can alter muscle __ and __ function resulting in increased excitatory activity of muscle spindles and a reduced inhibitory effect on golgi tendon organ.

Muscle spindle (rate in change of length)

Golgi tendon organ (inhibitory mechanism so too much force isn't produced)

Easy fix for muscle cramping is..

passive stretching

Two proposed theories of spasmodic involuntary muscle contractions (muscle cramps) are...

-Dehydration and electrolyte imbalance theory

-Altered neuromuscular control theory

Scientists reason that a potential treatment or prevention of muscle cramps is to send..

a strong inhibitory stimulus (via channel blockers) to the spinal cord to prevent motor neurons from firing

Skeletal muscle fibers are classified based on..

histochemical or biochemical characteristics

Two general categories of muscles are...

Slow, Type I (slow twitch)

Fast, Type II (fast twitch

-Type IIa

-Type IIx

Percentage of respective fiber types influenced by..

Genetics

Blood levels of hormones

Exercise habits

Three biochemical characteristics of muscle important to muscle function...

Oxidative capacity

Type of myosin isoform (ATPase activity)

Abundance of contractile protein with fiber ex: actin and myosin

Oxidative capacity is impacted by...

Mitochondrial density

Capillary density

Myoglobin within fiber

There are 3 different types of __ that exist in human skeletal muscle.

Fibers with high ATPase activity...

Myosin isoform

Degrade ATP quickly leading to enhanced muscle shortening

Large fibers with higher amounts of contractile proteins within the fiber (actin and myosin)...

Generate more force

Four characteristics are important when comparing contractile properties...

Maximal force production

Speed of contraction

Max power output

Efficiency contraction

Maximal force production

-Larger muscles..

-more actin and myosin...

Both produce more force with more actin and myosin allowing more cross-bridges

Speed of contraction

-Key factor for maximizing speed of contraction...

Myosin ATPase activity

*rate of cross-bridge recycling determines speed of contraction

Max power output determined by...

Force x shortening velocity

**high force/high shortening velocity=high power output

Efficiency of contraction

An efficient fiber requires ___ energy to perform work than a less efficient fiber.

Less

*ATP/force produced

Slow, Type I fibers have a large capacity for ___.

Aerobic metabolism

Reasons for Type I having a large capacity for aerobic metabolism...

-Greater mitochondrial density (oxidative potential)

-Greater capillary density

-Higher concentrations of myoglobin

-Slower shortening velocity (lower myosin ATPase activity)

-Lower force generating capacity (fewer actin-myosin filaments)

-More efficient due to lower rate of ATP turnover

Fast, Type IIx are referred to as __.

Fast-glycolytic

-fastest shortening speed of all human muscle fiber types

-high myosin ATPase activity

Other characteristics of Type IIx fibers...

-Fewer mitochondria (limited capacity for aerobic metabolism but more susceptible to fatigue)

-Richer in glycolytic enzymes (greater anaerobic capacity)

**less efficient than other fiber types

Highest power output of all fiber types are...

Fast, Type IIx

Referred to as fast-oxidative (or intermediate fibers) that contain biochemical and fatigue characteristics between Type IIx and Type I fibers...

Fast, Type IIa

*a mixture of all characteristics of each fiber type

highly successful 'power' athletes possess...

Endurance athletes possess...

Large percentage of fast fibers

Large percentage of slow fibers

**despite high correlation between slow fibers and VO2max, the percentage of slow fibers only explains 40% in VO2max variation between individuals

No change in muscle length is a __ contraction.

Isometric

Muscle contraction that involves muscle shortening...

Muscle contraction that involves lengthening...

Concentric contraction

Eccentric contraction

**dynamic muscle contractions

___ are capable of producing far greater force at fastest and slowest velocities.

Eccentric contractions

Amount of ___ is specific to joint angle.

Isometric force

___ is greatest at slowest velocities and less at faster velocities of shortening.

Concentric forces

A muscle twitch results from a ___. After stimulation __ exists corresponding to the depolarization of muscle fiber.

Single stimulus

Short latent period

Timing of phases is fiber type dependent with speed of shortening being greater in ___

Fast fibers

*SR releases Ca2+ at faster rate

*indicative of higher ATPase activity

Force generated in a single muscle fiber is related to...

number of myosin cross-bridges with actin

Amount of force generated in muscle contraction related to 4 factors...

1. Number and types of motor units

2. Initial length of muscle

3. Nature of neural stimulation

4. Prior contractile activity of the muscle

More motor units generate ___

Faster motor units generate. ____

Greater force

*size principle=smallest recruited first

Greater force

Force changes with changes muscle length which is related to...

overlap of actin and musoin

**longer than optimal resting length=overlap limited

**Shorter than optimal resting length=Z lines are very close to myosin filaments

Length tension relationship

As firing rate of motor neuron increases...

muscles do not have time to relax between stimuli (additive force) =summation

With an increased frequency of neural stimulation individual contractions become blended called...

tetanus

*continues until stimuli cease or muscle fatigues

A bout of fatiguing exercise (ex: prolonged and/or high intensity) results in decreased force production in subsequent contractions...

Prior contractile activity of th emuscle

Phenomenon of increase in muscle force production that occurs following a bout of non-fatiguing, submaximal exercise contractions...

Post Activation Potential

__ and __ results in a progressive loss of muscle mass, force production, and muscle strength.

Aging (>50 yrs.) and chronic disease

Age-related muscle loss is called..

Sarcopenia

*usually loss of fast fibers and gain in slow fibers, but resistance training can delay this

At any absolute force exerted by the muscle, the velocity of movement is greater in muscles that contain a..

high percentage of fast fibers compared to muscles possessing predominantly slow fibers

**fast fibers possess higher ATPase activity compared to slow fibers

Maximum velocity of muscle shortening is greatest at ___. Hence, the greatest speed of movement is generated at the ___.

Lowest forces

Lowest workloads

**down to actin and myosin filaments moving past each other at a fast rate

Peak power that can be generated is greater in muscle containing...

higher percentage of fast fibers

Peak power generated by any muscle increases with ___ of movement up to a movement speed of 200 to 300 degrees/sec.

Increasing velocities