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1.  Sarcomere - from Z line to Z line

2.  M line - the center of the thick filament

3. H band - only thick filament

4. I band - only thin filament

5. A band - the entire thick filament

      1.  Titin and nebulin

      2.  Myosin (thick filament)

      3.  Actin, tropomyosin, troponin (thin filament)

1.  Myosin, the thick filament protein, is an ATPase which is inhibited under certain conditions.

      2.  In presence of actin, myosin is a very active ATPase under certain conditions.

      3.  The energy from hydrolysis of one ATP molecule is used for each myosin – actin interaction – the net motion depends on changing affinities of myosin for actin.  The species of ATP/ATP hydrolysis bound to myosin determines myosin’s affinity for actin.  Each ATP hydrolysis, actin-myosin binding interaction =1 crossbridge cycle.

      4.  When muscle is depleted of ATP, myosin crossbridge stays bound to actin; this is muscle rigor mortis

      5.   In a living cell, the cycle is controlled at the level of binding of myosin+ADP+P to actin.

      6.  Ca²⁺ regulation (physiologic regulation)

      7.  ADP release is rate limiting (kinetics)

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1.   First step is crossbridge (“crossbridge” = myosin globular head) binding to actin (fig step 1)

2.  Second step is rotation of lever arm (2 arms may act independently) resulting in translational movement of thin filament past thick filament; at this stage ADP and Pi are released (fig step 2)

3.   Third step is ATP binding to myosin which decreases the affinity of myosin for actin and thus promotes detachment of crossbridge from thin filament (fig step 3)

4.   Fourth step is the arm swinging back to its original position (now opposite a new actin monomer) and hydrolysis of the ATP (but not dissociation of the ADP Pi).  ADP and Pi remain bound to crossbridge/myosin head (fig step 4)

Fill in the blank:

1. Myosin = _______ actin affinity

2. Myosin-ATP = _________ actin affinity

3. Myosin-ADP+Pi = ________ actin affinity

1. high

2. low

3. high

Describe the sliding-filament/cross-bridge hypothesis:

1.  How much does each sarcomere shorten?

2. Is crossbridge cycling synchronous or asynchronous?

3. Which lines/bands move?

4. What causes chemomechanical transduction?

1.   A single crossbridge cycle involves a 10 nm shortening.

2.  Contraction involves a population of crossbridges cycling asynchronously.  The asynchrony is probably due to the differences in repeat distances in thin filament.  Asynhrony allows muscle to maintain tone/force during contraction.

3.  Output also depends on polarity of the filaments and numbers of sarcomeres repeated.  Z-disks move towards thick filaments. (Z disks move toward M-line)

4.  Chemomechanical transduction appears to result from stored energy in crossbridge, due to ATP hydrolysis, liberated by a conformational change producing filament movement.

1.   Rise in myoplasmic Ca²⁺

        2.   Change in myofilament structure

      3.   Crossbridge binding to thin filament

      4.   After attachment; myosin heads change their conformation (tilt) using the engery stored in the high energy myosin – ADP Pi complex.  This conformation change produces or generates force moving the thin filament relative to the thick filament

      5.   Conformational change leads to release of ADP and Pi

      6.   ATP binds crossbridge, crossbridge detaches from thin filament

      7.   Decrease in Ca²⁺ concentration or removal of ATP will halt the cycle

Force length relationships

1. What happens if sarcomere length is too long?

2. What happens if sarcomere length is too short?

3.  What's the relationship between active stress and the overlap between filaments?

1.   Sarcomere length too long – not all crossbridges can interact with thin filament = submaximal force

      2.   Sarcomere length too short – thin filaments start to overlap at center of sarcomere à steric hindrance = submaximal force

      3.   Active stress proportional to the overlap between thick and thin filaments

Velocity-stress relationships

1. Define isotonic contraction

2.  Describe properties of shortening velocities

       1.   Isotonic contraction

            a.  Load or stress is held constant and then shortening velocity is measured

Light load → Rapid shortening

Heavy load → Slow shortening

      2.   Shortening velocities

            a.  Depends on number of sarcomeres in series

                  (end to end [—I—I—]

            b.  Myosin ATPase rate

            c.  Unloaded crossbridges can cycle at a maximal rate (Vo)

            d.  Vo depends on myosin

            e.  Vo proportional to myosin ATPase activity

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Define

1. isometric contraction

2. isotonic contraction

1. A constant length

2. A constant load

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*Blue line is optimum length, red line is shorter resting length.

*Max velocity is the same for all initial muscle lengths

*The contraction is "isometric" at zero velocity

*Shows the velocity of muscle shortening is faster during less load.

*A longer muscle can develop a greater tension than can a shorter muscle

Troponin

1. ratio to tropomyoisn

2. # and affinity of Ca bond

3. Troponin complex

4. Action of Ca bound

1.   Troponin is a thin filament protein attached to tropomyosin in 1:1 ratio (6-7 actin/1 Tm or Tn)

      2.   Each troponin binds 4 Ca²⁺ ions allosterically (2 Ca²⁺ with low affinity, 2 Ca²⁺ with high affinity)

      3.   Troponin is a complex:

Troponin – T (binds tropomyosin)

Troponin – C (binds Ca²⁺)

Troponin – I (binds actin)

      4.   Ca²⁺ binding to regulatory sites on troponin acts as a switch that regulates the number of crossbridges that are interacting with the thin filament

1.   This shift exposes the site on each 6-7 actin monomers to which crossbridges can bind and cycle.

      2.   The thin filament stays “on” allowing crossbridge cycling contraction until the sarcoplasmic reticulum lowers myoplasmic Ca²⁺ and Ca²⁺ dissociates from troponin

      3.   Without Ca²⁺ tropomyosin and troponins shift blocking further crossbridge attachements (i.e. Ca²⁺ turns on the troponin “switch”)

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Left side is when muscle is relaxed, right side is contracted

Define:

1. Cause of muscle weakness

2. Hypertrophy

3. Atrophy

4. Destructive myopathies

5. Examples of distal myopathies

1.  Most cases of muscle weakness – due to loss of contractile tissue

2.  Hypertrophy – increase in force via larger muscle diameter (more myofibrils)

3.  Atrophy – decrease in force via smaller muscle diameter (fewer myofibrils)

4.   Destructive myopathies muscle fibers replaced with fat and connective tissue, may sometimes mask the loss of muscle bulk (pseudohypertrophy)

5.   Distal myopathies (examples: titin, nebulin, troponin)

The sarcoplasmic reticulum calcium ATPase activity determines:

a. Calcium storage capacity

b. Maximum force

c. Maximum shortening velocity

d. Relaxation rate

e. Rate of calcium release

ATP promotes detachment of the cross bridge

from the thin filament by:

a. Modifying the interaction of tropomyosin with the actin

b. Blocking Ca2+ binding to troponin

c. Inhibiting the formation of actin filaments

d. Decreasing the affinity of myosin for actin

e. Direct phosphorlization

During an isometric contraction the peak

force is primarily dependent upon:

a. Load

b. Sarcomere length

 c. Myosin ATPase activity

 d. Cross-bridge cycling rate

 e. Calcium pump activity