Motor mechanisms II

Review questions

Binding of ATP to myosin head causes:• a) actin filament to slide

• b) Myosin head to detatch from actin filament

• C) Myosin head to cock

What will happen when ADP and Pi dissociate from myosin head?

• A) head will detatch from actin filament

• B) head will retract and pull actin filament

• C) head will bind ADP again

Ca++ induces muscle contraction by:• A) Causing actin monomers to assemble

into a filament

• B) making ATP available to bind to myosin head

• C) allowing myosin head to bind to actin filament

Muscle fiber types are the result of different types (isoforms) of

myosin

• Type I - slow fibers (aerobic)

• Type II - fast fibers (anaerobic)– Type IIx - very fast– Type IIa - intermediate between II x and I– Types present in other vertebrates, not humans

e.g., IIb in rats and other small mammals

TYPE I - SLOW TWITCH   Tonic muscles (darker:  red) - Leg muscles

TYPE II - (IIa & IIx) FAST TWITCH   Tetanic muscles (paler: white) - Pectoral muscles

slower contraction times (100-110 msec) faster contraction times (50 msec)

contain myoglobin (red) no myoglobin (white)

continuous use muscles - prolonged performance    for endurance performance ( marathoners)

one time use muscles - brief performances     for power & speed (sprinters) 

marathoner: 80% type I   &   20% type II sprinter: 20% type I  &  80% type II 

                     

best in long slow sustained contractions best in rapid (short) contractions 

not easily fatigued easily fatigued

more capillary beds, greater VO2 max  less capillary beds

smaller in size larger in size

lower glycogen content higher glycogen content

poor anaerobic glycolysis*  predominantly anaerobic glycolysis

    easily converts glycogen to lactate  w/o O2

*  predominant aerobic enzymes & metabolism some aerobic capacity

higher fat content lower fat content 

more mitochondria - Beta Oxidation high fewer mitochondria- Beta Oxidation low

poorly formed sarcoplasmic reticulum well formed sacroplasmic reticulum

slower release of Ca = slower contractions quick release of Ca = rapid contractions 

tropinin has lower affinity for Ca troponin - higher affinity for Ca

Changes between fiber types

• In response to weigh training and anaerobic exercise– IIx ---> IIa– I --> Iia

• Can IIa --> I?

• Can increase cross sectional area of fast fiber types by weigh training

  Weight Training and Different Myosin Types                sedentary people have higher amounts of IIx               active people have more IIa fibers                       heavy weight-load repetitions.....                        decreases Fast IIx fibers and converts them to Fast IIa fibers                             nuclei stop expressing IIx gene and express IIa genes                             after 1 month all IIx  -->  IIa    (muscle also become more massive)       Can we recruit   slow  --->   fast ?   maybe...                        but no good evidence to date for slow to fast recruitments.

Characteristics of other muscle types:

• Cardiac muscle - found only in the heart, striated, gap junctions allow direct electrical signaling between cells

• Smooth muscle - involuntary muscle, meshwork of actin and myosin, can contract more (greater shortening), but with less tension.

Muscle cell types

schematic:1) skeletal2) cardiac3) smooth

1

2

3

Skeletal muscle

Cardiacmuscle

http://www.youtube.com/watch?v=Cdl8Awn_lDY&feature=relatedhttp://www.youtube.com/watch?v=b1WD564sjWw&feature=related

Smooth muscle