15 & 17 October 2008•Muscle Physiology

–Properties of individual twitching myofibers–Fiber types–Motor units and whole muscle composition–Fatigue–Recruitment–Effects of training (King et al., 1999)–Creatine supplementation–Smooth muscle

•Innervation

•Single unit vs multi unit

•Lab next week: Frog Muscle Physiology•Test 2 Monday: Check email and website over the weekend for more information.

Isotonic = “same tension”

muscle shortens as tension exceeds load

Twitch

Duration depends on cytosolic Ca++

Contraction Time

Isot

onic

con

trac

tion

Classes of Myofibers based on Twitch Duration

• Fast twitch = rapid hydrolysis of ATP by Myosin means crossbridges cycle faster

• Slow twitch = slower hydrolysis, isozyme catalyzes the reaction slower

Classes of Myofibers based on Enzyme Profile

• Oxidative: many mitochondria, oxidative phosphorylation, many adjacent capillaries, with myoglobin, (red),fatigue resistant

• Glycolytic: few mitochondria, many glycolytic enzymes, large store of glycogen, few capillaries, little myoglobin (white),fatigue quickly

Fig. 09.24b

Fig. 09.03

Fast and Slow refer to Myosin-ATPase activity…determines rate of contraction

Type I

Type IIA

Type IIB

Isometric = “same length” …muscle contracts …develops tension but …does not shorten.

Load > tension

Twitch Duration

Will test in frog muscles during lab

Fig. 09.18

Be able to describe examples of isometric, isotonic, and lengthening contractions

Summation

Summation

Cytosolic Ca++

Produce this during lab with frog muscle

Fig. 09.21

True for Skeletal & Cardiac

Myofibers, but NOT for

smooth myofibers

Length-tension “curve”

Normal range

Factors determining Single Myofiber Tension

• Action Potential Frequency (summation)

• Sarcomere length (length-tension curve)

• Fiber diameter (# of thick and thin myofilaments; # of cross bridges)

• State of fatigue

But muscles consists of many myofibers, not all of which contract at the same time!

Fig. 09.13a

•Motor unit = motorneuron (mn) + all the myofibers it innervates.

•All myofibers of a motor unit are the same type:

Type I, Type IIA or Type IIB

Fig. 09.13b

• Smallest motor units consist of ~13 myofibers per mn in extraocular muscles of the eye– small motoneurons, easily depolarized, finely controlled movement

• Large motor units may have more than 1000 myofibers per mn, typical of gastrocnemius muscle– large motoneurons, more difficult to depolarize, recruited later

• Size principle: small mn’s most easily achieve threshold• Recruitment according to size principle

Motor Units

Fig. 09.26

Factors determining Whole Muscle Tension

• # myofibers per motor unit

• # of active motor units (recruitment)

Fuel for Skeletal Myofibers

Moderate exercise: Creatine phosphate, then oxidative phosphorylation (OP) from glycogen, then OP from blood glucose, then blood fatty acids. If intense; glycolysis.

                                                                                  

A 1998 Review on the Use of Creatine as a

Nutritional Supplement

Creatine

Reserve

Causes of fatigue

• High intensity, short duration exercise– Conduction failure in t-tubules– Lactic acid accumulation– Inhibition of cross bridge cycles by ADP

• Low intensity, long duration exercise– As above, and– Depletion of muscle glycogen– Low plasma glucose (hypoglycemia)– Dehydration

• Central command fatigue: “willpower”

Response to training

• Resistance trainingType II change enzyme profiles

more glycolytic enzymesType II add more actin and myosinType II increase CS area (hypertrophy)

• Endurance training– Type I increases vascularity– Type I get more mitochondria– Type I get slightly smaller– Type IIB convert to Type IIA

Fast and Slow refer to Myosin-ATPase activity…determines rate of contraction

Type I

Type IIA

Type IIBCheck King et al., 1999 to see if their results are as expected!