Download muscle2

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
Isotonic contraction
Contraction Time
Twitch
Duration depends on cytosolic Ca++
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
Type I
Fast and Slow refer
to Myosin-ATPase
activity…determines
rate of contraction
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
Length-tension “curve”
Fig. 09.21
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
Motor Units
• 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
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
Reserve
Creatine
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 training
Type II change enzyme profiles
more glycolytic enzymes
Type II add more actin and myosin
Type 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
Type I
Fast and Slow refer
to Myosin-ATPase
activity…determines
rate of contraction
Check King et al., 1999 to
see if their results are as
expected!
Type IIA
Type IIB