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U N I T II
Textbook of Medical Physiology, 11th Edition
GUYTON & HALL
Copyright © 2006 by Elsevier, Inc.
Chapter 6:
Contraction of Skeletal Muscle
Copyright © 2006 by Elsevier, Inc.
Anatomy of Skeletal Muscle
Gross organization:
Figure 6-1; Guyton & Hall
Copyright © 2006 by Elsevier, Inc.
Cellular Organization
Muscle fibers
• single cells
• multinucleated
• surrounded by the
sarcolemma
Myofibrils
• contractile elements
• surrounded by the
sarcoplasm
Cellular organelles - lie between
myofibrils (mitochondria,
sarcoplasmic reticulum etc.) Figure 6-1; Guyton & Hall
Copyright © 2006 by Elsevier, Inc.
Molecular Organization
Figure 6-1; Guyton & Hall
Copyright © 2006 by Elsevier, Inc.
The Sarcomere
A band
I bandZ disc
sarcomere
thick filament (myosin)
thin filament (actin)
titin (filamentous structural protein)
M line
H zone
Copyright © 2006 by Elsevier, Inc.
“Sliding Filament” Mechanism
Contraction results from the sliding action
of interdigitating actin and myosin
filaments
RELAXED:
CONTRACTED:
Copyright © 2006 by Elsevier, Inc.
F-actin• double-stranded helix
• composed of polymerized G-actin
• ADP bound to each G-actin
(active sites)
• myosin heads bind to active sites
tropomyosin• covers active sites
• prevents interaction
with myosin
troponin• I - binds actin
• T - binds tropomyosin
• C - binds Ca2+
The Actin Filament
− the I band filament
− tethered at one end at
the Z disc
− 1 m long: v. uniform
nebulin forms guide for
synthesisFigure 6-6; Guyton & Hall
Copyright © 2006 by Elsevier, Inc.
The Myosin Molecule:
• two heavy chains (MW 200,000)
• four light chains (MW 20,000)
• “head” region - site of ATPase activity
Figure 6-5; Guyton & Hall
Copyright © 2006 by Elsevier, Inc.
Mechanism of Muscle Contraction
Theory:
Binding of Ca2+ to
troponin results in a
conformational
change in
tropomyosin that
“uncovers” the active
sites on the actin
molecule, allowing for
myosin to bind.
Copyright © 2006 by Elsevier, Inc.
“Walk-Along” Theory
Figure 6-7; Guyton & Hall
Copyright © 2006 by Elsevier, Inc.
“Walk-Along” Theory
1. Myosin head attached
to actin
3. Myosin head “cocked”
(ADP and Pi bound)
hydrolysis
4. Myosin head attaches
to new site(ADP bound)
Pi
2. Myosin head releases(ATP bound)
ATP
POWER
STROKE
ADP
Copyright © 2006 by Elsevier, Inc.
Rigor mortis:
• state of contracture that occurs following death
• due to loss of ATP
RIGOR
ATP1. Myosin head attached
to actin
Copyright © 2006 by Elsevier, Inc.
Muscle Mechanics
Copyright © 2006 by Elsevier, Inc.
Length-Tension Relation for Skeletal Muscle
• Active tension cannot be measured
directly
• What can be measured?
(1) passive tension - tension required to
extend a resting muscle
(2) total tension - active tension and
passive combined
• Active is calculated from 1 & 2
(AT = TT – PT)
• Note that active tension falls away
linearly with increasing length
Length (proportion of resting length)
1.0 2.00
50
100
active tension
passive tension
total tension
Normal operating
range
Copyright © 2006 by Elsevier, Inc.
Length-Tension Relation – The Experiment
100
50
0
0 1x 2x
Length (proportion of resting length)Tensio
n(%
ag
e m
ax c
on
tra
ctio
n)
passive
Passive tension
Zero tension
1
stimulus
2
2
3
3
Total tension active
total
(preload)
(afterload = ∞ )
1
Copyright © 2006 by Elsevier, Inc.
Normal operating
range
Tension as a Function of Sarcomere Length
• Stress is used to compare
tension (force) generated by
different sized muscles
– stress = force/cross-sectional
area of muscle; units kg/cm2)
• In skeletal muscle, maximal
active stress is developed at
normal resting length ~ 2 m
• At longer lengths, stress
declines -
• At shorter lengths stress also
declines -
• Cardiac muscle normally
operates at lengths below
optimal length -
active
stress(tension)
sarcomere length ( m)
0
1
0 1 2 3 4
0.5
Copyright © 2006 by Elsevier, Inc.
Relationship of Contraction
Velocity to Load
no afterload:
• maximum velocity at
minimum load
increased afterload:
• contraction velocity
decreases
contraction velocity is zero
when afterload = max force
of contraction
A
B
A: larger, faster muscle (white muscle)
B: smaller, slower muscle (red muscle)
Copyright © 2006 by Elsevier, Inc.
Types of Skeletal Muscle- speed of twitch contraction -
• Speed of contraction determined by
Vmax of myosin ATPase.
– High Vmax (fast, white)
• rapid cross bridge cycling
• rapid rate of shortening
(fast fiber)
– Low Vmax (slow, red)
• slow cross bridge cycling
• slow rate of shortening
(slow fiber)
• Most muscles contain both types of
fiber but proportions differ
• All fibers in a particular motor unit
will be of the same type i.e., fast or
slow.
Figure 6-12; Guyton & Hall
Copyright © 2006 by Elsevier, Inc.
• fast and slow fibers show different resistance to fatigue
• slow fibers
– oxidative
• small diameter
• high myoglobin content
• high capillary density
• many mitochondria
• low glycolytic enzyme content
• fast fibers
– glycolytic
• large diameter
• low myoglobin content
• low capillary density
• few mitochondria
• high glycolytic enzyme content
forc
e (
% in
itia
l)
time (min)
50 60
Fast (white muscle)
Slow (red muscle)
Types of Skeletal Muscle- resistance to fatigue -
Copyright © 2006 by Elsevier, Inc.
What do the different types do?
• Fast, slow and intermediate twitch type muscle can be identified by histochemistry.
• In any muscle there will be a mixture of slow and fast fibers.
• Motor units containing slow fibers will be recruited first to power normal contractions.
• Fast fibers help out when particularly forceful contraction is required.
Different people have different proportions of these types.
There is little evidence that training alters these proportions in humans.
Fast-twitch slow-twitch
Marathon 18% 82%
Runners
Swimmers 26 74
Average 55 45
man
Weight 55 45
Lifters
Sprinters 64 37
Jumpers 63 37
Copyright © 2006 by Elsevier, Inc.
Conversion of Fiber Type- fast to slow -
• Anterior tibialis –– Predominantly fast twitch (upper)
– Stains light: few mitochondria
– Few, small capillaries
– Large fibers
• Electrical stimulation (10 Hz) via motor nerve (60 days)
– Stimulating fast muscle at the pace of a slow muscle converts fast twitch fibers to predominantly slow twitch
fibers (lower)
– Stains dark: more mitochondria
– Many, large capillaries
– Larger fibers
left AT
right AT
Copyright © 2006 by Elsevier, Inc.
Motor Unit:
• All fibers are same type (fast or
slow) in a given motor unit
• Small motor units (eg,larnyx, extraocular)
− as few as 10 fibers/unit
− precise control
− rapid reacting
• Large motor units (eg, quadriceps
muscles)
− as many as 1000 fibers/unit
− coarse control
− slower reacting
• Motor units overlap, which provides
coordination
• Not a good relation between fiber type
and size of motor unit
A collection of muscle fibers innervated by a single motor neuron
Copyright © 2006 by Elsevier, Inc.
Force summation: increase in contraction
intensity as a result of the
additive effect of
individual twitch
contractions
(1) Multiple fiber
summation: results from
an increase in the
number of motor units
contracting
simultaneously (fiber
recruitment)
Muscle Contraction - force
summation
(2) Frequency summation: results
from an increase in the frequency
of contraction of a single motor unit
Figure 6-13; Guyton & Hall
Copyright © 2006 by Elsevier, Inc.
Frequency Summation of
Twitches and Tetanus
• Myoplasmic Ca2+ falls (initiating relaxation) before development of maximal contractile force
• If the muscle is stimulated before complete relaxation has occurred the new twitch will sum with the previous one etc.
• If action potential frequency is sufficiently high, the individual contractions are not resolved and a „fused tetanus‟ contraction is recorded.
Myoplasmic [Ca2+]
Force
APTime (1 second)
Fused tetanus
Copyright © 2006 by Elsevier, Inc.
Muscle Remodeling - growth
hyperplasia
hypertrophy
lengthening
• Hypertrophy (common, weeks)
– Caused by near maximal force development (eg. weight lifting)
– Increase in actin and myosin
– Myofibrils split
• Hyperplasia (rare)
– Formation of new muscle fibers
– Can be caused by endurance training
• Hypertrophy and hyperplasia
– Increased force generation
– No change in shortening capacity or velocity of contraction
• Lengthening (normal)
– Occurs with normal growth
– No change in force development
– Increased shortening capacity
– Increased contraction velocity
Copyright © 2006 by Elsevier, Inc.
Muscle Remodeling - atrophy
atrophy with fiber loss
atrophy
• Causes of atrophy
– Denervation/neuropathy
– Tenotomy
– Sedentary life style
– Plaster cast
– Space flight (zero gravity)
• Muscle performance
– Degeneration of contractile proteins
– Decreased max force of contraction
– Decreased velocity of contraction
• Atrophy with fiber loss
– Disuse for 1-2 years
– Very difficult to replace lost fibers
weeks
months/
years