Download Neuromuscular Adaptation

SKELETAL MUSCLE
PHYSIOLOGY
Abraham D. Lee, Ph.D.,P.T.
Department of Physical Therapy
Office: Collier Building # 4206
Phone #: 419-383-3437
Email: [email protected]
Contents
1. Muscle structure & organization
2. Muscle fiber type
3. Muscle action
4. Muscle mechanics
5. Motor unit and its recruitment
6. Local muscle control
7. Muscle plasticity
8. Summary
Muscle organization
• Epimysium: wraps an entire muscle
• Perimysium: wraps a bundle of muscle fibers. This
bundle is called fascicle or fasciculus
• Endomysium: wraps an individual muscle fiber
• Sarcolemma: muscle membrane
• Myofibrils: contractile filaments
Myofibrils
• Thin filament
– Actin filaments
– Troponin
– Tropomyosin
• Thick filament
– Myosin: 4 light chains and 2 heavy chains
– Heavy chains
• Myosin head region: heavy meromyosin
• Myosin tail region: light memromysin
Muscle pennation
•
Longitudinal (non-pennated) architecture: muscle
fibers in parallel to the muscle force generating
axis
–
–
•
Example: biceps brachii, sartorius muscle
In these muscles fibers are said to be fusiform or spindle
shaped.
“Pennate” architecture: muscle fibers are oriented
at an angle or multiple angles relative to forcegenerating axis.
1) Unipennate: soleus-25 degree; vastus medialis-5
degree
2) Bipennate: gastrocnemius, rectus femoris
3) Multipennate: deltoid
Effect of pennation
Force loss
Space saving
Comparison b/n non-pennated & pennated muscle
Non-pennated
Pennated
•Contraction
•Fiber packing
w/ given volume
•Force loss due
to pennation
Fast
Less
Slow
More
No
Yes
•# fiber
Less
More
•Muscle force
Production
Less
Greater
•CSA
Less
Greater
Muscle Fiber Type
Muscle fiber type
• Type I,
– Slow-oxidative (SO) fibers
• Type IIa,
– Fast-oxidative-glycolytic (FOG) fibers
• Type IIb,
– Fast-glycolytic (FG) fibers
Characteristics of different fibers
#Mitochondria
Resistance to fatigue
Energy
ATPase activity
Vmax
Efficiency
•
•
•
•
•
•
•
Type I
H
H
A
L
L
H
L: low
H: high
M: moderate
A: aerobic
AN: anaerobic
HH: highest
Type IIa
H/M
H/M
A+AN
H
H
M
Type IIb
L
L
AN
HH
HH
L
Muscle composition in athletes
% Type I %Type IIa &IIb
Distance runners 70-80
20-30
Track sprinters
25-30
70-75
Weight lifters
45-55
45-55
Non-athletes
47-53
47-53
Will fiber type change with training?
Muscle Action
• Excitation-contraction coupling
• Type of muscle action
Excitation-Contraction Coupling
•Nerve impulse generation and propagation
•Neuromuscular junction transmission
•Muscle action potential propagation
•Ca2+ release from SR
•Ca2+ binding to troponin
•Interaction of myosin head and actin
•Cross bridge moves: tension development
•Ca2+ taken up to SR
•Ca2+ removal from troponin
•Relaxation
E-C coupling
DHPR: dihydropyridine receptors
RyR: ryanodine receptor
Other possible mechanism:
Inositol 1,4,5-triphosphate (InsP3)
InsP3 receptor activation
Ca2+ release from SR
May play a role in slow twitch
muscle in developmental stage
(Talon et al., Am. J. Physiol 282:
R1164-R1173, 2002)
E-C coupling
Sliding Filament Theory
Changes during shortening muscle
action
• Sarcomere length (distance between
two adjacent Z lines): shortens
• A band: no change
• I band: shortens
• H zone: shortens
Different type of muscle action
(contraction)
• Isometric action
• Isotonic action (dynamic action)
– Concentric action
– Eccentric action
Muscle mechanics
It deals with how muscle force is generated
and regulated.
Factors that affect muscle force generation
•
•
•
•
•
•
Rate of muscle stimulation
Muscle length
Joint angle
Speed of action (speed of contraction)
Muscle fiber type
# of MU recruitment
Rate of Muscle Stimulation
• Twitch:
• Tetanus:
Muscle twitch
Muscle tetanus
Effect of Muscle Length
Force-Length Relationship
• Isolated muscle
• In vivo human muscles
Force-Length Relationship
• Isolated muscle
Force-Length Relationship
• In vivo human muscles
– Two things are considered: muscle length
and joint angle
– In general, a group of muscles produces
more force (torque) when muscles are
lengthened before contraction. But some
muscles do not follow this rule.
Shoulder muscles
Shoulder flexors (anterior
deltoid): causes to flex
shoulder joint
180°
135°
Shoulder extensors(posterior
deltoid): causes to extend
90° shoulder joint
45°
40°
0°
Knee flexors
• A person is lying on the stomach (prone
position)
90°
120°
45°
0°
Lower leg
Thigh
Knee joint
Trunk
Hip joint
Knee flexors (hamstring muscles): causes to flex knee joint
Hip flexors
• A person is lying on the back (supine
position)
90°
120°
45°
0°
Lower leg
Thigh
Trunk
Knee joint hip joint
Hip flexors (iliopsoas, sartorius): causes to flex hip joint
Knee extensors
• A person is sitting on the bench
120° 90° 45°
0°
Knee extensors (quadriceps muscles): causes to extend knee joint
Elbow flexors
Elbow flexors (biceps brachii): causes to flex elbow joint
Force arm distance
Effect of Velocity (Speed of Action)
Force-velocity curve
Effect of Muscle Fiber
Effect of muscle fiber type on force
Muscle Power
Need to consider two factors:
1. Muscle force
2. Speed of action
Power-Velocity Relationship
Power = work/time = (force x
distance)/time = force x speed
Effect of muscle fiber type on power
Factors that affect muscle force/power generation
•
•
•
•
•
•
Rate of muscle stimulation
Muscle length
Joint angle
Speed of action
Muscle fiber type
# of MU recruitment
Motor Unit
How does an individual generate
appropriate force for a given task?
Motor Unit (MU)
Functional unit of movement
Motor Unit (MU)
•MU consists of
•Single -motor neuron
•Muscle fibers innervated by
the -motor neuron
Motor Unit (MU)
•Fast fatigable MU (FF)
•High twitch tension
•High fatigue index
•Fast fatigue resistant MU (FR)
•Intermediate twitch tension
•Intermediate fatigue index
•Slow MU (S)
•Low twitch tension
•Low fatigue index
Reasons for different twitch
tension in different MU
•Depends on number of muscle
fibers and fiber size
# muscle fiber: FF>FR> S
Size of fiber: FF>FR>S
Relationship b/n MU & Fiber type
MU
Fiber type
FF
FR
S
Fast glycolytic
Fast oxidative
Slow oxidative
Motor Unit
Muscle
• Biceps brachii
• Gastrocnemius
• First lumbrical
# neuron
774
580
98
# fibers/MU
750
1720
110
Motor Unit Recruitment
• Follows the size principle
– Small neuron cell body and axon activated first
– Larger cell body and axon recruited later
• Example:
S MU
FR MU
FF MU
100%
0%
% of effort level (Intensity of exercise)
Gradation of Muscle Strength
• By increasing # of MU recruited
• By increasing frequency of stimulation
Local Control of Muscle Action
• Muscle spindle: muscle length monitor
– Consists of 1) afferent nerves, 2) intrafusal fibers
& 3) γ(gamma)-motor neurons
• Golgi Tendon Organ: muscle tension monitor
Structure of muscle spindle
Action of muscle spindle
Nerve impulse pattern of afferent nerves
Rest
Stretch
Contraction Return to rest
Speed of stretch on impulse
discharge pattern
Clinical implications for individuals with spastic muscle?
Golgi Tendon Organ
•GTO#<spindle # in given muscle
•Composed of network of unmyelinated
nerve fibers enclosed by fine capsule
•Activated by either muscle stretch or
muscle contraction. More sensitive to
muscle contraction.
•Activates inhibitory interneuron in spinal
cord, which, in turn, inhibits -motor
neuron of contracting muscle (agonists).
Impulse discharge pattern of GTO
during stretch and contraction
Plasticity of Muscle
• Metabolic and morphological changes to
changes in stimulus
– Increased stimulus: exercise training
– Decreased stimulus: non-weight bearing, bed
rest and aging
Endurance training
• Mode: jogging, running, cycling,
swimming, etc
• Adaptations
– # of mitochondria
– size of mitochondria
– Oxidative enzyme activities
• Krebs cycle, beta-oxidation, ETS
– Some glycolytic enzymes
– Capillary density
Resistance training
• Strength
– Neural factor
– Muscle fiber enlargement
(hypertrophy)
33%
27%
38%
31%
5-6 month resistance training using triceps brachii
MacDougall et al, EJAP 43:25-34
6 wk
Resistance Training
Limb suspension
(Non-weight bearing)
Limb unloading on muscle strength and X-area
X-area
Knee extensor Strength
Dudley et al, in ACSM’s Resource Manual, p.201
Selective muscle atrophy with non-weight bearing
Dudley et al, in ACSM’s Resource Manual, p.201
Bed Rest
Muscle strength change with bed rest
(soleus and gastroc.).)
Dudley et al, in ACSM’s Resource Manual, p.203
Changes in skeletal muscles with aging
•
•
•
•
# of muscle fibers
Muscle area
Fiber type distribution
Muscle strength
McArdle et al, in Exercise Physiology, p639
Muscle fiber distribution with aging
Fiber Area
Age
%Type II
Type I
Type II
26.1
59.5
2944
3663
35.3
63.2
2854
3509
42.6
51.8
3133
3361
54.5
48.3
2877
2802
61.6
45.0
2264
2120
Muscle strength with aging
• A decline in muscle strength is associated
with a decrease in muscle mass.
• A decline in lower extremity muscle
strength is related to poor functional
performance
– walking ability, balance, stair-climbing
ability, falls
Trainability of skeletal muscles
with aging
Frontera et al, J. Appl. Physiol., 64:1038-1044, 1988
•Untrained old men (60-72 yrs)
•8 reps/set, 3 sets/day, 3days/week at 80% of
1 RM for 12 weeks training
•Thigh muscle X-area
•Knee extension and flexion strength.
Leg strength
Frontera et al
1 RM max
X-area of quadriceps
Frontera et al
Physician’s Role for Physical Activity
Summary
Know followings: names & functions
•Muscle structure: connective tissues, pennation,
myofibrils
•Muscle fiber characteristics
•Muscle action: E-C coupling
•Sliding filament theory: changes during contraction
•Muscle mechanics: force & power-length-velocity
•MU: elements, function, characteristics.
•Muscle action monitor: muscle spindle and GTO
•Changes in muscle in training
•Changes in muscle w/ suspension, bed rest aging
The End