Muscle Physiology
Lecture 3: Muscle fiber types and muscle receptors
Allied
Allied Health
Health Science
Science Physiology
Physiology
Dr.
Dr. Daniel
Daniel Ulrich
Ulrich
Trinity
Trinity College
College Dublin
Dublin
Lecture Outline
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Types of Skeletal Muscle Fibers
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Slow oxidative (Type I)
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Fast oxidative (Type IIA)
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Fast glycolytic (Type IIB)
Muscle receptors
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Muscle spindle
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Golgi tendon organs
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Types of Skeletal Muscle Fibers
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All skeletal muscle fibers alike:
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Excitation contraction coupling
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Crossbridge cycle
Differences in the speed of contraction:
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Fast-twitch fibers and
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Slow-twitch fibers
Differences in the primary mode of ATP production:
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Glycolytic fibers and
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Oxidative fibers
3 types: Slow oxidative, fast oxidative, and fast glycolytic
fibers
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Classification as Fast and Slow Fibers
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Dependent on rate of myosin ATPase activity
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ATP hydrolysis = rate limiting step of cycle
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Higher rate—faster crossbridge cycling
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Fast fibers: Myosin with fast ATPase activity
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Slow fibers: Myosin with slow ATPase activity
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Fast fibers contract two to three times faster than low fibers
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Fast fibers also relax more rapidly
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Slow fiber contractions last approximately
10 times longer than fast fiber
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Contraction Speed
fast twitch
intermediate twitch
slow twitch
Figure 12.23
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Oxidative Fibers
Glycolytic Fibers
Primary energy through
oxidative phosphorylation
Primary energy through
anaerobic glycolysis
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Many mitochondria
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Fewer mitochondria
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Myoglobin (red)
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Many glycolytic enzymes
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Many capillaries
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High glycogen stores
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Small diameter
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Use little oxygen—
anaerobic
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Resistant to fatigue
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Large diameter
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Quick to fatigue
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Oxidative and Glycolytic Fibers
Fiber types:
•Slow oxidative:
- Smallest size & force
•Fast oxidative:
- Intermediate size & force
•Fast glycolytic:
- Biggest size & force
Stain for mitochondria: oxidative fibers appear dark
Figure 12.24
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Distributions of Fiber Types in a Muscle
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One muscle = mixture of fiber types
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Proportions vary depending on function
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In single motor units—all muscle fibers of same type
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Example: Postural muscles have more slow oxidative
Type depends on innervation
Metabolic patterns can change, but not speed (myosin on fibers)
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Fast oxidative
Fast glycolytic
Recruitment Order
1.
2.
3.
Slow oxidative fibers
Fast oxidative fibers
Fast glycolytic fibers
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Skeletal Muscle Fiber Types
Table 12.1
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Muscle Fatigue
Fatigue
Decline in a muscle’s ability to maintain a
constant force of contraction during
repetitive stimulation
Causes of Muscle Fatigue
•Low intensity exercises: Depletion of
energy reserves
•High intensity exercises: Build up of
lactic acid.
•Strong and sustained contractions:
Compression of blood vessels.
•Very high intensity: Depletion of
acetylcholine (neuromuscular fatigue)
•Central fatigue = psychological fatigue
•Other possibilities: Build up of inorganic
phosphates, Changes in ion distribution
Figure 12.25
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Adaptation to Use
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Skeletal muscle adaptation to use
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No cell division
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Change in muscle size due to change in size of individual
cells
Disuse atrophy
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Denervation atrophy
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Decrease in size (lose myofibrils)
Motor neuron destroyed, so no excitation, atrophy due to
lack of use
Hypertrophy
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Increase in size (increase myofibrils)
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Increase production of actin and myosin
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Adaptation to Aerobic Exercise
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Endurance exercises = aerobic exercises
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Low to moderate intensity can be sustained
Increases oxidative capacity of muscle
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More mitochondria and bigger size
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Increase blood supply (capillaries)
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Decrease in diameter (easier for cells to get O2, but
force generating capacity decreases)
Fast glycolytic
Fast oxidative
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Adaptation to Anaerobic Exercise
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High intensity exercise
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Cannot be sustained
Increases ability of muscle to generate more tension
(strength)
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Increases amount actin and myosin
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Increases number of myofibrils
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Increases diameter of muscle fiber (increases force
generating capacity)
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Increases glycolytic enzymes
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Decreases oxidative capacity (decreases the size and
number of mitochondria)
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Decreases fatigue resistance
Fast oxidative
fast glycolytic
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Skeletal Muscle Receptors
• Muscle spindle
– Detect muscle length
• Golgi tendon organ
– Detect muscle tension
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Muscle Spindle
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Mucle:
Extrafusal fibers
– Contractile cells of the muscle
– Responsible for skeletal muscle
contraction
– Innervated by alpha motor neurons
Muscle spindle:
Intrafusal fibers
– Contractile elements of the muscle
spindle
– Adjust sensitivity of muscle to
stretch
– Innervated by gamma motor units
Type of sensory endings
– Annulospiral endings (Type Ia
afferents)
– Flower-spray endings (Type II
afferents)
Figure 12.28
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Muscle Spindle Responses
Figure 12.29
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Alpha / Gamma Coactiviation
Figure 12.30
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Golgi Tendon Organs
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Sensory capsules (type Ib
afferent) within tendons
Following muscle contraction,
tendon stretches and activates
GTO
– Increase in freq. of action
potentials of type Ib
afferent neurons
reflex
inhibition of muscle
– Protection against overactivity of muscle
Figure 12.31
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