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Skeletal Muscle Mechanics
Muscle fibers into whole muscle
Whole muscle tension:• Number of muscle fibers contracting• Tension developed by each fiber
Motor units:
Figure 8.15Page 269
Motor unit recruitment
Also influenced by fiber type!
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Whole muscle tension depends on:1. Frequency of stimulation
Figure 8.17 Page 271
Contractileactivity
Actionpotentials
Singletwitch
Twitchsummation
Tetanus
Stimulationceases orfatiguebegins
Whole muscle tension depends on:2. Length of fiber at onset of contraction
Figure 8.18
Page 272
Whole muscle tension depends on:3. Extent of fatigue4. Thickness of fiber
Duration of activity
Amount of motor unit recruitment
Fiber type
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Types of Contractions
1. Isotonic• Muscle tension remains constant as muscle
changes lengthConcentric & eccentric contractions
2. Isometric• Tension develops at constant length
3. Isokinetic• Fixed movement
Skeletal Muscle Metabolism
Major requirement of contraction-relaxation coupling is ATP…
1. Splitting of ATP by myosin ATPase
2. Binding of ATP to myosin
3. Active transport of Ca2+ back into sarcoplasmicreticulum
Must be in constant supply (readily available)
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Intramuscular ATP supplied by 3 means
1. Creatine Phosphate
2. Glycolysis
3. Oxidative Phosphorylation
Immediate energy source
Concentrations will drive reaction• Resting muscle: ~ 5x the amount of CP than ATP
Only 1 enzyme (rapid reaction)
Limited supply• Short bursts, high-intensity exercise
Creatine Phosphate:
CreatinePhosphate
ADP Creatine ATP+ +Creatine Kinase
Glycolysis:
No O2 requirement (like CP) ~ anaerobic
Continuous high-intensity exercise
Breakdown of glucose or glycogen:
2 ATPPyruvicacid
Pyruvicacid
Lactic acid
Oxidative phosphorylation
Mitochondria
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Oxidative Phosphorylation:
If energy requirement continues
Multiple steps• Time• Pathway fueled primarily by glucose & fatty acids
Occurs within the mitochondria (O2!!)• O2 comes from hemoglobin & myoglobin• Electron-transport chain
Yields 36 ATP (glucose), ~ 128 ATP (Fat)
Muscle fiber
Blood
Figure 8.23 Page 278
Fatigue:
1. Muscular• Increased concentration of Pi (inorganic
phosphate)• Accumulation of lactic acid (lactate)• Glycogen or glucose depletion
2. Neural (Central & Peripheral)• Psychological
3. O2 debt & nutrient depletion
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Skeletal Muscle Fibers
1. Slow-oxidative (Type I)• Mitochondrial density
More resistant to fatigue
2. Fast-oxidative (Type IIa)• Higher myosin ATPase activity
3. Fast-glycolytic (Type IIb)• Higher myosin ATPase activity
Endurance training
Changes in fibers:
Weight lifting
Drugs
Motor Control
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Input to Motor Neurons
1. Input from afferent neurons• Intervening interneurons (spinal reflexes)
2. Input from primary motor cortex
3. Input from brainstem
Figure 8.24Page 285
Cortical level
Subcortical level
Brain stem level
Spinal cord level
Periphery
Premotor and supplementary motor areas
Sensory areas of cortex
Primary motor cortex
Basal nuclei Thalamus
Brain stem nuclei(including reticular formation and vestibular nuclei)
Cerebellum
Afferent neuronterminals
Motor neurons
Peripheralreceptors
Muscle fibers
Other peripheral events,such as visual input
Sensory consequencesof movement
Movement
= Pathways conveying
afferent input
= Corticospinal motor system
= Multineuronal
motor system
Muscle Receptors
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Coordinated movement:
Learned muscle behaviors• CNS & muscle input• Proprioception
2 types of muscle receptors:1. Muscle spindles
• Stretch reflex (knee-jerk)
2. Golgi tendon organs• Respond to tension changes
Alpha motorneuron axon
Gamma motorneuron axon
Secondary (flower-spray)endings of afferentfibers
Extrafusal (“ordinary”)muscle fibers
Capsule
Intrafusal (spindle)muscle fibers
Contractile end portionsof intrafusal fiber
Noncontractilecentral portionof intrafusalfiber
Primary (annulospiral)endings of afferent fibers
Figure 8.25Page 286
Figure 8.26 (1) Page 287
Extrafusalskeletalmuscle fiber
Intrafusalmusclespindle fiber
Spinalcord
Afferent input from sensory endings of muscle spindle fiberAlpha motor neuron output to regular skeletal-muscle fiber
Stretch reflex pathwayGamma motor-neuron output to contractile end portions of spindle fiber
Descending pathways coactivating alpha and gamma motor neurons
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Relaxed muscle; spindlefiber sensitive to stretchof muscle
Contracted muscle inhypothetical situation ofno spindle coactivation;slackened spindle fibernot sensitive to stretchof muscle
Contracted muscle innormal situation of spindle coactivation;contracted spindle fibersensitive to stretch ofmuscle
Figure 8.26 (2)Page 287
Figure 8.27Page 288
Patellar tendon
Extensor muscle of knee(quadriceps femoris)
Musclespindle
Alpha motorneuron
