Download Skeletal Muscle Mechanics

Skeletal Muscle
Mechanics
Muscle fibers into whole muscle
Whole muscle tension:
• Number of muscle fibers contracting
• Tension developed by each fiber
Motor units:
Motor unit
recruitment
Figure 8.15
Page 269
Also influenced by fiber type!
1
Whole muscle tension depends on:
1. Frequency of stimulation
Tetanus
Contractile
activity
Single
twitch
Stimulation
ceases or
fatigue
begins
Twitch
summation
Action
potentials
Figure 8.17 Page 271
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 fatigue
4. Thickness of fiber
Duration of activity
Amount of motor unit recruitment
Fiber type
2
Types of Contractions
Isotonic
1.
•
Muscle tension remains constant as muscle
changes length
9 Concentric & eccentric contractions
Isometric
2.
•
Tension develops at constant length
Isokinetic
3.
•
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 sarcoplasmic
reticulum
Must be in constant supply (readily available)
3
Intramuscular ATP supplied by 3 means
1.
Creatine Phosphate
2.
Glycolysis
3.
Oxidative Phosphorylation
Creatine Phosphate:
¾
Immediate energy source
Creatine
+
Phosphate
¾
Creatine Kinase
ADP
Creatine +
ATP
Concentrations will drive reaction
• Resting muscle: ~ 5x the amount of CP than ATP
¾
Only 1 enzyme (rapid reaction)
¾
Limited supply
• Short bursts, high-intensity exercise
Glycolysis:
¾
No O2 requirement (like CP) ~ anaerobic
¾
Continuous high-intensity exercise
¾
Breakdown of glucose or glycogen:
Pyruvic
acid
Lactic
acid
Pyruvic
acid
Oxidative
phosphorylation
2 ATP
Mitochondria
4
Oxidative Phosphorylation:
¾
If energy requirement continues
¾
Multiple steps
• Time
• Pathway fueled primarily by glucose & fatty acids
¾
Occurs within the mitochondria (O2!!)
• O2 comes from hemoglobin & myoglobin
• Electron-transport chain
9Yields 36 ATP (glucose), ~ 128 ATP (Fat)
Muscle fiber
Blood
Figure 8.23 Page 278
Fatigue:
Muscular
1.
•
•
•
Neural (Central & Peripheral)
2.
•
3.
Increased concentration of Pi (inorganic
phosphate)
Accumulation of lactic acid (lactate)
Glycogen or glucose depletion
Psychological
O2 debt & nutrient depletion
5
Skeletal Muscle Fibers
Slow-oxidative (Type I)
1.
•
Mitochondrial density
9 More resistant to fatigue
Fast-oxidative (Type IIa)
2.
•
Higher myosin ATPase activity
Fast-glycolytic (Type IIb)
3.
•
Higher myosin ATPase activity
Changes in fibers:
Endurance
training
Weight
lifting
Drugs
Motor Control
6
Input to Motor Neurons
Input from afferent neurons
1.
•
Intervening interneurons (spinal reflexes)
2.
Input from primary motor cortex
3.
Input from brainstem
Premotor and
supplementary motor
areas
Cortical
level
Subcortical
level
Sensory
areas of
cortex
Basal
nuclei
Primary motor cortex
Thalamus
Brain stem
level
Cerebellum
=
Pathways
conveying
afferent input
Brain stem nuclei
(including reticular
formation and
vestibular nuclei)
Spinal cord
level
Afferent
neuron
terminals
=
Corticospinal
motor system
Motor
neurons
Muscle
fibers
Peripheral
receptors
Periphery
Movement
Other peripheral events,
such as visual input
Sensory consequences
of movement
=
Multineuronal
motor
system
Figure 8.24
Page 285
Muscle Receptors
7
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
Figure 8.25
Page 286
Capsule
Alpha motor
neuron axon
Intrafusal (spindle)
muscle fibers
Gamma motor
neuron axon
Contractile end portions
of intrafusal fiber
Secondary (flower-spray)
endings of afferent
fibers
Extrafusal (“ordinary”)
muscle fibers
Noncontractile
central portion
of intrafusal
fiber
Primary (annulospiral)
endings of afferent fibers
Extrafusal
skeletal
muscle fiber
Spinal
cord
Intrafusal
muscle
spindle fiber
Afferent input from sensory endings of muscle spindle fiber
Alpha motor neuron output to regular skeletal-muscle fiber
Stretch reflex pathway
Gamma motor-neuron output to contractile end portions of spindle fiber
Descending pathways coactivating alpha and gamma motor neurons
Figure 8.26 (1) Page 287
8
Figure 8.26 (2)
Page 287
Relaxed muscle; spindle
fiber sensitive to stretch
of muscle
Contracted muscle in
hypothetical situation of
no spindle coactivation;
slackened spindle fiber
not sensitive to stretch
of muscle
Extensor muscle of knee
(quadriceps femoris)
Contracted muscle in
normal situation of
spindle coactivation;
contracted spindle fiber
sensitive to stretch of
muscle
Muscle
spindle
Patellar tendon
Alpha motor
neuron
Figure 8.27
Page 288
9