Download Biol&241 9 Part2

Steps in Initiating Muscle Contraction
Motor
Synaptic
terminal end plate
Steps in Muscle Relaxation
T tubule Sarcolemma
Action
potential
reaches
T tubule
ACh released, binding
to receptors
Sarcoplasmic
reticulum
releases Ca2+
Active site
exposure,
cross-bridge
formation
Ca2+
Actin
Myosin
ACh broken down by AChE
Sarcoplasmic
reticulum
recaptures Ca2+
Active sites
covered, no
cross-bridge
interaction
Contraction
ends
Contraction
begins
Relaxation occurs,
passive return to
resting length
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Rigor mortis
•  “Stiff”, “death”
•  >3–4 hours
•  Dying cells take in calcium à cross bridge
formation
•  Cross bridge detachment requires ATP
•  No ATP generated to break cross bridges
•  Stops after proteins break down (48 - 60hrs)
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Tension Production and Contraction Types
•  To contract a muscle, many fibers have to
contract
•  Muscles are “split” into motor units
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Spinal cord
Motor
unit 1
Motor
unit 2
Spinal Nerves
Muscle
Muscle
fibers
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Branching axon
to motor unit
Tension Production and Contraction Types
•  Whenever we want contraction
•  Call on motor units
•  Motor Units can produce two tension patterns
•  Isotonic contraction
•  Isometric contraction
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Tension Production and Contraction Types
•  Isotonic Contraction
•  Skeletal muscle changes length
•  Resulting in
•  Motion!
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Tension Production and Contraction Types
•  Isometric Contraction
•  iso- = same, metric = measure
•  Skeletal muscle develops tension, but is
prevented from changing length
•  No motion
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Tension Production and Contraction Types
•  Tension Production by Muscles Fibers
•  muscle fiber : contracted or relaxed
•  Tension Depends on:
•  The frequency of stimulation
•  First let’s look at a twitch
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Twitch:
single
contraction
Tension
Maximum tension
development
Stimulus
Resting Latent Contraction
phase period
phase
1
2
Relaxation
phase
3
1. AP moves through sarcolemma; No tension
2. Ca2+ binds (crossbridges) àtension peaks
3. Ca2+ falls à active sites covered à tension falls
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Tension
Stimulus
Two ways to cause muscle response:
A. frequency
B. Strength of stimulus
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Tension
A. frequency
Relaxation
0
Stimulus
100
Apply BEFORE relaxation ends
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Time (ms)
200
300
Wave summation
= Stimulus
Time
Stimuli arrive before relax. ends
Causes increasing tension
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Tension
Maximal tension of a single twitch
Relaxation
0
Stimulus
100
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Time (ms)
200
300
Tension
Figure 9.15b A muscle's response to changes in stimulation frequency.
0
100
Time (ms)
Low frequency à incomplete tetanus
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200
300
Tension
Figure 9.15c A muscle's response to changes in stimulation frequency.
0
100
Time (ms)
200
High frequency à complete tetanus
POINT: Increase firing rate àincrease greater muscular force
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300
Tension
Stimulus
B. Strength of stimulus
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Stimulus voltage
Figure 9.16 Relationship between stimulus intensity (graph at top) and muscle tension (tracing below).
Stimulus strength
Maximal
stimulus
Threshold stimulus
1
2
3
4
7
5
6
Stimuli to nerve
8
9
10
Proportion of motor units excited
Strength of muscle contraction
Tension
Maximal contraction
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Time (ms)
Tension Production and Contraction Types
•  Motor Units and Tension Production
•  Recruitment
•  Slowly increase number of motor units stimulated
•  Leads to tension
•  à movement!
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Axons of
motor neurons
Motor
nerve
KEY
Motor unit 1
Motor unit 2
Motor unit 3
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SPINAL CORD
Muscle fibers
(Muscle cells)
Tension
Tension in tendon
Motor Motor Motor
unit 1 unit 2 unit 3
Time
Allows rest of individual motor units, but tendon tension constant
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Tension
The size
principle of
recruitment.
Time
Small
Fibers
medium
fibers
POINT: increase in force during weak contraction
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Education,
Inc. Inc.
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Pearson Education,
large
fibers
Motor Units
•  Muscles use both frequency increase and motor
unit partitioning to achieve:
•  Isotonic contractions
•  Isometric contractions
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Isotonic
Tendon
Two types:
If muscle tension > load:
Muscle shortens
(concentric contraction)
If muscle tension < load :
Muscle lengthens
(eccentric contraction)
Muscle
contracts
(concentric
contraction)
2 kg
2 kg
Muscle
tension
(kg)
Amount of
load
Muscle
relaxes
Peak tension
production
Contraction
begins
Resting length
Muscle
length
(percent
of resting
length)
Time
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Isometric
Amount of load
Muscle
tension
(kg)
Muscle
contracts
(isometric
contraction)
Muscle
relaxes
Peak tension
production
Contraction
begins
6 kg
Length unchanged
Muscle
length
(percent
of resting
length)
6 kg
Time
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Energy to Power Contractions
•  Muscles need lots of ATP
•  Why?
•  Cross bridges – think Rigor Mortis L
•  Ca2+ pumping
•  Na+/ K+ ion movement – think AP
•  So, where does ATP come from?
•  Stored reserves (1 way)
•  Synthesize it (3 ways)
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Energy to Power Contractions
•  ATP can be stored (very little)
•  Need a little more?
1.  Creatine phosphate (CP)
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Figure 9.19a Pathways for regenerating ATP during muscle activity.
CP
Creatine
kinase
Creatine
O2?
1 CP à 1 ATP
Short (15s)
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Energy to Power Contractions
• 
ATP Synthesis
• 
Cells produce ATP in two other ways
2.  glycolysis
3.  Cellular respiration
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Energy to Power Contractions
•  Glycolysis
•  primary energy source for peak muscular activity
•  b/c energy demands are extremely high
•  Use
•  Glycogen, glucose
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Figure 9.19b Pathways for regenerating ATP during muscle activity.
Glucose
O2?
1 C6H12O6 à 2 ATP
Glycolysis
Not as short (30-40s)
2
net gain
Released
to blood
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Pyruvic acid
Lactic acid
Energy to Power Contractions
•  Cellular respiration
•  primary energy source of resting muscles
•  Use:
•  Glycogen, glucose
•  fatty acids
•  AAs
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Figure 9.19c Pathways for regenerating ATP during muscle activity.
Glucose
O2?
1 C6H12O6 à 32 ATP
Long (Hours)
Fatty
acids
Amino
acids
Pyruvic acid
Aerobic respiration
in mitochondria
32
net gain per
glucose
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Figure 9.20 Comparison of energy sources used during short-duration exercise and prolonged-duration exercise.
Short-duration exercise
6 seconds
ATP stored
10 seconds
CP
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30–40 seconds
Prolonged-duration exercise
End of exercise
Glycolysis
Hours
Aerobic Respiration
Energy to Power Contractions
•  Muscle Fatigue
•  can no longer perform = fatigued
•  Results of Muscle Fatigue
•  Depletion of metabolic reserves
•  Low pH (lactic acid)
•  Muscle exhaustion and pain
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Energy to Power Contractions
•  The Recovery Period
•  time required to return to normal
•  Oxygen becomes available
•  Mitochondrial activity resumes
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Types of Muscles Fibers and Endurance
• 
Two Speeds of Skeletal Muscle Fibers
1.  Fast fibers
2.  Slow fibers
• 
Two ATP-forming types of Skeletal Muscle
Fibers
1.  Oxidative
2.  Glycolytic
•  Mix them up!
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Types of Muscles Fibers and Endurance
Slow
Fast
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Oxidative
Glycolytic
Slow Oxid
Slow Glyco
Fast Oxid
Fast Glyco
Types of Muscles Fibers and Endurance
Think: High Endurance
•  Slow Oxidative Fibers
•  slow to contract
•  Contains lots of myoglobin
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Myoglobin
Types of Muscles Fibers and Endurance
•  Fast Glycolytic Fibers
•  Contract very quickly
•  strong contractions, fatigue quickly
•  Lots of glycogen reserves
•  No Oxigen
•  Large diameter
•  Few mitochondria
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Dark Meat vs. White Meat
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