Muscle Physiology
Lecture 2: Mechanisms of force generation
Allied
Allied Science
Science Physiology
Physiology
Dr.
Dr. Daniel
Daniel Ulrich
Ulrich
Trinity
Trinity College
College Dublin
Dublin
Lecture Outline
I.
II.
Muscle metabolism
The Mechanics of Skeletal Muscle
Contraction
–
The Twitch
–
Factors affecting the force generated
by individual muscle fibers
–
Regulation of the force generated
by whole muscle
–
Velocity of shortening
2
Sources of ATP: Energy systems
3
Metabolic Pathways
Figure 12.11
4
Creatine Phosphate (and ATP) system
Creatine kinase
Creatine phosphate + ADP
Creatine + ATP
ATP ase
ATP + H2O
ADP + Pi + H+ + Energy
•
Creatine phosphate = first source of ATP (up to 30 sec)
•
Can provide 4–5 times the amount of ATP present in cell at rest
•
One step process—very rapid
•
Very limited amount—used up rapidly
•
Law of Mass Action:
–
Use of ATP drives the reaction to the right
5
Anaerobic or nonoxidative Glycolysis
•
When oxygen supply to muscle is limited (during intense exercise),
anaerobic glycolysis = primary source of ATP
•
Breakdown of glucose, O2 absent
•
High intensity exercise: 20-120 seconds
Glucose
•
•
2 lactic acid + 2 ATP
Rapid source of ATP
–
Only two ATP/glucose
–
Limited glucose availability
–
Build up lactic acid (burning sensation)
Occurs in cytosol
6
Oxidative Phosphorylation
•
Primary energy source for Light / Moderate exercise (> 2
minutes)
–
Muscle stores limited amount of glucose
as glycogen
•
–
Substrate of oxidative phosphorylation
up to 30 minutes
Glucose and fatty acids delivered to muscle
by blood
•
Dominant after 30 minutes
•
Oxygen supply is adequate
•
Occurs in mitochondria
7
Energy systems - Activities
Examples…
Activity / Sport
-Marathon (42km)
-Running 100m
-Running 1,500 m
-Running 5,000 m
-Running 10,000 m
-Downhill racing skiing
-Cross country skiing
-Swimming 50 m
-Swimming 200 m
-Swimming 1500 m
ATP / PC
(%)
98
20
10
5
98
92
30
10
Anaerobic
glycolysis
(%)
Oxidative
phosphorylation
(%)
5
95
2
25
70
80
55
20
15
2
5
2
65
20
95
5
70
8
The Mechanics of Skeletal Muscle Contraction
• The Twitch
• Factors affecting the force generated
by individual muscle fibers
• Regulation of the force generated
by whole muscle
• Velocity of shortening
9
Twitch Contraction
Contraction produced in a muscle fiber in
response to a SINGLE ACTION POTENTIAL
• Twitch is an all-or-nothing event for a given
muscle fiber at rest
• Twitch can be defined for a muscle fiber,
motor unit, or whole muscle
10
Phases of a Muscle Twitch
•Latent period
–Time from action potential in
muscle cell to onset of
contraction (few msec)
–Excitation-contraction
coupling
•Contraction phase
–Tension is increasing (10–100
msec)
–Crossbridge cycling
•Relaxation phase
–Tension is decreasing back to
zero (longer than contraction
phase)
Figure 12.12
–Calcium reuptake
11
Isometric vs Isotonic Contraction
Isometric Twitch
Contraction
•
Length constant
•
Contractile elements contract
generating tension
•
Load > Tension
–
Stretches series elastic
elements
–
Muscle does NOT
shorten, load not lifted
Isotonic Twitch Contraction
•
Constant tension
•
Tension > Load
–
Load is lifted as muscle
shortens
Figure 12.13
12
Effect of Load on Tension (Isotonic to Isometric)
Figure 12.14
13
Normal Muscle Activity
•
Some purely isometric contractions occur
•
Purely isotonic contractions are rare
– Even if load is constant, isometric precedes
isotonic phase of contraction
– Isometric continues (tension increases) until
tension exceeds load
– Then isotonic contraction begins
– Tension remains constant as muscle shortens
•
Load generally not constant
– Load is changing as limb position is changing
14
Graded Muscle Contractions
Depends on
•
Factors affecting the force or tension generated by
individual muscle fibers
– Frequency of stimulation
– Fiber diameter
– Changes in fiber length
– Extent of fatigue
•
Regulation of the force/tension generated
by whole muscle
– Number of fibers contracting: recruitment
15
Frequency of Stimulation
Increases in frequency of action potentials in
muscle fibers increases tension two ways
• Treppe
• Summation and tetanus
16
Treppe
Independent twitches follow one another closely: peak tension
increases to a constant level
Cause: Increase cytosolic calcium??
Figure 12.15
17
Basis of summation and tetanus: Isometric Twitch Duration
Basis of summation and tetanus:
•Action potential : Two msec
•Contraction : 10–200 msec
•Contractions can overlap and sum
Figure 12.16
18
Cause of Summation and Tetanus
•
•
•
Amount of tension developed depends on amount of
calcium bound to troponin
At high frequencies, release exceeds reuptake
– Calcium increases in cytosol
Eventually saturate system
– All troponin has calcium bound to it
– Crossbridge cycling maxed out
– Maximum tetanic contraction
Figure 12.17
19
Fiber Diameter
•
Force-generating capacity = inherent
ability of muscle to generate force
•
Force-generating capacity depends
on number of crossbridges in each
sarcomere and geometrical
arrangement of sarcomeres
•
–
More crossbridges/sarcomere
Æ more force
–
More sarcomeres in
PARALLELÆ more force
Number of thick and thin
filaments/area = constant
–
Fiber diameter does vary
–
Larger diameter Æ more
filaments Æ more force
20
Length-Tension Graph
Optimum Length
•
Resting length of muscle at which fiber
can develop greatest amount of
tension
•
Due to maximum overlap of thick
filament crossbridges and thin
filaments
Non-Optimum Length
•
•
Figure 12.18
Greater than optimum
– Decrease crossbridge overlap
Less than optimum
– Thin filaments overlap each
others
– Z lines contact thick filaments
In situ, most muscles are at optimum
length
21
Factors affecting whole muscle: Recruitment
Stimulating more muscle fibers to contract
•
•
•
•
•
Muscle = bundle of muscle fibers
More muscle fibers contracting,
greater overall muscle tension
Recruitment occurs at the level of the
motor unit
More fibers contracting Æ greater
tension
Recruit motor units
–
–
•
Muscles for delicate movements
–
•
Activate motor neuron activates
all muscle fibers in the motor unit
Increases in tension occur in
steps proportional to size of motor
unit
Small motor units
Muscles for strength
–
Large motor units
22
Motor Unit Sizes
•
•
•
•
Number of motor units vary in different muscles
Size of motor units vary
– Small for delicate movements
– Large for strength movements
Fiber diameter (and thus strength) varies in motor
unit
– Small for weaker movements
– Large for stronger movements
Size of motor unit and fiber diameter
often related
– Small motor units Æ small fibers
– Large motor units Æ large fibers
23
Motor Unit Size Principle
•
•
Small motor units recruited first
Large motor units recruited last
Figure 12.20
24
Muscle Shortening and Load
Figure 12.21
25
Load-Velocity Graph
Figure 12.22
26