Muscle Physiology
Outline
I.
II.
III.
IV.
V.
VI.
Skeletal Muscle Structure
Muscle Contraction: Cell Events
Muscle Contraction: Mechanical Events
Muscle Metabolism
Types of Skeletal Muscle Fibers
Smooth and Cardiac Muscles
Outline
I.
II.
III.
IV.
V.
VI.
Skeletal Muscle Structure
Muscle Contraction: Cell Events
Muscle Contraction: Mechanical Events
Muscle Metabolism
Types of Skeletal Muscle Fibers
Smooth and Cardiac Muscles
1- Skeletal Muscle Structure
– Muscle = group of fascicles
– Muscle fibers extend length of muscle from tendon to
tendon
Motor units
•
Motor unit: Composed of one
motor neuron and all the muscle
fibers that it innervates
•
There are many motor units in a
muscle
•
The number of fibers innervated
by a single motor neuron varies
(from a few to thousand)
•
The fewer the number of fibers per
neuron  the finer the movement
(more brain power)
•
Which body part will have the
largest motor units? The
smallest?
Components of a muscle fiber
Muscle fiber components
• Sarcolemma: muscle cell
membrane
• Sarcoplasma: muscle cell
cytoplasm
• Motor end plate: contact
surface with axon terminal
• T tubule: cell membrane
extension into the
sarcoplasm (to reach the
myofibrils)
• Cisternae: areas of the
ER dedicated to Ca++
storage (located on each
side of the T-tubules)
• Myofibrils: organized into
sarcomeres
Figure 12.2 (2 of 2)
The sarcomere
• The myofibrils are
organized into a repetitive
pattern, the sarcomere
• Myosin: thick filament
• Actin: thin filament
• Bands formed by pattern:
A and I and H bands
• Z line: area of attachment
of the actin fibers
• M line: Myosin fiber
centers
The sarcomere
Figure 12.5d
Myosin structure
• Many myosin molecules
per filament, golf club
shape
• Long tail topped by a
thickening: the head 
forms crossbridges with
the thin filament
• Presence of the enzyme,
ATPase in the head 
release energy for
contraction
Actin structure
• Formed by 3 different
proteins:
- globular (G) actins: bind to
myosin heads
- tropomyosin: long, fibrous
molecule, extending over
actin, and preventing
interaction between actin
and myosin
- troponin: binds reversibly to
calcium and able to move
tropomyosin away from the
actin active site
Figure 12.4
Outline
I.
II.
III.
IV.
V.
VI.
Skeletal Muscle Structure
Muscle Contraction: Cell Events
Muscle Contraction: Mechanical Events
Muscle Metabolism
Types of Skeletal Muscle Fibers
Smooth and Cardiac Muscles
2- Muscle contraction: Cell events
Figure 11.13
Synaptic events
• The AP reaches the axonal
bulb
• Voltage-gated calcium
channels open
• The influx of calcium in the
bulb activates enzymes
the vesicles containing
the neurotransmitter
molecule dock and release
the neurotransmitter in the
synapse
• The neurotransmitter for
skeletal muscles is always
acetylcholine
• The receptors on the
muscle fiber are cholinergic
receptors
• These receptors are
nicotinic (fast) acting
receptors
2- The Mechanism of Force Generation in Muscle
Figure 12.7
• http://www.black
wellpublishing.co
m/matthews/myo
sin.html
• http://www.ebsa.
org/npbsn41/intro
_muscle.html
Figure 12.6
Muscle relaxation
• Ach is removed from the
receptors by
acetylcholinesterase
• Ligand-gated Na+channels
close
• Na/K pumps reestablish the
RMP
• Ca++ ions leave troponin and
are brought back into the
cisternae (this process needs
energy)
• Tropomyosin moves back over
the actin active site
• The myosin heads release
their binding to actin
• The filaments passively move
back into resting position
Applications
• Myasthenia gravis: autoimmune disease where antibodies against
the Ach receptors are produced. Which consequences do you
expect?
• Muscular dystrophy: some proteins forming the muscle fibers are
abnormal. Which consequences do you expect?
• Curare binds to the Ach receptor without activating them. What are
the effect of curare on the skeletal muscle?
• The botulism toxin prevents the release of the neurotransmitter into
the synapse. What will be the consequence?
• Nerve gas inhibits acetylcholinerestase present in the synapse.
What will be the consequence?
•
• Rigor mortis: why does the body stiffen
shortly after death?
Outline
I.
II.
III.
IV.
V.
VI.
Skeletal Muscle Structure
Muscle Contraction: Cell Events
Muscle Contraction: Mechanical Events
Muscle Metabolism
Types of Skeletal Muscle Fibers
Smooth and Cardiac Muscles
3- Muscle contraction: Mechanical events
• 1 stimulation  1 twitch
• Muscle twitch: 3 phases:
- latent phase
- contraction phase
- relaxation phase
☻ do not confuse the AP
and the twitch!!!
Figure 12.16
Events during the twitch
• Latent phase: Stimulus to
beginning contraction: AP to
myosin binding to actin active
site
• Contraction phase: beginning
to end of muscle tension 
myosin heads slide along the
actin filaments
• Relaxation phase: peak
tension to no tension  Ca++
ions moved back into the
cisternae, tropomyosin moves
back over actin, myosin head
release actin and the filaments
move back into resting position
Figure 12.18
Isometric/isotonic contractions
• Isometric: muscle
contraction without
movement  no
muscle shortening
• Isotonic: muscle
contraction with
movement  muscle
shortens
Effect of consecutive stimuli: Treppe
• Treppe: gradual increase in
contraction intensity during
sequential stimulation
• Might be due to calcium ions
accumulating in the cytoplasm
with each stimulation
Figure 12.15
Summation and tetanus
• Summation: Rapid sequence of stimuli muscle
twitches fuse into each other, each subsequent one
being stronger that its precedent (due to Ca++?)
• Tetanus: very rapid sequence of stimuli: no relaxation
Figure 12.17
Recruitment
• An increase in force is
made possible by
recruiting more motor
units
• Muscles have various
sizes of motor units 
allows them to adjust the
size of the effort to be
made
• Activating motor units
alternatively allows the
muscle to sustain
contraction with minimal
fatigue
Figure 12.19
Outline
I.
II.
III.
IV.
V.
VI.
Skeletal Muscle Structure
Muscle Contraction: Cell Events
Muscle Contraction: Mechanical Events
Muscle Metabolism
Types of Skeletal Muscle Fibers
Smooth and Cardiac Muscles
IV- Muscle metabolism
• Muscle fibers use ATP (only
first few seconds) for
contraction
• ATP must then be generated
by the muscle cell:
- from creatine phosphate, first
- from glucose and glycogen
- from fatty-acids
ATP formation from the above
compound is possible if
oxygen is present (oxidative
phosphorylation: 36 ATP per
glucose)
Oxygen is delivered to the muscle
by myoglobin, a molecule with
high affinity to oxygen and
related to hemoglobin
If the effort is strong and
sustained, the muscle
might not have enough
oxygen delivered to it by
myoglobin  anaerobic
glycolysis with only 2 ATP
formed per glucose and
synthesis of lactic acid
Consequence of anaerobic
metabolism?
Figure 12.11
Muscle fatigue
• Muscle fatigue: a decline in the
ability of the muscle to sustain
the strength of contraction
• Causes:
- rapid build-up of lactic acid
- decrease in oxygen supply
- decrease in energy supply
(glucose, glycogen, fatty-acids)
- Decreased neurotransmitter at
the synapse
- psychological causes
Effects of exercise on the muscle
• Aerobic exercises: long
sustained exercises 
promote increased oxidative
capacity of the muscle fiber 
increased blood vessel supply,
increased mitochondria
• High intensity, short burst
exercise: increased glycolytic
activity  increased synthesis
of glycolytic enzymes,
increased synthesis of
myofibrils (increased muscle
size)
Outline
I.
II.
III.
IV.
V.
VI.
Skeletal Muscle Structure
Muscle Contraction: Cell Events
Muscle Contraction: Mechanical Events
Muscle Metabolism
Types of Skeletal Muscle Fibers
Smooth and Cardiac Muscles
V- Types of Muscle Fibers
• Various muscles contract at different speed 
composed of different types of muscle fibers
Figure 12.23
Basis for classification
• Velocity of contraction: slow vs fast
• Energy source: oxidative vs glycolytic
Oxydative
Primary energy through
oxidative
phosphorylation
– Many mitochondria
– Myoglobin (red)
– Small diameter
– Resistant to fatigue
Glycolytic
Primary energy through
anaerobic glycolysis
– Fewer mitochondria
– Many glycolytic
enzymes
– High glycogen stores
– Use little oxygen—
anaerobic
– Large diameter
– Quick to fatigue
• Which types of meat
are chicken breast
and duck breast?
• Why the difference?
Outline
I.
II.
III.
IV.
V.
VI.
Skeletal Muscle Structure
Muscle Contraction: Cell Events
Muscle Contraction: Mechanical Events
Muscle Metabolism
Types of Skeletal Muscle Fibers
Smooth and Cardiac Muscles
VI- Smooth and Cardiac Muscles
Skeletal
Cardiac
Smooth
Control
voluntary
unvoluntary
Unvoluntary
Neural input
somatic
ANS
ANS
Hormone
0
Epi
Epi/others
Ca++ prot
Troponin
Troponin
Calmodulin
Gap junctions No
Yes
Yes
Pacemaker
Yes
No
Appearance
No
Readings
• Chp. 12, p. 323-359