Download BE112A Topic 14: Skeletal Muscle

Skeletal Muscle
• Muscle is the material that really sets
biomechanics apart from other branches of
mechanics
• 3 Types:
–skeletal (voluntary)
–cardiac (involuntary)
–smooth (involuntary)
}striated
}striated
}unstriated
• The main difference between skeletal and
cardiac muscle is that skeletal muscle can exhibit
a sustained ‘tetanic’ contraction whereas
cardiac muscle ‘beats.’
Structure
of
Skeletal Muscle
Structure
of
Skeletal Muscle
Myofibril (1μm Ø):
• 100’s-1000’s per myofiber
• ~ 1500 thick filaments
• ~ 3000 thin filaments
Fascicle:
Surrounded by
collagen sheath
Muscle cell (fiber):
• 10-80  m 𝜙
• multinucleated
• sarcolemma
• sarcoplasm
• sarcoplasmic
reticulum
• striated
Myofibrils
•
•
•
•
1 μm Ø
~1500 thick filaments 12nm Ø
~3000 thin filaments 6 nm Ø
surrounded by sarcoplasmic
reticulum
• A (optically anisotropic) band
• I (optically isotropic) band
• Z-line
The Sarcomere
The Sarcomere
Anisotropic
Isotropic
~ 2.0 μm
to polarized light
Hexagonal Arrangement of
Myofilaments in Cross-Section
Myofilaments
• Thick myosin filaments (180
myosin molecules polymerized)
• Thin actin filaments
• They interdigitate to form the
SARCOMERE
Actin
• F-actin molecule
formed from G-actin
• Twisted with a period of
70 nm - double helix
• Two strands of
tropomyosin
• Globular troponin
molecules (with a
strong affinity for Ca2+)
attached to
tropomyosin strands,
every 7 G-actin
monomers
Myosin
• Subdomains:
Heavy Meromyosin HMM
Light Meromyosin LMM
S1-Head
• Myosin leads protrude in
pairs at 180º
• There are about 50 pairs
of cross-bridges at each
end of the thick filament
• The filament makes a
complete twist 310º every
3 pairs
• 14.3 nm internal between
pairs
The Crossbridge
Activation
• The trigger that stimulates
muscle contraction is calcium.
• It is bound at rest to the
sarcoplasmic reticulum (SR).
• When the myocyte is
depolarized, Ca2+ leaves SR
and binds to the troponin-C
subunit on the thin filaments.
• This causes tropomyosin to
move slightly removing a
steric restraint at the active
site on the thin filament.
• This switch is very sensitive to
Ca2+ concentration (mM).
Excitation-Contraction (EC) Coupling
• Muscle cells, like nerves, are excitable tissues.
• They have a negative resting membrane potential
owing to the imbalance of and K+ ions across the
semi-permeable membrane.
• This gradient is maintained by the Na-K pump
(ATPase).
+
[K ]
+
=
+o
– –
+
+
–
[K ]
+
–
+
–
= 140mEq / 1
i
+
4mEq / l
[Na ] o = 142mEq / l
+
+
+
+
–
–
+
–
+
+
–
+
–
–
[ Na ]i = 14mEq / l
Excitation-Contraction (EC) Coupling
• To stimulate excitable tissue: cell
membrane is locally depolarized to a
threshold level
• The depolarization amplifies transiently
due to changes in membrane permeability
to Na+ and K+
• This transient is the propagated as a wave
• Cell acts like a transmission line due to its
inherent intracellular conductivity and its
membrane capacitance
On each muscle fiber,
a nerve ending
terminates near the
center (at the muscle
end-plate).
Each nerve has from
2-3 to 150 fibers or
more depending on
the type of muscle.
At the end-plate is the
neuromuscular
junction.
Stimulus is transmitted
by neurotransmitter
Acetylcholine (mopped
up by cholinesterase).
The Motor Unit
Energy for Contraction
• Large amounts of ATP are hydrolysed to from ADP.
• The more work performed by the muscle, the greater
the amount of ATP that is cleaved: the Fenn effect.
• When the inhibitory effect of the troponin-tropomyosin
complex has been removed calcium binding, the heads
of the cross-bridges bind with exposed sites on the actin
filament.
• It is assumed that the bond between the head of the
cross-bridge and the active site of the actin filament
causes a conformational change in the head, thus
causing the head to tilt releasing energy that has
already been stored in crossbridge head during the
previous detachment.
Crossbridge Cycle
• Once the head of the cross-bridge is tilted,
conformational change in the head exposes a site
in the head where ATP can bind. One molecule of
ATP binds with the head, binding in turn causes
detachment of the head from the active site
• Once the head bound with ATP splits away from
the active site, the ATP is itself cleaved by a very
potent ATPase activity of the heavy meromyosin.
The energy released supposedly tilts the head
back to its normal condition and primes it for the
next “power stroke”.
Crossbridge Cycle
History of Muscle Contraction
Muscle Architecture
Flexor, Extensor, Belly, Tendon, Aponeurosis
Muscle Architecture:
Collagen Matrix
Muscle Architectures
Parallel (fusiform)
Convergent
Pennate
Circular (areolar)
Muscle Architecture
Figure 9.2:2 from textbook. Diagram illustrating
(a) parallel-fibered and (b) pinnate muscles. (c)
and (d) illustrate pinnate muscle contraction.
Muscle-Tendon Mechanics
• In most skeletal muscle, fibers attach to tendon plates, or
aponeuroses, which then become rounded tendons
• Interaction between muscle fiber and tendons cannot be
neglected
• Isolated tendon mechanical properties have been studied
• But we can not consider muscle or tendon properties in isolation
• Muscle length change is not the same as that of the muscletendon complex: muscle fibers of cats shortened by as much as
28% at the expense of the tendon during isometric contractions
• Muscle fibers do work when there is no net work done by the
whole muscle-tendon complex
• Ito et al., studied relationships between muscle architectural
parameters and tendon force using ultrasound
Masamitsu Ito, Yasuo Kawakami, Yoshiho Ichinose, Senshi Fukashiro, and
Tetsuo Fukunaga. Nonisometric behavior of fascicles during isometric
contractions of a human muscle. J Appl Physiol 1998;85(4):1230-1235
Muscle-Tendon Mechanics
LEFT: Schematic representation of experimental setup. A dynamometer was used
to fix the ankle joint angle at 20° plantarflexion from the anatomic position and
measure dorsiflexion torque. Right foot was firmly attached to the footplate of the
dynamometer with a strap. An ultrasonic probe was used to obtain sectional
images of the tibialis anterior muscle (TA). The TA is a bipennate muscle.
RIGHT: Longitudinal ultrasonic images of TA (right, proximal) at 20°
plantarflexion under relaxed (top) and moderately tensed (bottom) conditions.
Fascicle length (Lf) decreased and pennation angle (q) increased with force.
Cross-Sectional Area and Stress
Skeletal Muscle: Summary of Key
Points
• Skeletal muscle is striated and voluntary
• It has a hierarchical organization of myofilaments forming
myofibrils forming myofibers (cells) forming fascicles
(bundles) that form the whole muscle
• Overlapping parallel thick (myosin) and thin (actin)
contractile myofilaments are organized into sarcomeres in
series
• Thick filaments bind to thin filaments at crossbridges which
cycle on and off during contraction in the presence of ATP
• Nerve impulses trigger muscle contraction via the
neuromuscular junction
• The parallel and/or pennate architecture of muscle fibers
and tendons affects muscle performance