Muscle biophysics
Muscle
Tissue and/or cell specialized for
the generation of force and
movement.
Notice of a lecture presented
by Professor D.R. Wilkie to
the Institution of Electrical
Enginers in London.
Hungarians in muscle research
Albert Szent-Györgyi
Straub F. Brunó
András Szent-Györgyi
It can only pull, not push (...).
János Gergely
Katalin and Mihály Bárány
Ferenc Guba (fibrillin)
Skeletal muscle
Types of muscle
cardiac
myocyte
myoepithelial cell
smooth muscle cell
skeletal muscle fiber
Nucleus
Myofibrils
Myofibrils:
The organelle-level
structural and
functional units of
muscle.
Heart muscle
The sarcomere
A
I
sarcos: meat (Gr)
mera: unit
the smallest structural
and functional unit of
striated muscle.
myocyte:
capable of autonomous
contraction
Smallest structural and
functional unit:
sarcomere
Z-line
Titin
(spring
function)
Thin
filament
(actin)
M-line
Thick
filament
(myosin)
Z-line
Basic phenomena of muscle function I.
Basic phenomena of muscle function II.
2. Isotonic contraction
1. Isometric contraction
Complete tetanus
Force
(above fusion frequency)
Partial tetanus
Twitch
Time
Stimulus
Auxotonic contraction (simultaneous shortening and force generation)
Basic phenomena of muscle function III.
Energetics of muscle I.
Source of energy:
1. Work, Power
2. Force-velocity diagram
Fmax
W=Fs
P=Fs/t=Fv
Mg.ATP2-
+ H2O
Mg.ADP1-
Fmax = 30 N/cm2-muscle
+
Pi2-
+
H+
Type I fibers
* rich in mitochondria
* ATP generation by respiratory mechanisms
* slow fatigue
* rich in myoglobin: "red muscle"
* innervated by thin, slow nerves
* slow fiber
* dominates in postural muscles
F
Pmax (at appr. 30% vmax)
fast
P
W=0
W=0
v
vmax
slow
Type II fibers
* few mitochondria
* rich in glycogen
* ATP generation by glycolysis
* rapid fatigue due to lactate
* devoid of myoglobin: "white muscle"
* innervated by large, fast neurons
* fast fiber
* present in fast muscles
Energetics of muscle II.
Mechanisms of muscle shortening
32
4
Fenn effect
Phenomenological mechanism:
5
1
Liberation of heat increases with increasing speed of contraction
Speed of energy liberation
Sliding filament theory
Szarkomerhossz (m)
Fmax
Q+W
W
W=0
W=0
v
Andrew F. Huxley, Jean Hanson, Hugh E. Huxley
vmax
Molecular mechanisms of muscle contraction:
Cyclic, ATP-dependent actin-myosin interaction
Myosin II
The actin filament
37 nm
Regulatory Light
Chain (RLC)
“barbed” (+) end
ATP-binding
pocket
~7 nm thick, length in vitro exceeds 10 μm, in vivo 1-2 μm
Right-handed double helix.
Semiflexible polymer chain (persistence length: ~10 μm)
“pointed” (-) end
Neck
(lever)
Actin-binding
site
Structural polarity (“barbed”, “pointed” ends)
Tensile strength of actin: appr. 120 pN (N.B.: under isometric conditions up to 150
pN force may reach a filament.
Number of actin filaments in muscle: 2 x 1011/cm2-muscle cross section.
Essential Light
Chain (ELC)
Converter
domain
Myosin mutation - pathology
The myosin “cross-bridge” cycle
Mutational sites
in myosin motor
domain
Arg403Gln mutation: hypertrophic
cardiomyopathy
Elasticity of striated muscle
Models of muscle contraction
Experimental
model:
“in vitro motility
assay”
Structurel-functional model
Step size: 5,5 nm
(distance between neighboring
actin subunits)
Z-line
Titin
(spring
function)
M-line
Thin
filament
(actin)
Thick
filament
(myosin)
Z-line
Role of titin in sarcomere:
Limitation of A-band asymmetry
160
C
140
Mechanical
properties of titin
Erô (pN)
(pN)
Force
120
100
80
60
40
Adjustable spring and shock-absorber
B
20
A
0
0
area under hysteresis:
energy dissipated as
heat
D
E
1
2
3
4
5
6
Megnyúlás
(μm)
Extension (nm)
Contraction regulation in striated muscle
aktin
Excitation-contraction coupling