BS277 Biology of Muscle.
Lecture 1. Review of muscle structure and function.
Aim.
To review the structure (and function) of mammalian skeletal muscle.
Background reading.
Jones, D., Round, J. and de Haan, A. (2004) Skeletal muscle; from molecules to
movement. (Chapters 1-6) Churchill Livingstone.
Martini, F.H. (2009) Fundamentals of Anatomy and Physiology. (8th edition).
Chapter 10. Pearsons; Benjamin Cummings.
Randall, D.J., Burggren W and French, K. (2002), Eckert, Animal Physiology.
(5thEdition) Chapter 10, Freeman.
Widmaier,E.P., Raff, H. and Strang, K.T. (2006) Vander's human physiology : the
mechanisms of body function. (10th Edition) McGraw-Hill.
Anatomy of skeletal muscle (Fig. 10.1, Eckert; Fig. 10.1-10.3, Martini).

Describe the following structures and label diagrams to show how they contribute to the
structure of muscle.
1. Membranes and attachments.
a. Endomysium, perimysium, epimysium (fascia).
b. Tendon, aponeurosis, origin, insertion, periosteum.
2. The muscle cell (muscle fibre).
a. Fascicle (fasciculus), sarcomere.
b. Sarcolemma, T tubule, myofibril, sarcoplasmic reticulum, triad.
Overview of the structure of the Sarcomere (Figs 10.1-10.3, Eckert; Fig.10.5, Martini).

Describe the following structures and label diagrams to show how they contribute to
the structure of the sarcomere.
1. Banding pattern.
A band, I band, Z line, M line, H zone, zone of overlap.
2. Thick and thin myofilaments.
Actin, myosin.
3. Proteins that maintain their position (length).
Titin, actinin, myomesin, desmin (nebulin).
4. Proteins that regulate their interaction.
Troponin, tropomyosin.

Describe and explain the patterns of myofiaments that can be seen in transverse
sections of different parts of the sarcomere.
1. In the I band.
2. In the H zone.
3. In the zone of overlap.
The sliding filament model of muscle contraction.

Describe the microscopic observations that led to the development of the sliding
filament theory.

Describe and explain the experimental observations on the relationship between
isometric muscle tension and sarcomere length that support the idea that tension is
developed by interaction between myosin and actin.

Describe the cross bridge cycle and outline the experimental evidence that explains the
respective roles in the cycle of calcium and ATP.

Explain the difference between isometric contraction and rigor.
Muscular dystrophy

Describe the inherited disorder Duchenne Muscular Dystrophy and explain the role of
dystrophin.
Further reading.
Agarkova I, and Perriard J.C. (2005) The M-band: an elastic web that crosslinks thick filaments in the centre of the
sarcomere. Trends Cell Biol. 15 (9): 477-485
Berchtold, M.W. et al., (2000) Calcium ion in skeletal muscle; its crucial role for muscle function, plasticity and disease.
Physiological Reviews 80 (3), 1215-1265.
Blake,DJ., Weir, A., Newey, SE.,and Davies, KE. (2002) Function and Genetics of Dystrophin and Dystrophin-Related
Proteins in Muscle Physiol. Rev. 82, 291-329.
Carafoli, E. (2002) Calcium signalling: a tale for all seasons. Proceedings of the National Academy of Sciences USA 99
(3), 1115-122.
Gordon, A.M. et al (1966) The variation in isometric tension with sarcomere length in vertebrate muscle fibres. J. Physiol.
184, 170-192.
Gordon, A.M. et al. (2000) Regulation of Contraction in Striated Muscle. Physiol. Rev., 80, 853-924.
Huxley, H.E. (1969) The mechanism of muscle contraction. Science 164, 1356-1365.
Millman, B.M. (1998) The Filament Lattice of Striated Muscle. Physiol. Rev. 78, 359–391.
Olsen, E.N. (1992) Interplay between proliferation & differentiation within the myogenic lineage. Devl Biol.154, 261-272.
Pollard, T.D. (2000) Trends in Biochemical Sciences 25 (12), 607-611.
Trinick, J. & Tskhovrevoba, L; (1999) Titin; a molecular control freak. Trends Cell Biol. 9, 377-280.
Vale, R.D. and Milligan, R.A. (2000) The way things move. Science 288, (5463) 88-95.
Philosophical Transactions of the Royal Society B: Biological Sciences (2000)
355, (1396) 415-545 (Discussion meeting on molecular motors)
BS277#1.MHS.Oct09
BS277 Biology of Muscle.
Lecture 1. Review of muscle structure and function.
Aim.
To review the structure (and function) of mammalian skeletal muscle.
Background reading.
Jones, D., Round, J. and de Haan, A. (2004) Skeletal muscle; from molecules to
movement. (Chapters 1-6) Churchill Livingstone.
Martini, F.H. (2009) Fundamentals of Anatomy and Physiology. (8th edition).
Chapter 10. Pearsons; Benjamin Cummings.
Randall, D.J., Burggren W and French, K. (2002), Eckert, Animal Physiology.
(5thEdition) Chapter 10, Freeman.
Widmaier,E.P., Raff, H. and Strang, K.T. (2006) Vander's human physiology : the
mechanisms of body function. (10th Edition) McGraw-Hill.
Anatomy of skeletal muscle (Fig. 10.1, Eckert; Fig. 10.1-10.3, Martini).
Endomysium, perimysium and epimysium (fascia) of fibrous connective tissue fuse to form the
tendons or aponeuroses at the origins/insertions which, in turn, fuse with the periosteum and
penetrate the bone matrix. There is an extensive circulation and innervation.
Muscle fibres (Fig.10.2) are large syncitial cells (up to 100 x 50cm) that may run the entire length of
the muscle. They are organised into bundles (within elements of the perimysium) called fascicles
(fasciculae).
Sarcolemma and T tubules transmit the action potentials that initiate contraction.
Myofibrils are the characteristic organelles of skeletal muscle and contain thin myofilaments (~
microfilaments of the cytoskeleton) of actin and thick myofilaments of myosin. Each myofibril
comprises multiple (thousands) units of myofilament arrays called sarcomeres, 1.5-2.5 in length,
arranged in series and separated from each other by z-lines.
Myofibrils are associated with mitochondria, lipid droplets and glycogen granules. Each myofibril is
also surrounded by a fenestrated tube of sarcoplasmic reticulum (SR) that functions as a Ca++ store.
Separate sections of SR are associated with each sarcomere and terminally they make contact with
the T tubules to form triads (1 t-tubule + 2 sarcomere terminal cisternae).
Overview of the structure of the Sarcomere (Figs 10.1-10.3, Eckert; Fig.10.5, Martini).
Sarcomeres (~2.5um long) contain;
a. Thick and thin myofilaments.
b. Proteins that maintain their position.
c. Proteins that regulate their interaction.
1.
Thin myofilaments principally comprise actin polymers 6nm x 1. Thick myofilaments are
myosin bundles, 12nm x 1.6. The arrangement of the myofilaments in the sarcomere is
reflected by the striated appearance of the muscle;
A (anisotropic – light polarising) band = myosin. Encompassing the central M line, the
lighter H zone (wherein there is no overlap with the light myofilaments) and the terminal
zones of over-lap, generally associated with triads (2 per sarcomere).
I (isotropic – not light polarising) band contains non-overlapping parts of actin light
myofilaments in adjacent sarcomeres. I bands alternate with A bands. Central Z lines (discs)
define the boundaries of the sarcomere and anchor the thin myoflilaments. A sarcomere
comprises a central A band and 2 x ½ I bands.
2.
3.
The myofilaments and the adjacent myofibrils are maintained in their relative positions by
structural proteins, titin (aka connectin), nebulin, and desmin.
Thin myofilaments of actin, each subunit having a myosin binding site, are associated with a
strand of tropomyosin that covers these sites, and troponin that binds them together and also
binds Ca++.
The arrangement of the thin and thick myofibrils is best appreciated in cross section (Fig 10.3, Eckert;
Fig. 10.5, Martini).
I band.
Hexagonal arrays of actin thin myofilaments.
A band.
Each myosin thick myofilament surrounded by 6 actins; each actin by 3 myosins,
except in
H zone.
Only triangular/hexagonal arrays of myosin thick myofilaments.
Molecular structure of myofibril proteins (Figs 10.5-10.7, Eckert, Fig. 10.7, Martini)
Myofibrils can be digested into component filaments by homogenisation in Mg++ -ATP and EGTA
(Ca chelator) which breaks the cross links.
Thin myofilaments (actin).
F-(fibrillar, fibrous) actin is a polymer of G-(globular) actin subunits (42kD)
arranged in a double helix. The F actin is 6-8nM thick and 1u long and comprises
380 G monomers in frog sartorius. The polymer is stabilised by a cap of  -actinin
(37kD). Its length appears to be determined and maintained by a central core of
nebulin. Thin filaments are anchored to α-actinin in the Z line.
Associated regulatory proteins.
Tropomyosin.
Filaments in grooves in the actin helix.
Troponin.
A complex of globular proteins associated with tropomyosin and located at 40nm along the actin
filament (2 per turn of the helix).
Thick myofilaments (myosin).
Polymer of myosin, itself a polymer; a homodimer of heavy chains each associated
with a variable number of light chains. Each myosin “monomer” is 150nm long
comprising a 2nm thick fibrous tail + neck, and a globular head 20 x 4nm that has
actin binding and ATPase activity. The tail is 2 polypeptide helices coiled around
each other. These polypeptide are not helical in the head region and are associated
with a variable number of light chains that determine the enzyme characteristics.
In low salt the myosins spontaneously aggregate to form heavy myofilaments that are identical
to those found in vivo, 1.5u long and 12nm thick. There are about 300 myosins in the thick
filament of the frog sartorius.
Structural proteins (Fig 10.7, Martini).
Nebulin determines the length of the thin myofilament.
Titin (connectin) stabilises the position of the thick myofilaments and prevents overstretching
of the sarcomere. (Trinick and Tskhovrevoba, 1999)
α-actinin comprises the Z line and anchors actin and titin.
Myomesin, an elastic spring in the M-line stabilises thick myofilament lattice (Agarkova and
Perriard, 2005).
Z-Z intermediate filaments of desmin keep the myofibrils in register.
Other key proteins of the muscle fibre (Blake, et al., 2002)
Genetic diseases affecting the muscle fibre are common, and many are classified as muscular
dystrophies (“wrongly nourished”). In Duchenne MD, an X-linked recessive condition that
affects 1 in 3500 boys, muscle fibres begin to degenerate at an early age (2-5) and the
condition is normally fatal before 30. Muscle wasting ultimately results in respiratory failure.
DMD is characterised by the lack of the protein dystrophin, part of a transmembrane complex
that is associated internally with cytoskeletal actin and externally with the endomysium. Its
function is unclear, but if any of the components of the complex is defective, membrane
damage and poor calcium regulation lead to fibre degeneration, phagocytosis, transient
regeneration and ultimately fibrosis. There has been a suggestion that it might act like a shock
absorber to protect the membrane during muscle contraction when force is transmitted from
the myofibrils to the endomysium. The repair of the defect by gene therapy is a current
research priority.
The sliding filament model of muscle contraction.
An elegant example of the relationship between structure and function.
Light and electron microscopic study provided the basis for the sliding filament theory (Huxley,
1969).
 During muscle contraction A band length is constant. I band and H zone shorten or disappear.
 During relaxation and stretching, A band is constant, I band and H zone elongate.
 Thick and thin myofilaments do not change in length but the area of overlap increases during
contraction and decreases during relaxation.
The theory supposes that tension is developed by interaction between the myosin heads (crossbridges) and the actin filaments in the areas of overlap, causing the filaments to slide over each other.
This notion is supported by the observation that the isometric tension produced by an anchored fibre
is proportional to the degree of overlap of the heavy and light myofilaments (ie to the number of cross
bridges that are formed) Fig 10.14, Martini.
How do cross bridges produce force?
Myosin-actin binding  conformational change in x-bridge (myosin) head, stretching neck to exert
force, then head dissociates from actin, conformational change, rebinds to actin (myosin “rows” along
actins in both directions).
Biochemistry of cross bridge cycling.
i) ATP is involved.
Actomyosin (but not actin or myosin alone) has phosphatase activity.
Mg++ATP + actomyosin  actomyosin-Mg++ATP actomyosin-Mg++ADP + Pi
The myosin head has 1. an actin binding site,
2. an ATPase active site allosterically activated by actin binding.
++
++
If Mg ATP and Ca are present a mixture of actin and myosinactomyosin and contracts.
ii) Ca++ is essential too.
a). Glycerated (detergent-stripped) muscle fibres produce force (Fig 1) and develop ATPase activity
(Fig 2) only in the presence of Ca++. Tension development is only possible if both ATP and Ca++ are
present (Fig 3a). Relaxation is only possible in the absence of Ca++ if ATP is present (Fig 3b). ie
Ca++ is necessary for contraction, ATP is necessary for both contraction and relaxation.
b). Injection of Ca++ into intact muscle cell produces contraction.
c). Aequorin and Fura 1-injected muscle fibres illuminate, following neural stimulation, s before
tension development.
Role of Ca++.
Crossbridge attachment requires Ca++>10-7M. Such low concentrations in vitro achieved with EGTA
(ethylene glycol bis tetraacetic acid) that selectively chelates Ca++, not Mg++. The actin site for
myosin head attachment is sterically blocked by tropomyosin. The protein complex troponin,
attached to tropomyosin every 40nm, binds 4mols. Ca++, changes conformation and displaces
tropomyosin so that myosin head binding can occur.
Putting it together (Fig 10.12, Martini, Fig 11.2, Vander).
Provided that Ca++ is >10-7M step 1.
Myosin binding (step2) releases ADP +Pi and changes head conformation (step 3).
ATP binds and allosterically changes the myosin actin binding site so that detachment occurs (step 4).
Detachment allosterically alters the ATPase site so that dephosphorylation occurs, energising the
myosin head (step 5), that reattaches to the actin (normally at the next binding site)-step 1.
Relaxation and rigor.
When intracellular Ca++ is <10-7M the muscle relaxes if ATP is present. Depletion of ATP results in
“rigor”, as in rigor mortis. ( cramp).
Further reading.
Agarkova I, and Perriard J.C. (2005) The M-band: an elastic web that crosslinks thick filaments in the
center of the sarcomere. Trends Cell Biol. 15 (9): 477-485
Berchtold, M.W. et al., (2000) Calcium ion in skeletal muscle; its crucial role for muscle function,
plasticity and disease. Physiological Reviews 80 (3), 1215-1265.
Blake,DJ., Weir, A., Newey, SE.,and Davies, KE. (2002) Function and Genetics of Dystrophin and
Dystrophin-Related Proteins in Muscle Physiol. Rev. 82, 291-329.
Carafoli, E. (2002) Calcium signalling: a tale for all seasons. Proceedings of the National Academy
of Sciences USA 99 (3), 1115-122.
Gordon, A.M. et al (1966) The variation in isometric tension with sarcomere length in vertebrate
muscle fibres. J. Physiol. 184, 170-192.
Gordon, A.M. et al. (2000) Regulation of Contraction in Striated Muscle. Physiological Reviews, 80,
(2) 853-924.
Huxley, H.E. (1969) The mechanism of muscle contraction. Science 164, 1356-1365.
Millman, B.M. (1998) The Filament Lattice of Striated Muscle. Physiol. Rev. 78: 359–391.
Olsen, E.N. (1992) Interplay between proliferation and differentiation within the myogenic lineage.
Devl. Biol. 154, 261-272.
Pollard, T.D. (2000) Trends in Biochemical Sciences 25 (12), 607-611.
Trinick, J. and Tskhovrevoba, L; (1999) Titin; a molecular control freak. Trends Cell Biol. 9 377280.
Vale, R.D. and Milligan, R.A. (2000) The way things move. Science 288 (5463) 88-95.
Philosophical Transactions of the Royal Society B: Biological Sciences (2000)
355, (1396) 415-545 (Discussion meeting on molecular motors)
BS277#1.MHS.Oct09