Skeletal muscle is the most abundant tissue in the body and is critical for diverse functions, including locomotion, respiration, glucose homeostasis, thermoregulation and protection of bones and viscera. Muscle fibres are multinucleated muscle cells. Their number can vary greatly among muscles and we are talking tens of thousands and millions here (e.g. first lumbrical 10,250; sartorius 128,150; tibialis anterior 271,350; medial gastrocnemius 1,033,000 etc.) Length of the fibers also can differ markedly depending on the muscle. Muscle tissue contains substantial amount of water. When dried, only ~22% of wet weight is left.
Skeletal muscle is a network of muscle fibred linked together by a network of collagenous connective tissue. The network, endo-, peri and epi-mysium connect muscle fibres to tendons and bones. This connective tissue is important in transmission of generated force. Anchoring points between muscle fibres and collagenous network are called focal adhesions.
Functional unit of the contractile machinery of the skeletal muscle is muscle fibre. Fibres can vary greatly in length (1-400 mm) depending on the muscle. They can also vary in diameter (10-100 micrometers). Main components of the fibre (immediately relevant for contractile function): sarcolema (is an excitable membrane surrounding the fibre, propagates action potentials, AP); T-tubules enable inward propagation of AP. Destruction of T-tubules prevents core myofibrils in the core of the fibre from contracting. Velocity of inward spread of activation via T-tubules is ~7 cm/sec (at 20 degrees C) which is much slower than AP propagation along the sarcolema, ~600 cm/sec. Dihydropyridine receptors (DHP) are important proteins of T-tubules in the chain of events leading to muscle contraction. Sacroplasmic reticulum (SR) is a network of tubes and sacs surrounding myofibrils and located in close proximity to T-tubules. Serves as storage of Ca2+ ions and is important regulator of contractility; Ryanodine receptors (RYR) are calcium channels of the SR, located in close physical proximity with the DHP receptors of T-tubules. Another functionally important protein of SR is Calcium ATPase (Calcium pump). Sarcoplasm is fluid enclosed within the fibre, providing mileau for sources of energy and organels. Myofibril is a packed bundle of contractile and regulatory filaments running along the fibre.
The thick and thin filaments contain proteins which are essential for muscle contraction. Myofilaments are arranged in a certain pattern along myofibrils resulting in appearance of striation when muscle longitudinal sections are examined under the microscope. Striation pattern consists of A band, H zone, I band, Z lines (discs) and M lines. Changes of different bands between contraction and relaxation offered valuable information for the understanding of the nature of interactions between different structures.
Thin filament is composed of two helical strands of fibrous actin (F-actin) and two coiled strands of tropomyosin. F actin is a polymer of ~200 molecules of globular actin (G-actin). A G-acting molecule is a protein of 374 amino acids. Actin has a capacity to interact with myosin. Other components of thin filament are tropomyosin and troponin. They play regulatory role in interaction between actin and myosin. Two coiled strands of tropomyosin, each “sprinkled” with troponin complex at regular intervals, are located in the grove of the F-actin helix. Such positioning of tropomyosin prevents access of myosin to actin. Troponin consists of 3 sub-units: TN-T binds to tropomyosin; TN-I inhibits 4-to-7 G-actin molecules from interacting with myosin; TN-C can reversibly bind Ca2+ ions depending on the concentration of these ions. TN-C has 4 binding sites: 2 for Ca2+ and 2 for Ca2+ or Mg2+. Binding of Ca2+ ions to TN-C exposes actin for interaction with myosin. The most important functional component of the thick filament is myosin. Myosin molecule is composed of six proteins: two myosin heavy chains (MyHC) and one essential and one regulatory myosin light chain (MLC) associated with each MyHC. MyHC contain ATP- and actin-binding sites which are essential for muscle contraction.
Actin and myosin took the centre of the stage in the story of muscle contraction. However, there are other important “characters” of the “play” which need to be mentioned. Titin and nebulin are important proteins for maintaining the structure of sarcomere and specific spatial arrangement between thick and thin filaments. Titin spanning from the Z-line through the M band keeps thick filament in place and acts as a molecular spring. It is responsible for passive stiffness during stretch of the muscle. Nebulin is actin binding protein ancored to the Z-line and acting as a thin filament length ruler. Titin and nebuling are important components of the “frame” that contractile proteins of the sarcomere are assembled around.
For muscle contraction to occur an increase in intracellular calcium concentration is essential as well as presence of ATP. We will talk about that in the next couple of slides in more detail. Sarcomere is the basic contractile unit of muscle. If you understand contraction of one sarcomere you understand contraction of entire muscle. Number of cross-bridges (interactions between myosin head and actin filament) is the principal determinant of force produced by myofibril.
Muscle fibre develops max force at “optimal length”. Below or above this optimal length force is reduced. A ~1.65 micrometer B ~2.0 micrometer C ~2.25 micrometer (optimal length) D ~3.65 micrometer
ATP is needed to charge the cross bridges. Most of ATP are generated in mitochonria. Mitochondria escaped from these slides and images, but they are there. Myosin binds to actin in a weak-binding state, forming a cross-bridge. 2.ATP is hydrolyzed by myosin atpase to ADP and Pi. 3.Release of Pi is important for generation of the “power stroke”. It is thought that “power stroke” results in a 5-10 nm displacement of actin filament (the underlying mechanism of the sliding filament theory). 4.After “power stroke” each cross-bridge exerts ~2 pN force during the strong binding phase. 5.ADP dissociates and that concludes the strong binding phase. Yet myosin remains stuck to actin in rigor. Only ATP binding to myosin results in its dissociation from actin. Cycling is not synchronized among adjacent of corss-bridges. Depletion of ATP several hours after death results contracture called as rigor mortis because of the stalled cross-bridge cycle. Muscles remain in rigor until muscle proteins deteriorate (15-25 h later).
At rest actin-myosin interaction is inhibited by tropomyosin Ca2+is needed to unblock actin (Ca2+ disinhibition) Concentration of free Ca2+ is low in sacroplasm of resting muscle. However, there is a lot of it stored in SR. Action potential activates DHP receptors which in turn open the RYR receptors of SR for a few milliseconds. RYR receptors function as Ca2+ ion channels. As concentration of Ca2+increases (it can go up 100-fold between rest and tetanic contraction). When 4 Ca2+ ions bind to TN-C, conformational change in tropomyosin expose myosin-binding sites on actin. Then contraction can occur. Calcmium ions play a role of secondary messenger in excitation contraction coupling (ECC). Calcium ATPase keeps pumping Ca+2 ions back to SR (at a cost of 1 ATP per 2 ions). As cytoplasmic Ca2+ concentration decreases, they dissociate from TN-T and that leads to inhibition of interaction between actin and myosin. As a consequence muscle relaxes.
Muscle contractile properties can be studied in in vivo condition and also in isolated muscles and single muscle fibres. In vivo experiments in humans can provide information about the integrated function of the CNS and skeletal muscle (e.g. voluntary contraction force) and skeletal muscle alone (contraction evoked by electrical stimulation). In vitro models are used to study isolated muscle or even single muscle fibres. Such experiments provide information about variation in contractile properties between different muscles, different experimental conditions (e.g. temperature), species and help us to learn the fundamentals of contractility and understand factors influencing it.
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Skeletal Muscle I
October 24, 2013
Dr. Arimantas Lionikas
• Muscles and connective tissue
• Structure and components of
• Muscle contraction
1. Enoka R. Neuromechanics of human movement.
2008. Publishers: Human Kinetics, p. 205-213;
2. MacIntosh, B.R., Gardiner, P.F. McComas, A.J.
Skeletal muscle, 2nd edition. 2006. Publishers: Human
Kinetics, p. 151-160.
Muscles in numbers
• Humans have ~660 skeletal muscles
• Average muscle contains ~100,000
• Diameter of muscle fibres is 10 -100 µm
• Muscle fibre length is 1-400 mm
Ratio of fibre length / muscle length is 0.2 – 0.6
(Muscle fibres do not go from end to end of
Muscles and connective tissue
Skeletal muscles are connected to
bones by tendons
Muscles are enclosed by
epimysium (connective tissue)
Groups of muscle fibres form
muscle fascicles surrounded by
Muscle fibres are enlosed by
Connective tissue maintains
integrity of skeletal muscles and
is important in transmission of
force. Connective tissue makes
around 6% of total dry muscle
How are muscle fibres connected
to endomysium? Via focal
adhesions, consisting of a
number cytoskeletal anchor
Contractile machinery of the muscle
• Muscle fibres (∅10100 μm) are covered by
sarcolemma (≈7.5 nm
• T-tubules are
reticulum is a network
of tubes surrounding
• Sarcoplasm is fluid
enclosed within the fibre
• Myofibrils (∅ ~1 μm)
are packed bundles of
along the fibre
Essentials of muscle contraction
• Myofibril contains two
myofilaments, known as
think and thin filaments.
Each filament is
composed of several
– Thick filaments consist of
myosin and myosinbinding proteins: C
protein, H protein, M
– Thin filament is
composed of actin,
troponin complex (TNT, TN-I, TN-C
• Myofilaments within
myofibril are arranged in
a series of repeating
units, the sarcomere,
which is basic contractile
unit of muscle.
One end of thin filaments projects into the sarcomere while another connects
the Z line
Thin filaments contain troponin and tropomyosin proteins that participate in
blocking and unblocking of thin filaments. Because of this function troponin
and tropomyosin are referred to as regulatory proteins.
Thick filaments (myosin) are in the centre of sarcomere and overlap thin
filaments from both sides
A Closer Look: Myosin
Myosin light chains
Cytoskeletal proteins of the sarcomere
What holds actin and myosin in
Prado et al. J. Gen. Physiol. 2008:126:461-480
Titin acts as a molecular spring permitting return
of stretched sarcomeres to “optimal” length.
• Myosin heads drag thin
filaments from both ends
towards each other
• The distance between Z
lines shortens (sarcomere
• Shortening of sarcomeres in
series add up.
• Will variation in the number
of sarcomeres affect
contraction speed of a fibre?
• Cross bridges circulate between different states:
1) No binding; 2) Weak binding; 3) Power stroke; 4)
Strong binding; 5) Rigor
• Velocity of muscle shortening is determined by the rate
of transition between these states
Myosin binding site
Muscle contraction: in vivo & in vitro
• Ecc is eccentric (muscle lengthens)
• Con is concentric (muscle shortens)
• Max speed of
with a decrease in
external load (force)
• Muscle force is
greatest in lengthening
• Muscle cross-bridges
are stretched in
• Muscles contain significant amounts of connective
• Muscle fibres are muscle cells which contain
myofibrils with contractile elements
• Sarcomere is the basic contractile unit. Myosin
filaments in the middle of the sarcomere pull actin
filaments from both sides
• Muscle contraction is based on conformational
changes in the shape an orientation of myosin heads
due to ATP hydrolysis
• Speed of contraction decreases with an increase in
• Cytoskeletal proteins provide a frame for actin and