8 muscle i sr2002 2013 al

Skeletal Muscle I

SR2002

Dr. Arimantas Lionikas

October 24, 2013

Muscle IPlan

• Muscles and connective tissue• Structure and components of

muscle fibre• Muscle contraction

Reading list: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

fibres• Diameter of muscle fibres is 10 -100 m• Muscle fibre length is 1-400 mm • Note:

Ratio of fibre length / muscle length is 0.2 – 0.6(Muscle fibres do not go from end to end of muscles)

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 perimisium

• Muscle fibres are enlosed by endomysium

• Connective tissue maintains integrity of skeletal muscles and is important in transmission of force. Connective tissue makes around 6% of total dry muscle weight.

• How are muscle fibres connected to endomysium? Via focal adhesions, consisting of a number cytoskeletal anchor proteins.

Muscle shape and arrangement of fibres

Contractile machinery of the muscle• Muscle fibres (10-100

μm) are covered by sarcolemma (≈7.5 nm thick)

• T-tubules are invaginations of sarcolemma

• Sarcoplasmic reticulum is a network of tubes surrounding myofibrils

• Sarcoplasm is fluid enclosed within the fibre

• Myofibrils ( ~1 μm) are packed bundles of myofilaments running along the fibre

Essentials of muscle contraction• Myofibril contains two

myofilaments, known as think and thin filaments. Each filament is composed of several proteins.– Thick filaments consist of

myosin and myosin-binding proteins: C protein, H protein, M protein, myomesin

– Thin filament is composed of actin, tropomyosin and troponin complex (TN-T, TN-I, TN-C

• Myofilaments within myofibril are arranged in a series of repeating units, the sarcomere, which is basic contractile unit of muscle.

Sarcomere• 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

Myosin head

A Closer Look: Myosin

Myosin light chains

ATPase

Cytoskeletal proteins of the sarcomere

Prado et al. J. Gen. Physiol. 2008:126:461-480

Titin acts as a molecular spring permitting return of stretched sarcomeres to “optimal” length.

What holds actin and myosin in place?

• Myosin heads drag thin filaments from both ends towards each other

• The distance between Z lines shortens (sarcomere shortens)

• Shortening of sarcomeres in series add up.

• Will variation in the number of sarcomeres affect contraction speed of a fibre?

Muscle contraction

≈ 3.0

≈ 2.0

Force-length relationship (sarcomere)

Cross-bridge cycle• 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

1

2

3

4

5

Myosin

Ca2+ATPase

Sarcolema

Ca2+SR

T-tubule

DHP

RYR

Relaxation

Myosin

Myosin binding site

AP

Contraction

ECC

Muscle contraction: in vivo & in vitroElectrodes

Dynamometerstrap

Force

5 mm

Force–velocity relationshipMuscle fibre

• Max speed of shortening increases with a decrease in external load (force)

• Muscle force is greatest in lengthening (eccentric) contractions

• Muscle cross-bridges are stretched in eccentric contractions

Ecc. Con.

• Ecc is eccentric (muscle lengthens)• Con is concentric (muscle shortens)

Muscle ISummary

• Muscles contain significant amounts of connective tissue

• 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 resistance

• Cytoskeletal proteins provide a frame for actin and myosin filaments