Overview of Skeletal Muscle Contraction
Skeletal muscle fibers are very large, elongated cells (Fig 9.1). Roughly 80% of the content of each muscle fiber consists of long bundles of protein called myofibrils. The myofibrils, in turn, consist of two types of myofilament (Fig
9.2). One type of myofilament, called the thick filament, is composed of hundreds of molecules of a protein called myosin. The other type of myofilament, the thin filament, contains three different proteins: a structural protein called actin that can form bonds with myosin, a protein called tropomyosin that regulates binding between myosin and actin, and the calciumbinding troponin which regulates the position of tropomyosin. The two myofilaments are arranged in the myofibrils in distinctive repeated structures called sarcomeres. Each sarcomere contains a series of thin filaments at either end that partially overlap with thick filaments found in the center.
Muscle contracts through an ATP-driven
Fig 9.1. A micrograph of segments of skeletal muscle fibers. N = nucleus, CT = connective tissue, M = myofibrils. Note the alternating light and dark banding pattern created by the repeated sarcomeres along the lengths of the myofibrils. Image is from www.vms.hr
/atlas/ histology/08/ah08202.htm
Thin Filaments
Troponin
Tropomyosin
Actin
Myosin
Fig 9.2. Arrangement of myofilaments into sacromeres within a myofibril (above) and the structure of thick and thin filaments, illustrating the proteins that make up each.
interaction between actin and myosin called crossbridge cycling (Fig 9.3). First, the globular head of a myosin molecule extends laterally and binds with a complementary binding site on an actin molecule to form a bond called a crossbridge. Then, in a process called a power stroke, the globular head bends inward towards the center of the sarcomere, pulling the thin filament with it. The crossbridge then breaks, and the globular head of the myosin unbends, preparing the myosin molecule to repeat the process. As a result of many myosin molecules alternately binding the thin filaments and pulling them inward, the thin filaments are pulled over the thick filaments toward the center of the sarcomere, thus shortening the overall length of
Myosin Head Unbends
Myosin Binds to Actin,
Forming Crossbridge
Myosin Releases Actin,
Breaks Crossbridge
Power Stroke Pulls Thin
Filament over Thick
Fig 9.3. An outline of crossbridge cycling
Lab #9: Muscle Physiology
p.1
Without Calcium – Crossbridges Cannot Form
Fig 9.4. The sliding filament mechanism of muscle contraction. Myofibrils contract by the thick filaments pulling the thin towards the center of the sarcomere, increasing the degree of overlap between the thick and thin filaments.
each sarcomere and, in turn, the length of the muscle (Fig. 9.4).
Crossbridge cycling, and hence muscle contraction, can only occur under specific conditions. This is because normally troponin positions tropomyosin on top of myosin-binding
Somatic Motor Neuron
1
2
3
4
Myofibrils
Motor End Plate
Thin filaments
Thick filaments
Transverse Tubule
Sarcolemma
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Sarcoplasmic Reticulum
Fig 9.5. Excitation of a skeletal muscle fiber. The sarcoplasmic reticulum has been removed from the left side of the illustration to show the arrangement of the thick (myosin) and thin (actin) filaments in the sarcomeres of the myofibrils.
Skeletal muscle excitation typically occurs in the following series of events enumerated in the illustration: 1) The binding of acetylcholine from a somatic motor neuron to chemically gated ion channels on the motor end plate
(subsynaptic membrane) triggers an action potential in the sarcolemma. 2) The action potential propagates down the length of the muscle fiber. 3) When the action potential reaches the openings of transverse tubules, the depolarization is conducted
