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Lecture 4

BIOL 4510 Lecture 4 Cell Biology-revised-sept17.pdf

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BIOL 4510
Peter Backxx

BIOL 4510/KINE 4510 Some Basic Cell Biology ± Morphology of Muscle Cells Peter Backx There are 3 different muscle types: smooth muscle skeletal, and cardiac Although all 3 are capable of contracting, their morphology is quite different. - VNHOHWDO▯ DQG▯ FDUGLDF▯ PXVFOH▯ DUH▯ FDOOHG▯ VWULDWHG▯ PXVFOH«▯WKH\▯ KDYH▯ D▯ VWULDWHG▯ appearance due to the repeat of highly organized structures called sarcomeres along the length of these cells. Sarcomeres are made up of sarcoplasmic reticulum and contractile proteins that are arranged into very organized structures (discussed later) Skeletal Muscle The morphology of skeletal muscle is more complex (more organized) than the other 2 muscle types. Functional Organization Skeletal muscle has 3 layers of connective tissue Outer epimysium (epi ± on; mys ± muscle) Collagen fibers Central perimysium (peri ± around) Collagen fibers and elastic fibers; blood vessels and nerve 1 Inner endomysium (endo ± inside) Collagen fibers The collagen fibers of the epimysium, perimysium and endomysium come together at the end of muscle to form the tendon or aponeurosis (broad sheet of tendon). The muscle is divided into bundles (fascicles) which are surrounded by the perimysium Each muscle fascicle is divided into muscle fibers. Each muscle fiber is divided into myofibrils. Each skeletal muscle contains 100s ± 1000s of myofibrils Each myofibril is 0.5-2 µm diameter and can be as long as the entire length of the muscle (30 cm or more) Typical Muscle Fiber comprised of: Myofibrils ± contractile proteins; Mitochondria ± located on the periphery for fast twitch fibers and around individual myofibrils for slow twitch fibers Sarcolemma ± plasma membrane The sarcolemma forms specialized invaginations called transverse tubules or t-tubules Sarcoplasm - cytoplasm Sarcoplasmic reticulum (SR) is an internal organelle system comparable to the endoplasmic reticulum. The SR comes in close apposition with the t-tubules (terminal cisternae) forming dyads or triad. 2 Sarcoplasmic Reticulum 2+ The main function of the SR is to store Ca required for muscle contraction. It is in close proximity to the t-tubule system because of the role of sarcolemma and SR ion channels involved in muscle contraction: excitation-contraction coupling. Sarcomere The myofibrils are comprised of myofilaments (contractile proteins), primarily: Myosin - thick filaments Actin ± thin filaments Actin and myosin form a unique repeated pattern known as the sarcomere. Each myofibril contains ~10,000 sarcomeres end to end. Each sarcomere is ~ 1.6 ± 2.6 µm. Sarcomere length varies with the length of the skeletal muscle or the volume of the ventricle. The relevance of sarcomere length on muscle function will be discussed in a later lecture. But for now, it is worth mentioning the ³force-sarcomere length relationship 100 Myofilament 80 overlap 60 40 C ardiac M uscle Tension (% Max) (Starling) 20 0 1.0 1.4 1.8 2.2 2.6 3.0 3.4 3.8 Sarcomere Length (P m) 3 In electron micrographs, the sarcomere appears as a dark band (A band; anisotropic) and a light band (I band; isotropic). The A band is comprised of mainly the thick filaments (myosin), whereas the I band is made up of the thin filaments (actin) and titin. Titin keeps the myosin and actin aligned. Largest protein in the body M line (middle) ± center portion of each thick filament attached to its neighbor contains many proteins that hold thick filaments together in crystalline-like arrangement H zone (Heller ± bright) ± contains thick filaments but no thin filaments. Z disc (Zwischen ± between) contains many proteins and lots of actinin, which connects the thin filaments and titin to adjacent sarcomeres or to the extracellular matrix via integrins/adherins/dystrophin. Each sarcomere lies between 2 Z discs. Each sarcomere is encircled by the t-tubules. The triad lies at the border of the A band and I band (where the thick and thin filaments overlap). From the cross section of the myofibril at different locations of the sarcomere, one can observe the coordinated pattern of the thick and thin filaments. This patterning is important in force generation, which is accomplished by the myofilaments. 4 Due to the cross-striped pattern observed in the skeletal muscle formed by the myofilaments, skeletal muscle is referred to as striated muscle. This striated pattern is observed in both light and electron micrographs. Light micrograph Electron micrograph Nucleus Skeletal muscle is multinucleated, and have their nuclei located on the periphery. Skeletal muscle develops from mono-nucleated embryonic myoblast cells which fuse to form myotubes and eventually muscle. Some myoblast cells do not fuse, and form satellite cells. These cells can fuse later and differentiate with existing skeletal muscle. They are involved in normal muscle growth and regeneration of damaged muscle. 5 Skeletal Muscle: Fast, Intermediate and Slow Twitch Skeletal muscle is classified into fast, intermediate or slow twitch muscle, based on their speed of contraction. This is primarily dependent on the type of myosin ATPase expressed in these muscle types. Myosin ATPase hydrolyzes ATP required for force generation. There are a number of biochemical and morphological differences between these 3 muscle types.
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