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Physiology 2130
Anita Woods

Muscles Section 5.1 Objectives • Describe the structural components of skeletal muscle, including muscle fibers, myofibrils, myofilaments, sarcomere, and the arrangement of the thin and thick myofilaments. • Describe the structure of the thin filament and its associated proteins. • Describe the structure of the thick filament. • Describe the sliding filament theory and the interaction of thin and thick filaments. • Define excitation-contraction coupling and describe the sequences involved. • Describe the complete series of events involved in muscle contraction, starting with the release of calcium from the sarcoplasmic reticulum. • List four functions ofATP in muscle contraction. • Describe the mechanism responsible for rigor mortis. • Describe how the grading of muscle contraction is accomplished. Section 5.2 Introduction • Muscles:  Biological machines that utilize chemical energy from the breakdown and metabolism of food to perform useful work.  There are three kinds of muscle cells: 1) Skeletal muscle, which is used primarily for voluntary motion; 2) Smooth muscle, which is found within the walls of blood vessels, airways, various ducts, urinary bladder, uterus, and the digestive tract; 3) Cardiac muscle, which, as the name implies, is found in the heart.  The body contains over 600 different muscles.  These muscles perform three principal functions: 1) Movement 2) Heat production 3) Body support and posture Section 5.3 AWhole Look at the Structure of Muscle • Abreakdown of the structures follows:  Whole muscles are made up of bundles of fasciculi  Each fascicle is made up of groups of muscle cells or fibers.  Each muscle cell contains many bundles of myofibrils.  Each myofibril contains thin and thick myofilaments.  Thin myofilaments contain mostly the protein actin along with troponin and tropomyosin.  Thick myofilaments contain the protein myosin.  The interaction of thin and thick myofilaments results in muscle contraction. • Video: striated or skeletal muscle is attached to bone and makes movement possible.Amuscle structural pattern is a series of increasingly smaller parallel units. The muscle is composed of fascicles. Each fascicle consists of several fibres. Each fibre is an elongated cell with many nuclei. Within each fibre are myofibrils composed of thick and thin filaments made of protein. The regular arrangement of these filaments gives striated muscle its striped appearance. The basic functional unit of a muscle is the sarcomere - a section of the myofibril. One is shown here bordered by the crooked blue lines. Asarcomere is composed of thick filaments of myosin in red and thin filaments of actin in blue. Muscle contraction is a result of these thick and thin filaments sliding past one another Section 5.4 Structure of a Skeletal Muscle • The diagram below shows that a whole muscle, like the biceps muscle of the upper arm, is composed of groups of fasciculi surrounded by a white connective tissue called perimysium. • Each fascicle, in turn, is made up of bundles of muscle cells (also called muscle fibers). • Within each cell there are cylindrical bundles of myofibrils. • These myofibrils are composed of two types of myofilaments, which are the actual contractile elements of the cell. Section 5.5 Structure of a Muscle Cell • Muscle cells (or fibers) are one of the few cells in the body with more than one nucleus. • They are surrounded by the sarcolemma—the muscle cell membrane—over which the action potential is transmitted.  The sarcolemma has small tube-like projections called transverse (T) tubules that extend down into the cell. • These T tubules conduct the action potential deep into the cell where the contractile proteins are located. • Within the muscle cell are long cylindrical myofibrils that contain the contractile proteins of the muscle—the thin and thick myofilaments. • The myofibrils are surrounded by the sarcoplasmic reticulum (SR). This is a mesh-like network of tubes containing calcium ions (Ca ), which are essential for contraction. • At either end of, and continuous with the SR are the terminal cisternae, a membranous enlargement of the SR, which is close to the T tubule (where the action potential travels). Section 5.6 Thin Myofilament • Myofibrils contain two types of myofilaments. • The thin myofilaments are composed predominantly of the globular protein actin. • Each actin molecule contains a special binding site for the other contractile protein myosin. • Many actin molecules are strung together like the beads on a necklace and then twisted to form the backbone of the thin myofilaments. • Also found on the thin myofilaments are long protein strands called tropomyosin. • When the muscle is at rest, these proteins cover the binding sites for myosin. • Athird regulatory protein, called troponin, is made up of three subunits: 1. TroponinA- which binds to actin 2. Troponin T - which binds to the tropomyosin 3. Troponin C - which binds with Ca . ++ • At rest, the troponin complex holds the tropomyosin over the myosin binding sites. • When Ca binds to the troponin C unit, the tropomyosin is pulled off the myosin binding sites by the troponin. Section 5.7 Thick Myofilament • The second type of myofilament—the thick myofilament—is made up of the protein myosin. • Myosin:  This protein has a long, bendable tail and two heads that can each attach to the myosin binding sites on actin (as mentioned on the previous page).  The heads also have a site that can bind and split adenosine triphosphate (ATP).  It is the splitting ofATP that releases energy to the myosin that powers the contraction of the muscle.  Many myosin molecules are arranged to form one thick filament.  Next we will examine the arrangement of the thick and thin myofilaments, an extremely important concept. Section 5.8 Actin/Myosin Relationship • Groups of thin (actin) myofilaments and groups of thick (myosin) myofilaments are arranged in a repeating pattern (thick, thin, thick, thin, and so on) along the length of the myofibril from one end of the muscle cell to the other. • Each group of thin myofilaments extends outward in opposite directions from a central Z disk (also called a Z line), where they are anchored. • Similarly, groups of thick myofilaments extend outward from a central M line, where they are attached. • Each myofilament is parallel to the length of the myofibril and the muscle cell. • The region from one Z disk to another is called a sarcomere.  This is the smallest functional contractile unit of the muscle cell. • The repeating pattern of thin and thick myofilaments gives the muscle cell a banded or striated appearance.  This is why skeletal muscle is sometimes referred to as striated muscle. • The regions that contain thick filaments appear as dark bands called Abands. • The regions that contain only thin filaments appear lighter and are called I bands. Section 5.9 Muscle Contraction – The Sliding Filament Theory • The interaction between actin and myosin leads to muscle contraction. • The myosin undergoes a change in shape when the head of a myosin molecule attaches to the binding site on actin and forms a crossbridge. • This change in shape causes the myosin head to swing, producing the power stroke.  This power stroke is much like the stroke of an oar in the water—it propels the boat past the water.  In the muscle, the power stroke slides the actin past the myosin. • It is very important to realize that neither the thin nor the thick filaments shorten during a muscle contraction. Section 5.10 Excitation-Contraction Coupling and Muscle Contraction • Excitation-contraction coupling  Process by which an action potential in the cell membrane (sarcolemma) excites the muscle cell to produce a muscle contraction. • The action potential that was generated at the neuromuscular junction (discussed in module 4) will spread out over the sarcolemma and down the T-tubules into the core of the muscle cell. • This action potential travels very close to the sarcoplasmic reticulum (SR) and ++ will open Ca channe++, causing the release of Ca++ from the terminal cisternae of the SR. The Ca will bind to troponin C on the thin myofilaments, causing tropomyosin to uncover the myosin binding sites found on actin. • Myosin will now be able to attach to the actin and a power stroke will occur.  So Basically (from quiz): 1. An action potential travels down the t-tubules 2. Ca++ is released from the sarcoplasmic reticulum 3. Troponin binds to Ca++ 4. The myosin head attaches to actin 5. Acrossbridge is formed 6. Apower stroke occurs 7. The actin slides past the myosin Section 5.11 Relaxation of
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