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Muscle Physiology Page 1 of 17 Emojevwe’s Lecture note on physiology- Muscle Physiology Page 1 of 17 MUSCLE PHYSIOLOGY (PHS 213) By Emojevwe V. Course Objectives: To familiarize students with the principles and basic facts of Human Physiology and with some of the laboratory techniques and equipment used in the acquisition of physiological data. The emphasis will be on muscles physiology. Since the course will focus on muscle physiology, some cellular and molecular mechanisms will be discussed in order to present a current view of physiological principles of muscle. Where appropriate, basic chemical and physical laws will be reviewed in order to enhance and to promote student understanding. The laboratory component of the course is designed to reinforce the topics discussed in lecture, as well as to familiarize students with some of the laboratory techniques and equipment used in muscle research Student Learning Outcomes: Upon successful completion of lecture portion of this course, the students will be able to describe, identify, and/or explain: • Structure and function of skeletal muscle, including excitation-contraction coupling, sliding filament mechanism, force generation, and isometric versus isotonic contractions. • Structure and functions of the cardiovascular system, including the mechanical and electrical properties of cardiac muscle function. • Structure and functions of smooth muscles including contraction of smooth muscle Upon successful completion of the laboratory portion of this course, the students will be able to describe, identify, explain, perform, and/or measure: • Computer simulations of the membrane potential, action potential, and synaptic neurotransmission. • Skeletal muscle mechanics, and the electromyogram (EMG). Required Course Materials: • Any textbook of physiology (human or medical) published within the last four years will be appropriate. • Emojevwe’s Lecture Notes on Physiology. To obtain these notes as well as other supplementary materials, please visit the unimed portal Emojevwe’s Lecture note on physiology- Muscle Physiology Page 2 of 17 MUSCLE PHYSIOLOGY Muscle is an excitable tissue. The human body has over 600 muscles which perform many useful functions and help us in doing everything in day-to-day life. Classification of muscles: Muscles are classified by three different methods, based on different factors: I. Depending upon the presence or absence of striations; the muscles are divided into two groups: 1. Striated Muscle: Striated muscle is the muscle which has a large number of crossstriations (transverse lines). Skeletal muscle and cardiac muscle belong to this category. 2. Non-striated Muscle: Muscle which does not have cross-striations is called nonstriated muscle. It is also called plain muscle or smooth muscle. It is found in the wall of the visceral organs. II. Depending upon the control 1. Voluntary Muscle: Voluntary muscle is the muscle that is controlled by the will. Skeletal muscles are the voluntary muscles. These muscles are innervated by somatic nerves. 2. Involuntary Muscle: Muscle that cannot be controlled by the will is called involuntary muscle. Cardiac muscle and smooth muscle are involuntary muscles. These muscles are innervated by autonomic nerves. III. Depending upon the situation. 1. Skeletal Muscle: Skeletal muscle is situated in association with bones forming the skeletal system. The skeletal muscles form 40% to 50% of body mass and are voluntary and striated. These muscles are supplied by somatic nerves. Fibers of the skeletal muscles are arranged in parallel. In most of the skeletal muscles, muscle fibers are attached to tendons on either end. Skeletal muscles are anchored to the bones by the tendons. 2. Cardiac Muscle: Cardiac muscle forms the musculature of the heart. These muscles are striated and involuntary. Cardiac muscles are supplied by autonomic nerve fibers. 3. Smooth Muscle: Smooth muscle is situated in association with viscera. It is also called visceral muscle. It is different from skeletal and cardiac muscles because of the absence of cross striations, hence the name smooth muscle. Smooth muscle is supplied by autonomic nerve fibers. Smooth muscles form the main contractile units of wall of the various visceral organs. In the preceding sections, I will be introducing you to the physiology of the skeletal, cardiac and smooth muscles. Emojevwe’s Lecture note on physiology- Muscle Physiology Page 3 of 17 SKELETAL MUSCLE A skeletal muscle is a collection of muscle bundles or fascicles. Each fascicles is made up of a number cells or (myofibrils). Each muscle is cell compose of hundreds to thousands of myofibrils. Myofibrils are strings of the structural unit referred to as the sarcomere. The sarcomere is the functional unit of skeletal and cardiac muscle. The sarcomere contains the myofibrils; which are the proteins that are responsible for the contractile behavior of the muscle cell. In addition, the sarcomere contains structural proteins (which maintain the structural integrity of the sarcomere), as well as regulatory protein (which play an important role in the regulation of muscle contractile activity). A mature skeletal muscle cell is a long cell, which comes about through the fusion of many embryonic muscle cells. Therefore, skeletal muscle cells are among the largest cells in the body, whose length can be as long as tens of centimeters. The diameter of a typical skeletal muscle cell is about 100 µm. Skeletal muscle cells are multinucleated, and they contain numerous mitochondria. The plasma membrane of skeletal muscle cells (the sarcolemma) is specialized in that it has invaginations that run deep into the muscle cell. These invaginations are referred to as transverse tubules (or tubules). It is important to say that the membrane of the t-tubule is continuous with the sarcolemma. Thus, t-tubules serve to allow muscle action potentials to reach deep into the muscle cell. It is also important to recognize that the lumen of the t-tubule is the extracellular fluid. Within the cytoplasm of the skeletal muscle fiber (myoplasm or sarcoplasm), there are numerous specialized structures, which we need to understand. A highly specialized endoplasmic reticulum referred to as the sarcoplasmic reticulum is tightly wrapped around individual myofibrils and functions to store a high concentration of Ca2+. The release of Ca2+ from the sarcoplasmic reticulum is responsible for triggering muscular contraction. Another very important structure is the sarcomere. As mentioned above, the sarcomere is the functional unit of striated muscle, and it is discussed below. Fine Details of the Sarcomere The sarcomere is the functional unit of the muscle fiber. It is composed of overlapping units of two different filamentous proteins; the thick and the thin filaments. Movement of the thin filaments over the thick filaments brings about muscle shortening and force generation. Thus, the sarcomere is responsible for the contractile ability of muscle cells. In order to understand the function of the sarcomere, it is important that we first understand the protein composition of the sarcomere. The proteins found in the sarcomere can be placed into three proteins. As you read the following descriptions, be sure to examine the attached figures in order to achieve a better understanding. a. Contractile Proteins i. Myosin (gives rise to the thick filaments) ii. Actin (gives rise to the thin filaments) Emojevwe’s Lecture note on physiology- Muscle Physiology Page 4 of 17 b. Regulatory Proteins i. Tropomyosin (in the absence of Ca2+ , it covers the myosin –binding site of actin) ii. Troponin (Ca2+ sensor) c. Structural Proteins i. Titin (provide elasticity) ii. Nebulin (run closely with actin) iii. Dydrophine iv. Desmin (it binds z line with sarcomere) Now let us discuss these proteins Contractile Proteins As their name suggest, contractile proteins are in fact responsible for muscles shortening (contraction). Myosin is the protein that makes up the thick filaments of the sarcomere. Myosin has two structural domains; a head region and a tail region. The head and the tail regions are connected through a flexible neck region. The tail region of myosin is used to join many myosin molecules together in order to give rise to the thick filament. In skeletal muscle, approximately 250 mysin molecules are intertwined to form a thick filament. The myosin head has an actin-binding site to which ATP binds (see cross – Bridge Cycle below). In a thick filament, the myosin molecules are arrange so that the myosin heads are clustered at the ends (facing the Z disks), and the central region of the thick filament is a bundle of myosin tails. Actin is the protein that makes up the thin filaments of the sarcomere. An actin molecule is a globular protein (globular actin or G-actin). Many G-actin molecules polymerize to form filamentous actin (F-actin). In skeletal muscle, two F-actin polymers intertwine to form a thin filament. Each G-actin molecule has a myosin binding site. Thus, the interaction of actin and myosin take place via the interaction of myosin binding site of Gactin with the actin binding site of the myosin of the myosin head group. In order for the muscle to contract, the actin and the myosin molecules of the thin and thick filaments have to interact with one another. When the actin and myosin interact, they are said to be connected by cross-bridges. The cross-brides refer to the myosin head group that interact with a myosin-binding site on G-actin. Microscopic Details Under the light or electron microscope, the arrangement of the thick and thin filaments in a myofibril gives rise to a repeating pattern of alternating light and dark regions. One repeat in the pattern roughly corresponds to what is known as the functional unit of striated Emojevwe’s Lecture note on physiology- Muscle Physiology Page 5 of 17 muscle: the sarcomere. The special overlap of the thin and thick filaments within the sarcomere gives rise to the contractile and shortening capability of the muscle cell. The sarcomere has the following microscopic features: Z Disks: Two adjacent Z disk along the myofibril mark the boundaries of a single sarcomere.
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