Muscle Physiology - Part 2 - Smooth and Cardiac Muscle PDF

Part 2 – Smooth and Cardiac Muscle MUSCLE PHYSIOLOGY Marcus Machado, MSc, PhD [email protected] Learning Objectives At the end of the lecture, students should be able to: – Describe the 2 types of smooth muscle. – Understand the physical structure of smooth and cardiac muscle. – Describe the f...

Part 2 – Smooth and Cardiac Muscle MUSCLE PHYSIOLOGY Marcus Machado, MSc, PhD [email protected] Learning Objectives At the end of the lecture, students should be able to: – Describe the 2 types of smooth muscle. – Understand the physical structure of smooth and cardiac muscle. – Describe the function of the intercalated disks in the cardiac muscle. Smooth Muscle SMOOTH MUSCLE OF EACH ORGAN IS DISTINCTIVE: Different physical dimensions to adjust to the organs they are in; Organization into bundles or sheets to enable coordinated contractions for functions like digestion and blood flow regulation. Response to different stimuli, including neural, hormonal and local factors (O2, CO2, H2). Receives nerve signals from the autonomic nervous system (Sympathetic and Parasympathetic nervous system). Smooth muscle has various functions depending on the organ it is in, including peristaltic contraction, regulation of blood flow, and control of airway diameter Types of Smooth Muscle SINGLE-UNIT SMOOTH MUSCLE – Also called visceral, syncytial or unitary smooth muscle – Fibers are arranged in sheets or bundles Cell membranes are connected The force generated in one muscle fiber can be transmitted to the next Contract together as a single unit – Cell membranes are joined by many gap junctions Ions can flow freely from one muscle cell to the next Fibers contract together, Single-unit smooth muscle Neurotransmitters Acetylcholine Noroepinefrine RESPOND TO NEURAL, HORMONAL, AND/OR STRETCH STIMULI Single-unit smooth muscle Found in the gastrointestinal tract, bile ducts, ureters, uterus, and many blood vessels. More rudimentary contraction. Types of Smooth Muscle MULTI-UNIT SMOOTH MUSCLE – Discrete, separate smooth muscle fibers – Each muscle fiber contracts independently – Innervated by a single nerve-ending – Repond to neural stimuli (ANS) Types of Smooth Muscle Found in ciliary muscles of the eyes Iris and pupil muscle of the eyes Base of hair follicles RESPONSIBLE FOR FINER CONTROL Particularities of Smooth Muscle Fusiform Smooth muscle has only one nucleous at the cell’s center. Smooth muscle does not contain myofibrils or sarcomere, but it contains thick and thin fillaments, and contract via a sliding filament mechanism. Particularities of Smooth Muscle Smooth muscle does not have the same striated arrangement of actin and myosin filaments The actin filaments are attached to dense bodies, which are analogous to the Z-discs in striated muscle sarcomeres. Some bodies are attached to cell membrane Some bodies are dispersed inside the cell Myosin filaments are inserted among the actin filaments Particularities of Smooth Muscle Myosin filaments have side-polar cross-bridges – They are arranged so that the bridges on one side bend in one direction and those on the other side bend in the opposite direction – This configuration allows the myosin to pull an actin filament simultaneously in opposite directions – Smooth muscle can contract as much as 80% of their length Skeletal muscle only 30% Particularities of Smooth Muscle Low frequency of cross-bridges cycles – Low energy to mantain the contraction - 1/10 to 1/300 of the energy necessary to contract skeletal muscle. – Once smooths muscle iniciated the contraction, a low energy is needed to keep the tonic contraction for hours. – Some smooth muscle has a tonic contracting, that can last for hours or days. Particularities of Smooth Muscle Sarcoplasmic reticulum is slightly developed – SR is not the major source of Ca2+ for smooth muscle contraction – extracellular fluid is! SR is not present in all smooth muscle fibers – Lies near the cell membrane in some larger smooth muscle cells – Small invaginations of the cell membrane, called CAVEOLAE, that make contact with the surface of SR. Rudimentary T-Tubule. Is believed to excite calcium release from the SR. – The more extensive the SR in the smooth muscle fiber, the more rapidly it contracts Smooth Muscle Contraction Smooth muscle uses calmodulin as a calcium-binding protein to regulate the contraction process instead of troponin. 1. Calcium-bound calmodulin, resulting in its activation. 2. Activated calmodulin then activates an enzyme called myosin light chain kinase (MLCK). MLCK phosphorylates a specific region of myosin. 3. Phosphorylated myosin binds to actin, forming a cross-bridge between myosin and actin allowing the muscle cell to contract. 4. When the ion calcium concentration is reduced, myosin phosphatase removes the phosphate from the myosin light chain, causing muscle relaxation Key Points General Characteristics of Smooth Muscle w Smooth muscle can be divided into two types: 1) Multiunit smooth muscle; 2) Single-unit smooth muscle. w Dense bodies are small, dense structures found within the cytoplasm of smooth muscle cells. They serve as anchoring points for actin filaments. These dense bodies are functionally analogous to the Z-discs found in skeletal muscle. w The arrangement of actin, myosin, and dense bodies in smooth muscle allows for the coordinated contraction and relaxation of the muscle. Key Points General Characteristics of Smooth Muscle w Smooth muscle contracts involuntarily without conscious control. It is found in the walls of various organs and structures throughout the body, such as blood vessels, the digestive system, respiratory system, and reproductive system. w Smooth muscle performs essential functions, such as regulating blood pressure, propelling food through the digestive tract, and facilitating the movement of air in the lungs. w Compared to skeletal muscle, smooth muscle exhibits slower contraction and relaxation times. The slower kinetics of smooth muscle contraction and relaxation are important for sustained contractions, such as maintaining muscle tone in blood vessels or in the uterus. Cardiac Muscle Cardiac muscle tissue is found only in the heart! Cardiac means “relating to the heart”. Cardiac muscle cells are called cardiomyocytes or cardiocytes. These cells make up the myocardium, the muscle layer of the heart. The myocardium contracts to pump blood throughout the body, supplying organs and tissue with oxygen and nutrients. Heart and Cardiovasular System Purpose The heart pumps blood through the organs of the body to facilitate the exchange of gases, fluid, electrolytes, and heat between cells and the outside environment. Cardiac Muscle The structure of cardiac muscle shares some similarities with skeletal muscle – Fibers are striated – Myofibrils are made up of actin and myosin filaments Similar organization of the sarcomere – Contains a less-developed Sarcoplasmic Reticulum – T-tubules (also release Ion Calcium) Cardiac Muscle CARDIAC MUSCLE DIFFERS FROM SKELETAL MUSCLE IN SEVERAL WAYS. – Contraction is involuntary (controlled via the ANS) – Fibers are shorter and branched – Usually uninucleated – Interconnected by intercalated disks – Generates their action potential (pacemaker fibers) Cardiac Muscle THE CARDIAC ACTION POTENTIAL IS NOT INITIATED BY NERVOUS ACTIVITY. It is generated by a group of specialized cells known as pacemaker cells. These cells have automatic action potential generation capability. Are found in the sinoatrial node in the right atrium. They produce roughly 60–100 action potentials every minute. Cardiac Muscle CARDIAC MUSCLE IS A FUNCTIONAL SYNCYTIUM, WORKING AS A SINGLE UNIT – Greek: Syn = together + Kytos = cell – Do not fuse into a single multinucleated fiber during embryonic development like skeletal muscle does (morphological syncytium) – During development, cardiac myocytes bifurcate and form branches that connect to neighboring myocytes. – Fibers remain separated as distinct cells with their respective sarcolemma – Electrically connected through intercalated disks Intercalated Discs and GAP Junctions Individual myocytes connect to each other by way of specialized cell membranes called intercalated discs. GAP junctions within these intracellular regions serve as low-resistance pathways between cells, permitting cell-to-cell conduction of electrical (ionic) currents. Desmosome – Is a cell structure specialized for cell-to-cell adhesion. – They are found in high numbers in tissues that are subject to a lot of mechanical forces. – Provide mechanical strength and stability. Excitaton-Contraction Coupling – Occur similarly to the skeletal muscle. – Because SR in the myocytes is less developed than in the skeletal muscle, Ca++ ions also enter the cell during the action potential through long–lasting Calcium channels located on the external sarcolemma and T-tubules. Key Points General Characteristics of a Cardiac Muscle w Cardiac muscle is a specialized type of muscle found in the heart. It consists of elongated cells called cardiomyocytes, which are striated due to the organized arrangement of actin and myosin filaments. w Cardiac muscle contracts involuntarily, but the contraction is controlled via ANS. w Intercalated discs are unique structures found between adjacent cardiac muscle cells. They contain specialized cell-to-cell junctions called desmosomes and gap junctions. Key Points General Characteristics of a Cardiac Muscle w Desmosomes provide mechanical strength and stability, allowing for the transmission of force during contraction. w Gap junctions permit the passage of ions between cells, enabling electrical coupling. w The excitation-contraction coupling occurs similarly to that observed in skeletal muscle. Calcium binding to Troponin C produces a conformation change in the troponin-tropomyosin complex that exposes a myosinbinding site on the actin.