Muscle Physiology- Pt 3.docx

- **Muscles: General information** - Muscles are contractile tissue. - **Muscles** have the following **functions**: locomotion, respiration, digestion, swallowing, parturitions, blood and lymphatic circulation, and generating body heat. - **Muscles** have the...

- **Muscles: General information** - Muscles are contractile tissue. - **Muscles** have the following **functions**: locomotion, respiration, digestion, swallowing, parturitions, blood and lymphatic circulation, and generating body heat. - **Muscles** have the following **properties**: - Contractility - Contractility is the ability to shorten. - Excitability - Excitability is the capacity to receive and respond to stimuli. - Extensibility - Extensibility is the ability to be stretched. - Elasticity - Elasticity is the ability to return to the original shape after being stretched. - **Muscle types** include skeletal muscle, cardiac muscle, and smooth muscle. - Cardiac and smooth muscle make up 10% of body mass. - **Skeletal muscle** - Skeletal muscle makes up 70% of body mass. - Skeletal muscle produces fast contractions which are under voluntary control. - Skeletal muscle can be found in the trunk of the body, the head, neck, and extremities. - Skeletal; muscle has striations. - **Smooth muscle** - Smooth muscle produces slow contractions which are under involuntary control. - Smooth muscle can be found in viscera and blood vessels. - Smooth muscle lacks striations. - **Cardiac muscle** - Cardiac muscle produces fast contractions which are under involuntary control. - Cardiac muscle can be found in the heart. - Cardiac muscles have striations. - **Skeletal Muscle: General Info** - Skeletal muscle is crucial for body movement as it is attached to bones (typically) via tendon where it moves and supports the skeleton. - Skeletal muscle is considered to be under voluntary (conscience) response because it is stimulated by motor nerves. - Body movement is due to skeletal muscle contraction across movable joints. - Joints tend to have one or more skeletal muscles attached to it, influencing the joint to extend (increase its' angle) or flex (decrease its' angle). - **Skeletal Muscle: Organization** - **Epimysium** is a sheath of connective tissue surrounding skeletal muscle. - A **fascicle** is a small bundle or cluster of muscle fibers (cells). - **Perimysium** is connective tissue extending from the epimysium which surrounds each fascicle. - **Endomysium** is connective tissue extending from the perimysium which surrounds individual muscle fibers and is attached to the sarcolemma. - The **sarcolemma** is a thin cell membrane which encloses skeletal muscle fibers (cells). - At the end of muscle fibers, the sarcolemma fuses with tendon fibers and collects into bundles to form muscle tendons. - Tendons connect muscles to bone. - **Muscle fibers** are composed of long, cylindrical, multinucleated muscle cells. - The pull of muscle fibers is transmitted by the perimysium, epimysium, and endomysium to the tendon or aponeurosis attached to the bones. - **Myofibrils** are present by the hundreds or thousands within each muscle fiber. - Each myofibril is composed of a linear series of repeating sarcomeres. - **Sarcomeres** are the basic contractile unit within striated muscle fibers, giving them the appearance of striations. - Sarcomeres are found between Z lines (AKA: Z disks). - Sarcomeres contain protein myofilaments (thick and thin). - **Myofilaments** are directly responsible for muscle contraction and can be further classified as thick and thin filaments. - **Thin Filaments** - The thin filaments are actin, troponin, and tropomyosin. - The thin filament composition is of 2 intertwined helical strands of actin protein and 2 strands of tropomyosin protein. - **Troponin** - Troponin is located intermittently along the tropomyosin-actin strand. - Troponin has an affinity for calcium ions. - **Thick Filaments** - The thick Filament is myosin. - The **myosin** filament contains a tail of intertwined helices and 2 globular heads where 1 head binds to ATP and the other binds to actin. - Myosin heads can flex and relax. - Myosin acts as an ATPase enzyme by using ATP as the source of energy for contractions. - Approximately 500 myosin heads form cross bridges that interact with actin to shorten the sarcomere. - **Titin Filamentous Molecules** - Titin filamentous molecules keep myosin and actin filaments in place. - Titin filamentous molecules are very elastic filamentous molecules which have 1 end attached to the end of the Z disk and another part attached myosin. - **Myofilament: Organization** - Myosin and actin filaments partially interdigitate which result in the myofibrils to have alternating light and dark bands, also known as striations. - **I bands** are the light bands (thin filaments) which are isotropic to polarized light. - **A bands** are the darker bands (thick filaments) which are anisotropic to polarized light. - The end of actin filaments are attached to **Z disks** (AKA: Z lines), forming the periphery of the sarcomere and transversely bisecting the I bands. - The **H-zone** is the light line in the center of the sarcomere from the thick filaments. - The **M-line** is composed of thick filaments and accessory proteins and can be found inside the H-zone, forming the center of the sarcomere. - **Skeletal Muscle: Fiber Types** - **Type 1 (slow)** muscle fibers are red fibers. - Type 1 fibers are slow twitch oxidative muscle fibers, meaning that they highly rely on oxidative metabolism. - Type 1 fibers are slow contracting and fatigue resistant. - Type 1 fibers are rich in mitochondria, myoglobin, and capillaries which cause this fiber type to have a red appearance. - Type 1 fibers are plentiful within muscles that have functions involving slow, prolonged activity. - For example, type 1 fibers are plentiful within muscles that help maintain body posture. - **Type 2 (fast)** muscle fibers are white fibers. - Type 2 muscle fibers are fast twitch glycolytic muscle fibers. - Type 2 fibers are fast contracting and are more fatigable fibers, in comparison to Type 1 fibers. - Type 2 fibers rely on glycolytic metabolism and have less mitochondria than type 1 fibers. - Type 2 fibers lack myoglobin, causing it to have a white pigmentation. - Type 2 muscle fibers can be further classified has "Type 2A" and "Type 2B." - **Type 2A Fiber** - Type 2A fibers use mixed oxidative-glycolytic metabolism and are fast contracting WITH fatigue resistance. - These properties cause Type 2A fibers to act as an intermediate between Type 1 and Type 2B fibers. - Dog muscles are solely composed of type 2A fibers, allowing them to have strong oxidative capacity. - **Type 2B Fiber** - Type 2B fibers use glycolytic metabolism (causing them to rely on glycogen for energy) and are fast contracting, fast-fatiguing fibers. - Muscles that are designed for sprinting have a large composition of type 2B fibers. - The proportion of type 1 vs type 2 muscle fibers varies from muscle to muscle. - **Muscle Cell: Organelles** - **Sarcoplasmic Reticulum** - The sarcoplasmic reticulum is a specialized endoplasmic reticulum which manages calcium storage, release, and reuptake- aiding in muscle contraction. - The sarcoplasmic reticulum in lager in cells with faster contracting fibers, such as the Type 2 (white) fibers. - **Mitochondria** - The mitochondria is present in large numbers within muscle cells and lay parallel to myofibrils. - Mitochondria is more prevalent within slow fibers, such as Type 1 (red) fibers. - The mitochondria supplies the contracting myofibrils with large supplies of ATP to be used for energy. - **T Tubules** - The T Tubules are tubules that contain periodic invagination within the sarcolemma and are arranged transversely to myofibrils. - T Tubules contain ECF (extra cellular fluid). - T Tubules allow the plasma membrane of the muscle fiber to carry depolarization of action potentials to the interior of the muscle fiber. - In mammals, T Tubules are located at junctions between A bands and I bands of the sarcomere (2 per sarcomere in longitudinal view). - Each T Tubule is flanked by 2 terminal cisternae of the sarcoplasmic reticulum, resulting in the formation of **Triad structures**. - **Myoglobin** - Myoglobin is a protein composed of a single polypeptide chain with only 1 **HEME** group (also known as an oxygen binding site). - In comparison to **hemoglobin** which is composed of 4 polypeptide chains and has 4 oxygen binding sites. - Myoglobin is located primarily in the straited muscles of vertebrates and is found exclusively in skeletal and cardiac muscle. - Myoglobin constitutes up to 5-10% of all cytoplasmic proteins. - Myoglobin is a member of the globin superfamily which also includes hemoglobin. - Myoglobin serves as a local oxygen reservoir where it can temporarily provide oxygen whenever blood oxygen supply is low during periods of intense muscular activity. - The body will use myoglobin as an oxygen storage protein within muscles, as myoglobin can bind and release oxygen depending on the oxygen concentration within the cell. - Myoglobin can also function in the homeostasis of nitric oxide and the detoxification of reactive oxygen species. - The total amount of myoglobin with the body depends on the animals' body weight, degree of muscle development, and myoglobin concentration within the muscles. - Red muscles are very rich in myoglobin, while white muscles have a lower supply of myoglobin.