5 1 Muscle Structure and Function All eukaryotic cells have a cytoskeleton and motor proteins that allow their cells to move and move intracellular components Recall that the three components of the cytoskeleton are microtubules microfilaments and intermediate filaments Microtubules and microfilaments work with various motor proteins to elicit cellular movement In a single animal the cytoskeletal and motor proteins can have multiple isoforms versions of a single protein due to slight variations in DNA sequence pre mRNA processing or post translational modification The diversity of cytoskeletal and motor proteins and the number of possible combinations allow the cell to perform a wide range of tasks with few components For example the African claw toed frog can rapidly change the color of its skin via the network of microtubules in its melanophores cells that produce the dark pigment molecule eumelanin In response to melanophore stimulating hormone MSH the microtubules in the melanophores disperse the eumelanin throughout the cells causing the skin to appear black In response to melatonin the microtubules aggregate the melatonin causing the skin to appear merely spotted with black Microtubules and the motor proteins kinesin and dynein are used to transport vesicles throughout the cell These cellular highways carry empty vesicles to wherever their contents are being synthesized then deliver those contents to their destination The figure below shows the importance of vesicle trafficking in neurons where microtubules carry vesicles between the soma and the axon terminal to be refilled and unloaded sometimes through meters of axon Microtubules in cellular appendages like cilia and flagella allow the cells in the respiratory tract to propel mucus over the surface to expel contaminants and sperm to swim through the female reproductive tract to fertilize an egg All eukaryotic cells contain a cytoskeleton and motor proteins but animals use these same cellular components to make muscle cells myocytes Myocytes are specialized in that they can dramatically rapidly change shape When a myocyte gets shorter it is called contraction the opposite of contraction is relaxation when the myocyte returns to its baseline length By attaching to the tissues around it the myocyte can exert contractile force on the body The contractile force that these cells generate is necessary for many of the processes and behaviors that occur in a multi cellular animal Each muscle in the body is composed of multiple muscle cells and we can group muscles into two types smooth muscle and striated muscle These groupings are based on differences in appearance which are due to differences in the arrangement of the cytoskeleton inside the myocytes that make up the muscle Striated muscle includes both cardiac muscle and skeletal muscle Recall the arrangement of muscles involved in the patellar reflex Because the quadriceps and biceps muscles are on opposite sides of the femur they work as an antagonistic pair moving the lower leg in opposite directions These muscles are attached at the base of the femur one to the front of the patella the other to the fibula in the back such that shortening of each muscle pulls on either the front or back of the lower leg eliciting movement Regardless of appearance all myocytes are able to dramatically rapidly change in length they are all capable of contracting shortening and relaxing lengthening Note that lengthening is a passive process The muscle cell does not push itself outward but lengthens by relaxing Myocytes contract due to interactions between the motor proteins actin and myosin in an arrangement called the sliding filament model Actin is a globular roughly spherical protein that polymerize into long chains of actin which is how the thin microfilaments of the cytoskeleton are formed when it is referred to as F actin When actin exists as a globular monomer it is referred to as G actin Thus G actin can polymerize to form F actin In the cell this assembly process is spontaneous and does not require any energy from the cell These so called thin filaments will be used by the myocyte to change its length Myosin is a rope like protein with a head on one end attached via a neck that can move pivoting the myosin head back and forth In the myocyte multiple ropes of myosin of various lengths are aligned to form a bundle of myosin chains Two bundles are attached at the tail ends to form a thick filament with a region of tails in the middle and regions of heads on either end The thin and thick filaments are arranged parallel to one another with an array of actin surrounding a two headed bundle of myosin The basic mechanism of myocyte shortening is that the thin and thick filaments slide past each other Neither filament changes length the cell shortens because the thin filaments slide inward along the thick filaments To do this the myosin heads attach to the actin monomers and pivot at the neck pulling the filaments of actin inward To pull the thin filament myosin heads store the chemical energy in ATP and convert it to kinetic energy 1 ATP binds a myosin head 2 Myosin hydrolyzes ATP into ADP and inorganic phosphate and stores the chemical bond energy 3 Myosin extends its head forming the high energy conformation and binds the actin releasing the ADP and the phosphate 4 The stored energy is converted into kinetic energy as the myosin neck bends pulling the thin filament the power stroke 5 Myosin head remains bound to actin until more ATP is available which will initiate another cycle of contraction As long as ATP is available the actin myosin cross bridges will break and reform pulling the thin filaments inward bit by bit The exact molecular mechanism of contraction differs between smooth and striated muscle cells but in all cells it is controlled by the level of Ca2 in the cytosol We will look more at how Ca2 mediates contraction in each type of muscle Smooth muscle is found in areas that require slow sustained contractions over large areas This is accomplished with layers of muscle cells that are connected via gap junctions allowing the network of cells to exert a synchronized contraction The individual myocytes contract via actin and myosin but the filaments are not arranged uniformly which is why the muscle appears smooth instead of striped In smooth muscle cells the thin and thick filaments are scattered throughout the cytoplasm and anchored to the membrane by adhesion plaques This allows
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