10.7-10.9

• Similar to skeletal muscle, cardiac muscle is striated and organized into sarcomeres, possessing the same banding organization as skeletal muscle. However, cardiac muscle fibers are shorter than skeletal muscle fibers and usually contain only one nucleus, which is located in the central region of the cell.
• Cardiac muscle fibers cells also are extensively branched and are connected to one another at their ends by intercalated discs.
• intercalated disc: allows the cardiac muscle cells to contract in a wave-like pattern so that the heart can work as a pump.
• Intercalated discs are part of the sarcolemma and contain two structures important in cardiac muscle contraction: gap junctions and desmosomes. A
• A gap junction forms channels between adjacent cardiac muscle fibers that allow the depolarizing current produced by cations to flow from one cardiac muscle cell to the next. This joining is called electric coupling, and in cardiac muscle it allows the quick transmission of action potentials and the coordinated contraction of the entire heart.
• heart. This network of electrically connected cardiac muscle cells creates a functional unit of contraction called a syncytium.
• The remainder of the intercalated disc is composed of desmosomes. A desmosome is a cell structure that anchors the ends of cardiac muscle fibers together so the cells do not pull apart during the stress of individual fibers contracting
• Although cardiac muscle cannot be consciously controlled, the pacemaker cells respond to signals from the autonomic nervous system (ANS) to speed up or slow down the heart rate. The pacemaker cells can also respond to various hormones that modulate heart rate to control blood pressure.
• The wave of contraction that allows the heart to work as a unit, called a functional syncytium, begins with the pacemaker cells. This
• This group of pacemaker cells is self-excitable and able to depolarize to threshold and fire action potentials on their own, a feature called autorhythmicity; they do this at set intervals which determine heart rate. Becase
• Another feature of cardiac muscle is its relatively long action potentials in its fibers, having a sustained depolarization “plateau.”
• The plateau is produced by Ca++ entry though voltage-gated calcium channels in the sarcolemma of cardiac muscle fibers.
• This sustained depolarization (and Ca++ entry) provides for a longer contraction than is produced by an action potential in skeletal muscle.
• Unlike skeletal muscle, a large percentage of the Ca++ that initiates contraction in cardiac muscles comes from outside the cell rather than from the SR.
• Smooth muscle is also present in the eyes, where it functions to change the size of the iris and alter the shape of the lens; and in the skin where it causes hair to stand erect in response to cold temperature or fear.
• Smooth muscle fibers are thousands times shorter than skeletal muscle fibers
• Smooth muscle fibers are spindle-shaped (wide in the middle and tapered at both ends, somewhat like a football) and have a single nucleus; they range from about 30 to 200 μm, and they produce their own connective tissue, endomysium.
• Although they do not have striations and sarcomeres, smooth muscle fibers do have actin and myosin contractile proteins, and thick and thin filaments. These thin filaments are anchored by dense bodies.
• A dense body is analogous to the Z-discs of skeletal and cardiac muscle fibers and is fastened to the sarcolemma. Calcium
• A dense body is analogous to the Z-discs of skeletal and cardiac muscle fibers and is fastened to the sarcolemma. Calcium ions are supplied by the SR in the fibers and by sequestration from the extracellular fluid through membrane indentations called calveoli.
• Because smooth muscle cells do not contain troponin, cross-bridge formation is not regulated by the troponin-tropomyosin complex but instead by the regulatory protein calmodulin.
• 1. In a smooth muscle fiber, external Ca++ passing through opened Ca channels in the sarcolemma, and additional Ca++ released from SR, bind to calmodulin.
  2. The Ca++-calmodulin complex then activates myosin kinase, which, in turn, activates the myosin heads by phosphorylating them.
  3. The heads can then attach to actin-binding sites and pull on the thin filaments. The thin filaments also are anchored to the dense bodies.
  4. When the thin filaments slide past the thick filaments, they pull on the dense bodies, which then pull on the intermediate filaments networks throughout the sarcoplasm.
• fiber. Smooth muscle fibers have a limited calcium-storing SR but have calcium channels in the sarcolemma (similar to cardiac muscle fibers) that open during the action potential along the sarcolemma.
• T-tubules are not needed in smooth muscles due to their small diameter
• The influx of extracellular Ca++ ions, which diffuse into the sarcoplasm to reach the calmodulin, accounts for most of the Ca++ that triggers contraction of a smooth muscle cell.
• Smooth Muscle

  Muscle contraction continues until ATP-dependent calcium pumps actively transport Ca++ ions back into the SR and out of the cell. However, a low concentration of calcium remains in the sarcoplasm to maintain muscle tone. This remaining calcium keeps the muscle slightly contracted, which is important in certain tracts and around blood vessels.
• Some smooth muscle can also maintain contractions even as Ca++ is removed and myosin kinase is inactivated/dephosphorylated. This can happen as a subset of cross-bridges between myosin heads and actin, called latch-bridges, keep the thick and thin filaments linked together for a prolonged period, and without the need for ATP. This allows for the maintaining of muscle “tone” in smooth muscle that lines arterioles and other visceral organs with very little energy expenditure.
• factors. In certain locations, such as the walls of visceral organs, stretching the muscle can trigger its contraction (the stress-relaxation response).
• there is a series of neurotransmitter-filled bulges called varicosities as an axon courses through smooth muscle, loosely forming motor units (
• ). A varicosity releases neurotransmitters into the synaptic cleft. Also,
• Also, visceral muscle in the walls of the hollow organs (except the heart) contains pacesetter cells.
• A pacesetter cell can spontaneously trigger action potentials and contractions in the muscle.
• varicosities - "boutons"
• Smooth muscle is organized in two ways: as single-unit smooth muscle, which is much more common; and as multiunit smooth muscle.
• Single-unit muscle has its muscle fibers joined by gap junctions so that the muscle contracts as a single unit. This type of smooth muscle is found in the walls of all visceral organs except the heart (which has cardiac muscle in its walls), and so it is commonly called visceral muscle. Because
• . Because the muscle fibers are not constrained by the organization and stretchability limits of sarcomeres, visceral smooth muscle has a stress-relaxation response. This means that as the muscle of a hollow organ is stretched when it fills, the mechanical stress of the stretching will trigger contraction, but this is immediately followed by relaxation so that the organ does not empty its contents prematurely. This
• fill. The smooth muscle around these organs also can maintain a muscle tone when the organ empties and shrinks, a feature that prevents “flabbiness” in the empty organ. In general, visceral smooth muscle produces slow, steady contractions that allow substances, such as food in the digestive tract, to move through the body.
• Multiunit smooth muscle cells rarely possess gap junctions, and thus are not electrically coupled. As a result, contraction does not spread from one cell to the next, but is instead confined to the cell that was originally stimulated.
• Stimuli for multiunit smooth muscles come from autonomic nerves or hormones but not from stretching. This type of tissue is found around large blood vessels, in the respiratory airways, and in the eyes.
• Unlike other muscle, smooth muscle can also divide to produce more cells, a process called hyperplasia.