Cardiac and Smooth Muscle Claudia Stanescu Ph D Office Hours in Gittings 108 Tuesday 10 11 and Thursday 1 2 or by appointment Objectives 1 Compare and contrast cardiac muscle and skeletal muscle 2 Know that cardiac muscle has no motor units all cells contract contraction is relatively slow and initiated by pacemaker cells 3 Understand the role of smooth muscle 4 Know the basic structure of smooth muscle no sarcomeres and the general steps of contraction Cardiac and Smooth Muscle All types of muscle skeletal cardiac and smooth generate tension The underlying mechanisms in all three rely on variations of two common themes 1 Sliding filaments actin myosin 2 Regulation of cytoplasmic Ca2 Ca inside the cell primarily from SR but also from outside cell SM mostly outside cell regardless Ca inside cell must increase for contractionres Cardiac Muscle 1 Function what does heart muscle do Pumps Blood involves contraction of muscle around a confined volume increasing the pressure so the fluid can be pushed through the circulatory system Cardiac Muscle 2 Structural Features compared to skeletal muscle a Smaller cells branched b Less extensive T tubule and SR system c Myofibrils organized into sarcomeres d Extensive cell to cell interactions Intercalated Discs containing Gap Junctions Desmosomes Microscopic Anatomy of Cardiac Muscle Cardiac Muscle 3 Functional Issues Similarities to skeletal muscle the Sliding Filament Model still applies Crossbridge cycling Ca2 interaction with thin filament regulatory proteins Cardiac Muscle 3 Functional Issues Differences compared to skeletal muscle SR Ca2 is not sufficient to support contraction there is an obligatory need for extracellular Ca2 entry of extracellular Ca2 required on a beat to beat basis Cardiac Muscle 3 Functional Issues Differences compared to skeletal muscle Contraction is NOT initiated by an electrical signal from the nervous system cardiac cells show autorhythmicity electrical and contractile activity started by pacemaker cells spontaneous generation of APs No Motor Units every cell contracts with every beat the electrical signal moves from cell to cell through gap junctions Every contraction is a twitch NO TETANUS Cardiac Muscle 3 Functional Issues Differences compared to skeletal muscle Nervous input influences the rate strength of contraction acts on pacemaker cells acts on Ca2 delivery Cardiac Muscle 3 Functional Issues Differences compared to skeletal muscle The single twitch of cardiac muscle has a very long time course 100 s of millisec reflects characteristics of the cardiac action potential Smooth Muscle located in digestive system bladder blood vessels etc SLOWEST to contract 1 Function control diameter of tissue tubes blood vessels gut etc Many types of smooth muscle Common structural features 1 small single nucleus 2 lacks a clearly organized structure NO SARCOMERES no extensive T tubule SR system smooth no striation doesn t have stripes works differently Ca comes from outside cell t tubules rudimentary contraction of smooth muscle is SLOWER Smooth Muscle Still there are thick myosin and thin actin filaments with a rudimentary overlapping organization actin thin crossbridges can form filaments cross bridge and sliding actions still apply football shape dense bodies some connected to membranes act like anchors for filaments thin overlap with thick still get cross bridge but not at neat dense bodies instead of z disc myosin thick filaments caveoli act like t tubules fill with Ca reach deeper into cell causes slower contraction imagine number of cells lined up to form tube tube becomes smaller in diameter constricts pull in cell and twists slightly Ca controls contraction in both cardiac and smooth but the generation is different Regulation of cytoplasmic Ca2 is STILL the key the generation of tension in smooth muscle but that regulation is very different from that in skeletal and cardiac muscle Ca2 DOES NOT interact with regulatory proteins on the thin filament to permit myosin head groups to bind in smooth muscle Ca has slightly different role in SM comes from outside due to no SR outside Ca2 Ca channel Calmodulin binds Ca2 enzymes inactivate head groups initially activates regulatory enzymes cytoplasmic myosin head group regulatory myosin head not read to bind must be activated enzymes activates myosin head actin is read to bind myosin light chain kinase The level of tension reflects a balance between the number of active and inactive head groups activated head group can now form crossbridge and generate tension increases in Ca2 cell increases the number of active head groups and so increases tension