Smooth muscle has thin filaments that are composed of actin and tropomyosin, similar to skeletal muscle. However, the cellular content of actin and tropomyosin in smooth muscle is double that of striated muscle. Unlike striated muscle, smooth muscle does not have troponin and nebulin, but it does contain two proteins called caldesmon and calponin. The roles of these proteins are not fully understood, but they do not seem to be essential for cross-bridge cycling. Calponin and caldesmon may play a role in regulating the contractility of smooth muscle, potentially by inhibiting actomyosin ATPase activity. The majority of the cytoplasm in smooth muscle is filled with thin filaments that are roughly aligned along the cell's long axis. The amount of myosin in smooth muscle is only a quarter of that in striated muscle. Small groups of three to five thick filaments are aligned and surrounded by numerous thin filaments. These groups of thick and thin filaments are connected to dense bodies or areas and represent the equivalent of the sarcomere. The contractile apparatus of adjacent smooth muscle cells is mechanically coupled through the links between membrane-dense areas. The cytoskeleton in smooth muscle cells serves as an attachment point for the thin filaments and allows for the transmission of force to the ends of the cell. Unlike skeletal muscle, the contractile apparatus in smooth muscle is not organized into myofibrils, and Z lines are absent. The ellipsoidal dense bodies in the cytoplasm and the dense areas along the sarcolemma serve as attachment points for the thin filaments and contain α-actinin, a protein also found in the Z lines of striated muscle. Intermediate filaments, which are larger in diameter than thin filaments but smaller than thick filaments, are prominent in smooth muscle. These intermediate filaments link the dense bodies and areas, creating a cytoskeletal network. The intermediate filaments are composed of protein polymers of desmin or vimentin. The activity of smooth muscle can be controlled by various factors, including hormones, autonomic nerves, pacemaker activity, and drugs. Similar to skeletal and cardiac muscle, smooth muscle contraction relies on the presence of calcium ions (Ca++), and the aforementioned factors induce smooth muscle contraction by increasing the concentration of intracellular calcium ions. However, unlike skeletal and cardiac muscle, action potentials in smooth muscle are highly variable and not always necessary to initiate contraction. Additionally, certain agents can increase intracellular calcium ion concentration and consequently contract smooth muscle without altering the membrane potential. Fig. 14.5 illustrates different types of action potentials in smooth muscle and the corresponding changes in force. An action potential in smooth muscle can result in a slow twitch-like response, and the forces produced by these twitches can accumulate during periods of repetitive action potentials, resembling tetany in skeletal muscle. This pattern of activity is characteristic of single-unit smooth muscle in many visceral organs.
The thin filaments of smooth muscle have an actin and tropomyosin composition and structure similar to that in skeletal muscle. However, the cellular content of actin and tropomyosin in smooth muscle is about twice that of striated muscle. Smooth muscle lacks troponin and nebulin but contains two proteins not found in striated muscle: caldesmon and calponin. The precise roles of these proteins are unknown, but they do not appear to be fundamental to cross-bridge cycling. It has been suggested that both calponin and caldesmon may regulate the contractility of smooth muscle. (in part by inhibiting actomyosin ATPase activity). Most of the myoplasm is filled with thin filaments that are roughly aligned along the long axis of the cell. The myosin content of smooth muscle is only a fourth that of striated muscle. Small groups of three to five thick filaments are aligned and surrounded by many thin filaments. These groups of thick filaments with interdigitating thin filaments are connected to dense bodies or areas (
Fig. 14.4
Fig. 14.3A–B ) and represent the equivalent of the sarcomere. The contractile apparatus of adjacent cells is mechanically coupled by the links between membrane-dense areas.
The cytoskeleton in smooth muscle cells serves as an attachment point for the thin filaments and permits transmission of force to the ends of the cell. In contrast to skeletal muscle, the contractile apparatus in smooth muscle is not organized into myofibrils, and Z lines are lacking. The functional equivalents of the Z lines in smooth muscle cells •Fig. 14.4 Apparentorganizationofcell-to-cellcontacts,cytoskeleton,andmyofilamentsinsmoothmusclecells.Smallcontractileelementsfunctionallyequivalenttoasarcomereunderliethesimilaritiesinmechanicsbetweensmoothandskeletalmuscle.Linkagesconsistingofspecializedjunctionsorinterstitialfibrillarmaterialfunctionallycouplethecontractileapparatusofadjacentcells.Densebodies,thefunctionalequivalentofZlinesinstriatedmuscle,areinterconnectedbyintermediatefilaments.Themyofilamentsareorientedlargelyinparallelwiththelongitudinalaxisofthecell,thoughanobliqueorientationhasbeenobservedinsomearteries.
are ellipsoidal dense bodies in the myoplasm and dense areas that form bands along the sarcolemma (see
Figs. 14.3A–B 14.4 ). These structures serve as attachment points for the thin filaments and contain α-actinin, a protein also found in the Z lines of striated muscle. Intermediate filaments with diameters between those of thin filaments (7 nm) and thick filaments (15 nm) are prominent in smooth muscle. These filaments link the dense bodies and areas into a cytoskeletal network. The intermediate filaments consist of protein polymers of desmin or vimentin.
Control of Smooth Muscle Activity
The contractile activity of smooth muscle can be controlled by numerous factors, including hormones, autonomic nerves, pacemaker activity, and a variety of drugs. Like skeletal or cardiac muscle, contraction of smooth muscle is dependent on Ca++ , and the agents just listed induce smooth muscle contraction by increasing intracellular [Ca++]. However, in contrast to skeletal or cardiac muscle, action potentials in smooth muscle are highly variable and not always needed to initiate contraction. Moreover, several agents can increase intracellular [Ca++] and hence contract smooth muscle without changing the membrane potential.
Fig. 14.5 shows various types of action potentials in smooth muscle and the corresponding changes in force. An action potential in smooth muscle can be associated with a slow twitch-like response, and the twitch forces can summate during periods of repetitive action potentials (i.e., similar to tetany in skeletal muscle). Such a pattern of activity is characteristic of single-unit smooth muscle in many viscera.