Smooth Muscles PDF

SMOOTH MUSCLE Structure.  Smooth(visceral) muscle is arranged in circular layers around hollow organs (e.g., esophagus, respiratory airways) and blood vessels (including the aorta but not the heart); contraction reduces the size of these structures.  The cells are spindle shaped....

SMOOTH MUSCLE Structure.  Smooth(visceral) muscle is arranged in circular layers around hollow organs (e.g., esophagus, respiratory airways) and blood vessels (including the aorta but not the heart); contraction reduces the size of these structures.  The cells are spindle shaped.  The actin-myosin myofilaments are not arranged into sarcomeres, so cells are nonstriated in appearance.  The absence of sarcomeres enables smooth muscle to contract even when the cells are enormously stretched (i.e., smooth muscle contraction is not limited by the length-tension relationship).  The sarcoplasmic reticulum is loosely arranged within the cells, and there are no T tubules.  Cells do contain dense bodies, structures analogous to the Z disks found in skeletal muscle Structure and Function  At a cellular level, smooth muscle can be described as an involuntary, non-striated muscle. Smooth muscle consists of thick and thin filaments that are not arranged into sarcomeres giving it a non-striated pattern.  Smooth muscle contraction is slow and sustained because the ATPase activity is relatively slow compared to skeletal muscles.  Both circular and longitudinal smooth muscle layers occur in the tubular digestive tract, the ureters (which transport urine), the ductus deferentia (which transport sperm cells), and the uterine tubes (which transport ova).  The alternate contraction of circular and longitudinal smooth muscle layers in the intestine produces peristaltic waves, which propel the contents of these tubes in one direction. Action potentials in smooth muscles  Smooth muscle(SMC) action potentials are unique in that membrane potential acts to initiate or modulate contraction. As such, graded membrane response can become stimulated by multiple factors, including local humoral factors, circulating hormones, or mechanical stimulation like stretching of the cells.  Many SMCs are activated by membrane depolarization that opens L-type Ca2+voltage-operated, Na+ and T type Ca2+ gated channels allowing external Ca2+ to flood into the cell to trigger contraction. This depolarization is induced either by ionotropic receptors (vas deferens) or a membrane oscillator (bladder and uterus).  Smooth muscle have tropomyosin but lacks troponin instead calmodulin takes the role. Instead of using a steric mechanism, Ca2+ works through a biochemical cascade.  Although smooth muscle cells do not contain sarcomeres (which produce striations in skeletal and cardiac muscle), they do contain a great deal of actin and some myosin, which produces a ratio of thin to thick filaments of about 16 to 1 (in striated muscles the ratio is 2 to 1).  Unlike striated muscles, in which the thin filaments are relatively short (extending from a Z disc into a sarcomere), the thin filaments of smooth muscle cells are quite long.  They attach either to regions of the plasma membrane of the smooth muscle cell or to cytoplasmic protein structures called dense bodies, which are analogous to the Z discs of striated muscle.  The myofilaments and dense bodies are so numerous that they occupy as much as 90% of the volume of a smooth muscle cell. In smooth muscle, the myosin proteins of the thick filaments are stacked vertically so that their long axis is perpendicular to the long axis of the thick filament.  In this way, the myosin heads can form cross bridges with actin all along the length of the thick filaments. This is different from the horizontal arrangement of myosin proteins in the thick filaments of striated muscles, which is required to cause the shortening of sarcomeres.  The arrangement of the contractile apparatus in smooth muscle cells, and the fact that it is not organized into sarcomeres, is required for proper smooth muscle function.  Smooth muscles must be able to contract even when greatly stretched— in the urinary bladder, for example, the smooth muscle cells may be stretched up to two and a half times their resting length.  The smooth muscle cells of the uterus may be stretched up to eight times their original length by the end of pregnancy.  Striated muscles, because of their structure, lose their ability to contract when the sarcomeres are stretched to the point where actin and myosin no longer overlap.  Types 1. Single-unit (unitary or visceral) smooth muscle  The predominant type of smooth muscle in the body, located in the gastrointestinal tract, bladder, uterus, and ureters Functions as a syncytium. Types of   Low-resistance channels between cells (gap junctions) transmit nerve impulses, causing the Smooth contraction of many cells at once.  A unique quality of gastrointestinal smooth muscles muscle is the rhythmic fluctuation of membrane potential (slow waves) that gives rise to spike potentials, which can cause muscle contraction (i.e., they function as a pacemaker)  Although slow waves are the primary regulator of single-unit smooth muscle, activity can be modified substantially through the autonomic nervous system. Type 2- Multiunit smooth muscle  Located in the iris, ciliary muscle of the lens, arrector pili of the skin, and vas deferens.  Similar to skeletal muscle in that each muscle fiber is innervated, and therefore functions, separately.  Gap junctions are absent.  Because there is no pacemaker activity, regulation of multiunit smooth muscle is dependent on the autonomic nervous system. Mechanism of contraction  Slow waves give rise to spike potentials, which stimulate cell contraction.  The initial phase of contraction is triggered by an increase in cytoplasmic calcium, released from the sarcoplasmic reticulum, as occurs in skeletal muscle.  Sustained contraction is mediated by continued influx of Ca2+ into the cytoplasm from the interstitium, through voltage-gated calcium channels on the cell membrane.  Calcium combines with the protein in plasma-calmodulin to form the calcium-calmodulin (Ca2+-CaM) complex and activates myosin light-chain kinase (MLCK).Troponin is not present in smooth muscles.  MLCK in turn phosphorylates the myosin cross-bridges, exposing binding sites for actin.  Actin and myosin then form cross-bridges that contract the muscle cell.  Relaxation occurs when Ca2+ has been pumped back into the sarcoplasmic reticulum such that the Ca2+-CaM complex can no longer be formed. Regulation of contraction  Most smooth muscle has intrinsic pacemaker activity, but smooth muscle activity can be modulated by the autonomic nervous system (i.e., it is generally not under voluntary control).  Sympathetic and parasympathetic nerves are distributed to all organ systems in the body and stimulate smooth muscle activity in many organs at once.  For example, in the fight-or-flight response, sympathetic stimulation causes a myriad of responses such as pupillary dilation, dilation of coronary arteries, decreased intestinal motility, and bronchial dilation.  In general, parasympathetic stimulation has the opposite effects Autonomic innervation of smooth muscles.  The neural control of skeletal muscles differs significantly from that of smooth muscles. A skeletal muscle fiber has only one junction with a somatic motor axon, and the receptors for the neurotransmitter are located only at the neuromuscular junction.  By contrast, the entire surface of smooth muscle cells contains neurotransmitter receptor proteins. Neurotransmitter molecules are released along a stretch of an autonomic nerve fiber that is located some distance from the smooth muscle cells.  The regions of the autonomic fiber that release transmitters appear as bulges, or varicosities, and the neurotransmitters released from these varicosities stimulate a number of smooth muscle cells. Since there are numerous varicosities along a stretch of an autonomic nerve ending, they form synapses “in passing”—or synapses en passant —with the smooth muscle cells. T Autonomic Innervation of Smooth Muscles  The neural control of skeletal muscles differs significantly from that of smooth muscles. A skeletal muscle fiber has only one junction with a somatic motor axon, and the receptors for the neurotransmitter are located only at the neuromuscular junction.  By contrast, the entire surface of smooth muscle cells contains neurotransmitter receptor proteins. Neurotransmitter molecules are released along a stretch of an autonomic nerve fiber that is located some distance from the smooth muscle cells.  The regions of the autonomic fiber that release transmitters appear as bulges, or varicosities, and the neurotransmitters released from these varicosities stimulate a number of smooth muscle cells.  Since there are numerous varicosities along a stretch of an autonomic nerve ending, they form synapses “in passing”—or synapses en passant —with the smooth muscle cells. SKELETAL MUSCLE CARDIAC MUSCLE SMOOTH MUSCLE Striated; actin and myosin arranged in Striated; actin and myosin arranged Not striated; more actin than myosin; actin sarcomeres in sarcomeres inserts into dense bodies and cell membrane Well-developed sarcoplasmic reticulum Moderately developed sarcoplasmic Poorly developed sarcoplasmic reticulum; and transverse tubules reticulum and transverse tubules no transverse tubules Contains troponin in the thin filaments Contains troponin in the thin Contains calmodulin, a protein that, when filaments bound to Ca2+ , activates the enzyme myosin light-chain kinase Ca2+ released into cytoplasm from Ca 2+ enters cytoplasm from Ca2+ enters cytoplasm from extracellular sarcoplasmic reticulum sarcoplasmic reticulum and fluid, sarcoplasmic reticulum, and perhaps extracellular fluid mitochondria Cannot contract without nerve Can contract without nerve Maintains tone in absence of nerve stimulation; denervation results in stimulation; action potentials stimulation; visceral smooth muscle muscle atrophy originate in pacemaker cells of produces pacemaker potentials; heart denervation results in hypersensitivity to stimulation Muscle fibers stimulated independently; Gap junctions present as Gap junctions present in most smooth no gap junctions intercalated discs muscles