Smooth Muscles Structure & Contraction PDF

Smooth Muscles  Smooth Muscles  Smooth Muscles are non-striated and involuntary muscles  These are present in almost all the organs in the form of sheets, bundles or sheaths around other tissues Structures in which smooth Muscles fibers are present Wall of organs like esop...

Smooth Muscles  Smooth Muscles  Smooth Muscles are non-striated and involuntary muscles  These are present in almost all the organs in the form of sheets, bundles or sheaths around other tissues Structures in which smooth Muscles fibers are present Wall of organs like esophagus, stomach and intestine Ducts of digestive glands Trachea, bronchial tube and alveolar ducts of respiratory tract Ureter, urinary bladder and urethra in excretory system Wall of blood vessels in circulatory system Functions of Smooth Muscles They regulate the blood pressure and blood flow in the vessels In Cardiovascular System Functions of Smooth Muscles Contraction and relaxation of Smooth Muscle fibers of the air passage alter the diameter of air passage and regulate the inflow and outflow of air In Respiratory System  They help in movement of food substances, mixing of food substances with digestive juices, absorption of digested material In Digestive System  They regulate the renal blood flow and glomerular filtration In Renal System  Structure of Smooth Muscle Smooth Muscles are fusiform or elongated cells These fibers are generally very small in length Nucleus is single and elongated and it is centrally placed Myofibrils and Sarcomere: Well-defined myofibrils and sarcomere are absent in smooth muscles So the alternate dark and light bands are absent  Myofilaments and Contractile Proteins Contractile proteins in Smooth Muscles are actin, myosin and tropomyosin Thick and thin filaments are present in Smooth Muscles However these filament are not arranged in orderly fashion as in skeletal muscles  Dense Bodies: Dense bodies are the special structures of Smooth Muscle fibers to which the actin and tropomyosin molecules are attached The dense bodies are scattered all over the sarcoplasm in the network of intermediate filaments, which is formed by the protein desmin Because of this, Smooth Muscle fibers twist like corkscrew during contraction Adjacent Smooth Muscle fibers are bound together at dense bodies Converging and Tendons: Smooth Muscle fibers are covered by connective tissues But the tendons and aponeurosis are absent Sarcotubular System: Sarcotubular System in Smooth Muscle fibers is in the form of network T-tubules are absent and L tubules are poorly developed  Types of Smooth Muscle fibers Single-Unit or Visceral Smooth Muscle fibers Multiunit Smooth Muscle fibers  Single-Unit or Visceral Smooth Muscle fibers These are the fibers with inter- connecting connecting gap junctions The gap junctions allow rapid spread of action potential throughout the tissue  Feature of Single-Unit Smooth Muscle fibers Muscle fibers are arranged in sheets or bundles Cell membrane of adjacent fibers fuses at many points to form gap junctions Through the gap junctions, ions move freely from one cell to the other Thus a functional syncytium is developed The syncytium contracts as a single unit Distribution of Single-Unit Smooth Muscle fibers Visceral Smooth Muscle fibers are in the walls of the organs such as gastrointestinal organs, uterus, ureters, respiratory tract etc. Multiunit Smooth Muscle fibers These are the fibers without inter- connecting connecting gap junctions Muscle fibers are individual fibers Each Muscle fiber is innervated by a single nerve Each Muscle fiber has got an outer membrane made up of glycoprotein Control of these Muscle fibers is mainly by nerve signals  Distribution of Multiunit Smooth Muscle fibers Multiunit Smooth Muscle fibers are in Ciliary muscles of the eye, iris of the eye, nictitating membrane, arrector pili and smooth muscles of the blood vessels and urinary bladder Electrical Activity in single- unit Smooth Muscle fibers Usually 30-40 smooth muscle fibers are simultaneously depolarized, which leads to development of self- propagating action potential Resting Membrane Potential: Resting Membrane Potential in visceral smooth muscles is very unstable and ranges between -50 and -75 mV Types of Action Potential Spike potential Spike Action Potential Potential Initiated by with Plateau Slow-Wave Rhythm Spike potential: It appears similar to that of skeletal muscles However it is different from the spike potential in skeletal muscles in many ways In smooth muscles, the average duration is 30-50 milliseconds Its amplitude is very low and it does not reach the isoelectric base Spike Potential Initiated by Slow-Wave Rhythm Sometimes the slow-wave rhythm of resting membrane potential initiates the spike potentials, which leads to contraction of muscles The spike potential appears rhythmically a rate of one or two spikes at the peak of each slow-wave The spike potential initiated by the slow-wave rhythmic contractions of smooth muscles These types of potentials are self- excitatory and contract themselves without any external stimuli Action Potential with Plateau This type of action potential starts with rapid depolarization as in the case of skeletal muscle But repolarization does not occur immediately The muscle remains depolarized for long period of time about 100-1,000 milliseconds This type of action potential is responsible for sustained contraction of smooth muscle fibers Ionic Basis of Action Potential The important difference between action potential in skeletal and smooth muscles lie in the ionic basis of depolarization In skeletal muscles, the depolarization occurs due to opening of sodium channels and entry of sodium ions from extracellular fluid into the muscle fibers But in smooth muscle, the depolarization is due to entry of calcium ions rather than sodium ions  Cont…. Unlike the fast sodium channels, the calcium channels open and close slowly It is responsible for prolong action potential with plateau in smooth muscles Electrical Activity in Multiunit Smooth Muscle fibers  Electrical Activity of Multiunit Smooth Muscles Electrical changes leading to contraction of multiunit smooth muscles are triggered by nervous stimuli Nerve endings secrete the neurotransmitters like acetylcholine and noradrenaline Neurotransmitters depolarize the membrane of smooth muscle fiber slightly leading to contraction  Cont…. The action potential does not develop This type of depolarization is called local depolarization or excitatory junctional potential This local depolarization travels throughout the entire muscle fiber and cause contraction  Contractile process in Smooth Muscle As compared to skeletal muscles the contraction and relaxation process is slow in Smooth Muscles The latent period is so long The total twitch period is very long and it is about 1-3 seconds  Molecular Basis of Smooth Muscle Contraction The process of contraction and relaxation is very slow in Smooth Muscles because of poor development of L tubules So the calcium ions, which are responsible for excitation-contraction coupling, must be obtained from the extracellular fluid It makes the process slow Molecular Basis of Smooth Muscle Contraction Calcium + Calmodulin Calcium-Calmodulin complex Activation of myosin light chain kinase Phosphorylation of Myosin Activation of myosin ATPase Muscular Contraction  Calcium-Calmodulin Complex Stimulation of ATPase activity of myosin in smooth muscle is different than in skeletal muscles In Smooth Muscles the myosin has to be phosphorylated for the activation of myosin ATPase Phosphorylation of Myosin occurs in the following manners  Cont…. Calcium, which enters the sarcoplasm from the extra cellular fluid combines with a protein called Calmodulin and forms calcium- Calmodulin complex It activates Calmodulin-dependent myosin light chain kinase This enzyme in turn causes phosphorylation of myosin followed by activation of myosin ATPase Now the sliding of the actin filaments starts  Cont…. Phosphorylated myosin gets attached to the actin molecule for longer period It is called latch-bridge mechanism It is responsible for sustained contraction of muscle with expenditure of little energy Relaxation of muscles occurs due to dissociation of Calcium- Calmodulin complex Neuromuscular Junction in Smooth Muscle Well defined Neuromuscular Junctions are absent in smooth muscles The nerve fibers do not end in the form of endplate Instead these nerve fibers end on smooth muscle fibers in three different ways 1: Diffused Junctions: Nerve fibers diffuse on the sheet of smooth muscle fibers without making any direct contact with the muscle The diffused nerve fibers form diffused junctions, which contain neurotransmitters Neurotransmitters are released into the matrix, which coats the smooth muscle fibers From here neurotransmitters enters the muscle fibers 2. Varicosities Junctions: In smooth muscle fibers, the axon terminal ends in the form of many varicosities The varicosities have vesicles which contain the neurotransmitters Neurotransmitters is released from varicosities through their wall into the muscle fiber 3. Gap Junction: In some of multiunit smooth muscle fibers, a gap is present between varicosities and the membrane of smooth muscle fibers It resembles the synaptic cleft in skeletal muscles The width of this gap is 30-40nm This gap is called contact junction Smooth Muscle are Controlled by Nervous factors Hormonal factors Nervous factors: Smooth Muscles are supplied by both sympathetic and parasympathetic nerves They antagonize each other and control the activities of Smooth Muscles Humoral factors: Activity of Smooth Muscle is also controlled by humoral factors, which includes hormones, neurotransmitters and other humoral factors  Hormones and Neurotransmitters Hormones and Neurotransmitters which acts on smooth muscles are Acetylcholine Antidiuretic Hormone Adrenaline Angiotensin II, III and IV Endothelin Histamine Oxytocin Serotonin Other Humoral Factors Humoral Factors which relax the smooth muscles Lack of oxygen Excess of carbon dioxide Increase in hydrogen ion concentration Adenosine Lactic acid Excess of potassium ions Decrease in calcium ions Nitric oxide Thanks