Smooth Muscle Introduction .
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| Smooth Muscle Structure . |
[1]. Smooth muscles are non-striated (plain) and involuntary muscles.
[2]. These muscles are present in almost all the organs in the form of sheets, bundles or sheaths around other tissues.
[3]. Smooth muscles form the major contractile tissues of various organs.
Smooth Muscle Distribution .
Structures in which smooth muscle fibers are present includes :
[1] . Wall of organs like esophagus, stomach and intestine in the gastrointestinal tract
[2] . Ducts of digestive glands
[3] . Trachea, bronchial tube and alveolar ducts of respiratory tract
[4] . Ureter, urinary bladder and urethra in excretory system
[5] . Wall of the blood vessels in circulatory system
[6]. Arrector pilorum of skin
[7] . Mammary glands, uterus, genital ducts, prostate gland and scrotum in the reproductive system
[8] . Iris and ciliary body of the eye.
Functions of Smooth Muscle .
Smooth muscles are concerned with very important functions in different parts of the body.
In the Cardiovascular System .
Smooth muscle fibers around the blood vessels regulate blood pressure and blood flow through different organs and regions of the body.
In the Respiratory System .
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 Digestive System .
[1]. Smooth muscle fibers in digestive tract help in movement of food substances, mixing of food substance with digestive juices, absorption of digested material and elimination of unwanted substances.
[2]. Sphincters along the digestive tract regulate the flow of materials.
In Renal System .
[1]. Smooth muscle fibers in renal blood vessels regulate renal blood flow and glomerular filtration.
[2]. Smooth muscles in the ureters propel urine from kidneys to urinary bladder through ureters.
[3]. Smooth muscles present in urinary bladder help voiding urine to the exterior.
In Reproductive System .
[1]. In males, smooth muscle fibers facilitate the movement of sperms and secretions from accessory glands along the reproductive tract.
[2]. In females, these muscles accelerate the movement of sperms through genital tract after sexual act, movement of ovum into uterus through fallopian tube, expulsion of menstrual fluid and delivery of the baby.
Structure of Smooth Muscle .
[1]. Smooth muscle fibers are fusiform or elongated cells.
[2]. These fibers are generally very small, measuring 2 to 5 microns in diameter and 50 to 200 microns in length.
[3]. Nucleus is single and elongated and it is centrally placed.
[4]. Normally, two or more nucleoli are present in the nucleus .
Myofibrils and Sarcomere .
[1]. Well-defined myofibrils and sarcomere are absent in smooth muscles. So the alternate dark and light bands are absent.
[2]. Absence of dark and light bands gives the non-striated appearance to the smooth muscle.
Myofilaments and Contractile Proteins .
[1]. Contractile proteins in smooth muscle fiber are actin, myosin and tropomyosin. But troponin or troponin-like substance is absent.
[2]. Thick and thin filaments are present in smooth muscle. However, these filaments are not arranged in orderly fashion as in skeletal muscle.
[3]. Thick filaments are formed by myosin molecules and are scattered in sarcoplasm.
[4]. These thick filaments contain more number of cross bridges than in skeletal muscle.
[5]. Thin filaments are formed by actin and tropomyosin molecules.
Dense Bodies .
[1]. Dense bodies are the special structures of smooth muscle fibers to which the actin and tropomyosin molecules of thin filaments are attached.
[2]. The dense bodies are scattered all over the sarcoplasm in the network of intermediate filaments, which is formed by the protein desmin .
[3]. Some of the dense bodies are firmly attached with sarcolemma.
[4]. The anchoring of the dense bodies, intermediate filaments and thin filaments make the smooth muscle fiber shorten when sliding occurs between thick and thin filaments.
[5]. Another interesting feature is that the dense bodies are not arranged in straight line. Because of this, smooth muscle fibers twist like corkscrew during contraction.
[6]. Adjacent smooth muscle fibers are bound together at dense bodies.
[7]. It helps to transmit the contraction from one cell to another throughout the tissue.
Covering and Tendons .
Smooth muscle fibers are covered by connective tissue. But the tendons and aponeurosis are absent.
Sarcotubular System
[1]. Sarcotubular system in smooth muscle fibers is in the form of network. ‘
[2]. T’ tubules are absent and ‘L’ tubules are poorly developed .
Types of Smooth Muscle Fibers .
Smooth muscle fibers are of two types:
1. Single-unit or visceral smooth muscle fibers
2. Multiunit smooth muscle fibers.
Single Unit or Visceral Smooth Muscle Fibers .
[1]. Single-unit smooth muscle fibers are the fibers with interconnecting gap junctions.
[2]. The gap junctions allow rapid spread of action potential throughout the tissue so that all the muscle fibers show synchronous contraction as a single unit.
[3]. Single unit smooth muscle fibers are also called visceral smooth muscle fibers.
Features of single-unit smooth muscle fibers:
1. Muscle fibers are arranged in sheets or bundles
2. Cell membrane of adjacent fibers fuses at many points to form gap junctions.
[2a]. Through the gap junctions, ions move freely from one cell to the other. Thus a functional syncytium is developed.
[2b]. The syncytium contracts as a single unit. In this way, the visceral smooth muscle resembles cardiac muscle more than the skeletal muscle.
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 .
[1]. Multiunit smooth muscle fibers are the muscle fibers without interconnecting gap junctions.
[2]. These smooth muscle fibers resemble the skeletal muscle fibers in many ways.
Features of multiunit smooth muscle fibers:
[1] . Muscle fibers are individual fibers
[2] . Each muscle fiber is innervated by a single nerve ending
[3] . Each muscle fiber has got an outer membrane made up of glycoprotein, which helps to insulate and separate the muscle fibers from one another
[4] . Control of these muscle fibers is mainly by nerve signals
[5] . These smooth muscle fibers do not exhibit spontaneous contractions.
Distribution of Multiunit Smooth Muscle Fibers .
Multiunit muscle fibers are in ciliary muscles of the eye, iris of the eye, nictitating membrane (in cat), arrector pili and smooth muscles of the blood vessels and urinary bladder.
Electrical Activity in Single Unit Smooth Muscle .
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| Electrical activities in smooth muscle A = Slow-wave rhythm of resting membrane potential C = Spike potential initiated by slow wave rhythm |
[1]. Usually 30 to 40 smooth muscle fibers are simultaneously depolarized, which leads to development of self propagating action potential.
[2]. It is possible because of gap junctions and syncytial arrangements of single-unit smooth muscles.
Resting Membrane Potential .
Resting membrane potential in visceral smooth muscle is very unstable and ranges between –50 and –75 mV. Sometimes, it reaches the low level of –25 mV.
Cause for Unstable resting membrane Potential -Slow Wave Potential .
[1]. The unstable resting membrane potential is caused by the appearance of some wave-like fluctuations called slow waves.
[2]. The slow waves occur in a rhythmic fashion at a frequency of 4 to 10 per minute with the amplitude of 10 to 15 mV .
[3]. The cause of the slow-wave rhythm is not known. It is suggested that it may be due to the rhythmic modulations in the activities of sodium-potassium pump.
[4]. The slow wave is not action potential and it cannot cause contraction of the muscle. But it initiates the action potential .
Action Potential .
Three types of action potential occur in visceral smooth muscle fibers:
1. Spike potential .
2. Spike potential initiated by slow-wave rhythm .
3. Action potential with plateau.
1. Spike Potential .
[1]. Spike potential in visceral smooth muscle appears similar to that of skeletal muscle. However, it is different from the spike potential in skeletal muscles in many ways.
[2]. In smooth muscle, the average duration of spike potential varies between 30 and 50 milliseconds.
[3]. Its amplitude is very low and it does not reach the isoelectric base. Sometimes, the spike potential rises above the isoelectric base (overshoot).
[4]. The spike potential is due to nervous and other stimuli and it leads to contraction of the muscle.
2. Spike Potential Initiated by Slow-wave Rhythm .
[1]. Sometimes the slow-wave rhythm of resting membrane potential initiates the spike potentials, which lead to contraction of the muscle.
[2]. The spike potentials appear rhythmically at a rate of about one or two spikes at the peak of each slow wave.
[3]. The spike potentials initiated by the slow-wave rhythm cause rhythmic contractions of smooth muscles.
[4]. This type of potentials appears mostly in smooth muscles, which are self-excitatory and contract themselves without any external stimuli. So, the spike potentials initiated by slow-wave rhythm are otherwise called pacemaker waves.
[5]. The smooth muscles showing rhythmic contractions are present in some of the visceral organs such as intestine.
3. Action Potential with Plateau .
[1]. This type of action potential starts with rapid depolarization as in the case of skeletal muscle. But, repolarization does not occur immediately.
[2]. The muscle remains depolarized for long periods of about 100 to 1,000 milliseconds.
[3]. This type of action potential is responsible for sustained contraction of smooth muscle fibers. After the long depolarized state, slow repolarization occurs.
Tonic Contraction of Smooth Muscle Without Action Potential .
[1]. Smooth muscles of some visceral organs maintain a state of partial contraction called tonus or tone. [2]. It is due to the tonic contraction of the muscle that occurs without any action potential or any stimulus.
[3]. Sometimes, the tonic contraction occurs due to the action of some hormones.
Ionic Basis of Action Potential .
[1]. The important difference between action potential in skeletal muscle and smooth muscle lies in the ionic basis of depolarization.
[2]. In skeletal muscle, the depolarization occurs due to opening of sodium channels and entry of sodium ions from extracellular fluid into the muscle fiber.
[3]. But in smooth muscle, the depolarization is due to entry of calcium ions rather than sodium ions. [4]. Unlike the fast sodium channels, the calcium channels open and close slowly.
[5]. It is responsible for the prolonged action potential with plateau in smooth muscles.
[6]. The calcium ions play an important role during the contraction of the muscle.
Electrical activity in multi-unit smooth muscle
[1]. Electrical activity in multiunit smooth muscle is different from that in the single unit smooth muscle.
[2]. Electrical changes leading to contraction of multiunit smooth muscle are triggered by nervous stimuli.
[3]. Nerve endings secrete the neurotransmitters like acetylcholine and noradrenaline.
[4]. Neurotransmitters depolarize the membrane of smooth muscle fiber slightly leading to contraction. The action potential does not develop.
[5]. This type of depolarization is called local depolarization or excitatory junctional potential (EJP).
[6]. This local depolarization travels throughout the entire smooth muscle fiber and causes contraction .
[7]. Local depolarization is developed because the multiunit smooth muscle fibers are too small to develop action potential.
Contractile Process in Smooth Muscle .
[1]. Compared to skeletal muscles, in smooth muscles, the contraction and relaxation processes are slow.
[2]. The latent period is also long. Thus, the total twitch period is very long and it is about 1 to 3 seconds.
[3]. In skeletal muscle, the total twitch period is 0.1 sec.
Molecular Basis of Smooth Muscle Contraction .
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| Molecular basis of smooth muscle contraction . |
[1]. The process of excitation and contraction is very slow in smooth muscles because of poor development of ‘L’ tubules (sarcoplasmic reticulum).
[2]. So, the calcium ions, which are responsible for excitation-contraction coupling, must be obtained from the extracellular fluid.
[3]. It makes the process of excitation-contraction coupling slow.
Calcium-calmodulin Complex .
[1]. Stimulation of ATPase activity of myosin in smooth muscle is different from that in the skeletal muscle.
[2]. In smooth muscle, the myosin has to be phosphorylated for the activation of myosin ATPase.
[3]. Phosphorylation of myosin occurs in the following manner:
1. Calcium, which enters the sarcoplasm from the extracellular fluid combines with a protein called calmodulin and forms calcium-calmodulin complex .
2. It activates calmodulin-dependent myosin light chain kinase
3. This enzyme in turn causes phosphorylation of myosin followed by activation of myosin ATPase
4. Now, the sliding of actin filaments starts.
[4]. Phosphorylated myosin gets attached to the actin molecule for longer period.
[5]. It is called latch-bridge mechanism and it is responsible for the sustained contraction of the muscle with expenditure of little energy.
[6]. Relaxation of the muscle occurs due to dissociation of calcium-calmodulin complex.
Length-Tension Relationship – Plasticity .
[1]. Plasticity is the adaptability of smooth muscle fibers to a wide range of lengths.
[2]. If the smooth muscle fiber is stretched, it adapts to this new length and contracts when stimulated. Because of this property, tension produced in the muscle fiber is not directly proportional to resting length of the muscle fiber.
[3]. In other words, Starling’s law is not applicable to smooth muscle. Starling’s law is applicable in skeletal and cardiac muscles and the tension or force of contraction is directly proportional to initial length of fibers in these muscles.
[4]. The property of plasticity in smooth muscle fibers is especially important in digestive organs such as stomach, which undergo remarkable changes in volume.
[5]. In spite of plasticity, smooth muscle fibers contract powerfully like the skeletal muscle fibers. Smooth muscle fibers also show sustained tetanic contractions like skeletal muscle fibers.
Neuromuscular Junction in Smooth Muscle .
[1]. Well-defined neuromuscular junctions are absent in smooth muscle.
[2]. The nerve fibers (axons) do not end in the form of endplate.
[3]. Instead, these nerve fibers end on smooth muscle fibers in three different ways:
(A) . Nerve fibers diffuse on the sheet of smooth muscle fibers without making any direct contact with the muscle.
(A.1).The diffused nerve fibers form diffuse junctions, which contain neurotransmitters. Neurotransmitters are released into the matrix, which coats the smooth muscle fiber. From here the neurotransmitters enter the muscle fibers
(B) . In some smooth muscle fibers, the axon terminal ends in the form of many varicosities. The varicosities have vesicles, which contain the neurotransmitter.
(B.1) . Neurotransmitter is released from varicosities through their wall into the muscle fiber
(C). In some of the multiunit smooth muscle fibers, a gap is present between varicosities and the membrane of smooth muscle fibers, which resembles the synaptic cleft in skeletal muscle.
(C.1). The width of this gap is 30 to 40 nm. This gap is called contact junction and it functions as neuromuscular junction of skeletal muscle.
Control of Smooth Muscle .
Smooth muscle fibers are controlled by:
1. Nervous factors
2. Humoral factors.
Nervous Factors .
[1]. Smooth muscles are supplied by both sympathetic and parasympathetic nerves, which antagonize (act opposite to) each other and control the activities of smooth muscles.
[2]. However, these nerves are not responsible for the initiation of any activity in smooth muscle.
[3]. The tonus of smooth muscles is also independent of nervous control.
Humoral Factors .
Activity of smooth muscle is also controlled by humoral factors, which include hormones, neurotransmitters and other humoral factors.
Hormones and Neurotransmitters .
[1]. Action of the hormones and neurotransmitters depends upon the type of receptors present in membrane of smooth muscle fibers in particular area.
[2]. The receptors are of two types, excitatory receptors and inhibitory receptors.
[3]. If excitatory receptors are present, the hormones or the neurotransmitters contract the muscle by producing depolarization.
[4]. If inhibitory receptors are present, the hormones or the neurotransmitters relax the muscles by producing hyperpolarization.
[5]. Hormones and neurotransmitters, which act on smooth muscles are:
1. Acetylcholine .
2. Antidiuretic hormone (ADH) .
3. Adrenaline .
4. Angiotensin II, III and IV .
5. Endothelin .
6. Histamine .
7. Noradrenaline .
8. Oxytocin .
9. Serotonin.
Other Humoral Factors .
[1]. Humoral factors other than the hormones cause relaxation of smooth muscle fibers.
[2]. Humoral factors which relax the smooth muscles:
1. Lack of oxygen
2. Excess of carbon dioxide
3. Increase in hydrogen ion concentration
4. Adenosine
5. Lactic acid
6. Excess of potassium ion
7. Decrease in calcium ion 8. Nitric oxide (NO), the endothelium-derived relaxing factor (EDRF).
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