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Muscle tissue carries out many functions in our bodies including propulsion of food we eat along gastrointestinal tract; expulsion of the waste we produce; changing the amount of air that enters the lung and pumping blood to body tissues.
-There are three types of muscle tissue: skeletal, smooth and cardiac.
Functions of Skeletal Muscle
1. Body movement is due to contraction of muscles attached to bones.
2. Maintenance of posture - Muscles stabilize joints and help maintain the body's posture.
3. Protection and support - Muscles are arranged along the walls of the abdominal and pelvic cavity. They protect the internal organs and support normal position.
4. Storage and movement of materials - Sphincters circular muscle bands that contract and relax to regulate passage of material. This allows such things as voluntary expulsion of feces and urine.
5. Heat production - Heat is produced by energy required for muscle contraction. It is continuously generate heat to maintain body temperature. We shiver when cold to generate heat!
Characteristics Skeletal Muscle Tissue
Muscle has unique properties that allow it to carry out its role in the body:
1.Excitability refers to the ability to respond to stimuli which in muscle is usually a neurotransmitter released by a nerve cell. This chemical gives the muscle cell the "message to contract".
2.Contractility refers to the ability of a muscle cell or fiber to shorten producing movement. This property sets muscle apart from other tissues.
3.Extensibility refers to the ability of muscle fibers to be stretched or extended.
Elasticity refers to the ability of muscle fibers to contract or extend and return to their original length.
4. Conductivity refers to and electrical change traveling along the plasma membrane. This is initiated in response to neurotransmitter binding.
picture of skeletal muscle slide 6-8
Muscle is responsible for all body movements. Muscle tissue is composed of cells called fibers which contain contractile cytoplasmic filaments. Humans have three types of muscle tissue: skeletal, smooth and cardiac. Skeletal consists of long, multinucleate cells with cross striations. We can consciously control the action of skeletal muscle so we call it voluntary muscle. Cardiac muscle which is found in the heart, has branched fibers with intercalated disks which mark the boundaries between cells. This muscle is involuntary. You do not consciously control your heart beat! Smooth muscle consists of spindle shaped cells which surround tubes such as in the digestive system. These cells have one nucleus and are involuntary.
Anatomy of Skeletal Muscle: Connective Tissue
Skeletal muscle fibers are organized into bundles, termed fascicles. Muscles are composed of fibers, connective tissue, blood vessels, and nerves. We have examined muscle fibers so let's look at connective tissue.
There are three concentric layers of connective tissue: epimysium, perimysium, and endomysium. These provide protection, sites for blood vessel and nerve distribution and means of attachment to the skeleton or other structures.
1.Epimysium is a layer of dense irregular connective tissue surrounding an intact muscle.
2.Perimysium is a fibrous connective tissue layer surrounding groups of muscle fibers or fascicles.
3.Endomysium is a layer of reticular fibers surrounding individual muscle fibers.
Anatomy of Skeletal Muscle
Most skeletal muscle cross joints and are attached to bone in at least two places. Muscles attachments may be direct in which the epimysium of the muscle is fused to the periosteum of the bone or indirect in which attachment is by means of a tendon or sheet like aponeurosis. Tendons are cordlike structures composed of dense regular connective tissue. They attach the muscle to bone, skin or another muscle. Aponeuroses are thin, flattened sheets of dense irregular tissue.
-Deep fascia is an additional sheet of dense irregular connective tissue external to the epimysium. It separates individual muscles and binds together muscles with similar functions. It contains nerves, blood vessels, and lymph vessels and fills spaces between muscles.
- Superficial fascia is superficial to deep fascia. Composed of areolar and adipose connective tissue, it separates muscles from skin.
Skeletal muscles are vascularized by extensive blood vessels. They are innervated by motor neurons.
Microscopic Anatomy of a Skeletal Muscle Fiber
Each muscle fiber is a long cylindrical , multinucleate cell. The multiple nuclei are located just beneath the sarcolemma or muscle cell membrane. The nuclei appear to be pushed to one side. The fibers are 10 to 100 micrometers long. (Think about the length of some of your leg muscles.) The cytoplasm of the cell is termed sarcoplasm and contains glycogen which serves as an energy storage compound, and myoglobin which binds oxygen. Fibers contain the usual organelles, plus myofibrils, a sarcoplasmic reticulum and T-tubules.
-The interior of the cell is filled with myofibrils which are rod like contractile elements. They make up most of the muscle volume and push the nuclei to the edges of the cell. The arrangement of myofibrils within a muscle fiber is such that a perfectly aligned repeating series of dark A bands and light I bands is evident.
-As we saw, the plasma membrane of a skeletal muscle fiber is called the sarcolemma. Invaginations of the sarcolemma are called T-tubules, or transverse tubules. Na+/ K+ pumps along sarcolemma and T-tubules create concentration gradients for Na+ and K+. Three Na+ are pumped out while two K+ are pumped in. The resting membrane potential is maintained by pumps. The inside of the cell is relatively negative in comparison to outside. This is responsible for excitability of skeletal muscle fibers. Voltage-gated Na+ channels and voltage-gated K+ channels are also present. These are necessary for propagation of the electrical change along the sarcolemma.
The sarcoplasmic reticulum is an internal membrane complex similar to the smooth endoplasmic reticulum. It surrounds bundles of contractile proteins. Terminal cisternae are blind sacs of sarcoplasmic reticulum which serve as reservoirs for calcium ions. They combine in twos with central T-tubule to form triads.
Sarcoplasmic reticulum (continued)
-Ca2+ pumps embedded in sarcoplasmic reticulum
move Ca2+ into sarcoplasmic reticulum
stored bound to specialized proteins, calmodulin and calsequestrin
-Voltage-gated Ca2+ channels
open to release Ca2+ from sarcoplasmic reticulum into sarcoplasm
causes muscle contraction
Anatomy of Skeletal Muscle:
Microscopic Anatomy: Muscle Fibers and Myofibrils
Myofibrils are long cylindrical structures that extend the length of muscle fiber.
They compose 80% of volume of muscle fiber. Each fiber is made up of hundreds to thousands myofilaments. There are two types of these bundles of protein filaments: thick and thin.
-Thick filaments are assembled from bundles of the protein molecules, myosin. Each myosin protein has two intertwined strands. Each strand has a globular head and elongated tail, the tails point toward the center of the thick filaments and the heads point toward edges of thick filaments. The head has a binding site for actin (thin filaments) and is where ATP attaches and is split.
-Thin filaments are primarily composed of two strands of the protein actin twisted around each other. Many small spherical molecules called globular actin are connected to form a fibrous strand called filamentous actin. Globular actin has a myosin binding site where the myosin head attaches during contraction. Tropomyosin covers the myosin binding sites in a noncontracting muscle. Troponin a globular protein attached to tropomyosin is the binding site for Ca2+. Together they form the troponin-tropomyosin complex.
Anatomy of Skeletal Muscle:
Organization of a Sarcomere
A sarcomere is the smallest contractile unit of a muscle and is defined as the distance between 2 Z- disks. The sarcomere is composed of thick and thin myofilaments composed of contractile protein. Each sarcomere contains 1 A-band and 2 semi I bands. Thick filaments are found at center of sarcomere and thin between thick and attached at Z-disk. I Bands contain thin filaments only. A-bands contain thick with thin in between and the H-zone contains only thick. Thin filaments contain actin, tropomyosin and troponin. Thick contain all of the myosin.
-The M line is a protein meshwork structure at center of H zone which serves as an attachment site for thick filaments.
Overlapping filaments form alternating patterns of light and dark regions. Each thin filament has three thick filaments forming a triangle at its periphery.
Other structural and functional proteins are found in the sarcomeres:
-Connectin is a protein extending from Z discs to M line. It extends through the core of each thick filament and stabilizes the position of thick filaments.
-Nebulin an actin-binding protein possibly helps to organize the sarcomere. part of I band of the sarcomere
-Dystrophin anchors myofibrils adjacent to sarcolemma to sarcolemma proteins. We see abnormal structure or amounts of this protein in muscular dystrophy.
Structures Associated With Energy Production
Muscles have a with high ATP requirement and thus possess abundant mitochondria for aerobic cellular respiration. They use glycogen stores for readily available fuel molecules. Creatinine phosphate (a molecule unique to muscle tissue) provides a means of supplying ATP anaerobically.
-Myoglobin, another molecule unique to muscle tissue, is a reddish globular protein similar to hemoglobin. It binds oxygen when muscle is at rest and releases it during muscular contraction. This provides additional oxygen to enhance aerobic cellular respiration.
Innervation of Skeletal Muscles: Motor Unit
The motor unit is the nerve-muscle functional unit. A motor unit is simply a motor neuron and all of the muscle fibers it supplies. When the nerve is stimulated, all of the muscle fibers linked to it contract. This could be 150 muscle fibers or as few as 4. Large weight bearing muscle have large motor units while muscle with fine control have small ones. The muscle fibers in a single motor unit are not clustered together in a muscle. Therefore, contraction of one motor unit might cause weak contraction of the entire muscle.
The junction between an axon and a muscle fiber is termed a neuromuscular junction. Each axon terminal forms a neuromuscular junction with a muscle fiber. Each muscle fiber connected to motor neuron from brain which stimulates it to contract. The structure of neuromuscular junction involves:
-Synaptic knob which contain synaptic vesicles containing the neurotransmitter acetylcholine. It has Ca2+ pumps embedded in plasma membrane. These
establish a calcium gradient, with more outside the neuron. There are voltage-gated Ca2+ channels in membrane. Ca2+ flows down concentration gradient if opened.
Vesicles normally repelled from membrane of synaptic knob because both normally negatively charged.
-Synaptic cleft is the space between the axon and muscle fiber. Acetylcholinesterase is located here. This enzyme breaks down acetylcholine molecules after their release into synaptic cleft.
-Postsynaptic membrane is the sarcolemma of the muscle fiber. This is called a motor end plate and contains receptors for acetylcholine. Plasma membrane protein channels
-Are opened by binding of Ach allowing Na+ to enter and K+ to exit.
Physiology of Skeletal Muscle Contraction
During muscle contraction:
-Protein filaments within sarcomeres interact
-Tension is exerted on portion of skeleton where muscle attached
-Contracting fiber decreases in length
Neuromuscular Junction: Excitation of a Skeletal Muscle Fiber
First Physiological Event:
-Muscular fiber excitation by motor neuron occurs at the neuromuscular junction. This results in release of ACh and subsequent binding of ACh receptors.
-Release of ACh from synaptic knob. Merging of synaptic vesicles with synaptic knob membrane is triggered by binding of Ca2+. Exocytosis of ACh into synaptic cleft occurs.
-Binding of ACh at motor end plate. ACh diffuses across the synaptic cleft and binds with ACh receptors within motor end plate causing excitation of muscle fiber
Skeletal Muscle Contraction—Sarcolemma, T-Tubules, Sarcoplasmic Reticulum: Excitation-Contraction Coupling
The Second physiological event is Excitation-contraction coupling which links skeletal muscle stimulation to the events of contraction. It consists of three events:
1. Development of end-plate potential at motor end plate. Binding of ACh to ACh receptors on motor end plate causes receptors to open. This allows Na+ to rapidly diffuse into muscle fiber and allows K+ to slowly diffuse out. There is a net gain of positive charge inside fiber which reverses the electrical charge difference at motor end plate. The reverse is termed an end plate potential (EPP)
2. Initiation and propagation of action potential along sarcolemma. Action potential triggered by EPP. First, the inside of sarcolemma becomes relatively positive due to influx of Na+ from voltage-gated channels. This is termed depolarization. Then, the inside of sarcolemma returns to the resting potential due to an outflux of K+ from voltage-gated channels. This is termed repolarization. The action potential is propagated along sarcolemma and T-tubules. Inflow of Na+ at the initial portion of sarcolemma causes adjacent regions to experience electrical changes initiating voltage-gated Na+ channels in this region to open. The action potential is propagated down the sarcolemma and t-tubules. The refractory period is the time between depolarization and repolarization. During this period the muscle is unable to be restimulated.
3. Release of Ca2+ from sarcoplasmic reticulum Opening of voltage-gated Ca2+ channels found in terminal cisternae of sarcoplasmic reticulum is triggered by action potential. Ca2+ diffuses out of cisternae and into sarcoplasm. It now interacts with thick and thin filaments.
Physiology of Skeletal Muscle Contraction—Sarcomere: Crossbridge Cycling
The third physiological event is binding of Ca2+ and crossbridge cycling.
This results in muscle contraction. Calcium binds to subunit of troponin. This causes a conformation change in troponin. The troponin-tropomyosin complex moves exposing the myosin binding sites of actin.
Crossbridge Cycling involves four repeating steps:
1. Crossbridge formation-
myosin heads in the ready position
attach to exposed myosin binding sites on actin
results in formation of a crossbridge between thick and thin filament
2. Power stroke-
swiveling of the myosin head, termed power stroke
pulls thin filaments a small distance past thick filaments
ADP and Pi released
3) Release of myosin head-
binding of ATP to binding site of myosin head
causes release of myosin head from actin
4) Reset myosin head-
ATP split into ADP and Pi by ATPase
enzyme on myosin head
provides energy to "cock" the myosin head
-Crossbridge cycling continues as long as Ca2+ is present to keep the myosin binding sites exposed. This results in sarcomere shortening into a contracted state, disappearance of H zone and narrowing or disappearance of I band. The Z discs closer together and thick and thin filaments are the same length.
Physiology of Skeletal Muscle Contraction: Skeletal Muscle Relaxation
Events in muscle relaxation involve termination of the nerve signal in the motor neuron. This prevents further release of ACH. ACH is removed from the receptor by acetylcholinesterase. No further action potential generated. Already released calcium continuously returned by calcium pumps. Remaining calcium transported back into storage. Troponin returns to its original shape. Tropomyosin now moves over myosin binding sites on actin preventing crossbridge formation.
Skeletal Muscle Metabolism: Supplying Energy for Skeletal Muscle Contraction
There are three ways to generate ATP in a skeletal muscle fiber:
1. Immediate supply via the phosphagen system
2. Short-term supply via anaerobic cellular respiration
Long-term supply via aerobic cellular respiration
The Phosphagen System
-The phosphagen system provides an Immediate supply of ATP. This system uses ATP already present in skeletal tissue providing enough energy for about 5 sec of maximal exertion. Myokinase transfers phosphate from one adenosine diphosphate to another yielding ATP and adenosine monophosphate.
-Creatinine phosphate can supply ATP in skeletal muscle only. Creatinine kinase
transfers Pi from creatine phosphate to ADP yielding creatine and ATP. This provides an additional 10 to 15 seconds of energy. The process is reversed during rest.
Short-term Supply of ATP: Anaerobic Cellular Respiration
A short-term supply of ATP can be provided by anaerobic cellular respiration. This process occurs in cytosol and does not require oxygen. Glucose from glycogen or provided through the blood is converted to two pyruvate molecules releasing 2 ATP per glucose molecule. Pyruvate normally enters mitochondria for aerobic cellular respiration. If there is insufficient oxygen, increasing amounts are converted to lactate Which can be taken up by heart to be used as fuel. It can also be taken up by the liver for gluconeogenesis.
Long-term Supply of ATP: Aerobic Cellular Respiration
-Aerobic cellular respiration occurs within mitochondria. This process in which pyruvate is oxidized to carbon dioxide requires oxygen. Chemical bond energy is transferred to NADH and FADH2. Energy is used to generate ATP by oxidative phosphorylation with
-34 net ATP produced. ATP is also generated from triglycerides with longer fatty acid chains. This is the preferential fuel molecule for most skeletal muscle tissue and oxygen is required.
Energy Supply and Varying Intensity of Exercise
The source of ATP is dependent on the intensity and duration of the exercise.
E.g., in a 50-meter sprint
ATP supplied primarily by phosphagen system
ATP supplied initially by phosphagen system
then primarily by anaerobic cellular respiration
ATP supplied by all three
primarily supplied by aerobic processes after first minute
-Oxygen debt is the amount of additional oxygen that must be inhaled following exercise to restore pre-exercise conditions. The additional oxygen required to replace oxygen on hemoglobin and myoglobin, replenish glycogen, replenish ATP and creatine phosphate in phosphagen system and convert lactic acid back to glucose (in the liver).
Skeletal Muscle Fiber Types: Criteria for Classification of Muscle Fiber Types
Muscle fibers categorized by the type of contraction generated and the primary means used for supplying ATP.
Type of contraction generated:
Contractions differ in power, speed, and duration. Power is related to the diameter of the muscle fiber. Larger muscle fibers produce more powerful contractions. Different types of contraction are generated by different fiber types:
have fast variant of myosin ATPase
initiate contraction more quickly following stimulation
produce contraction of shorter duration
produce a strong contraction
greater power and speed than slow-twitch fibers
have slow variant of myosin ATPase
Means for Supplying ATP
-Oxidative versus Glycolytic Fibers
Oxidative fibers use aerobic cellular respiration and have features that support this such as extensive capillaries, large numbers of mitochondria, and a large supply of myoglobin (impart red appearance). These features them allow to continue contracting for long periods. These fibers are also called fatigue-resistant.
-Glycolytic fibers use anaerobic cellular respiration and have fewer structures needed for aerobic cellular respiration. These fibers have a white appearance due to lack of myoglobin and large glycogen reserves for anaerobic respiration. They tire easily after a short time of sustained activity and are also termed fatigable.
Skeletal Muscle Fiber Types: Classification of Muscle Fiber Types
Three types of skeletal muscle fibers:
1. Slow Oxidative Fibers
-also called type I fibers
-about half diameter of other fibers
-contains slow ATPase
-contractions slower and less powerful
-ATP supplied primarily though aerobic cellular respiration
-can contract long periods of time without fatigue
-appear red due to large amounts of myoglobin
2. Fast Oxidative Fibers
-also called intermediate or type IIa
-least numerous of types
-contain fast ATPase
-produce fast, powerful contraction
-primarily aerobic respiration, but delivery of oxygen lower
-contain myoglobin, but less than slow oxidative
3. Fast glycolytic fibers:
-also called fast anaerobic
fibers or type IIb
-most prevalent of types
-contain fast ATPase
-provide power and speed
-ATP primarily anaerobic
-can contract only for short bursts
-appear white due to lack of myoglobin
See Table 10.1: Structural and Functional Characteristics of Different Types of Skeletal Muscle Fibers
Distribution of Muscle Fiber Types
One sees a mixture of muscle fiber types in skeletal muscle. Most muscles have a mixture of all types. In the eye, we see high percentage of fast glycolytic fibers required for swift, brief contractions. In postural muscles, we see a high percentage of slow oxidative fibers which are needed to contract continually to help maintain posture.
Variation of muscle fiber types in individuals
-Long distance runners
higher proportion of slow-oxidative fibers in legs
higher percentage of fast-glycolytic fibers
-Determined primarily by genes
-Determined partially by training
Measurement of Skeletal Muscle Tension
-Muscle tension is the force generated when a skeletal muscle is stimulated to contract. This can be measured in several types laboratory experiments. One uses a specimen of the gastrocnemius muscle with the sciatic nerve excised from a frog. The muscle is used to produce a myogram or graphic recording of changes in muscle tension. This is often depicted as a muscle twitch. The muscle twitch is divided into three periods:
1. Latent period is the period after the stimulus before contraction begins. During this time there is no change in fiber length. This period represents the time needed to initiate tension in the fiber.
2. Contraction period begins as power strokes pull thin filaments increasing muscle tension. It is shorter duration than the relaxation period.
3.Relaxation period begins with release of crossbridges resulting in a decrease in muscle tension.
Measurement of Skeletal Muscle Tension—Changes in Stimulus Intensity: Motor Unit Recruitment
Let's discuss an experiment to demonstrate motor unit recruitment. We will stimulate a gastrocnemius muscle repeatedly with each stimulation at greater voltage. With each increase in voltage a greater number of motor units contract. Tension increases until all motor units stimulated (point of maximum contraction). This increase in tension with increased stimulus is termed recruitment.
-Recruitment helps explain how muscles can exert varying levels of force in spite of the all-or-none law. (muscle fiber contracts maximally or not at all). The difference in force and precision can be varied by changing the number of motor units. If we reduce the number of motor unit activated, less force is exerted. If a greater number of motor units is activated, more force is exerted.
Measurement of Muscle Tension—Changes in Stimulus Frequency
At a frequency between 20 and 40 per second, relaxation is not occurring this summation of contractile forces is called wave summation or temporal summation. With further increases there is less time for relaxation. Tension increases resulting in incomplete tetany.
-At 40 to 50 stimulations per second, contractions fuse to form continuous contraction called tetany. If stimulation continues, muscle reaches fatigue. We see a decrease in muscle tension from repetitive stimulation
Muscle Tone refers to a constant state of slight contraction which muscles maintain to be ready to respond to stimulus. If the muscle is stimulated, it initiates contraction.
-Muscle tension is the force a muscle exerts on an object during contraction. Load refers to the weight the object exerts on a muscle. We usually describe two types of contractions:
- In isotonic contractions, the muscle shortens and moves the load. In isometric contractions, there is no muscle shortening. This occurs when muscle attempts to move too heavy a load. If you tried to lift a building, the muscles would contract but would not cause movement. (See next 2 slides)
Isotonic Contraction - The muscle contracts and shortens.
Isometric contraction - The muscle contracts but does not shorten
because the load is too heavy.
The influence of muscle length on tension is termed the length-tension relationship. Tension is dependent on length at stimulation. A graphical presentation of this is seen in the length-tension curve. A fiber at resting length generates maximum contractile force because there is optimal overlap of thick and thin filaments.
-If the fiber is already contracted, one sees a weaker contraction when stimulated because the sliding filaments are limited in movement. If the fiber is already contracted or overly stretched, one sees a weaker contraction when stimulated because there is minimal thick and thin filament overlap for crossbridges.
(See next slide)
Muscle fatigue results when a muscle runs out of ATP. Lactic acid produced by anaerobic metabolism lowers the pH of the muscle. Ionic imbalances occur. This results in muscle aches and fatigue. In muscle fatigue, we see state of continuous contraction because no ATP is available to bind myosin. To return muscle to the resting state, ATP must be produced aerobically. Vigorous exercise may cause oxygen debt in which an extra amount of oxygen must be taken in to restore a muscle to normal. In order for this to occur:
1. ATP and creatine phosphate level must return to normal.
2. Oxygen reserves must be replenished.
3. Lactic acid must be converted to pyruvate.
4. Glycogen stores must be replenished.
Effects of Exercise
Changes in muscle from a sustained exercise program can result in hypertrophy or increase in skeletal muscle size. This results from repetitive stimulation of fibers. One sees more mitochondria, larger glycogen reserves, increased ability to produce ATP and more myofibrils that contain larger number of myofilaments. Hyperplasia may is an increase in the number of muscle fibers. This may occur in a limited way with exercise
-Effects of lack of exercise include atrophy which causes decrease in muscle tone and power. It is initially reversible, but dead fibers are not replaced. With extreme atrophy, there is permanent loss of muscle function muscle because muscle fibers are replaced with connective tissue.
Cardiac Muscle Tissue
Components of cardiac muscle tissue include cardiac muscle cells which are arranged in thick bundles within heart wall. These branching cells are shorter and thicker than skeletal muscle fibers. They have one or two nuclei and are striated and contain sarcomeres. Cardiac muscle cells have large numbers of mitochondria and use aerobic respiration.
Intercalated discs are junctions joining cardiac cells. They are composed of desmosomes and gap junctions.
The autorhythmic pacemaker is responsible for repetitious, rhythmic heartbeat. It stimulates cardiac muscle cells. The rate and force of heartbeat controlled by autonomic nervous system.
-Components of cardiac muscle tissue (continued)
junctions joining cardiac cells
composed of desmosomes and gap junctions
responsible for repetitious, rhythmic heartbeat
stimulates cardiac muscle cells
rate and force of heartbeat controlled by autonomic nervous system
Smooth muscle is composed of spindle-shaped fibers with a diameter of 2-10 m and lengths of several hundred m. It lacks the coarse connective tissue sheaths of skeletal muscle, but has a fine endomysium. It is organized into two layers (longitudinal and circular) of closely opposed fibers. Smooth muscle is found in the walls of hollow organs (except the heart) and has essentially the same contractile mechanisms as skeletal muscle.
Microscopic Anatomy of Smooth Muscle
In smooth muscle, the sarcoplasmic reticulum is less developed than in skeletal muscle and lacks a specific pattern. T tubules are absent. Plasma membranes have pouchlike infoldings called caveoli.
-Ca2+ is held in the extracellular space near the caveoli, allowing for rapid mobilization when channels are opened. There are no visible striations and no sarcomeres, but thin and thick filaments are present.
-The proportion and organization of myofilaments in smooth muscle is different from skeletal muscle. The ratio of thick to thin filaments is much lower than in skeletal muscle. Thick filaments have heads along their entire length, and there is no troponin complex. Thick and thin filaments are arranged diagonally, causing smooth muscle to contract in a corkscrew manner. Noncontractile intermediate filament bundles attach to dense bodies (analogous to Z discs) at regular intervals.
-Whole sheets of smooth muscle exhibit slow, synchronized contraction. They contract in unison, in part due to electrical coupling with gap junctions. Action potentials are transmitted from cell to cell. Some smooth muscle cells act as pacemakers and set the contractile pace for whole sheets of muscle. These are self-excitatory and depolarize without outside stimulation.
Contraction of Smooth Muscle - note intermediate
filaments and dense bodies.
Smooth Muscle Tissue: Microscopic Anatomy
If we compare myofilaments of smooth muscle and skeletal muscle, we see that smooth muscle cells have myosin heads along entire length rather than at the ends only, like skeletal muscle. Smooth muscle can form additional crossbridges and can "latch on" to actin and remain attached without ATP. This is termed the latchbridge mechanism.
-Have actin and tropomyosin, but no troponin instead we see calmodulin which is a protein that binds Ca2+ to form a complex. Myosin light-chain kinase (MLCK) is an enzyme activated by Ca2+-calmodulin complex. It phosphorylates the smooth muscle myosin head causing activation of ATPase activity.
-Myosin light-chain phosphatase is an enzyme that dephosphorylates the myosin head. It inactivates ATPase and is required for relaxation.
Characteristics of Smooth Muscle Contraction
Let's examine initiation and duration of contraction. In smooth muscle contraction, we see a long latent period due to the need to phosphorylate the myosin head and variations in ATPase activity. This long duration is also due to the slowness of calcium pumps, the need for dephosphorylation of the myosin head and the latchbridge mechanism.
-Smooth muscle contraction generally doesn't require rapid onset. However, it requires ability to remain in contracted state such as in maintaining continuous tone in visceral walls.
-Smooth muscle is fatigue-resistant. Energy requirements are low compared to skeletal muscle. ATP is supplied through aerobic cellular respiration. The cell can maintain contraction without ATP through the latchbridge mechanism. Smooth muscle can contract for extended periods without fatigue.
-Smooth muscle exhibits a broader length-tension curve than skeletal muscle. It lacks limitations due to Z discs. Myosin heads present in center of thick filaments. It can contract forcefully when compressed to half resting length, when stretched to double resting length or at resting length. For example, stretching of urinary bladder wall allows emptying bladder regardless of amount of urine.
Smooth Muscle Tissue:
Control via the Autonomic Nervous System
One cannot voluntarily control the contraction of smooth muscle.
The response of smooth muscles is dependent on neurotransmitter released. For example, smooth muscle within the bronchioles contracts in response to ACh but relaxes in response to norepinephrine.
Smooth Muscle Tissue:
Controlling Smooth Muscle
The response to stretch is not continuous if a stretch is prolonged. In the stress-relaxation response, when smooth muscle is "stressed" by being stretched, it responds at first by contracting and after a time, relaxes.
Smooth muscle may be stimulated to contract by:
-lower oxygen concentration
-increased carbon dioxide levels
Smooth Muscle Tissue:
Multiunit smooth muscle cells receives stimulation to contract individually. They are
found in the iris and ciliary muscles of the eye, arrector pili muscles in skin, larger air passageways in respiratory system and the walls of larger arteries. The cells arranged into motor units. The degree of contraction is dependent on number of motor units activated. Greater tension is produced as more units stimulated.
Smooth Muscle Tissue:
Single-unit smooth muscle are stimulated to contract in unison. Most in this category form two or three sheets linked by gap junctions. Single-unit smooth muscle is found in the walls of digestive, urinary, and reproductive tracts, portions of the respiratory tract and most blood vessels.
-Stimulation of single-unit smooth muscle occurs through swellings of autonomic neurons that pass close to smooth muscle cells. These swellings are called varicosities. Synaptic vessels contain one type of neurotransmitter. Receptors are scattered across the sarcolemma. Numerous smooth muscle cells stimulated simultaneously and stimulation is spread from cell to cell via gap junctions. This results in synchronous contraction of cells as a unit.
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