Muscles 2

3 Types of muscle Tissue Skeletal muscle tissue striations; long, cylindrical cells called muscle fibers; multinucleate cells Cardiac muscle tissue striations; branching cells form junctions called intercalated discs; uninucleate cells Smooth muscle tissue No striations; spindle-shaped cells; uninucleate cells Each muscle tissue type with connective tissue wrappings, blood vessels, nerve fibers = muscle as an organ

Figure 9.2: Connective tissue sheaths of skeletal muscle, an organ (b) (a) Bone Perimysium Endomysium Blood vessel Muscle fiber (cell) Fascicle (wrapped by perimysium) Endomysium (between fibers) Epimysium Tendon Epimysium Muscle fiber in middle of a fascicle Perimysium Blood vessel Endomysium

Skeletal Muscle (organ) Each muscle fiber ( = skeletal muscle cell) is wrapped in a delicate CT membrane called ENDOMYSIUM Fascicle – consists of a group of endomysium-covered muscle fibers wrapped in a coarse CT membrane called PERIMYSIUM Skeletal muscle – consists of a group of fascicles wrapped in a tough CT membrane called EPIMYSIUM

Patterns of Arrangement of Fascicles in skeletal Muscles Muscle fibers in a skeletal muscle form bundles called fascicles . The muscle fibers in a single fascicle are parallel, but the organization of fascicles in the skeletal muscle can vary, as can the relationship between the fascicles and the associated tendon. The different patterns of fascicle organization form parallel muscles, convergent muscles, pennate muscles, circular muscles Parallel Muscles In a parallel muscle, the fascicles are parallel to the long axis of the muscle. Most of the skeletal muscles in the body are parallel muscles. The biceps brachii muscle is a parallel muscle with a central body. When a parallel muscle contracts, it gets shorter and larger in diameter. A skeletal muscle cell can contract until it has shortened by roughly 30-50 percent. Because the fibers in a parallel muscle are parallel to the long axis of the muscle, when the fibers contract together, the entire muscle shortens by the same amount. If the muscle is 10 cm long, the end of the tendon will move 3-5 cm when the muscle contracts. Source: IFBB.com

Pattern of Arrangement, continued Convergent Muscles In a convergent muscle, the muscle fibers are spread over a broad area, but all the fibers converge at one common attachment site. The muscle fibers typically spread out, like a fan or a broad triangle, with a tendon at the apex. The pectoralis major muscle is a good example of it. A convergent muscle has versatility, because the stimulation of only one portion of the muscle can change the direction of pull. Then, convergent muscle fibers pull in different directions rather than all pulling in one same direction. Pennate Muscles In a pennate muscle, the fascicles form a common angle with the tendon. Because the muscle cells pull at an angle, contracting pennate muscles do not move their tendons as far as parallel muscles do. But a pennate muscle contains more muscle fibers--and, as a result, produces more tension--than does a parallel muscle of the same size. If all the muscle fibers are on the same side of the tendon, the pennate muscle is unipennate. More commonly, a pennate muscle has fibers on both sides of the tendon. Such a muscle is called bipennate. The rectus femoris muscle, for example, is bipennate If the tendon branches within a pennate muscle, the muscle is said to be multipennate. The triangular deltoid muscle of the shoulder is multipennate. Circular Muscles also referred to as sphincters the fascicles are arranged in concentric rings; they surround external body openings Source: IFBB.com

Patterns of Arrangements of Fascicles in skeletal Muscles Circular arrangement – the orbicularis oris and orbicularis oculi - muscles around the mouth and eyes, respectively

Attachment sites for each skeletal muscle Skeletal muscles span joints and they have at least 2 attachment sites – the ORIGIN and the INSERTION The bone that moves (the movable bone) when the skeletal muscle contracts is known as the INSERTION; and the bone that does not move ( the immovable bone) is the ORIGIN . Hence, when the skeletal muscle contracts, the insertion moves toward the origin. Example: Brachioradialis Origin (O) - lateral supracondylar ridge at distal end of the humerus Insertion(I) – base of styloid process of radius Refer to tables in the lab manual and in the textbook ( pages 329-379)

Direct Skeletal Muscle Attachments DIRECT ATTACMENTS - the epimysium of the skeletal Muscle is fused directly to the periosteum

Naming of Skeletal Muscles 1. Location of the muscle – name indicates region of the body its found in or bone its attached to. Example: Frontalis 2. Shape of the muscle. Example Deltoid – roughly triangular 3. Relative size – maximus (largest) – Gluteus maximus; longus (long)- palmaris longus 4. Direction of muscle fibers/fascicles in reference to the midline of the body or the longitudinal axis of a limb bone. Examples: rectus ( straight)- rectus abdominis; transversus ( right angle) – transversus abdominis 5. Number of origins/heads– biceps ( 2 origins) – biceps brachii; triceps ( 3 origins/heads)-triceps brachii; quadrceps (4 heads) – quadriceps femoris 6. Location of the attachments – their points of origin and insertion and the first part of the name indicated the origin. Example: sternocleidomastoid – origin is the sternum and the clavicle is the insertion point 7. Action – the name indicates the movement the muscle produces. Example: adductor – adductor longus for adduction of the thigh Some muscles are named using more than one of the above criteria

Figure 9.2a: Connective tissue sheaths of skeletal muscle, p. 283. (a) Bone Perimysium Endomysium Blood vessel Muscle fiber (cell) Fascicle (wrapped by perimysium) Endomysium (between fibers) Epimysium Tendon INDIRECT ATTACHMENT- the Connective tissue wrappings of the skeletal muscle extends as a tendon or an aponeurosis to anchor the muscle to bone, cartilage or fascia

Figure 9.3a-c : Microscopic anatomy of a skeletal muscle fiber , p. 285. Nuclei Fiber Nucleus Light I band Dark A band Sarcolemma Mitochondrion H zone (b) Myofibril (a) (c) Thin (actin) filament Thick (myosin) filament Z disc Z disc

Microscopic Anatomy of Skeletal Muscle Fiber Each muscle fiber contains: Myofibrils – rod-like structures that run the entire length of the muscle fiber; 80% of the volume of the muscle fiber is occupied by the myofibrils Myoglobin – a red pigment that binds and stores oxygen Inclusions - glycosomes contain glycogen Mitochondria – for aerobic respiration to produce energy Sarcoplasm – cytoplasm of the muscle fiber Sarcoplasmic reticulum(SR ) – specialized smooth endoplasmic reticulum that stores/releases calcium into the sarcoplasm; the expanded ends of SR are called TERMINAL CISTERNAE Sarcolemma – plasma membrane of muscle fiber Transverse tubules(T-tubules) – involutions of the sarcolemma into the sarcoplasm

Triad Composed of a transverse tubule in between 2 terminal cisternae: Terminal cisterna- Ttubule -Terminal cisterna Function: for the release of calcium ions into the sarcoplasm when the sarcolemma depolarizes

Figure 9.5: Relationship of the sarcoplasmic reticulum and T tubules to myofibrils of skeletal muscle, p. 288. Myofibril Myofibrils Triad Tubules of sarcoplasmic reticulum Sarcolemma Sarcolemma Mitochondrion I band I band A band H zone Z disc Z disc Part of a skeletal muscle fiber (cell) T tubule Terminal cisterna of the sarcoplasmic reticulum M line

The Myofilaments Each myofibril contains smaller structures called MYOFILAMENTS – 2 main types: MYOSIN and ACTIN Thick filaments – 16nm in diameter; composed of the protein MYOSIN – 300 myosin molecules form a dark band called the A band Each myosin consists of a tail and 2 globular heads . The myosin globular heads have binding sites for actin, binding sites for ATP and contains the enzyme ATPase Thin filaments – 8nm in diameter; anchored by the Z lines; Thin filaments contain 3 different proteins : Actin – contains the binding sites for the myosin heads Tropomyosin – a rod-shaped regulatory protein that spirals around the actin and blocks myosin binding sites on actin in a relaxed skeletal muscle Troponin – a three-polypeptide complex namely TnC – binds calcium ions TnT – binds to tropomyosin TnI - inhibitory subunit that binds to actin Elastic filaments are composed of the protein tictin - they extend from Z lines through the A bands to attach to the M line thus, anchoring the A bands in place within the sarcomere 300 myosin

Figure 9.4: Composition of thick and thin filaments, p. 286. (b) (c) (d) (e) (a) Heads Flexible hinge region Tail Myosin head Troponin complex Tropomyosin Actin Thin filament Thick filament Thin filament (actin) Thick filament (myosin) Myosin heads Myosin molecule Portion of a thick filament Portion of a thin filament Longitudinal section of filaments within one sarcomere of a myofibril Transmission electron micrograph of part of a sarcomere

Figure 9.11: Role of ionic calcium in the contraction mechanism, p. 294. (a) (b) (c) (d) Actin Actin Tnl TnT Tropomyosin Myosin binding sites Troponin complex TnC Myosin head Myosin binding site Additional calcium ions bind Additional calcium ions bind to TnC Myosin head Actin Overview Troponin Tropomyosin Myosin head Plane of (a) Plane of (d) + Ca 2+

Sarcomeres Structural units of skeletal muscle = each skeletal muscle is composed of repeated units arranged end to end called sarcomeres The distance between 2 successive Z lines = a sarcomere Components of a Sarcomere: A band = thick filament M line = line that bisects and anchors the A bands Thin filaments alternating with “A” bands; the alternating pattern of the thick and thin filaments results in the characteristic striated appearance of skeletal muscle Z lines (= Z discs) – anchor the thin filaments H zone – middle region of the A band not overlapping with the thin filaments I bands – regions of the thin filaments not overlapping with the A band Z M Z

Figure 9.3c-e: Microscopic anatomy of a skeletal muscle fiber, p. 285. I band Z disc Z disc I band A band H zone (c) (d) (e) Thin (actin) filament Thick (myosin) filament Thin (actin) filament Elastic (titin) filaments Z disc Z disc M line M line Sarcomere Thick (myosin) filament I band thin filaments only H zone thick filaments only M line thick filaments linked by accessory proteins Outer edge of A band thick and thin filaments overlap

The sliding Filament Mechanism of Muscle Contraction States that the sliding of the thin filaments past the A bands results in muscle contraction According to this mechanism when a muscle contracts there is more overlap between the thin filaments and the A bands: H zone decreases or disappears I bands decrease or disappear Sarcomere length shortens = Skeletal muscle shortens ( contracts) However, the lengths of the A bands and the thin filaments remain the same – they do not shorten 1 2 3

Figure 9.6: Sliding filament model of contraction , p. 289. A Z Z I I A Z Z A Z Z H 1 2 3

Excitation-Contraction Coupling What stimulates skeletal muscles to contract? When they are stimulated by activated MOTOR NEURONS Motor neurons conduct impulses to skeletal muscles. Motor neuron makes contact with skeletal muscle fibers via its AXONAL TERMINALS Each axonal terminal innervates one muscle fiber in the skeletal muscle to form the NEUROMUSCULAR JUNCTION The motor neuron and all the skeletal muscle fibers it innervates via its axonal terminals is called a MOTOR UNIT

Figure 9.7a: The neuromuscular junction, p. 290. (a) Action potential Axon terminal at neuromuscular junction Sarcolemma of the muscle fiber Nucleus Myelinated axon of motor neuron

Figure 9.7: The neuromuscular junction, p. 290. (a) (b) (c) Axon terminal of a motor neuron Junctional folds of the sarcolemma at motor end plate Part of a myofibril Mitochondrion Synaptic cleft T tubule Binding of Ach to receptor opens Na + /K + channel Acetylcholinesterase Synaptic cleft ACh molecules Fusing synaptic vesicle Synaptic vesicle Acetic acid Choline Axon terminal Action potential Axon terminal at neuromuscular junction Sarcolemma of the muscle fiber Nucleus Na + K + Myelinated axon of motor neuron Ca 2+

Figure 9.7b : The neuromuscular junction, p. 290. (b) Axon terminal of a motor neuron Junctional folds of the sarcolemma at motor end plate Part of a myofibril Mitochondrion Synaptic cleft T tubule Synaptic vesicle Ca 2+

The Neuromuscular Junction The junction between the axonal terminal of a motor neuron and a skeletal muscle fiber separated by a small space called the neuromuscular cleft (synaptic cleft) Each muscle fiber has only one neuromuscular junction The highly folded region of the sarcolemma of the muscle fiber at the neuromuscular junction is called the Motor End Plate – express acetylcholine receptors on the surface A motor neuron and all the muscle fibers it innervates via its axonal terminals is called a motor unit ; motor units come in different sizes – small, medium, large

Figure 9.13: A motor unit consists of a motor neuron and all the muscle fibers it innervates , p. 296. (b) (a) Spinal cord Motor neuron cell body Muscle Branching axon to motor unit Muscle fibers Nerve Motor unit 1 Motor unit 2 Muscle fibers Motor neuron axon Axon terminals at neuromuscular junctions

Sequence of events in Excitation-Contraction Coupling Motor neuron is activated Axon of motor neuron generates and transmits action potential to the axonal terminals Results in the release of the neurotransmitter, ACETYLCHOLINE from vesicles in the axonal terminals into the neuromuscular cleft Acetylcholine binds to acetylcholine receptors on the motor end plate to cause depolarization which leads to the generation of action potential at the motor end plate The action potential spreads across the entire sarcolemma and into the T-tubules of the triads Results in the release of calcium ions from the terminal cisternae of the triads into the sarcoplasm Calcium ions bind to TnC which results in a conformational change and the removal of tropomyosin from blocking the myosin-binding sites on actin

Figure 9.10: Excitation-contraction coupling, p. 292. ADP P i Net entry of Na + initiates an action potential which is propagated along the sarcolemma and down the T tubules. T tubule Sarcolemma SR tubules (cut) Synaptic cleft Synaptic vesicle Axon terminal ACh ACh ACh Neurotransmitter released diffuses across the synaptic cleft and attaches to ACh Action potential in T tubule activates voltage-sensitive receptors, which in turn trigger Ca 2+ release from terminal cisternae of SR into cytosol. Calcium ions bind to troponin; troponin changes shape, removing the blocking action of tropomyosin; actin active sites exposed. Contraction; myosin heads alternately attach to actin and detach, pulling the actin filaments toward the center of the sarcomere; release of energy by ATP hydrolysis powers the cycling process. Removal of Ca 2+ by active transport into the SR after the action potential ends. SR Tropomyosin blockage restored, blocking myosin binding sites onactin; contraction ends and muscle fiber relaxes. Ca 2+ Ca 2+ Ca 2+ Ca 2+ Ca 2+ Ca 2+ Ca 2+ Ca 2+ Ca 2+ Ca 2+ 1 2 6 5 4 3

Sequence of events –continued With the tropomyosin blockade ended, activated myosin heads = cross bridges bind to the accessible myosin-binding sites on actin Myosin heads are activated when the myosin heads are attached by ADP +Pi (resulting from ATPase hydrolysis of ATP into ADP + Pi) When the ADP and Pi dissociate from the cross bridges, the attached cross bridges change their orientation termed the POWERSTROKE from a right angle to a bent position pulling the attached thin filament inward into the H zone toward the M line = sliding of thin filaments which results on skeletal muscle contraction Cross bridge detachment - NEW ATP molecules bind to the attach myosin heads to cause them to detach . Lack of new ATP results in skeletal muscle contracture termed RIGOR MORTIS - occurs when an individual dies and ATP synthesis ceases - actin and myosin are irreversibly cross linked and skeletal muscles remain contracted Muscle fatigue is a physiological inability of a stimulated skeletal muscle to contract due to ATP DEFICIT – rate of ATP production lags behind ATP demand.

Figure 9.12: The cross bridge cycle, p. 295. ATP ADP ADP ATP hydrolysis ADP ATP P i P i Myosin head (high-energy configuration) Myosin head attaches to the actin myofilament, forming a cross bridge. Thin filament As ATP is split into ADP and P i , the myosin head is energized (cocked into the high-energy conformation). Inorganic phosphate (P i ) generated in theprevious contraction cycle is released, initiating the power (working) stroke. The myosin head pivots and bends as it pulls on the actin filament, sliding it toward the M line. Then ADP is released. Myosin head (low-energy configuration) Thick filament A s new ATP attaches to the myosin head, the link between Myosin and actin weakens, and the cross bridge detaches. 1 2 3 4

Factors that affect the strength/force of skeletal muscle contraction Size of motor units activated – larger motor units generate more force than smaller motor units Number of motor units activated – force increases as the number of motor units activated increases Recruitment = smaller motor units are activated first followed by larger motor units Frequency of skeletal muscle activation – force increases as the rate of stimulation by motor neurons increases The length of the sarcomeres prior to contraction – sarcomeres at the optimum length generate the maximum force; sarcomere length below the optimum length ( shortened sarcomeres) results in decreased force; sarcomere length greater than the optimum length ( stretched sarcomeres) results in decreased force of contraction

Figure 9.21: Factors influencing force, velocity, and duration of skeletal muscle contraction, p. 304. (a) (b) (c) Increased contractile force Large number of muscle fibers activated Large muscle fibers Asynchronous tetanic contractions Muscle and sarcomere length slightly over 100% of resting length Predominance of fast glycolytic (fatigable) fibers Predominance of slow oxidative (fatigue-resistant) fibers Small load Increased contractile velocity Increased contractile duration

2 Main Categories of Skeletal Muscle Contraction Isometric Contraction : force(tension) generated by the muscle is increasing at a constant muscle length ( “isometric” = same length); occurs when the weight exceeds the force generated by the muscle Isotonic Contraction : muscle shortens at a relatively constant force ( isotonic = same force); force generated by skeletal muscle exceeds the weight so the skeletal muscle contracts and work is done, such as lifting the weight.

Figure 9.20 : Methods of regenerating ATP during muscle activity , p. 303. Creatine Energy source: CP Oxygen use: None Products: 1 ATP per CP, creatine Duration of energy provision: 15s (a) Direct phosphorylation [coupled reaction of creatine phosphate (CP) and ADP] (b) Anaerobic mechanism (glycolysis and lactic acid formation) (c) Aerobic mechanism (aerobic cellular respiration) Energy source: glucose Oxygen use: None Products: 2 ATP per glucose, lactic acid Duration of energy provision: 30–60 s. Energy source: glucose; pyruvic acid; free fatty acids from adipose tissue; amino acids from protein catabolism Oxygen use: Required Products: 38 ATP per glucose, CO 2 , H 2 O Duration of energy provision: Hours O 2 O 2 ATP ATP net gain Glucose (from glycogen breakdown or delivered from blood) Pyruvic acid Glycolysis in cytosol Glucose (from glycogen breakdown or delivered from blood) Pyruvic acid 2 ATP net gain per glucose 38 Lactic acid Released to blood O 2 H 2 O O 2 Fatty acids Amino acids CP ADP Aerobic respiration in mitochondria CO 2

Sources of ATP to support skeletal muscle contraction Stored ATP – used first Creatine Phosphate (CP) – ATP produced from direct phosporylation of ADP by CP using the enzyme Creatine Phosphatase: CP + ADP------------  ATP + creatine Aerobic catabolism of glucose – produces the most ATP Anaerobic catabolism of glucose – pyruvic acid is converted into LACTIC ACID which reduces blood pH and contributes to muscle fatigue How is the ATP generated used in skeletal muscle Contraction 1. ATP is hydrolyzed by ATPase to produce ADP and Pi to activate the myosin heads 2. ATP is required for crossbridge detachment 3. ATP is required for the sequestration of calcium ions back into the SR for storage (active transport)

3 Skeletal Muscle Fiber Types 3 major types of skeletal Muscle Fiber types based on: Speed of Contraction – 2 types: slow fibers and fast fibers due to the speed at which Myosin ATPase hydrolyzes ATP Major Pathway for ATP production - 2 ways: aerobic respiration = oxidative fibers types anaerobic respiration using more glycogen = Glycolytic fiber types. Based on the above criteria there are 3 major skeletal muscle types: i) Slow Oxidative Fibers ii) Fast Oxidative Fibers iii) Fast Glycolytic fibers

Structural and Functional Characteristics of the 3 Types of Skeletal Muscle Fibers Slow Oxidative Fast Oxidative Fast Glycolytic Characteristics Speed of contraction can figure all this out based on the name of the process Activity of Myosin ATPase again, same as above Pathway for ATP synthesis oxidative = aerobic glycolytic = anaerobic Myoglobin content NEED OXYGEN SO oxidative = high glycolytic = low Color red red-pink white Number of Mitochondria need more in oxidative so oxidative = high and low in glycolytic Number of Capillaries how do you get O2 for ATP? CAPILLARIES…. So oxidative = many instead of few Fiber Diameter smaller fibers diffuse O2 better, so low, intermediate, LARGE Recruitment order (small to large) 1 st 2 nd 3 rd Glycogen content low low high Rate of fatigue slow intermediate high because of lactic acid production Activities suited for endurance intermediate activities (sprinting) burst of energy activity Rate of fatigue

Effect of Exercise on Skeletal Muscles Active muscles may increase in size or strength based on which of the 2 major types of exercise used: Aerobic or endurance exercise ( jogging,biking,swimming) changes in the skeletal muscles: increase in capillaries increase in mitochondria increase in myoglobin content Overall, endurance improved = increase in stamina Resistance exercise ( weight lifting, isometric exercises where muscle are pitted against immovable objects) changes in the skeletal muscles: Increase in the size of skeletal muscles = hypertrophy of skeletal muscles occur due to increase in the number of myofibril within each muscle fiber Increase in glycogen content Overall, bulky muscles generate more force = increase in muscle strength

Figure 9.25: Innervation of smooth muscle , p. 311. Smooth muscle cell Mucosa Varicosities Autonomic nerve fiber Varicosity Synaptic vesicles Mitochondrion Submucosa Serosa Muscularis externa

Smooth Muscle Smooth muscle, the organ, consists of smooth muscle tissue surrounded by endomysium, infiltrated with blood vessels and autonomic nerve fibers. Most smooth muscles form sheets in the walls of hollow organs in the tracts of the body. Smooth muscles contract to move substances down the body’s tracts Structural differences between Smooth muscle and Skeletal muscle i) Smooth muscle lacks striations because the thick filaments and thin filaments are not arranged in alternating pattern but rather, diagonally ii) Smooth muscle lacks sarcomeres iii) Thin filaments in smooth muscle lack troponin; calcium binds to a regulatory protein called CALMODULIN iv) Smooth muscle cells lack Z lines v) Smooth muscle cells contain DENSE BODIES that anchor the thin filaments vi) Smooth muscle cells contain INTERMEDIATE FILAMENTS that resist tension vii) Sarcolemma of smooth muscle cells lack T-tubules rather, the sarcolemma has shallow cavities called CAVEOLAE (caveoli) that contain extracellular fluid rich in calcium

Excitation-Contraction Coupling in Smooth Muscle Smooth muscle can be excited to contract by : Pacemaker cells are intrinsic to the smooth muscle; they spontaneously depolarize generating action potentials to stimulate smooth muscle contraction Chemicals such as hormones stimulate smooth muscle to contract or relax Autonomic nerve fibers; the bulbous swellings of the autonomic fibers called VARICOSITIES form junctions with smooth muscle called DIFFUSE JUNCTIONS. When the autonomic nerve fibers are activated , the neurotransmitter released may stimulate the smooth muscle to contract or to relax

Figure 9.27: Sequence of events in excitation-contraction coupling of smooth muscle, p. 313. ATP P i P i P i P i ADP Calcium ions (Ca 2+ ) enter the cytosol from the ECF or from the scant SR. Ca 2+ binds to and activates calmodulin. Ca 2+ Ca 2+ Ca 2+ Sarcoplasmic reticulum Plasma membrane Activated calmodulin activates the myosin light chain kinase enzymes. Inactive calmodulin Activated calmodulin Inactive kinase Activated kinase Extracellular fluid Cytoplasm The activated kinase enzyme catalyzes transfer of phosphate to myosin heads, activating the myosin head ATPases. Phosphorylated myosin heads form cross bridges with actin of the thin filaments and shortening occurs. Inactive myosin molecule Activated (phosphorylated) myosin molecule Thin myofilament Thick filament Cross bridge activity ends when phosphate is removed from the myosin heads by phosphorylase enzymes and intracellular Ca 2+ levels fall. 1 2 3 4 5 6

Sequence of events in the mechanism of Smooth Muscle Contraction Pacemaker activity, a chemical or autonomic nerve fiber activation leads to increase in intracellular calcium – calcium ions enter the smooth muscle cells from the extracellular fluid in the caveolae and also by release from poorly-developed sarcoplasmic reticulum Calcium binds to Calmodulin Calcium-Calmodulin complex activates myosin light chain kinase( MLCK ) Activated MLCK phosphorylates/activates the myosin heads to form cross bridges - attach to the always accessible myosin binding sites on actin (although tropomyosin is present in thin filaments, it does not block myosin binding sites on actin) Upon attachment of cross bridges, sliding of thin filaments occurs = shortening of smooth muscle cells = smooth muscle contraction Cross bridge detachment occurs when the enzyme, phosphorylase , removes the phosphate from the myosin heads

2 Types of Smooth Muscle Based on smooth muscle fiber arrangement, innervation and responsiveness to stimuli i) Single-Unit Smooth Muscle: Composed of circular and longitudinal smooth muscle sheets Innervated by autonomic nerve fibers Electrically-coupled by gap junctions hence, single-unit smooth muscle contracts as a unit Stimulated to contract by chemicals Exhibit pacemaker activity ii) Multiunit Smooth Muscle: Composed of individual smooth muscle fibers Lacks gap junctions – smooth muscle cells contract independently of each other Innervated by the autonomic nerve fibers Stimulated to contract by chemicals No pacemaker activity

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