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Muscles &Muscle Tissue Chapter 10

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Functions of Skeletal Muscles 1.Produce skeletal movement 2.Maintain body position 3.Support soft tissues 4.Guard body openings 5.Maintain body temperature

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Functional Characteristics of Muscle Excitability (irritability) Can receive and respond to stimuli. Stimuli can include nerve impulses, stretch, hormones or changes in the chemical environment. Contractility the ability to shorten with increasing tension. Extensibility the ability to stretch. Elasticity the ability to snap back (recoil) to their resting length after being stretched.

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Three types of muscle Skeletal Smooth Cardiac

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Characteristics of Skeletal Muscle Striated Multinucleate (it is a syncytium) Voluntary Parallel fibers

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Organization of Connective Tissues Figure 101

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Formation of Skeletal Muscle Fibers Skeletal muscle cells are called fibers

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Organization of Skeletal Muscle Fibers

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Anatomy of a myofibril

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A Triad Is formed by 1 T tubule and 2 terminal cisternae

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Sarcomeres Figure 104

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Muscle Striations A striped or striated pattern within myofibrils: alternating dark, thick filaments (A bands) and light, thin filaments (I bands)

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M Lines and Z Lines M line: the center of the A band at midline of sarcomere Z lines: the centers of the I bands at 2 ends of sarcomere

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Zone of Overlap The densest, darkest area on a light micrograph Where thick and thin filaments overlap

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The H Zone The area around the M line Has thick filaments but no thin filaments

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Titin Are strands of protein Reach from tips of thick filaments to the Z line Stabilize the filaments

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Sarcomere Structure

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Summary of skeletal muscle anatomy: muscles are made of fascicles

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Fascicles are made of fibers, fibers are made of myofibrils

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Figure 106 (1 of 5) Level 1: Skeletal Muscle

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Level 2: Muscle Fascicle Figure 106 (2 of 5)

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Level 3: Muscle Fiber Figure 106 (3 of 5)

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Level 4: Myofibril Figure 106 (4 of 5)

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Level 5: Sarcomere Figure 106 (5 of 5)

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Fibrils are divided into sarcomeres, sarcomeres are made of myofilaments

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Myofilaments are made of protein molecules

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A Thin Filament

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4 Thin Filament Proteins 1.F actin: is 2 twisted rows of globular G actin the active sites on G actin strands bind to myosin

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4 Thin Filament Proteins 2.Nebulin: holds F actin strands together

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4 Thin Filament Proteins 3.Tropomyosin: is a double strand prevents actinmyosin interaction

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4 Thin Filament Proteins 4.Troponin: a globular protein binds tropomyosin to G actin controlled by Ca 2+

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Troponin and Tropomyosin Figure 107b

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A Thick Filament Figure 107c

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Thick Filaments Contain twisted myosin subunits Contain titin strands that recoil after stretching

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The Mysosin Molecule Figure 107d

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Muscle Contraction: the Sliding Filament Theory Muscle contraction requires: Stimulus the generation of an action potential. Crossbridge formation interaction between the thick and thin myofilaments. This is triggered by Ca ++ ions released from the sarcoplasmic reticulum. Energy ATP to energize the myosin molecules.

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Sliding Filaments

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Skeletal Muscle Contraction

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T- tubules supply the stimulus, Sarcoplasmic Reticulum supplies the Ca ++, Mitochondria supply the ATP.

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The Process of Contraction Neural stimulation of sarcolemma: causes excitationcontraction coupling Cisternae of SR release Ca 2+ : which triggers interaction of thick and thin filaments consuming ATP and producing tension

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Skeletal Muscle Innervation

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Figure 1010c

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The Neuromuscular Junction Is the location of neural stimulation Action potential (electrical signal): travels along nerve axon ends at synaptic terminal

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A neuromuscular junction (NMJ).

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The actual synapse acetylcholine

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Synaptic Terminal Releases neurotransmitter (acetylcholine or ACh) Into the synaptic cleft (gap between synaptic terminal and motor end plate)

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The Neurotransmitter Acetylcholine or ACh: travels across the synaptic cleft binds to membrane receptors on sarcolemma (motor end plate) causes sodiumion rush into sarcoplasm is quickly broken down by enzyme (acetylcholinesterase or AChE)

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Action Potential Generated by increase in sodium ions in sarcolemma Travels along the T tubules Leads to excitationcontraction coupling

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ExcitationContraction Coupling Action potential reaches a triad: releasing Ca 2+ triggering contraction Requires myosin heads to be in cocked position: loaded by ATP energy

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Exposing the Active Site

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The Contraction Cycle

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5 Steps of the Contraction Cycle 1.Exposure of active sites 2.Formation of cross-bridges 3.Pivoting of myosin heads 4.Detachment of cross-bridges 5.Reactivation of myosin

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Show the animation

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A Review of Muscle Contraction

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Excitation-Contraction coupling Stimulus or excitation is required for muscles to contract. In skeletal muscle, the stimulus is from a motor neuron. The stimulus is in the form of an action potential. This action potential starts at the neuromuscular junction (NMJ).

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Excitation-contraction coupling

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Show NMJ animation

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Micrograph of an NMJ

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A Synapse Synaptic vesicles

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Tension and Sarcomere Length Figure 1014

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LengthTension Relationship Number of pivoting cross-bridges depends on: amount of overlap between thick and thin fibers Optimum overlap produces greatest amount of tension: too much or too little reduces efficiency

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LengthTension Relationship Normal resting sarcomere length: is 75% to 130% of optimal length

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Frequency of Stimulation A single neural stimulation produces: a single contraction or twitch which lasts about 7100 msec Sustained muscular contractions: require many repeated stimuli

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Tension in a Twitch Length of twitch depends on type of muscle

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Myogram A graph of twitch tension development Figure 1015b (Navigator)

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3 Phases of Twitch 1.Latent period before contraction: the action potential moves through sarcolemma causing Ca 2+ release

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3 Phases of Twitch 2.Contraction phase: calcium ions bind tension builds to peak

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3 Phases of Twitch 3.Relaxation phase: Ca 2+ levels fall active sites are covered tension falls to resting levels

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Treppe A stair-step increase in twitch tension Figure 1016a

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Treppe Repeated stimulations immediately after relaxation phase: stimulus frequency < 50/second Causes a series of contractions with increasing tension

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Wave Summation Increasing tension or summation of twitches

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Wave Summation Repeated stimulations before the end of relaxation phase: stimulus frequency > 50/second Causes increasing tension or summation of twitches

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Incomplete Tetanus Twitches reach maximum tension Figure 1016c

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Incomplete Tetanus If rapid stimulation continues and muscle is not allowed to relax, twitches reach maximum level of tension

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Complete Tetanus Figure 1016d

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Complete Tetanus If stimulation frequency is high enough, muscle never begins to relax, and is in continuous contraction

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Comparative speed of different muscles

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Tension Produced by Whole Skeletal Muscles Depends on: internal tension produced by muscle fibers external tension exerted by muscle fibers on elastic extracellular fibers total number of muscle fibers stimulated

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Motor Units in a Skeletal Muscle Figure 1017

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Motor Units in a Skeletal Muscle Contain hundreds of muscle fibers That contract at the same time Controlled by a single motor neuron

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Recruitment (Multiple Motor Unit Summation) In a whole muscle or group of muscles, smooth motion and increasing tension is produced by slowly increasing size or number of motor units stimulated

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Maximum Tension Achieved when all motor units reach tetanus Can be sustained only a very short time

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Sustained Tension Less than maximum tension Allows motor units to rest in rotation

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2 Types of Skeletal Muscle Tension 1.Isotonic contraction 2.Isometric contraction

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Isotonic Contraction Figure 1018a, b

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Isotonic Contraction Skeletal muscle changes length: resulting in motion If muscle tension > resistance: muscle shortens (concentric contraction) If muscle tension < resistance: muscle lengthens (eccentric contraction)

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Isometric Contraction Figure 1018c, d

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Isometric Contraction Skeletal muscle develops tension, but is prevented from changing length Note: Iso = same, metric = measure

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Resistance and Speed of Contraction

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Are inversely related The heavier the resistance on a muscle: the longer it takes for shortening to begin and the less the muscle will shorten

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ATP and Muscle Contraction Sustained muscle contraction uses a lot of ATP energy Muscles store enough energy to start contraction Muscle fibers must manufacture more ATP as needed

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ATP and CP Reserves Adenosine triphosphate (ATP): the active energy molecule Creatine phosphate (CP): the storage molecule for excess ATP energy in resting muscle

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Recharging ATP Energy recharges ADP to ATP: using the enzyme creatine phosphokinase (CPK) When CP is used up, other mechanisms generate ATP

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Energy Storage in Muscle Fiber Table 102

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ATP Generation Cells produce ATP in 2 ways: aerobic metabolism of fatty acids in the mitochondria anaerobic glycolysis in the cytoplasm

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Aerobic Metabolism Is the primary energy source of resting muscles Breaks down fatty acids Produces 34 ATP molecules per glucose molecule

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Anaerobic Glycolysis Is the primary energy source for peak muscular activity Produces 2 ATP molecules per molecule of glucose Breaks down glucose from glycogen stored in skeletal muscles

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Anaerobic Metabolism: a losing proposition

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Energy Use and Muscle Activity At peak exertion: muscles lack oxygen to support mitochondria muscles rely on glycolysis for ATP pyruvic acid builds up, is converted to lactic acid

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Muscle Metabolis m

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Figure 1020b

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Muscle Metabolism Figure 1020c

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Results of Muscle Fatigue 1.Depletion of metabolic reserves 2.Damage to sarcolemma and sarcoplasmic reticulum 3.Low pH (lactic acid) 4.Muscle exhaustion and pain

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The Recovery Period The time required after exertion for muscles to return to normal Oxygen becomes available Mitochondrial activity resumes

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The Cori Cycle The removal and recycling of lactic acid by the liver Liver converts lactic acid to pyruvic acid Glucose is released to recharge muscle glycogen reserves

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Oxygen Debt After exercise: the body needs more oxygen than usual to normalize metabolic activities resulting in heavy breathing

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Muscle Performance Power: the maximum amount of tension produced Endurance: the amount of time an activity can be sustained Power and endurance depend on: the types of muscle fibers physical conditioning

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3 Types of Skeletal Muscle Fibers 1.Fast fibers 2.Slow fibers 3.Intermediate fibers

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Fast Fibers Contract very quickly Have large diameter, large glycogen reserves, few mitochondria Have strong contractions, fatigue quickly

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Slow Fibers Are slow to contract, slow to fatigue Have small diameter, more mitochondria Have high oxygen supply Contain myoglobin (red pigment, binds oxygen)

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Intermediate Fibers Are mid-sized Have low myoglobin Have more capillaries than fast fiber, slower to fatigue

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Fast versus Slow Fibers Figure 1021

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Comparing Skeletal Muscle Fibers

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Muscles and Fiber Types White muscle: mostly fast fibers pale (e.g., chicken breast) Red muscle: mostly slow fibers dark (e.g., chicken legs) Most human muscles: mixed fibers pink

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Muscle Hypertrophy Muscle growth from heavy training: increases diameter of muscle fibers increases number of myofibrils increases mitochondria, glycogen reserves

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Muscle Atrophy Lack of muscle activity: reduces muscle size, tone, and power

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Structure of Cardiac Tissue Cardiac muscle is striated, found only in the heart Figure 1022

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7 Characteristics of Cardiocytes Unlike skeletal muscle, cardiac muscle cells (cardiocytes): are small have a single nucleus have short, wide T tubules

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7 Characteristics of Cardiocytes have no triads have SR with no terminal cisternae are aerobic (high in myoglobin, mitochondria) have intercalated discs

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Intercalated Discs Are specialized contact points between cardiocytes Join cell membranes of adjacent cardiocytes (gap junctions, desmosomes)

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Functions of Intercalated Discs Maintain structure Enhance molecular and electrical connections Conduct action potentials

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Coordination of Cardiocytes Because intercalated discs link heart cells mechanically, chemically, and electrically, the heart functions like a single, fused mass of cells

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4 Functions of Cardiac Tissue 1.Automaticity: contraction without neural stimulation controlled by pacemaker cells 2.Variable contraction tension: controlled by nervous system

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4 Functions of Cardiac Tissue 3.Extended contraction time 4.Prevention of wave summation and tetanic contractions by cell membranes

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Structure of Smooth Muscle Nonstriated tissue Figure 1023

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Comparing Smooth and Striated Muscle Different internal organization of actin and myosin Different functional characteristics

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8 Characteristics of Smooth Muscle Cells 1.Long, slender, and spindle shaped 2.Have a single, central nucleus 3.Have no T tubules, myofibrils, or sarcomeres 4.Have no tendons or aponeuroses

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8 Characteristics of Smooth Muscle Cells 5.Have scattered myosin fibers 6.Myosin fibers have more heads per thick filament 7.Have thin filaments attached to dense bodies 8.Dense bodies transmit contractions from cell to cell

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Functional Characteristics of Smooth Muscle 1.Excitationcontraction coupling 2.Lengthtension relationships 3.Control of contractions 4.Smooth muscle tone

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ExcitationContraction Coupling Free Ca 2+ in cytoplasm triggers contraction Ca 2+ binds with calmodulin: in the sarcoplasm activates myosin light chain kinase Enzyme breaks down ATP, initiates contraction

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LengthTension Relationships Thick and thin filaments are scattered Resting length not related to tension development Functions over a wide range of lengths (plasticity)

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Control of Contractions Subdivisions: multiunit smooth muscle cells: connected to motor neurons visceral smooth muscle cells: not connected to motor neurons rhythmic cycles of activity controlled by pacesetter cells

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Smooth Muscle Tone Maintains normal levels of activity Modified by neural, hormonal, or chemical factors

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Smooth Muscle

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Varicosities

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Skeletal Smooth Diameter 10 - 100 m3 - 8 m Connective tissueEpi-, Peri- & Endomysium Endomysium only SRYes, complexBarely, simple T - tubulesyesno Sarcomeresyesno Gap Junctionsnoyes voluntaryyesno NeurotransmittersAcetylcholine (Ach) Ach, epinephrine, norepinephrine, et al RegenerationVery littleLots, for muscle

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Future governors of Califorina?