Chapter 9 Muscle Tissue

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Chapter 9 Muscle Tissue

• Lab exam next Thurs. 2/12
• CPR practice, essay entry complete this morning,
  remainder due next Tuesday
• Knee dissection Tuesday2/5

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Objectives

• Discuss organization and functions of muscle as
  an organ including all tissue types
• Describe the structural modifications of muscle
  cells and their functional significance
• Describe the neuromuscular junction and events
• Discuss the sliding filament theory
• Describe a motor unit and neural control of
  muscle
• Explain the link between anatomy physiology
  exemplified in the length/tension curve
• Compare and contrast the three types of skeletal
  muscle cells and relate these to muscular
  performance
• Compare and contrast the three types of muscle
  tissue
• Discuss developmental changes of muscle
• Apply knowledge of levers to human skeletal
  muscles(if time)

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Muscle Function

• Movement including moving substances within
  body
• Contract against resistance
• Skeletal move against bone
• Cardiac move against fluid blood
• Smooth move against other contents
• Muscles also maintain posture stabilize joints
• Regulate organ volume
• Generate heat

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Functional Characteristics of Muscle Tissue

• Contractility the ability to shorten forcibly
  is the unique feature of muscle
• Excitability, or irritability the ability to
  receive and respond to stimuli, have action
  potentials like neurons
• Extensibility the ability to be stretched or
  extended
• Elasticity the ability to recoil and resume the
  original resting length

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Skeletal Muscle Organs

• Organs include muscle tissue, blood vessels,
  nerve fibers, and connective tissue
• The three connective tissue wrappings are
• Epimysium an overcoat of dense regular CT that
  surrounds the entire muscle
• Perimysium fibrous CT that surrounds groups of
  muscle fibers called fascicles
• Endomysium fine sheath of CT composed of
  reticular fibers surrounding each muscle fiber

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Skeletal Muscle Nerve and Blood Supply

• Each muscle is served by at least one nerve,
  artery, and vein
• Each skeletal muscle fiber is supplied with a
  nerve ending that controls contraction a
  neuromuscular junction
• Contracting fibers require continuous delivery of
  oxygen and nutrients via arteries
• Wastes must be removed via veins

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Skeletal Muscle Attachments

• Muscles span joints and are attached in at least
  two places
• When muscles contract the movable bone, the
  muscles insertion moves toward the immovable
  bone the muscles origin (i.e., origin is
  stationary flawed concept, but customary)
• Muscles attach
• Directly epimysium of the muscle is fused to
  the periosteum of a bone
• Indirectly (more common) CT wrappings extend
  beyond the muscle as ropelike tendon or sheetlike
  aponeurosis

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Microscopic Anatomy of a Skeletal Muscle Fiber

• Each fiber is a long, cylindrical cell with
  multiple nuclei just beneath the sarcolemma
• Fibers are 10 to 100 ?m in diameter, and up to
  hundreds of centimeters long
• Each cell is a syncytium produced by fusion of
  myoblasts (embryonic cells)
• Sarcoplasm has a unique oxygen-binding protein
  called myoglobin
• Fibers contain the usual organelles plus
  myofibrils, sarcoplasmic reticulum, and T tubules

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Myoblasts
Syncytium
Note satellite cells
Muscle Cell Formation
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Sarcoplasmic Reticulum (SR)

• SR is an elaborate smooth endoplasmic reticulum
  that mostly runs longitudinally and surrounds
  each myofibril
• Paired terminal cisternae form perpendicular
  cross channels
• Functions in the regulation of intracellular
  calcium levels
• Elongated tubes called T tubules penetrate into
  the cells interior at each A bandI band
  junction
• T tubules associate with the paired terminal
  cisternae to form triads

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T Tubules

• T tubules are continuous with the sarcolemma
• They conduct impulses to the deepest regions of
  the muscle
• These impulses signal for the release of Ca2
  from adjacent terminal cisternae

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Myofibrils

• Myofibrils are densely packed, rodlike
  contractile elements
• They make up most of the muscle volume
• The arrangement of myofibrils within a fiber is
  such that a perfectly aligned repeating series of
  dark A bands and light I bands is evident

Figure 9.2b
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Sarcomeres

• The smallest contractile unit of a muscle
• The region of a myofibril between two successive
  Z discs
• Composed of myofilaments made up of contractile
  proteins
• Myofilaments are of two major types thick and
  thin

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Myofilaments Banding Pattern

• Thick filaments extend the entire length of an
  A band
• Thin filaments extend across the I band and
  partway into the A band
• Z-disc coin-shaped sheet of proteins
  (connectins) that anchors the thin filaments and
  connects myofibrils to one another

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Myofilaments Banding Pattern

• Thin filaments do not overlap thick filaments in
  the lighter H zone
• M lines appear darker due to the presence of the
  protein desmin
• Elastic filaments of protein titin

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Ultrastructure of MyofilamentsThick Filaments

• Each myosin molecule has a rodlike tail and two
  globular heads
• Tails two interwoven, heavy polypeptide chains
• Heads two smaller, light polypeptide chains
  called cross bridges

Figure 9.3a, b
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Ultrastructure of MyofilamentsThick Filaments

• Thick filaments are composed of the protein
  myosin
• Myosin heads contain
• 2 smaller, light polypeptide chains that act as
  cross bridges during contraction
• Binding sites for actin of thin filaments
• Binding sites for ATP
• ATPase enzymes

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Ultrastructure of Myofilaments Thin Filaments

• Thin filaments are chiefly composed of protein
  actin
• Each actin molecule is a helical polymer of
  globular subunits called G actin
• The subunits contain the active sites to which
  myosin heads attach during contraction
• Tropomyosin (filamentous protein) and troponin
  are regulatory subunits bound to actin

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Arrangement of Filaments in a Sarcomere

• Longitudinal section within one sarcomere

Figure 9.3d
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Sliding Filament Mechanism of Contraction

• Thin filaments slide past the thick ones so that
  the actin and myosin filaments overlap to a
  greater degree
• In the relaxed state, thin and thick filaments
  overlap only slightly
• Upon stimulation, myosin heads bind to actin and
  sliding begins (interactive physiology page 17)
• Each myosin head binds and detaches several times
  during contraction, acting like a ratchet to
  generate tension and propel the thin filaments to
  the center of the sarcomere
• As this event occurs throughout the sarcomeres,
  the muscle shortens

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The Sliding Filament Theory
Figure 9.7 Changes in a Sarcomere
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Sarcomere Structure
Figure 9.4b Sarcomere Structure
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Regulation of Contraction

• In order to contract, a skeletal muscle must
• Be stimulated by a nerve ending (NMJ)
• Propagate an electrical current, or action
  potential, along its sarcolemma (T-tubules)
• Have a rise in intracellular Ca2 levels, the
  final trigger for contraction (SR)

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Nerve Stimulus of Skeletal Muscle

• Skeletal muscles are stimulated by motor neurons
  of the somatic nervous system
• Axons of these neurons travel in nerves to muscle
  cells
• Axons of motor neurons branch profusely as they
  enter muscles
• Each axonal branch forms a neuromuscular junction
  with a single muscle fiber

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Neuromuscular Junction

• The neuromuscular junction is
• Axonal endings, which have small membranous sacs
  (synaptic vesicles) that contain the
  neurotransmitter acetylcholine (ACh)
• The motor end plate of a muscle, which is a
  specific part of the sarcolemma that contains ACh
  receptors that helps form the neuromuscular
  junction
• Though exceedingly close, axonal ends and muscle
  fibers are always separated by a space called the
  synaptic cleft

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Neuromuscular Junction
Figure 9.8a, b
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Neuromuscular Junction

• When a nerve impulse reaches the end of an axon
  at the neuromuscular junction
• Voltage-regulated calcium channels open and allow
  Ca2 to enter the axon
• Ca2 inside the axon terminal causes axonal
  vesicles to fuse with the axonal membrane
• This fusion releases ACh into the synaptic cleft
  via exocytosis
• ACh diffuses across the synaptic cleft to ACh
  receptors on the sarcolemma
• Binding of ACh to its receptors initiates an
  action potential in the muscle

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Neuromuscular Junction
Figure 9.8c
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Summary
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Excitation-Contraction Coupling
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Motor Unit The Nerve-Muscle Functional Unit

• A motor unit is a motor neuron and all the muscle
  fibers it supplies
• The number of muscle fibers per motor unit can
  vary from four to several hundred
• Muscles that control fine movements (fingers,
  eyes) have small motor units

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Motor Unit The Nerve-Muscle Functional Unit

• Large weight-bearing muscles (thighs, hips) have
  large motor units
• Muscle fibers from a motor unit are spread
  throughout the muscle therefore, contraction of
  a single motor unit causes weak contraction of
  the entire muscle

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Neuromuscular Junction
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Muscle Tone

• Muscle tone
• The constant, slightly contracted state of all
  muscles, which does not produce active movements
• Keeps the muscles firm, healthy, and ready to
  respond to stimulus
• Spinal reflexes account for muscle tone by
• Activating one motor unit and then another
• Responding to activation of stretch receptors in
  muscles and tendons

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Force of Contraction

• The force of contraction is affected by
• The number of muscle fibers contracting the
  more motor fibers in a muscle, the stronger the
  contraction
• The relative size of the muscle the bulkier
  the muscle, the greater its strength

Figure 9.19a
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Force of Contraction

• Frequency of stimulation
• Length-tension relationships
• Series-elastic elements the noncontractile
  structures in a muscle
• Degree of muscle stretch muscles contract
  strongest when muscle fibers are 80-120 of their
  normal resting length

Figure 9.19a
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Length/Tension Relationship
Maximum force generated between 80-120 of
resting length, interactive physiology page
16 Think of cross-bridge mechanism for explanation
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Muscle Fiber Type Functional Characteristics

• Speed of contraction determined by speed in
  which ATPases split ATP
• The three types of fibers are slow and fast and
  intermediate
• ATP-forming pathways
• Oxidative fibers use aerobic pathways
• Glycolytic fibers use anaerobic glycolysis
• These two criteria define three categories slow
  oxidative fibers, fast oxidative fibers, and fast
  glycolytic fibers

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Training

• Slow, oxidative respond to endurance training.
  Diameter changes little.
• Fast, oxidative respond to strength and power
  training. Diameter increases.
• Intermediate can take on characteristics of fast
  or slow, depending on type of training.

In what birds do you expect to find FT? And ST?
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Muscle Hypertrophy

• Exercise causes
• An increase in the number of mitochondria
• An increase in the activity of muscle spindles
• An increase in the concentration of glycolytic
  enzymes
• An increase in the glycogen reserves
• An increase in the number of myofibrils
• The net effect is an enlargement of the
  muscle(hypertrophy)
• Disuse causes atrophy
• A decrease in muscle size
• A decrease in muscle tone

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Developmental Aspects

• Muscle tissue develops from embryonic mesoderm
  called myoblasts
• Multinucleated skeletal muscles form by fusion of
  myoblasts forming a syncytium
• The growth factor agrin stimulates the clustering
  of ACh receptors at newly forming motor end
  plates
• As muscles are brought under the control of the
  somatic nervous system, the numbers of fast and
  slow fibers are also determined

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Developmental Aspects

• Cardiac myoblasts do not fuse but develop gap
  junctions at an early embryonic stage
• Most smooth muscle follows the same pattern of
  gap junctions rather than fusion

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Developmental Aspects After Birth

• Muscular development reflects neuromuscular
  coordination
• Development occurs head-to-toe, and
  proximal-to-distal
• Peak natural neural control of muscles is
  achieved by midadolescence
• Athletics and training can improve neuromuscular
  control

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Developmental Aspects Male and Female

• There is a biological basis for greater strength
  in men than in women
• Womens skeletal muscle makes up 36 of their
  body mass
• Mens skeletal muscle makes up 42 of their body
  mass
• These differences are due primarily to the male
  sex hormone testosterone
• With more muscle mass, men are generally stronger
  than women
• Body strength per unit muscle mass, however, is
  the same

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Homeostatic Imbalance Age Related

• With age, connective tissue increases and
  myofibrils, glycogen and myoglobin decrease
• Muscles become stringier and more sinewy
• By age 80, 50 of muscle mass is lost
  (sarcopenia), and myosatellite cells decrease
• Regular exercise reverses sarcopenia
• Aging of the cardiovascular system affects every
  organ in the body
• Atherosclerosis may block distal arteries,
  leading to intermittent claudication and causing
  severe pain in leg muscles

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Developmental Aspects Regeneration

• Cardiac and skeletal muscle become amitotic, but
  can lengthen and thicken (hypertrophy)
• Myoblastlike satellite cells of skeletal muscle
  show very limited regenerative ability (Cardiac
  tissue lacks satellite cells)
• Smooth muscle has good regenerative ability
  (hyperplasia)

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Levers

• F1L1 F2L2
• MassForce Llength
• There are several ways to increase the force
  efficiency of a lever
• increasing the length of the in-lever arm
• decreasing the length of the out-lever
• or doing both of the above

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See-saw
Wheelbarrow Less distance
Hotdog tongs most common, least mechanical
advantage, more force, more speed/distance
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The Arm is a Lever and Fulcrum System
Figure 12-21b
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The Lever-Fulcrum System Amplifies the Load
Distance Traveled and the Speed of Movement
Figure 12-22
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Smooth Muscle

• Composed of spindle-shaped fibers diameter of
  2-10 ?m and lengths of several hundred ?m
• Lack the coarse CT sheaths of skeletal muscle,
  but have fine endomysium

Figure 9.23
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Smooth Muscle

• Are generally organized into two layers
  (longitudinal and circular) of closely apposed
  fibers
• Found in walls of hollow organs (except the heart)

Figure 9.23
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Innervation of Smooth Muscle

• Most smooth muscle lacks neuromuscular junctions
• Innervating nerves have bulbous swellings called
  varicosities
• Varicosities release neurotransmitters into wide
  synaptic clefts called diffuse junctions

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Microscopic Anatomy of Smooth Muscle

• SR is less developed than in skeletal muscle and
  lacks a specific pattern (no cisterns)
• T tubules are absent
• Plasma membranes have pouchlike infoldings called
  caveoli
• Ca2 is sequestered in the extracellular space
  near the caveoli, allowing rapid influx when
  channels are opened
• There are no visible striations and no sarcomeres
• Thin and thick filaments are present

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Proportion and Organization of Myofilaments in
Smooth Muscle

• Ratio of thick to thin filaments (12) is much
  lower than in skeletal (16) or cardiac (14)
• Thick filaments have heads along their entire
  length
• There is no troponin complex

Figure 9.25
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Proportion and Organization of Myofilaments in
Smooth Muscle

• Thick and thin filaments are arranged diagonally,
  causing smooth muscle to contract in a corkscrew
  manner

Figure 9.25
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Proportion and Organization of Myofilaments in
Smooth Muscle

• Noncontractile intermediate filament bundles
  attach to dense bodies (analogous to Z discs) at
  regular intervals

Figure 9.25
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Contraction of Smooth Muscle

• Whole sheets of smooth muscle exhibit slow,
  synchronized contraction
• They contract in unison, reflecting their
  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
• Are self-excitatory and depolarize without
  external stimuli

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Contractile Mechanism

• Actin and myosin interact according to the
  sliding filament mechanism
• The final trigger for contractions is a rise in
  intracellular Ca2
• Ca2 is released from the SR and from the
  extracellular space
• Ca2 interacts with calmodulin and myosin light
  chain kinase to activate myosin

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Special Features of Smooth Muscle Contraction

• Unique characteristics of smooth muscle include
• Smooth muscle tone
• Slow, prolonged contractile activity
• Low energy requirements
• Response to stretch

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Response to Stretch

• Smooth muscles exhibits a phenomenon called
  stress-relaxation response in which
• Smooth muscle responds to stretch only briefly,
  and then adapts to its new length
• The new length, however, retains its ability to
  contract
• This enables organs such as the stomach and
  bladder to temporarily store contents

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Types of Smooth Muscle Single Unit

• The cells of single unit smooth muscle, commonly
  called visceral muscle
• Contract rhythmically as a unit
• Are electrically coupled to one another via gap
  junctions
• Often exhibit spontaneous action potentials
• Are arranged in opposing sheets and exhibit
  stress-relaxation response

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Types of Smooth Muscle Multiunit

• Multiunit smooth muscles are found
• In large airways to the lungs
• In large arteries
• In arrector pili muscles
• In the internal eye muscles
• Characteristics include
• Rare gap junctions
• Infrequent spontaneous depolarizations
• Structurally independent muscle fibers
• A rich nerve supply, which, with a number of
  muscle fibers, forms motor units
• Graded contractions in response to neural stimuli

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Muscle Comparison Summary
Table 12-3