muscular system histology and physiology l.
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MUSCULAR SYSTEM: Histology and Physiology. Introduction. Types and Features of Muscle Tissue smooth muscle : cardiac muscle : muscle :. skeletal. General Characteristics of Muscle contractility excitability. extensibility. elasticity. General Functions of Muscle. 1.

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MUSCULAR SYSTEM: Histology and Physiology

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  • Types and Features of Muscle Tissue
    • smooth muscle:
    • cardiac muscle:
    • muscle:


General Characteristics of Muscle
    • contractility
    • excitability



skeletal muscle structure
Skeletal Muscle: Structure


  • Skeletal muscles are composed of skeletal muscle fibers, connective tissue, blood vessels and nerves.
Muscle fiber , is a skeletal muscle cell
    • single, cylindrically shaped cell
    • multiple nuclei, located peripherally
    • development
      • develop from
      • myoblasts fuse to form muscle cells
      • grow into skeletal muscle to form neuromuscular junction
        • One neuron per skeletal muscle fiber
      • numbers of striated muscle cells remains relatively constant after birth, but they may enlarge


Motor nerves

size of muscle fibers/cells
    • 1 - 40 mm long
    • 10 - 100 microns in diameter
    • size of fibers varies with size of muscle
  • muscle fibers are (banded appearance)




  • Muscle Anatomy
    • - plasma membrane of muscle cell
    • also a delicate layer of CT with reticular fibers
    • surrounds each muscle fiber outside the external lamina
    • CT layer
    • surrounds a bundle of muscle fibers, a


muscle fasciculus




  • a relatively thick layer of dense irreg. collagenous CT
  • surrounds the many that make up a muscle
  • covers the entire surface of the muscle


  • fascia
    • a layer of fibrous CT outside the
    • separates individual muscles
    • may surround muscle groups
    • fascia of an individual muscle is also called the epimysium
these CT layers are continuous with each other and with the tendons and CT sheaths of the bones
  • functions of CT
    • holds muscles together
    • provides a passageway to the muscle cells for blood vessels and nerves
      • tendon or sheetlike aponeurosis
      • epimysium fused to periosteum

indirect attachment

direct attachment

Muscle Fibers, Myofibrils, & Myofilaments
    • Muscle cell components
      • sarcolemma


    • threadlike structures, 1 - 3 micrometers in diameter
    • extend from one end of the muscle fiber to another
    • composed of protein myofilaments
      • - thin filaments
      • - thick filaments




Z line to Z

  • sarcomere
    • a highly organized unit of myofilaments
    • join end to end with other sarcomeres to form myofibrils
    • extends from line (line=disc)
  • Movie

I (isotropic) band

  • banding due to arrangements of actin & myosin microfilaments
      • light band
      • includes a Z lines (disk)
      • consists only of actin filaments
      • extends on either side of Z line to myosin myofilaments
A (anisotropic) band
    • dark band
    • extends the length of myosin myofilaments within the sarcomere
H zone
    • a smaller band located within the center of each A-band
    • only myosin myofilaments are present, no overlapping with actin


  • M line
    • a dark band in the middle of the H zone
    • consists of delicate filaments that attach to the center of the myosin myofilaments
    • holds myosin myofilaments in place
titin (elastic filament)
    • an elastic filament anchoring myosin myofilament at M line to Z disc
    • resists excessive stretching or sarcomere
Actin myofilaments
    • two strands of fibrous actin (F-actin) for double helix
      • which are polymers of about 200 globular ( ) monomers



active site

  • two strands are arranged in a double helix
  • each G-actin monomer has an to which myosin molecules can bind during muscle contraction


  • molecules
    • an elongated molecule that winds along the groove of the F-actin double helix
    • each molecule covers 7 G-actin active sites


  • composed of three subunits
    • one with a high affinity for actin (TnI)
    • one with a high affinity for tropomyosin (TnT)
    • one with a high affinity for ions (TnC)
  • spaced between ends of tropomyosin molecules in groove between F-actin double helix


in presence of Ca2+, troponin is activated
    • troponin activation results in it displacing tropomyosin deeper into the “groove” in F-actin double helix
    • this exposes the actin active sites
Myosin myofilaments
    • composed of elongated, club-shaped, myosin molecules
    • two parts of myosin molecule
      • rods (tails)
      • with ATPase


rod-like portions wound together
  • about 100 myosin molecules per myosin myofilament
  • point of attachment between head and rod is hinged
  • myosin head forms cross bridge (contact) to actin’s active site

striated pattern

  • other
    • A bands and I bands of parallel myofibrils are aligned to produce seen with a microscope
    • many
    • many glycogen granules & lipid droplets


transverse (T) tubules
    • tube like invaginations of the sarcolemma
    • project into the muscle fiber and wrap around the sarcomeres where the actin and myosin microfilaments overlap
    • lumen is continuous with exterior of muscle fiber
    • filled with fluid


sarcoplasmic reticulum
    • highly specialized smooth ER
    • near the T tubules, enlarged to form terminal cisternae
    • is a T tubule and 2 terminal cisternae
    • transports ions from sarcoplasm to its lumen



clinical focus
Clinical Focus
  • Muscular dystrophy
  • Muscular atrophy


sliding filament theory
Sliding Filament Theory
  • Contracting Muscle
    • actin and myosin myofilaments don't change length but slide past one another during muscle contraction.
    • cross bridges form between heads of myosin and active sites on actin, release, and then reform, causing the actin myofilaments at each end of the sarcomere to slide past the myosin microfilaments toward the H zone
    • I bands and H zones become more narrow during contraction; H zone may disappear
    • A bands remain constant in length
    • sarcomere shortens
Slidng filament theory 1
  • Slidng filament theory 2
neuromuscular junctions
Neuromuscular Junctions
  • Motor neurons
    • specialized nerve cells that propagate action potentials to skeletal muscle fibers at a relatively high velocity
    • most motor neurons enter skeletal muscles with the blood vessels branching when they reach the
    • each motor neuron innervates more than muscle fiber
    • a branch of the motor neuron innervates one muscle fiber, forming a neuromuscular junction or synapse near the center of the muscle fiber



Neuromuscular junction structure
    • presynaptic (axonal) terminal
    • postsynaptic terminal (motor end plate)
    • synaptic cleft
    • synaptic vesicles:

Release acetylcholine

quick review
Quick Review
  • Membrane Transport 2
    • Active processes
      • Move substances “uphill” or against their concentration gradient; most often is the energizer for this process
      • Active Transport is the mechanism
        • ; an mechanism


Na+ - K+ ATPase


Na+ - K+ ATPase 1
  • Na+ - K+ ATPase 2
resting membrane potential
Resting Membrane Potential
  • The cell membrane is more permeable to some molecules than to others.
  • Membrane Potentials
    • A , or electrical potential due to the separation of charges by the cell membrane.
    • Resting Membrane Potential occur in all cells due to
    • RMP ranges from -20 to -200 millivolts (mV)
      • state of the cell


Concentration Gradients

Active Transport Pumps

Electrical Gradients


Inside of the cell is electrically neutral
  • Outside of the cell is electrically neutral
  • But,
    • higher outside of cell
      • Membrane ~impermeable to Na+
    • higher inside of cell
      • Membrane ~permeable to K+
      • K+ diffuses down its concentration gradient
    • This concentration difference make a loss of positive charges inside the cell
    • maintains this “imbalance” How?



Na+ - K+ ATPase


Electric charges

  • But there is more,
    • reinforce the ion concentration on the inside and outside of the cell
      • Electrical forces resist K+ diffusion out of the cell
    • An produces the RMP

electrochemical gradient

Fig. 9.7

Action Potential tracing
    • Stimulus
      • Na+ channel open -> Na+ influx
      • K+ channel begin to open late in phase
      • Na+ channel close
      • K+ channel open
    • (after potential)
      • K+ leaks until Na+-K+ ATPase reestablish RMP

Depolarization phase

Repolarization phase


regulation skeletal muscle fiber
Regulation - Skeletal Muscle Fiber
  • Regulation of Contraction
    • Nerve Stimulus & Neuromuscular Junction
      • has cell body in CNS (brain & spinal cord)

Motor neuron


Action potential

  • generated at cell body travels along axon
    • Axon branch to muscle fiber
    • Muscle fiber innervated by one motor neuron
    • is a motor neuron and all muscle fibers it stimulates
      • fine muscle control ->few fibers/unit
      • coarse movement ->may fibers/unit

Motor unit


Axonal terminal

  • Neuromuscular junction formed by axonal terminal and muscle fiber (motor end plate)
    • (presynaptic membrane)
      • Synaptic vesicles ->
    • Synaptic cleft
    • Junctional folds of sacrolemma (post synaptic membrane)
      • ACh receptors

acetylcholine (ACh)

Nerve AP
    • Nerve AP arrives at the presynaptic terminal causing voltage-gated Ca2+ ion channels to open
      • Ca2+ influx -> into synaptic cleft
      • diffusion of ACh across synaptic cleft
  • Movie

exocytosis of ACh

Muscle fiber AP
    • 2-ACh binds chemically gated ACh receptor-channel
      • opens ACh receptor-channel -> Na+ influx -> of sacrolemma
      • graded potential across sarcolemma



Voltage-gated Na+

  • Propagation of AP
    • channels open -> AP propagation
  • ACh in synaptic cleft broken down by (AChE)
    • acetic acid ->broken down by cells
    • choline ->uptake into axon terminal & reincorporated into ACh




  • Excitation-Contraction Coupling
    • Muscle fiber causes muscle fiber contraction
    • AP propagated along the sarcolemma, causes wave to spread along the sarcolemma of the T tubules
    • Depolarization of the T tubule membrane
      • triggers opening of voltage-gated Ca2+ channels to open in sarcoplasmic reticulum
      • Ca2+ influx into sarcoplasm from sarcoplasmic reticulum
  • Movie


Repolarizationrestores the polarized state
    • voltage-gated Na+ channels close
    • voltage-gated K+ channels open
    • Na+ - K+ pump restores electrochemical gradient


  • sarcoplasmic Ca2+ binds (TnC)
    • activated troponin moves deeper into the F-actin groove, and exposes G-actin active sites
  • G-actin active site exposure allows cross-bridge attachment of myosin head


contraction skeletal muscle fiber
Contraction – Skeletal Muscle Fiber
  • Sliding Filament Mechanism of Contraction
    • Myosin myofilament head has hydrolyzed ATP -> ADP & P (that remain attached)
      • energy transfer “cocks” myosin (energizes) head




  • Myosin head attaches to a active site
    • Cross bridge formed
    • phosphate is released
    • ADP may remain attached to myosin head
Energized myosin head bends & “pulls” on the actin myofilament
    • Actin “stroked” past myosin myofilament
    • ADP released from de-energized head
    • myosin head remains fixed to actin’s active site

power stroke

Myosin myofilament head ATPase hydrolyze ATP -> ADP & P (that remain attached)
    • myosin head from actin’s active site
    • energy transfer “cocks” myosin (energizes) head
  • Mechanism repeats


  • Movie2
  • Movie3
Sliding Filament Mechanism Questions
    • Do actin myofilaments change in overall length?
    • Do myosin myofilaments change in overall length?
    • What are the structures that form a cross-bridge?
    • How many times can cross-bridges be formed during one muscle fiber contraction?
    • Which myofilaments are stationary (fixed), and which myofilaments move during a muscle fiber contraction?
    • Describe the appearance of the banding in a relaxed muscle fiber?
    • During a muscle fiber contraction, how does the appearance of the H-zone change?
    • During a muscle fiber contraction, how does the appearance of the A-band change?
    • During a muscle fiber contraction, how does the appearance of the I-band change?
    • During a muscle fiber contraction, how does the appearance of the Z-discs change?
    • During a muscle fiber contraction, how does the appearance of the sarcomere change?
    • How does this relate to motor unit size and the amount of “fine” or “coarse” control over muscle movement?


  • All-or-None Principle
    • An action potential (AP) is all-or-none.
      • depolarization occurs:
        • insufficient cation influx will not trigger opening of voltage-gated cation channels
        • a local depolarization, but no propagation (AP)
        • threshold, a membrane potential at which an AP is produced as a result of depolarization, is not “reached”
        • NONE = no AP -> no muscle fiber contraction
      • depolarization occurs
        • Cation influx trigger opening of voltage-gated cation channels
        • threshold is reached, an AP is propagated
        • ALL = AP -> muscle fiber contraction
      • A stronger-than-threshold stimulus produces an AP of the same magnitude -> therefore produces an identical contraction




  • Fig. 11.22
  • Refractory Period
    • early in AP
    • NO additional stimulus will generate another AP
  • Refractory Period
    • following abs. ref. p., late repolarization phase, many open K+ channels causes hyperpolarization
    • only a stronger-than-threshold stimulus can initiate another AP


The Muscle Twitch and Development of Muscle Tension
    • A single AP causes a brief muscle fiber contraction followed by relaxation
      • Contraction -> tension -> transferred to CT -> movement
Graded Muscle Responses
    • Myogram is tracing of a muscle contraction, not an AP
      • Latent (lag) phase
        • delay betw. AP, depolarization, & start of mechanical events
        • mechanical events of contraction
        • tension generated
      • Relaxation phase
        • cell repolarization to polarized state
        • tension decreases

Contraction phase

Multiple Motor Unit Summation
    • Each motor unit responds in an All-or-None fashion
    • A whole muscle is capable of producing an increasing amount of tension as the number of stimulated increases
      • Recruitment of motor units
        • threshold stimulus -> 1st muscle fiber contractions; few motor units
        • maximal stimulus -> ~all motor units recruited

motor units

Multiple Wave Summation
    • AP frequency increasing (slowly)
      • Muscle tension increases; Why?
    • AP frequency increasing (rapidly)
      • Muscle tension increases; Why?
      • Incomplete tetany vs. Complete tetany
Treppe: The Staircase Effect
    • Maximal stimulus at a (low and constant) frequency that allow for complete relaxation between stimuli
      • second contraction produces greater tension than the first, and the third contraction greater tension than the second
Muscle Tone
    • Relatively constant tension produced by a muscle for long periods as a result of asynchronous contraction of motor units
  • Types of Contraction
    • Isometric contractions
    • contractions
      • Concentric contractions
      • Eccentric contractions




  • Length-Tension Relationship
    • muscle not stretched
      • tension produced is
    • muscle is severely stretched
      • tension produced is
    • optimally stretched
      • tension produced is maximal
      • number of cross-bridges that can form is maximal


muscle metabolism
Muscle Metabolism
  • Providing Energy for Muscle Contraction
    • Stored ATP
      • 4-6 seconds
    • Direct Phosphorylation of ADP by Creatine Phosphate
      • 15 seconds
      • 1 ATP/CP
    • Anaerobic Glycolysis and Lactic Acid Formation
      • 30-60 seconds
      • 2 ATP/glucose or lactic acid
    • Aerobic Respiration
      • Hours
      • 36 ATP/glucose
Oxygen Debt
    • Which pathway for ATP regeneration requires oxygen?
    • Lactic acid buildup, ATP & CP store depletion result in ->
      • Oxygen debt is the time required for the body to remove lactic acid and regenerate ATP & CP stores
      • How? Increased

oxygen debt




  • Fatigue
    • Muscle fatigue
      • relative deficit of
      • lactic acid accumulation (pain) and
        • ion loss in sweat
        • Na+ - K+ pump “less efficient”
      • contracture results; Why?
    • Psychological fatigue
      • “the flesh (muscle) is still willing, but the spirit is not!”
  • Heat Production During Muscle Activity
    • ~40% waste energy in the form of heat is generated by muscles

ion imbalances

Velocity and Duration of Contraction
    • Muscle Fiber Type
      • Slow oxidative fibers
      • Fast oxidative fibers
      • Fast glycolytic fibers
    • For each fiber type:
    • What is the primary pathway for ATP synthesis?
    • What is the relative myglobin content? Why?
    • What is the fibers relative color? Why?
    • What is the relative amount of mitochondria?
    • What is the relative concentration of capillaries?
    • What is the relative rate of fatigue?
    • What is the relative speed of fiber contract?
    • What type of activity is the each fiber best suited?
general principles
General Principles
  • Tendons: Attach muscles to bones
    • Aponeurosis: A very broad tendon
  • Muscles
    • Origin or head: Muscle end attached to more stationary of two bones
    • Insertion: Muscle end attached to bone with greatest movement
    • Belly: Largest portion of the muscle between origin and insertion
Synergists: Muscles that work together to cause a movement
    • Prime mover: Plays major role in accomplishing movement
    • Agonist: Muscle causing an action when contracts
    • Antagonist: A muscle working in opposition to agonist
Fixators: Stabilize joint/s crossed by the prime mover
  • Muscle shapes:
    • unipinnate; bipinnate; multipinnate
    • parallel; circular
    • convergent
    • quadraangular; trapazoidal; triangular; rhomboidal; fusiform
    • digastric; bicipital
  • Muscles are named according to:
    • Location: pectoralis gluteus, brachial
    • Size: maximus, minimus, longus, brevis
    • Shape: deltoid, quadratus, teres
    • Orientation: rectus
    • Origin and insertion: sternocleidomastoid, brachioradialis
    • Number of heads: biceps, triceps
    • Function: abductor, adductor, masseter
muscle movements
Muscle Movements
  • Muscle contractions are a pull or force by relative positions of
    • Lever: Rigid shaft or bone
    • Fulcrum: Pivot point or joint
    • Weight or resistance
classes of levers
Classes of Levers
  • Class I
    • Fulcrum between force and weight
    • Seesaw or head movement
  • Class II
    • Weight is between fulcrum and pull
    • Wheelbarrow, standing on toes
  • Class III
    • Pull located between fulcrum and weight
    • Person using a shovel
    • Most common
  • Explain the events that influence the width of each band of a sarcomere when a muscle goes through the sequence of stretching, contracting, and then relaxing.
  • Describe and compare the three types of muscle tissue (skeletal, cardiac, smooth) on the basis of their microscopic structure, location, function, and innervation.
  • What is the difference between a strain and a sprain?
  • Describe the sliding filament mechanism (theory) of muscle contraction.
smooth muscle this may be covered only in part or not at all
Smooth Muscle*This may be covered only in part or not at all.
  • Arrangement and Microscopic Structure of Smooth Muscle Fibers
    • Basic Characteristics
      • Shape?
      • Nucleus?
      • Striations?
      • Sarcomeres?
      • CT coverings?
      • SR?
      • sheets


Filaments & Sarcolemma
    • Intermediate fibers connect to imbedded in the sarcolemma
      • Transfer tension of myofilaments to sarcolemma and endomysium
    • No
    • many F-actin filaments
    • few Myosin myofilaments
    • (small pockets), no T tubule

dense bodies



Contraction of Smooth Muscle
    • Regulation of Contraction
      • regulation
        • Spontaneous depolarization due to local stimulus
        • “Pacemaker” cells communicate stimulus via gap junctions
      • nervous system branches
        • Diffuse synaptic junctions -> ACh released
      • regulation
        • Hormones




Mechanism and Characteristics of Contraction
    • Stimulus -> local, ANS, hormone -> AP
    • Ca2+ channel opens in sarcolemma
    • Ca2+ binds -> activates
    • Activated calmodulin binds myosin kinase -> activates
    • Activated transfer phosphate from ATP to myosin head -> activates
    • Cycle of cross-bridge formation, movement, detachment, and cross-bridge formation occurs
    • Relaxation occurs when removes phosphate from myosin


myosin kinase

myosin phosphatase

Special Features of Smooth Muscle Contraction
    • Basic Features
      • Slow, prolonged contractions
      • Peristalsis
      • Low energy requirement
    • Response to Stretch
      • Stretch -> can increase contraction -> increase tension
    • Hyperplasia


  • Types of Smooth Muscle
    • Smooth Muscle (visceral muscle)
      • Contracts as a unit and rhythmically
        • gap junctions
      • Spontaneous AP
        • local control mechanisms
    • Smooth Muscle
      • Muscle fiber must be stimulated (ANS)
        • few gap junctions