Download The Muscular System

The Muscular System
Chapter 8
Types of muscle & function
• Skeletal- 40-50% of total body weight- voluntary
– mostly movement of bone & body parts
– Stabilizing body positions
• Cardiac- only in heart- involuntary
– Heart only
– Develops pressure for arterial blood flow
• Smooth- grouped in walls of hollow organs
– Sphincters regulate flow in tubes
– Maintain diameter of tubes
– Move material in GI tract and reproductive organs
Muscle Functions
• Produce body movements
• Stabilize body positions
• Regulate organ volume
• Moving substances internally
• Producing heat
Skeletal Muscle Tissue
• Muscle includes: muscle fibers, connective
tissue, nerves & blood vessels
• Wrapped in Epimysium
• Perimysium surrounds fiber bundles called
fascicles
• Endomysium surrounds each individual
fiber
Skeletal Muscle Tissue
• Well-supplied with blood vessels and nerves
• Terminal of a neuron on each muscle fiber
Figure 8.1
Muscle Histology
• elongated cylindrical cells = muscle fibers
• plasma membrane = sarcolemma
• Transverse (T- tubules) tunnel from
surface to center of each fiber
• Multiple nuclei lie near surface
• Cytoplasm = sarcoplasm
Figure 8.2a
Muscle histology (cont.)
• Throughout sarcoplasm is sarcoplasmic
reticulum
– Stores Calcium ions
• Sarcoplasm contains myoglobin
– Red pigmented protein related to Hemoglobin
that carries oxygen
• Along entire length are myofibrils
• Myofibrils made of protein filaments
– Come in thick and thin filaments
Figure 8.2b
Sarcomere
•
•
•
•
Filaments overlap in repeating patterns
Unit structure is called sarcomere
Separated by Z-discs
Darker area = A-band associated with thick
filaments
• H-zone has no thin filaments
• I-band has thin filaments no thick filaments
Figure 8.2c
Figure 8.3a
Figure 8.3b
Functional Structure
• Thick filament (myosin) has moveable heads
• Thin filaments (actin) are anchored to Z-discs
– Contain myosin binding sites for myosin head
– Also contain tropomyosin & troponin
• Tropomyosin blocks myosin binding site at rest
Sliding Filament Mechanism
• During contraction myosin heads bind actin
sites
• Pull and slide actin molecules (and Z-discs)
toward H-zone
• I-bands and H-zones narrow
• Sliding generates force and shortens
sarcomeres and thus fibers.
Figure 8.4
Neuromuscular Interaction
• Nerve signal triggers muscle action
potential
• Delivered by motor neuron
• One neuron can trigger 1 or more fibers at
the same time
• Neuron plus triggered fibers = motor unit
Neuromuscular Junction
• neuronal ending to muscle fiber =
Neuromuscular junction
• Synaptic end bulbs (at neuron terminal)
– Release neurotransmitter
• Muscular area = Motor end plate
• Between is synaptic cleft
Figure 8.5
Action at NMJ
1. Release of acetylcholine (ACh)
–
Diffuses across cleft
2. Activation of ACh receptors
3. Generation of Muscle Action Potential
Repeats with each neuronal action potential
4. Breakdown of ACh
Contraction Trigger
• Muscle action potential=> Ca2+ release from
Sacroplasmic Reticulum (SR)
• Ca2+ binds to troponin =>
• Moves tropomyosin off actin sites =>
• Myosin binds & starts cycle
Contraction Cycle
• Myosin binds to actin & releases phosphate group
(Forming crossbridges)
• Crossbridge swivels releasing ADP & shortening
sarcomere (Power stroke)
• ATP binds to Myosin => release of myosin from
actin
• ATP broken down to ADP & Pi => activates
myosin head to bind and start again
• Repeats as long as Ca2+ concentration is high
Figure 8.6
Relaxation
• Breakdown of Ach to stop muscle Action
potentials
• Ca2+ ions transported back into SR lowering
concentration=>
– This takes ATP
• tropomyosin covers actin binding sites
Figure 8.7
Muscle Tone
• Even at rest some motor neuron activity
occurs = Muscle Tone
• If nerves are cut fiber becomes flaccid
(very limp)
Metabolism
• Rapid changes from very low ATP
consumption to high levels of consumption
• Creatine phosphate (high energy store)
• Fast & good for ~ 15 sec
Figure 8.8a
Glycolysis
• Break down glucose to 2 pyruvates getting
2 ATPs
• If insufficient mitochondria or oxygen
pyruvate => lactic acid
• Get about 30-40 seconds more at max.
Figure 8.8b
Aerobic Cellular Respiration
• Production of ATP in mitochondria
• Requires oxygen and carbon substrate
• Produces CO2 and H2O and heat.
Fatigue
• Inability to contract forcefully after
prolonged activity
• Limiting factors can include:
– Ca2+
– Creatine Phosphate
– Oxygen
– Build up of acid
– Neuronal failure
Oxygen Use After Exercise
• Convert lactic acid back to glucose in liver
• Resynthesize Creatine Phosphate and ATP
• Replace oxygen removed from myoglobin
Control of Muscle Contraction
• Single Action Potential(AP) =>twitch
– Smaller than maximum muscle force
• Total tension of fiber depends on frequency of
APs (number/second)
– Require wave summation
– Maximum = tetanus
• Total tension of muscle depends on number of
fibers contracting in unison
– Increasing numbers = Motor unit recruitment
Figure 8.9
Figure 8.10
Fiber types
• Slow oxidative (SO)- small diameter & red
– large amounts of myoglobin and mitochondria
– ATP production primarily oxidative
– Fatigue resistant-
• Fast oxidative- glycolytic (FOG)
– Large diameter = many myofibrils
– Many mitochondria and high glycolytic capacity
• Fast glycolytic fibers (FG)
– white, fast & powerful and fast fatiguing
– For strong, short term use
Recruitment
• Muscle contractions only use the fibers
required for the work
• Recruited in order: SO=>FOG=>FG
• if force is constant and the muscle shortens =
Isotonic Contraction
• If length is constant and the force varies =
Isometric Contraction
– The latter is often a postural muscle activity
Effects of Exercise
• SO/FG fiber ratio genetically determined
– High FG => sprinters
– High SO=> marathoners
• Endurance exercise gives FG=> FOG
– Increased diameter and numbers of
mitochondria
• Strength exercise increases size & strength
of FG fibers
Cardiac Muscle
• Striated, short fibers and branched
• Single central nucleus; Cells joined by gap
junctions & desmosomes
• Thickened joint area called intercalated discs
• Some cardiac muscles generate own APautorhythmicity
• Involuntary
Cardiac muscle
• No nerve- internal pacemaker
• Ca2+- from S.R. and extracellular space
• separate cells with gap junctions -> electrical
connections
Figure 15.2b
Smooth muscle
•
•
•
•
Involuntary
In internal organs
Filaments not regular so not striated
Visceral (single unit) type or
– Form sheets and are autorhythmic
– Contract as a unit
• Multi-unit type– each has own nerve and can contract independently
Smooth Muscle
•
•
•
•
Graded contractions and slow responses
Often sustain long term tone
Often triggered by autonomic nerves
modulated chemically, nerves, by
mechanical events (stretching)
Figure 8.11
Aging
• Like bone there is a slow progressive loss of
skeletal muscle mass
• Relative number of SO fibers tends to
increase
Movement
• Move one bone relative to another
• Origin => most stationary end
– Location where the tendon attaches
• Insertion => the most mobile end
– Location where tendon inserts
• Action => the motion or function of the muscle
Figure 8.12
Movement (cont.)
• Generally arranged in opposing pairs
– Flexors- extensors; abductors- adductors
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•
•
•
•
The major actor = Prime mover or agonist
The one with opposite effect = antagonist
Synergists- help prime mover
Fixators- stabilize origin of prime mover
Role of muscle varies with motion
Naming Terms-Table 8.2
• Direction relative to body axes
– e.g. Lateralis, medialis (medius), intermedius, rectus
• Specific regions
– e.g. abdominus, Brachialis, cleido, oculo-, uro-,
• Origin
– e.g. biceps, triceps, quadriceps
• Shape
– e.g. deltoid, orbicularis, serratus, trapezius
Names (Cont.)
• Other features
– e.g. alba, brevis, longus, magnus, vastus
• Actions
– e.g. abductor, adductor, flexor, extensor
• Specific references
– e.g. Buccinator (trumpeter), Sartorius (like a
tailor)
Figure 8-13a
Figure 8-13b
Figure 8.14
Figure 8.15
Figure 8.16
Figure 8.17
Figure 8.18
Figure 8.19
Figure 8.20
Figure 8.21ab
Figure 8.21c
Figure 8.22
Figure 8.23a
Figure 8.23b
Figure 8.24ab
Figure 8.24cd