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The Muscular System
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Unit 1
Chapter 8
Types of muscle & function
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• 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
Sphincters regulate flow in tubes
Maintain diameter of tubes
Move material in GI tract and reproductive organs
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• Smooth- grouped in walls of hollow organs
Muscle Functions
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• Moving substances internally
• Producing heat
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• Produce body movements
• Stabilize body positions
• Regulate organ volume
• Muscle includes: muscle fibers,
connective tissue, nerves & blood
vessels
• Wrapped in Epimysium
• Perimysium surrounds fiber
bundles called fascicles
• Endomysium surrounds each
individual fiber
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Skeletal Muscle Tissue
Skeletal Muscle Tissue
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• Well-supplied with blood vessels and
nerves
• Terminal of a neuron on each
muscle fiber
Figure 8.1
Muscle Histology
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• 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
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Figure 8.2a
Muscle histology (cont.)
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• Throughout sarcoplasm is
sarcoplasmic reticulum
Stores Calcium ions
• Sarcoplasm contains myoglobin
• Along entire length are myofibrils
• Myofibrils made of protein filaments
Come in thick and thin filaments
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Red pigmented protein related to
Hemoglobin that carries oxygen
Figure 8.2b
Sarcomere
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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
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Figure 8.2c
Figure 8.3a
Figure 8.3b
Functional Structure
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Contain myosin binding sites for myosin
head
Also contain tropomyosin & troponin
• Tropomyosin blocks myosin binding site
at rest
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• Thick filament (myosin) has moveable
heads
• Thin filaments (actin) are anchored to Zdiscs
Sliding Filament Mechanism
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• 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.
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Figure 8.4
Neuromuscular Interaction
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• 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
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Neuromuscular Junction
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• neuronal ending to muscle fiber =
Neuromuscular junction
• Synaptic end bulbs (at neuron
terminal)
• Muscular area = Motor end plate
• Between is synaptic cleft
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Release neurotransmitter
Figure 8.5
Action at NMJ
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1. Release of acetylcholine (ACh)
Diffuses across cleft
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2. Activation of ACh receptors
3. Generation of Muscle Action Potential
Repeats with each neuronal action
potential
4. Breakdown of ACh
Contraction Trigger
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• Muscle action potential=> Ca2+
release from Sacroplasmic Reticulum
(SR)
• Ca2+ binds to troponin =>
• Moves tropomyosin off actin sites
=>
• Myosin binds & starts cycle
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• 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
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Contraction Cycle
Figure 8.6
Relaxation
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• Breakdown of Ach to stop muscle
Action potentials
• Ca2+ ions transported back into SR
lowering concentration=>
• tropomyosin covers actin binding
sites
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This takes ATP
Figure 8.7
Muscle Tone
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• Even at rest some motor neuron
activity occurs = Muscle Tone
• If nerves are cut fiber becomes
flaccid (very limp)
• Rapid changes from very low ATP
consumption to high levels of
consumption
• Creatine phosphate (high energy
store)
• Fast & good for ~ 15 sec
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Metabolism
Figure 8.8a
• 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.
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Glycolysis
Figure 8.8b
Aerobic Cellular Respiration
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• Production of ATP in mitochondria
• Requires oxygen and carbon
substrate
• Produces CO2 and H2O and heat.
Fatigue
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• Inability to contract forcefully after
prolonged activity
• Limiting factors can include:
Creatine Phosphate
Oxygen
Build up of acid
Neuronal failure
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Ca2+
• Convert lactic acid back to glucose in
liver
• Resynthesize Creatine Phosphate
and ATP
• Replace oxygen removed from
myoglobin
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Oxygen Use After Exercise
Control of Muscle Contraction
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• Single Action Potential(AP) =>twitch
Smaller than maximum muscle force
• Total tension of fiber depends on
frequency of APs (number/second)
• Total tension of muscle depends on
number of fibers contracting in unison
Increasing numbers = Motor unit
recruitment
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Require wave summation
Maximum = tetanus
Figure 8.9
Figure 8.10
Fiber types
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• Slow oxidative (SO)- small diameter &
red
large amounts of myoglobin and mitochondria
ATP production primarily oxidative
Fatigue resistantLarge diameter = many myofibrils
Many mitochondria and high glycolytic capacity
• Fast glycolytic fibers (FG)
white, fast & powerful and fast fatiguing
For strong, short term use
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• Fast oxidative- glycolytic (FOG)
• 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
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Recruitment
Effects of Exercise
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• SO/FG fiber ratio genetically determined
High FG => sprinters
High SO=> marathoners
Increased diameter and numbers of
mitochondria
• Strength exercise increases size &
strength of FG fibers
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• Endurance exercise gives FG=> FOG
Cardiac Muscle
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• 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
AP- autorhythmicity
• Involuntary
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Cardiac muscle
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• No nerve- internal pacemaker
• Ca2+- from S.R. and extracellular space
• separate cells with gap junctions ->
electrical connections
Figure 15.2b
Smooth muscle
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Form sheets and are autorhythmic
Contract as a unit
• Multi-unit typeeach has own nerve and can contract
independently
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• Involuntary
• In internal organs
• Filaments not regular so not striated
• Visceral (single unit) type or
• Graded contractions and slow
responses
• Often sustain long term tone
• Often triggered by autonomic nerves
• modulated chemically, nerves, by
mechanical events (stretching)
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Smooth Muscle
Figure 8.11
Aging
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• Like bone there is a slow progressive
loss of skeletal muscle mass
• Relative number of SO fibers tends
to increase
Movement
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Location where the tendon attaches
• Insertion => the most mobile end
Location where tendon inserts
• Action => the motion or function of the
muscle
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• Move one bone relative to another
• Origin => most stationary end
Figure 8.12
Movement (cont.)
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• Generally arranged in opposing pairs
• 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
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Flexors- extensors; abductors- adductors
Naming Terms-Table 8.2
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• Direction relative to body axes
e.g. Lateralis, medialis (medius), intermedius,
rectus
• Specific regions
e.g. abdominus, Brachialis, cleido, oculo-, uro-,
e.g. biceps, triceps, quadriceps
• Shape
e.g. deltoid, orbicularis, serratus, trapezius
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• Origin
Names (Cont.)
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• Other features
e.g. alba, brevis, longus, magnus,
vastus
• Actions
• Specific references
e.g. Buccinator (trumpeter), Sartorius
(like a tailor)
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e.g. abductor, adductor, flexor, extensor
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