Unit 5 Notes
To Review…
• Muscle surrounded by
epimysium
• Fascicle surrounded
by perimysium
• Individual muscle
fibers (cells)
surrounded by
endomysium
Let’s zoom in on an individual
muscle fiber…
Note: The sarcolemma (cell membrane) is surrounded
by endomysium (connective tissue).
Let’s zoom in on a myofibril…
Definitions
Sarcolemma: the specific name for the
plasma membrane of a muscle cell;
surrounded by endomysium.
Definitions
Myofibril: contractile organelles found in
the cytoplasm of muscle cells.
Definitions
Sarcomere: tiny contractile units that make
up the myofibril; aligned end-to-end.
Definitions
Myofilaments: filaments composing the
myofibrils; the two types are actin (thin) and
myosin (thick).
Sarcoplasmic reticulum: specialized smooth
ER that surrounds myofibrils; stores calcium
and releases on demand
There are also multiple nuclei and
mitochondria scattered throughout the muscle
fiber, surrounding the myofibrils.
So, in summary, as you work your
way smaller in a muscle….
• Muscle – surrounded by epimysium
• Fascicle – surrounded by perimysium
• Muscle fiber (cell) – surrounded by
endomysium/sarcolemma
• Myofibril – sections called sarcomeres,
surrounded by sarcoplasmic reticulum,
mitochondria, and nuclei
• Myofilaments – 2 types are actin & myosin
The light band is also called the I band, and has a
dark midline interruption called the Z disc.
The Z disc marks the end of the sarcomere, and
is formed by a disc-like membrane.
The dark band is also called the A band, and has
a lighter central area called the H zone (which
contains the M line).
The H zone lacks thin filament, so it looks a bit
lighter than the rest of the A band.
Notice that the I and A bands are responsible for
the striations of skeletal muscle tissue!
So, striations are formed by the locations of
actin and myosin filaments.
Zooming in on a sarcomere…
The bands of myofilaments are actually formed
by many myofilaments packed together.
THICK FILAMENTS…
• Contain myosin protein
• ATPase help split ATP to generate power for
muscle contraction
• Form the “A” band
• Have projections that are myosin “heads” – called
cross bridges
• Cross bridges link thick and thin filaments
together in contraction of muscle
THIN FILAMENTS…
• Contain actin protein
• Also contain tropomyosin protein filaments (which
block specific binding sites) and troponin protein
• Thin filaments anchored to the Z disc (a disc-like
membrane separating the sarcomeres)
• The zone where thin filaments do not exist in the
sarcomere is called the H zone
What functional properties allow a
muscle to perform its duties?
• Irritability
–Ability to receive and respond to a
stimulus
• Contractility
–Ability to shorten when adequate
stimulus is received
What functional properties allow a
muscle to perform its duties?
• Conductivity
–Ability for impulse to travel along
plasma membrane of muscle cell
• Elasticity
–Ability to recoil and resume original
length
What role does the nervous system
play in muscle movement?
• Motor Unit – one
neuron and all
the skeletal
muscle cells it
stimulates
Within a motor unit…
• Muscle Fibers
• Axons
• Axon Terminals
(neuromuscular
junctions)
Nerve endings and muscle fibers
don’t physically touch…
• Neuromuscular
juction – where axon
terminals match up
with muscle fibers
• Snyaptic cleft –
space between nerve
endings and muscle
fibers; chemical
impulses travel here
between nerve
endings and muscle
What steps occur to stimulate
muscle movement?
• 1. Nerve impulse
reaches axon
terminals
• 2. Chemical
Neurotransmitter
(ACh – acetylcholine)
released
• 3. ACh diffuses
across synaptic cleft
and attaches to
receptors
What steps occur to stimulate
muscle movement?
• 4. ACh causes the
sarcolemma to become
temporarily permeable to
Na+
• 5. Na+ rush into the
muscle cell
• 6. Excess of positive
ions creates electric
current (action potential)
• 7. Muscle contracts
(another whole set of
steps!)
• So, we know how muscle contraction is
stimulated… but now we need to know the
steps that help muscle contraction to
happen!
• Called the Sliding Filament Theory
The Sliding Filament Theory
• Muscle fibers activated by nervous system
due to action potential
• Calcium ions (Ca+2) happen to be released
– Do you remember which structure releases
those calcium ions???
The Sliding Filament Theory
• Muscle fibers activated by nervous system
due to action potential
• Calcium ions (Ca+2) happen to be released
– Do you remember which structure releases
those calcium ions???
– THE SARCOPLASMIC RETICULUM
The Sliding Filament Theory
• Release of Ca+2 allows troposin & tropomyosin
to stop blocking binding sites on the actin…
which allows the cross-bridges on the thick
filaments to attach to the binding sites on the
thin filaments
• Let the sliding begin!
http://highered.mcgrawhill.com/sites/0072495855/studen
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http://highered.mcgrawhill.com/sites/0072495855/studen
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yofilament_contraction.html
The Sliding Filament Theory
• Energized by energy from
ATP, cross-bridges attach
and detach from thin
filaments
• Works like an oar to keep
moving thin filaments
closer and closer together
(Attach, pull, detach!)
http://highered.mcgrawhill.com/sites/0072495855/stu
dent_view0/chapter10/animati
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Neurotransmitter released diffuses
across the synaptic cleft and attaches
to ACh receptors on the sarcolemma.
Axon terminal
Synaptic
cleft
Synaptic
vesicle
Sarcolemma
T tubule
1
ACh
ACh
ACh
Net entry of Na+ Initiates
an action potential which
is propagated along the
sarcolemma and down
the T tubules.
Ca2+
Ca2+
SR tubules (cut)
SR
Ca2+
Ca2+
2
ADP
Pi
6
Action potential in
T tubule activates
voltage-sensitive receptors,
which in turn trigger Ca2+
release from terminal
cisternae of SR
into cytosol.
Ca2+
Ca2+
Tropomyosin blockage restored,
blocking myosin binding sites on
actin; contraction ends and
muscle fiber relaxes.
3
Ca2+
5
Ca2+
Ca2+
Calcium ions bind to troponin;
troponin changes shape, removing
the blocking action of tropomyosin;
actin active sites exposed.
Removal of Ca2+ by active transport
into the SR after the action
potential ends.
Ca2+
4
Contraction; myosin heads alternately attach to
actin and detach, pulling the actin filaments toward
the center of the sarcomere; release of energy by
ATP hydrolysis powers the cycling process.
The Sliding Filament Theory
• As this process is
happening in every
sarcomere throughout
the muscle, the muscle
itself is contracting!
• The whole series of
events (beginning with
the nervous system
signal) takes just a few
thousandths of a
second!!!
The Sliding Filament Theory
• Notice in the contracted
muscle, the H zone has
disappeared
• The I band has
shortened significantly
(all that’s left is the Z
disc)
• The A band (the dark
striations!) have stayed
the same thickness
Where’s all this energy coming
from?
• As a reminder, energy comes from
ATP because of breaking a
phosphate bond
• Breaking a bond releases energy
• When this energy is used by your
body, it releases heat
• Because ATP is the only energy
source that can be used to move the
cross-bridges back and forth (which
contract the muscle), ATP must be
regenerated continuously
ATP Regeneration – 3 Sources
• Direct phosphorylation of ADP by creatine
phosphate
– When ATP used, changes to ADP
– Creatine phosphate adds that missing
phosphorous back on!
– PROBLEM: only makes 1 ATP at a time… so
not very much. And, only supplies energy for
15-20 seconds of activity!
– Your body will always do this, but it’s not very
effective. Therefore, we have to have other ways of
supplying energy…..
ATP Regeneration – 3 Sources
• Aerobic respiration
– Occurs in the mitochondria
– Glucose broken down to pyruvic acid
(releasing 2 ATP), and then into carbon
dioxide and water (releasing 34 ATP)
– 36 ATP made for 1 glucose! A lot of energy!
And, can supply energy for hours at a time!
– PROBLEM: NEEDS OXYGEN
– But what if you’re out of oxygen??? Then your
muscles will begin……..
ATP Regeneration – 3 Sources
• Anaerobic glycolysis and lactic acid formation
– Glucose broken down to pyruvic acid,
releasing 2 ATP
– If oxygen present, process continues to the
rest of aerobic respiration…
– BUT… if oxygen is inadequate, or muscle
activity is intense, pyruvic acid is instead
changed to lactic acid
– PROBLEM: Buildup of lactic acid is not good…
promotes muscle fatigue and soreness. And,
only supplies energy for 30 seconds of activity!
ATP Regeneration
• 95% of ATP produced through aerobic
respiration
• C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O + ATP
• If you don’t have proper blood circulation
or breathing, muscles can’t get oxygen
needed for aerobic respiration
• If they can’t get oxygen, they can’t
produce enough ATP, which means
muscles can’t contract!
Muscle Fatigue
• A muscle is fatigued when it is unable to
contract even though it is being stimulated –
means you don’t have ATP to move the
cross-bridges!
• Lack of oxygen can cause…
– Lactic acid buildup (anaerobic glycolysis)
– ATP supply low (production can’t keep up
with usage)
– Muscle will contract less and less
effectively, eventually stopping contraction
completely
Muscle Fatigue
• FYI: When you
breathe heavy after
physical activity, your
muscles are trying to
get enough oxygen
for aerobic respiration
to replace all of the
ATP you used!
Muscle Contraction
• Isotonic contractions
–Myofilaments slide, shortening
the muscle
–Movement occurs – bending
knee, rotating arms, smiling
Muscle Contraction
• Isotonic contractions
– Myofilaments slide, shortening the muscle
– Movement occurs – bending knee, rotating
arms, smiling
• Isometric contractions
– Myofilaments trying to slide, but can’t – just
building up tension (crossbridges are
“rowing”, but actin is not moving together)
– Movement doesn’t occur – object too heavy to
lift, push against wall
Muscle Tone
• Muscle tone – sustained partial contraction
of a muscle; muscle stays firm, healthy,
and ready for action
• Muscle inactivity can lead to muscle
weakness and wasting (this is why Range
of Motion exercises on bedridden people
is important!)
Effect of Exercise on Muscles
• Aerobic exercises include…
–Running
–Jogging
–Biking
–Elliptical
Effect of Exercise on Muscles
• Increase endurance of muscles because
muscle cells will form more mitochondria
and store more oxygen (meaning more
energy for the muscles)
• Also – improves body metabolism,
improve digestion, enhance coordination,
strengthens skeleton, heart & lungs more
efficient
• Muscles do NOT increase in size!
Effect of Exercise on Muscles
• Resistance exercises include…
– Pushing against wall
– Contracting muscles (likes gluteus maximus)
– Lifting weights
• Does increase muscle size!
– Due to enlargement of individual muscle cells
(makes more myofilaments)
– You don’t add more muscle cells – you just
bulk up the ones you already have!!!
–
• NEED BOTH TYPES OF EXERCISES IN ANY
TRAINING PROGRAM!