SM 1 Flashcards
Sarcomeres Shorten During Contraction
• Zone of Overlap —
• I-Band —
• H-Zone —
Increases
Decreases
Decreases
The thin filament
is composed of actin (with G-actin molecules, the active site which binds myosin), tropomyosin, and troponin (which binds actin, tropomyosin, and calcium).
The myosin (thick) filament
has multiple cross-bridges where the “heads” can bind to the G-actin molecule. Myosin also functions as an ATPase enzyme.
Dystrophin Protein connects thin filaments to
glycoproteins in sarcolemma
Dystrophin-Glycoprotein Complex provides
scaffolding
for sarcomeres
Muscular Dystrophies (5)
a. Duchenne
b. Beckers
c. Myotonic
d. Oculopharyngeal
e. Limb Girdle
The alpha motor neuron
releases – which binds to
a
ACh
nicotinic ACh receptor (NM)
on the muscle fiber
Botulinum toxin A inhibits the
release of
ACH at the neuromuscular junction. Botox can be used in dentistry (Bruxism, Sialorrhea, Masseteric Hypertrophy, etc.
For contraction to occur, the intracellular calcium
in the muscle fiber must —
increase
in resting muscle,
tropomyosin prevents a
strong bond between the
myosin head and G-actin
molecules
When troponin binds to cytosolic Ca++, tropomyosin is pulled away from the myosin binding site, and allows for the
power stroke
Sarcoplasmic Reticulum (SR):
modified ER that sequesters Ca2+
Transverse (T)-Tubules:
invaginations of sarcolemma
Terminal Cisternae:
Portion of SR that contact T-tubules
Calcium is released from the Sarcoplasmic Reticulum (2)
a.The AP travels down the membrane, down T-tubules, & activates
voltage-sensitive dihydropyridine (DHP) receptors on the T-
tubules. These in turn open calcium channels (ryanodine
receptors) on the SR. Calcium goes from SR to the sarcoplasm.
b.The intracellular structure of myocytes ensures spread of action
potential (and calcium) throughout the cell
ATP is Necessary for Contraction (3)
- ATP binding to the myosin head breaks the cross-bridge
(connection between actin and myosin). - Energy released from ATP hydrolysis by the myosin head
provides energy for cocking the myosin head (myosin is
now in the high energy form). - Release of inorganic phosphate from the myosin head
provides energy for the POWER STROKE (myosin head
pulling actin towards the center of the sarcomere). This
shortens the sarcomere.
Muscle cells only have enough ATP for ~– twitches.
8
Both (2) produce ATP for muscle
fibers.
Aerobic and Anaerobic Metabolism
However, the contribution of each in a specific muscle fiber depends on (2)
(1) the metabolic enzymes are present in the cell (ex. glycolytic
fibers versus oxidative fibers)
(2) the intensity of the exercise.
Sources of ATP (3)
aerobic/anaerobic metabolism
phosphocreatine
Measurement of Creatine
Kinase (CK) levels in the
blood is done to determine if
damage to muscle tissue (skeletal and cardiac) has occurred (ex. heart attack or muscular dystrophy). Different isoforms of CK are found in skeletal versus cardiac muscle.
Fatigued muscles: (3)
1.have decreased tension generation,
2.take longer to contract
3.relax more slowly and may not completely
relax.
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Theories for fatigue: (6)
- Change in membrane potential
- Decreased ACH
- Blockage of blood flow
- Central Fatigue
- Increased metabolic byproducts
- Depleted glycogen
Both (2) must be present for cross-bridge
cycling.
Calcium and ATP
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Cross-Bridge Cycling (Sliding Filament Theory) (5)
A. Rigor State: myosin and actin are tightly bound
B. ATP binds myosin, decreases its affinity for
actin, and the two separate
C. Myosin head moves in the direction of the Z line,
ATP is hydrolyzed
D. Myosin binds the next actin (one closer to Z line)
and power stroke occurs (pulls actin toward the
M Line)
E. ADP is released and the actin and myosin
resume the brief rigor state
Relaxation of Skeletal Muscle (2)
- The alpha motor neuron must stop firing
* Cytosolic (intracellular) calcium concentrations must decrease
Cytosolic (intracellular) calcium concentrations must decrease (3)
A. Calcium ATPases on SR remove calcium from
cytosol
B. Tropomyosin moves, and covers actin’s myosin-
binding site
C. Actin slowly slides back to its original resting place
and the sarcomere returns to its original length.
For relaxation to occur, — must be removed,
but — must be present to release myosin from
actin. Otherwise, rigor state is maintained.
Rigor Mortis?
Calcium
ATP
Diversity of Skeletal Muscle Fibers
3 Types
Slow-Twitch: Type I
Fast-Twitch:
Oxidative-Glycolytic: Type IIA
Glycolytic: Type IIB
Changes in Size (3)
- Hypertrophy
- Atrophy
- Sarcopenia
The thickness of the jaw muscles decreases
significantly with
age, which is caused primarily by a
decrease in the cross-sectional area of their
fibers. These
changes might be explained by the
progressive reduction in the number and the
total duration of activity bursts per day with
age.
Certain Characteristics of Skeletal Muscle Fibers
Can Change in Response to Use (2)
changes in size
changes in fiber types
Human jaw muscles are different from
other skeletal muscle. Human jaw
closing muscles are composed of a
relatively homogenous mixture of
type I
and II fibers, the type II fibers being
much smaller than the type I
The fibre-type composition of the jaw muscles also changes with age. In the jaw-closing muscles of elderly subjects, the proportion of pure type -- fibres decreases, while the proportion of pure type -- fibres and that of hybrid fibres increase
I
II
Motor Units:
a motor unit is the alpha motor neuron
and the muscle fibers it innervates
Motor units are recruited in order of size (2)
a. Small motor units are
recruited first
b. Smallest motor units
control fewer fibers
An increase in the number of motor units activated increases the total tension produced by contraction of a muscle—
SPATIAL
SUMMATION
— Recruitment
Asynchronous
Small motor units (X) are composed of
slow-twitch
oxidative fibers. They have the lowest threshold for
firing and are recruited first.
Larger motor units (Z) are composed of
fast-twitch
glycolytic fibers. They have the highest threshold
and are recruited last
Motor units in jaw muscles are
restricted to specific areas of the
jaw muscles, which permits
differential control of separate
muscle portions.
Types of Contraction: the amount of (2) determines the type of
contraction
load and the
force the muscle generates
ISOMETRIC CONTRACTION.
Force produced is
less than the load; no movement.
ISOTONIC CONTRACTION.
Force produced is
great enough to move a load
The force of contraction increases until the isometric contraction becomes an
isotonic
contraction—
NOT always
possible!
The contraction with the -- gram load is isometric because the muscle is not strong enough to move the load and shorten. The maximum isometric contraction only produces -- grams of tension.
20
17
ncrease total force by (2)
(1) increasing the frequency of fiber activation
and/or
(2) increasing the number of muscle fibers contracting (motor
unit recruitment).
An increase in the frequency of α motor neuron
stimulation will increase the amount of
tension produced.
The tension produced in response
to each action potential will sum if the muscle
has not yet completely relaxed.
For a single muscle twitch (one muscle fiber), the
tension developed is altered by
sarcomere length
Optimal length is where there is the best degree of overlap
between the
thick and thin filaments.
At the optimal length the greatest number of actin/myosin cross- bridges can form, which results to
maximal tension
production.
Tension is reduced if the muscle (2)
is not
stretched enough or if it
stretched too much.
The velocity of contraction (distance moved/time)
depends on the
load a fiber is contracting against.
The greater the load, the
– the speed of
contraction.
SLOWER
Load isn’t the only factor that dictates the speed of contraction. — dictates it, too. For example,
Fiber Type
Type I fibers have an isoform of myosin with
slower ATPase activity while type II fibers have fast myosin ATPase.
Skeletal Muscle Reflexes (2)
A. Stretch Reflex (Muscle Receptors)
B. Golgi Tendon Reflex (Tendon
Receptors)
Extrafusal Fibers
Skeletal muscle fibers/cells that produce the
contraction.
Alpha Motor Neuron:
Efferent neuron that
releases ACh and causes contraction of
the extrafusal fiber (skeletal muscle).
Muscle Spindle
Small structure within the extrafusal fibers that
contains Intrafusal Fibers that have sensory
nerve endings wrapped around them that are
sensitive to CHANGES IN MUSCLE LENGTH.
Gamma Motor Neuron:
Efferent neuron
that causes contraction of intrafusal
fibers so they mimic what the EF fibers
are doing.
Jaw --- have a lot of muscle spindles. Jaw --- have few to none.
closers
openers
Components of the Muscle Spindle:
sensitive to
muscle length
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The Stimulus for the Muscle Spindle Reflex is Stretch
- When the muscle stretches, the sensory fibers of the muscle
spindle are squeezed. - Afferent information enters the spinal cord and
a. Activates the alpha motor neuron.
i. This stimulates muscle (extrafusal fiber) contraction.
ii. There is also simultaneous inhibition of the alpha motor
neuron of antagonistic muscles.
b. Also activates the gamma motor neuron.
i. This stimulates intrafusal fiber contraction.
ii. If the intrafusal fibers did not contract, the sensory fibers
would not be able to sense a further change in muscle
length since they would be slack.
The Effect of Muscle Spindle Activation is
Contraction.
The reflex has dynamic (immediate) and static (maintains
tone—constant contraction) reflex components
Anytime the alpha motor neuron is activated, the – motor
neuron is also activated.
gamma
Anytime the alpha motor neuron is activated, the gamma motor neuron is also activated. This allows the muscle spindle to maintain
sensitivity
to changes in
muscle length.
if the intrafusal fibers did not contract, the sensory fibers would not
be able to sense a further change in
muscle length since they
would be slack.
Golgi Tendon Organs:
Mechanosensitive receptors found at the junction of tendons and muscle. Sensitive to a change in FORCE.
The Golgi Tendon
Organ Reflex is a
— reflex.
protective
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The Stimulus for the Golgi Tendon Organ
(GTO) Reflex is Tendon Stretch (2)
1. Tendons stretch in response to contraction (particularly isometric —maximal—contractions) 2. Extreme stretch of the tendon will squeeze the GTO and afferent neurons will send information into the spinal cord a. Stimulates an inhibitory interneuron. b. This neuron decreases the activity of the alpha motor neuron c. Skeletal muscle contraction is decreased (relaxation)
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control of mastication (4)
• A mix of voluntary, reflex and central (subconscious)
processes.
• Requires coordination of muscles controlling lips, tongue
and cheeks.
• Voluntary control is usually bilateral, although people tend
to favor one side of the mouth over the other.
• Central component; mastication is a cyclical movement
with develops early in life. It is controlled by the Central
Pattern Generator (CPG) of the brainstem, which when
stimulated, elicits rhythmic, coordinated activation and
inactivation of jaw-closers and jaw-openers.
The — cannot by itself adjust muscle force to
deal with changing conditions that occur when different foods are
chewed. Reflexes are important modifiers of force.
Central Pattern Generator
Input from higher cortical regions can regulate
the CPG frequency via the
Corticobulbar
Pathway
Jaw opening reflex (2)
– Pain inhibits the alpha motor neuron of jaw closing muscles such as
when you bite down on your tongue, a metal spoon, with your
incisors
– Jaw opens
Jaw jerk reflex (3)
– Strong tap to the chin stretches the jaw closers – The jaw closers respond by contraction so the jaw closes – Stretch reflex
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How do you determine the force needed to
take a bite of something hard?
• CPG starts chewing by activating jaw closing muscles
• When the jaw closing muscles connect with the food (ex.
carrot), they initially meet with resistance:
– The force of contraction is initially insufficient to
overcome the load (carrot) – isometric contraction
– Intrafusal fibers in muscle spindle are still contracting and
stretching the muscle spindle so this signals further
contraction of jaw closing muscles
• Constant feedback from the muscle spindle in the jaw
closing muscles is sufficient to overcome the load of the
carrot — isotonic contraction
With subsequent chewing strokes, the response is greater and
matches the load of the food with appropriate force more —.
quickly
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Protection by Muscle Spindles
• To crack a nut, a tremendous amount of force is required.
Once the nut cracks, there is potential to damage the intra-
oral structures.
• Muscle spindles decrease the contraction in these
situations.
– When the nut cracks, the force is now greater than the
load (nut).
– At this point, there is more slack in the muscle spindle
which leads to less activation of the muscle (due to
decreased alpha/gamma co-activation).
Powerful isometric contractions stimulate both jaw closing and
opening muscles. The jaw openers keep the jaw from
snapping shut.
