SK 1 Skeletal Muscle and Reflexes Flashcards
what is the anatomical organization of a skeletal muscle
-whole muscle
-muscle fascicles
-muscle fibers (cells)
-myofibrils
-myofilaments (thick and think) arranged in sarcromeres
what is a sarcomere
functional unit of cardiac and skeletal muscle
what is the dark band
the A band
what is the light band
the I band
what creates the striated appearance in skeletal and cardiac muscle
alternating A bands and I bands
what happens to the I band, the H zone and the A band in muscle contraction
I band: shortens
H zone: shortens
A band: stays the same
what does the I band contain
thin filaments
what does the A band contain
entire length of thick filaments
what does the H zone contain
only thick filaments
what is anchored at the Z line
thin filaments
what does the M line link
the central regions of thick filaments
how do sarcomeres change in length during contraction
they shorten
what is the thin filament composed of
actin, tropomyosin and troponin
what does actin contain
G-actin molecules, the active site that binds myosin
what does troponin bind
actin, tropomyosin and calcium
what regulates when contraction can happen
troponin and tropomyosin
what is the thick filament made of
myosin
what are the 3 globular proteins of troponin
T, C and I
what is the dystropin-glycoprotein complx
dystrophin protein connects thin filaments to glycoproteins in sarcolemma
what does the dystropin-glycoprotein complex do
provides scaffolding for sarcomeres
what are the types of muscular dystrophy
-duchenne
-beckers
-myotinic
-oculopharyngeal
-limb girdle
what is similar and different about duchennes MD and Beckers MD
similar: both produce less dystrophin
difference: duchennes produces much less dystrophin than beckers
what happens in myotonic MD, what type of inheritence and when is onset
muscle contracts and struggles to relax, autosomal dominant, most common form of MD in adult
what NT does the alpha motor neuron release and what does it bind to
releases AcH which binds to a nicotonic receptor on the muscle fiber
how does botox work
inhibits the release of ACH at the NMJ
what can botox be used for in dentistry
bruxism, sialorrhea, masseteric hypertrophy
what are the 3 phases of muscle twitch
latent, contraction, relaxation
what is the latent period of muscle twitch
time between AP and when muscle starts producing force
what is required to occur before contraction can happen
intracellular calcium in muscle fiber must increase
what does tropomyosin do in resting muscle
prevent a strong bond between the myosin head and G actin molecules
what happens when troponin binds to calcium
tropomyosin is pulled away from the myosin binding site and allows for the power stroke
what is calcium released from
sarcoplasmic reticulum
what are T-tubules and what do they do
invaginations of sarcolemma that allow for APs to be transmitted deep in the cell
what are terminal cisterna
portion of SR that contract T tubules
what is the mechanism of action of calcium release from the SR
AP travels down the membrane and T tubules and activates voltage sensitive dihydropyridine (DHP) receptors on the T tubules. these open calcium channels (ryanodine receptors) on the SR
what is the mechanism of action with ATP in muscle contraction
-ATP binding to the myosin head breaks the cross bridge between actin and myosin
- energy released from ATP hydrolysis by the myosin head provides energy for cocking the myosin head
- release of inorganic phosphate from the myosin head provides energy fr the power stroke which shortens the sarcomere
muscle cells only have enough ATP for ~ ___ twitches
8
what does the source of ATP in a muscle fiber depend on
-the metabolic enzymes are present in the cell (glycolytic vs oxidative fibers)
-the intensity of the exercise
what are the sources of ATP
-aerobic and anaerobic metabolism
- phosphocreatine
what does creatine kinase blood levels measure
if damage to muscle tissue has occurred
fatigued muscles:
-have decreased tension generation
- take longer to contract
- relax more slowly and may not completely relax
what are the theories for fatigue
- change in membrane potential
-decreased ACH
-blockage of blood flow - central fatigue
- increased metabolic byproducts (lactate)
-depleted glycogen
what must be present for cross bridge cycling
calcium AND ATP
explain cross bridge cycling (sliding filament theory)
-rigor state: myosin and actin are tightly bound
-ATP binds myosin, decreases its affinity for actin and the two separate
-myosin head moves in the direction of the Z line, ATP is hydrolyzed
- myosin binds the next actin and power stroke occurs
-ADP is released and the actin and myosin resume the brief rigor state
what is required to relax skeletal muscle
-alpha motor neuron must stop firing
- cytosolic calcium concentrations must decrease
how do cytosolic calcium concentrations decrease to relax skeletal musclr
-calcium ATPases on SR remove calcium from cytosol
-tropomyosin moves and covers actins myosin binding site
- actin slowly slides back to its original resting place and the sarcomere returns to its original length
why doesnt ATP need to be removed with calcium to relax skeletal muscle
ATP must be present to release myosin from actin
what are the 3 types of skeletal muscle fibers
-slow twitch (type 1)
-fast twitch- oxidative glycolytic (type 2A) and glycolytic (type 2B)
describe the color, glycolytic capacity, oxidative capacity and fatigue of type 1 fibers
color-red
-glycolytic capacity- moderate
-oxidative capacity-high
-fatigue- slow
describe the color, glycolytic capacity, oxidative capacity and fatigue of type 2A fibers
color-red
-glycolytic capacity- high
-oxidative capacity-moderate
-fatigue- fast resistant
describe the color, glycolytic capacity, oxidative capacity and fatigue of type 2B fibers
color-white
-glycolytic capacity- high
-oxidative capacity-low
-fatigue- fast fatigable
how can skeletal muscle fibers change in response to use
in size and fiber types
how do fibers change in size
hypertrophy, atrophy, sarcopenia
what are human jaw closing muscle fiber types
mixture of type 1 and type 2 fibers, the type 2 fibers are much smaller than the type 1
how does the jaw muscles change with age
thickness decreases and type 1 fibers number decreases and type 2 increase
what is a motor unit
the alpha motor neuron and the muscle fibers it innervates
what are motor units recruited in order of
size, small recruited first (type 1) but small motor units control fewer fibers
what does an increase in the number of motor units activated increase
the total tension produced by contraction of a muscle
what are small motor units composed of
slow twitch oxidative fibers
which muscle fibers are the smallest
type 1
which muscle fibers have the lowest threshold for firing
type 1
what are larger motor units composed of
fast twitch glycolytic fibers
what determines the type of contraction
the amount of load and the force the muscle generates
what is isometric contraction and what is the result
force produced is less than the load resulting in no movement
what is isotonic contraction and what is the result
force produced is great enough to move a load
has the muscle shortened in isometric contraction
no, but sarcomeres shorten to generate force
when does the contraction become isometric
when the muscle cannot move the load anymore
what does the force of contraction do the cross bridges and force
increases both
how do you increase total force
-increasing the frequency of fiber activation (AP)
-increasing the number of muscle fibers contracting (motor unit recruitment)
for a single muscle twitch, the tension developed is altered by ____
sarcomere length
what is optimal length
the best degree of overlap between thick and thin filaments
- greatest number of actin/myosin cross bridges can form resulting in maximal tension production
what does the velocity of contraction depend on
the load a fiber is contracting against and the type of fibers
the greater the load the ___ the speed of contraction
slower
what are the skeletal muscle reflexes
-stretch reflex (muscle receptors)
- golgi tendon reflex (tendon receptors)
what fibers are found in the muscle spindle
1a fiber (sensory) and A gamma (motor)
what sensory fibers are found in the golgi tendon
1b fiber
what are extrafusal muscle fibers
skeletal muscle fibers/cells that produce the contraction
what motor neuron is in the extrafusal fibers and what does it do
alpha motor neuron- efferent neuron that releases Ach and causes contractino of the extrafusal fiber (skeletal muscle)
what is the muscle spindle
small structure within the extrafusal fibers that contain intrafusal fibers that have sensory nerve endings wrapped around them that are sensitive to changes in muscle length
what motor neurons are in the muscle spindle and what does it do
gamma motor neuron: efferent neuron that causes contraction of intrafusal fibers to mimic what the EF fibers are doing
do jaw openers or jaw closers have a lot of muscle spindles
jaw closers
what is the muscle spindle sensitive to
changes in muscle length
what is the stimulus for the muscle spindle reflex
stretch
what is the mechanism of action for muscle spindle reflex stimulus
-sensory fibers get squeezed in muscle spindle when stretched
- afferent information enters the spinal cord
- activates alpha motor neuron to stimulate extrafusal fiber contraction and inhibition of alpha motor neuron to antagonistic muscles
-activates the gamma motor neuron to stimulate intrafusal fiber contraction
what would happen if intrafusal fibers did not contract
the sensory fibers would not be able to sense a further change in muscle length since there would be slack
what is the effect of muscle spindle activation
contraction
describe the muscle spindle reflex components
dynamic (immediate) and static (maintains tone- constant contraction)
anytime the alpha motor neuron is activated what else is activated
the gamma motor neuron
where are mechanosensitive receptors found at in golgi tendon organs
at the junction of tendons and muscle
what are mechanoreceptors sensitive to in golgi tendon organs
changes in force
what type of reflex is the golgi tendon organ reflex
protective
what is the mechanism of action of the golgi tendon reflex
-extreme stretch of the tendon will squeeze the GTO and afferent neurons will send information into the spinal cord
- stimulates an inhibitory interneuron
- this neuron decreases the activity of the alpha motor neuron
-skeletal muscle contraction is decreased
describe control of mastication
-mix of voluntary, reflex, and subconscious processes
- voluntary control is usually bilateral although people favor one side of the mouth over the other
what is mastication controlled by
central pattern generator of the brainstem
what are the neural components of mastication
CPG regulated by corticobulbar pathway
where is the CPG located
in pons and medulla
what nerve does the CPG use to send signals
trigeminal
describe the jaw opening reflex
pain inhibits the alpha motor neuron of jaw closing muscles such as when you bite down on your tongue or spoon with incisors, jaw opens as a result
describe the jaw jerk reflex
-strong tap to the chin stretches the jaw closers
-jaw closers respond by contraction so the jaw closes
-stretch reflex
how do you determine the force needed to take a bite of something hard
-CPG starts chewing by activating jaw closing muscles
- the force of contraction is initally insufficient to overcome load (isometric)
- intrafusal fibers in muscle spindle are contracting and stretching the muscle spindle and signals further contraction of jaw closing muscles
-constant feedback from the muscle spindle in the jaw closing muscles is sufficient to overcome the load (isotonic)
how do muscle spindles have protective action
-when nut cracks (for example) the force is now greater than the load
-there is more slack in muscle spindle now which leads to less activation of the muscle due to decreased alpha/gamma co-activation
what distance separates the occlusal surfaces at rest
3-8 mm
what happens to the jaw when you jog or run
force is applied to the mandible
- which each step there is a down movement of the mandible in relation to the head
- this movement stretches the muscle spindle which causes a reflex contraction of the jaw closing muscles
what keeps the jaw in the rest position
muscle spindles
