muscles Flashcards
what is the organization of skeletal muscle?
myofilaments -> sarcomeres -> myofibrils -> muscle fiber -> fascicle (bundle of muscle fiber) -> muscle
epimysium
dense collagenous connective tissue that surrounds entire muscle
perimysium
collagenous connective tissue that surrounds bundles of muscle fibers called fascicles
endomysium
fine sheath of connective tissue composed of reticular fiber that surrounds individual muscle
myotendinous junction
finger-like extensions of the muscle fibers insert into the connective tissue of the tendon
basal lamina
specialized layer of extracellular matrix that supports and separates epithelial cells from the underlying connective tissue
how does the basal lamina provide structural support to the cell?
it binds to the myofiber via dystroglycan-containing complex
how does basal lamina connect to the actin (cytoskeleton)?
basal lamina -> transmembrane protein alpha-dystroglycan then beta -> dystrophin -> actin
myofibers
long, cylindrical and striated
multinucleated with nuclei located at the periphery (at the edge)
myofibrils
aligned in parallel
separated by mitochondria and sarcoplasmic reticulum
composed of filaments called myofilaments
sarcoplasmic reticulum
smooth endoplasmic reticulum
forms an interconnected network of tubules
surrounds each myofibrils
forms chambers called terminal cisternae on either side of the t-tubules
terminal cisternae
chamber formed by SR on either side of the t-tubules
when is Ca2+ stored and released?
stores Ca2+ when muscle is at rest
released in the sacroplasm when muscle is stimulated
t-tubules
deep invagination of sacrolemma
perpendicular to the length of myofiber
how many terminal cisternae are next to the t-tubule?
two tc and a single t-tubule form a triad junction
how are Ca2+ released?
action potential runs along the sarcolemma, reaches the T-tubules, and activates DHPRs.
DHPRs are mechanically coupled to RyRs, which release Ca²⁺ from the sarcoplasmic reticulum into the sarcoplasm, where it initiates muscle contraction.
which side does the myosin head face and why?
it faces outwards because the actin binding site and myosin ATPase site is on it (for contraction to occur)
what happens when Ca2+ is at low concentration?
myosin binding sites on actin are masked by tropomyosin
TnI
TnC
TnT
TnI - binds to actin
TnC - binds to Ca2+
TnT - binds to myosin
what is the movement of troponin upon stimulation?
SR releases Ca2+ into the sarcoplasm
Ca2+ binds to troponin -> changes its conformation
shift in the position of tropomyosin
unmasking of myosin binding sites
myosin binding to actin
what happens to the actin when muscle contracts?
slides but doesn’t shorten
overlap with myosin
what happens to the myosin when muscle contracts?
shortens and overlap with actin
are skeletal muscle voluntary or involuntary? striated or smooth?
voluntary and striated
what is a motor end plate?
a specialized domain of the sarcolemma
highly excitable
responsible for initiating the action potential that propagates across the myofiber -> muscle contraction
what is a neuromuscular junction?
the junction/synapse of the axon terminal of a motor neuron with the motor end plate
how does the electrical signal travel from the brain/spinal cord to the muscle to cause contraction?
- signal originates in the brain/spinal cord
- action potential travels down the axons of motor neuron
- reaches the axon terminal at the neuromuscular junction
- ACh are released into the synaptic cleft
- ACh binds to the receptor on the motor end place and depolarize muscle membrane
- action potential propagates across sarcolemma into the t-tubules
- t-tubules signals SR to release Ca2+ into muscle fiber
- Ca2+ bind to proteins in muscle fiber allowing actin and myosin to slide past each other
where are ACh receptors located?
they are located on the sarcolemma (PM of muscle cell), at the motor end plate
are axons myelinated?
they are generally myelinated outside the muscle cell and unmyelinated at the end of the axon at the neuromuscular junction
where are the ACh neutransmitter stored in?
in the synaptic vesicles
is the junction of muscle fiber folded?
yes
how is Ca2+ restored after muscle contraction?
they are pumped back from the sarcoplasm to the SR via ATPase
what happens in duchenne muscular dystrophy?
the link between the actin cytoskeleton and the basal lamina (with dystrophin and dystroglycan complex) in lost
-> disorganized sarcomeres
where is the cardiac muscle located?
in the heart where it forms a thick layer called myocardium
is cardiac muscle striated and voluntary?
it is striated and involuntary
how are the characteristics of cardiac fibers and how are they joined together?
they are short and branched striated fibers and joined by intercalated disks
zonula adherens
anchor actin myofilament of the terminal sarcomeres to the PM (transverse part of cardiac muscle)
desmosomes
bind fibers together preventing their separation during contraction cycles (transverse part of cardiac muscle)
gap junction in cardiac muscle
rapid flow of information between fibers
how many t-tubule and terminal cisternae are present in cardiac muscle?
one t-tubule and one terminal cisternae that forms a diad
what are the characteristics of smooth muscle?
found in the GI tract, respiratory tract, around blood vessels
involuntary
non-striated fibers and spindle-shaped and tapered
how many nucleus are in smooth muscle?
mono-nucleated
(one central nucleus per fiber)
is there t-tubules and SR cisternae for smooth muscle?
no
tunica media
it is made up of smooth muscle cells and elastic tissue
how does the tunica media differ in arteries and veins?
tunica media is thicker in arteries to sustain higher blood pressure
what are the 2 types of smooth muscle layer?
longitudinal - push food down
circular - constricts to prevent food from going up
what are the contractile apparatus of smooth muscle?
consist of myosin and actin filaments attached to dense bodies in the sarcoplasm or PM plaques
what do smooth muscles have instead of t-tubules?
caveolae - small flasked-shaped invaginations of the membrane
