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MED221A/B Lectures > MSK 14 - Skeletal muscle 1 > Flashcards

MSK 14 - Skeletal muscle 1 Flashcards

1
Q

what are the 4 roles of skeletal muscle

A

produce force and movement necessary for life

provide support

produce heat

regulate glucose homeostasis

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2
Q

what is the morphological classification of muscles - based on structure

A

striated = skeletal and cardiac muscle

non striated = smooth muscle

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3
Q

is skeletal muscle voluntary or involuntary movements

A

voluntary

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4
Q

what happens at the neuromuscular junction in terms of AP and what does it cause

A

nerve action potential arrives via alpha motor neuron to the neuromuscular junction and AP spreads along fibres leading to excitation contraction coupling

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5
Q

what happens in excitation contraction coupling in terms of Ca2+

A

calcium has to be released very quickly to floor the cytosol in order to enable contractive filaments to be activated by cross bridge cycle and force production

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6
Q

what are the 2 myofilament proteins found in a sarcomere

A

myosin and actin

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7
Q

what is a muscle cell called

A

myofibre

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8
Q

what is the smallest unit of a muscle

A

myofibres

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9
Q

what is wrapped around the myofibre

A

endomysium

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10
Q

what is beneath the endomysium

A

plasma membrane - sarcolemma

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11
Q

what is a bundle of myofibres called in a muscle

A

fascicles

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12
Q

what is wrapped around a fascicle

A

perimysium

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13
Q

what wraps around a bundle of fascicles

A

epimysium - outer connective tissue

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14
Q

what is the structure of skeletal muscle fibres in terms of innervation, branched/unbranched and shape

A

innervated via neural networks

unbranched long cylindrical (varying lengths)

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15
Q

how are skeletal muscle fibres arranged relative to one another and where are they connected to one another

A

stacked side by side but not connected to one another apart from at the ends of the muscles fibres to the tendon

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16
Q

why are skeletal muscles multinucleated

A

during formation of skeletal muscle cells precursor myoblasts that each have a nucleus and fuse together to form muscle fibre

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17
Q

do myofibres have connections to other myofibres

A

no

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18
Q

describe T tubules in terms of what they are, their orientation and what they do

A

T tubules are extensions of the sarcolemma which can be transverse or longitudinal

APs propagate along surface of sarcolemma and down the T tubules which activates contraction and synchronises activation

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19
Q

what does the sarcoplasmic reticulum do

A

store calcium ions

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20
Q

what does calcium ions do

A

triggers cross bridge cycle

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21
Q

what is a myofibril a bundle of

A

a bundle of contractile proteins

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22
Q

what is the A band of the myofibril made up of and what is its appearance

is it isotropic/anisotropic

A

made up of myosin filaments (dark/thick)

anisotropic

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23
Q

what is the I band of the myofibril made up of and what is its appearance

is it isotropic/anisotropic

A

thin actin filaments

isotropic

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24
Q

what holds the I band in position on a myofibril

A

Z disc

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25
what does the Z disc hold in position
the I band
26
where is the M line in a myofibril and what does it do
middle of the H zone gives contractile proteins structure
27
what is a sarcomere where are its borders
one contractile unit of myofibiril from z disc to z disc
28
what are the junctional areas of the sarcoplasmic reticulum near the T tubules called
terminal cisterna
29
what is the name of the structure of the 2 terminal cisternae and the T tubule
triad
30
where are T tubules located in relation to the myofibril zones
at the junction of overlap between the A and I bands
31
what is calsequestrin and what does it do
protein within the sarcoplasmic reticulum that binds the Ca2+ (can bind aorund 40 Ca2+ per protein)
32
what receptors does calsequestrin link to and what does this do
links to ryanodine receptors that are release channels from the SR
33
what does the triad allow
the very small gap between the T tubule and terminal cisternae allows AP excitation to be transferred to the Ca2+ release channels (Ryanodine receptors) at the SR junctional regions
34
what are the 4 sarcomeric proteins that are associated with the thin actin filaments
titin troponin nebulin tropomyosin
35
what does titin do
holds the filamentous actin (F actin) in place and connects Z disc to thick filaments/myosin acts as spring because during contractions, Z disc moves closer to one another so titin protein coils up and holds thick filaments in position
36
what structures does the titin protein connect
Z discs and thick/myosin filaments
37
thin actin filaments are held together where at each end of the sarcomere
at the Z disc
38
what does the hinge region of the myosin filament allow
allows movement of the head by 45 degrees and causes Z discs to move closer together
39
what do the myosin heads contain
contains ATPase and binding sites for actin/thin filaments
40
how many heads are on each myosin filament
2
41
what is the actin chain made up of
globular G-actin molecules
42
what structures on actin contain the myosin binding sites
troponin
43
what is troponin
troponin C is on the actin filaments and contains the myosin binding sites
44
what holds the G-actin molecules together
nebulin
45
what does nebulin do
holds the G-actin molecules together
46
what does tropomyosin do
covers myosin binding sites on actin filaments
47
what covers myosin binding sites on actin filaments
tropomyosin
48
what happens to tropomyosin at rest
at rest there is a low Ca2+ concentration so tropomyosin blocks the myosin binding sites on actin and so stops force production when youre at rest
49
what happens to troponin C when there is Ca2+ bound to it
there is movement of troponin I and troponin T which moves aside tropomyosin and exposes the myosin binding sites on actin
50
what band does myosin form
A band
51
what is the orientation of myosin filaments in terms of their tail and head what does this mean for the different zones of a myofibiril
tail faces into the M line, repeated staggered array of heads so H zone has no myosin heads = no cross bridge possible in that area
52
what is the actin filament composed of
composed of globular (G) actin molecules linked to two strands twisted into a helix to form filamentous (F) actin
53
is actin held at fixed lengths
yes by proteins
54
what band does actin filaments make up
I bands
55
how are tropomyosin orientated relative to the F actin
rod shaped Tm lies along each F actin in a groove
56
what other proteins is tropomyosin associated with and what do they comprise
3 proteins TnT, TnC, TnI comprises the troponin complex
57
of TnT, TnC, TnI where does Ca2+ bind
on TnC
58
what does nebulin do to help the F actin filaments
aligns twisted F actin filaments
59
in the troponin complex what does TnT do
Troponin tropomyosin positions the complex on the tropomyosin molecule and holds tropomyosin in place
60
in the troponin complex what does TnC do
troponin calcium contains the Ca2+ binding sites
61
how many Ca2+ binding sites are there on TnC and what is the affinity strength
up to 4 Ca2+ binding sites for each TnC - some are high affinity some are low affinity
62
in the troponin complex what does TnI do
troponin inhibitor binds actin and inhibits the myosin head from binding to the actin binding site when cytosolic Ca2+ is low
63
what two components make up the Ca2+ sensitive switch
Tn complex (TnC, TnT, TnI) and Tropomyosin
64
what does the Tn complex and Tm create
the Ca2+ sensitive switch
65
what happens when Ca2+ is high and bound to TnC binding site what needs to be present for binding to continue
it moves tropomyosin and starts the cross bridge cycle as long as there is ATP then all myosin heads wants to bind to myosin binding site
66
the ability of a muscle fibre to develop force depends on what
the interaction of contractile proteins actin and myosin
67
myosin has a high affinity for actin binding site when what is bound
when ADP.P is bound
68
when myosin is bound with ADP.P what effect does this have on the affinity for actin binding site
increases affinity
69
in the relaxed state what orientation are the myosin heads in and what is the binding site like
myosin heads at 90 degrees tropomyosin blocks the binding site from myosin head
70
describe the sequence of events of how a muscle shortens due to actin and myosin in the cross bridge cycle
Ca2+ levels increase in cytosol Ca2+ binds to troponin troponin-Ca2+ complex pulls tropomyosin away from actin myosin binding site myosin binds strongly to actin and completes power stroke - swing of myosin head brings Z discs closer together in sarcomere actin filament moves and muscle shortens
71
describe the release of myosin from the myosin actin binding site and how this happens - 5 steps
ATP binds to myosin and myosin head dissociates from actin ATPase of myosin head hydrolyses ATP to ADP and Pi, myosin head remains bound myosin head swings over and binds to new actin molecule at 90 degrees (relaxed state) release of Pi initiates power stroke, myosin head rotates and filaments slide end of power stroke myosin releases ADP and remains in rigor process repeats
72
what happens if there is no ATP and the end of the cross bridge cycle what is this state called
myosin head remains tightly bound to actin - rigor state
73
what are the 2 roles of ATP in the cross bridge cycle
ATP binding to myosin breaks link between actin and myosin allowing cycle to repeat ATP hydrolysis provides energy from movement and force development in the cross bridge cycle
74
when does relaxation come about for a muscle
when Ca2+ concentration in cytosol decreases
75
how is Ca2+ taken back up and where does it go
taken back up into longitudinal sarcoplasmic reticulum membrane by ATPase (calcium pump) once back in SR, Ca2+ binds to calsequestrin
76
when at rest and cytosolic Ca2+ levels are low, what inhibits cross bridge cycle
thin filament proteins troponin (TnC, TnI, TnT) and tropomyosin
77
cross bridge cycling occurs and generates force as long as what two conditions are met
as long as Ca2+ remains high and ATP is present
78
what structure hangs in the space between the T tubule and the junctional SR at the triad
the extracellular portion of the ryanodine receptor (electron dense feet region of ryanodine receptor)
79
what do the ryanodine receptors do
link proteins in T tubule membrane dihydropyridine receptor (DHPR)
80
how does the DHPR and ryanodine receptors interact
Ap down the T tubule depolarises the membrane around the DHPR and causes an interaction between the feet regions of the ryanodine recpetors and DHPR opens the ryanodine receptors
80
what type of receptor is the DHPR
voltage gated Ca2+ channels in sarcolemma
81
are DHPRs functional Ca2+ channels in skeletal muscle why
no because there is no effective transfer of Ca2+ across the T tubule membrane
82
what are ryanodine receptors (RyR)
Ca2+ release channels of SR
83
what does it mean when we say that troponin C binds up to 4 Ca2+ cooperatively
when one or 2 sites bind Ca2+, the other sites bind Ca2+ more easily
84
how many high affinity and low affinity binding sites are there on troponin C
2 high affinity 2 low affinity
85
what type of RyR is in skeletal muscle
RyR1
86
where do the RyRs exist
at the terminal cisternae of junctional SR
87
what inhibits skeletal RyR activation
cytosolic Mg2+
88
how does voltage sensor DHPR activation overcomes Mg2+ inhibition
sense depolarisation of membrane around DHPR and causes them to interact with RyR and opens them
89
what overcomes the Mg2+ inhibition of RyR
DHPR activation
90
what are the benefits of voltage dependent excitation contraction coupling - 4 reasons
rapid response doesnt depend on sarcolemal membrane channels opening and current flow no reliance on sarcolemme diffusion muscle contraction can occur in abscence of extracellular Ca2+

MED221A/B Lectures (23 decks)

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