muscles Flashcards
what muscle in the body makes the greatest force
masseter
what are the 3 types of muscle and is each on striated or not
1) skeletal (“striated” as are striped as shown on image) = voluntary (neural)
2) cardiac (“striated” as are striped)
3) smooth (NOT “striated”)
explain the structure of muscles in the oesophagus and chewing
- striated muscle at v top
- goes down into smooth muscle into the gut which is NOT striated
- so only the muscle at top is voluntary muscle
what is
a) similar
b) different
in the different muscle types
a) molecular components
b) molecular / cellular organisation
what can we see on a cross section of skeletal muscle
around the muscle fascicle
- layer of connective tissue around the fassicles (perimysium)
- around each muscle fibre theres an inner layer of connective tissue (endomysium ; endo for inside)
4) around whole muscle fibre itself theres an outer layer of connective tissue (epimysium; epi for outside)
5) in the muscle there are lots of blood vessels and nerves
what can be seen on a cross section through a single muscle fibre
1) surrounded by layer of endomysium
2) capillaries run v close to the muscle fibre in this connective tissue layer
3) satellite cell
4) LOTS of MYOFIBRILS inside it
what are the 3 layers of connective tissue in a muscle
1) epimysium
2) perimysium
3) endomysium
how do capillaries act around muscle fibres
accommodate stretching / shortening as the muscle relaxes and contracts (capillaries are quite stretchy as is the endomysium itself)
where are capillaries found in muscles
rich network, surround the muscle fibres so blood is well vascularised
at what increments are striations in skeletal muscle fibres
1 every 2.5um
unlike most cells in the body, muscle cells are what
multinucleated (cell has multiple nuclei, peripheral nuclei)
- formed by fusion of lots of mononucleated cells together to make a muscle fibre
why are muscle cells multinucleated
ie bicep muscle = 30cm long
not possible to have a SINGLE cell w 1 nucleus that is 30cm long
there is 1 nucleus every ___ striations / sarcomeres
10/15 (35um)
each nucleus has its own
microdomain
explain the process of multinucleated cell formation
proliferating myoblasts (mononucleated cells) line up and fuse together into long muscle stretches (muscle fibres)
what happens if myoblasts do not fuse with the muscle fibre
form the SATELLITE CELLS
- sit next to the muscle membrane under the connective tissue / basal lamina of muscle fibres
what are satellite cells
stem cells of muscle
responsible for growth + regeneration of muscle fibre
what can be seen on low power electron micrograph of a muscle fibre
- regular organisation of proteins into sarcomeres
- sarcomeres organised into myofibrils
- myofibrils = long connections of sarcomeres that run across, lots of them aligned w respect to one another (them lined up is what gives striation effect)
- Lots of ‘myofibrils’ –(>90%) of the muscle. Repeating structure – the muscle sarcomere
what can be seen on a zoomed in pic of a single myofibril
1) both ends of the sarcomere = the Z disk
2) one Z disk is connected to the next one + the adjacent myofibril (this repeats)
3) so they look lined up (not perfectly lined up but close)
4) most of striation comes from repeating sarcomeres along muscle fibre from one end to other (tendons at either end + alignment of myofibrils across the muscle fibre)
explain the terminology we use when talking about muscles
- Terminology (“sarco” from Greek “flesh”) so instead of saying
1) plasmalemma for muscle membrane we say sarcolemma
2) Sarcolemma = plasmalemma
3) Sarcoplasmic reticulum = endoplasmic reticulum
4) Sarcoplasm = cytoplasm
5) Sarcomere – (méros = part)
define myofibril
longitudinal contractile unit composed of sarcomeres arranged in series
describe the structure of a muscle sarcomere
- Z-disks = in centre of light region called the I BAND
- z-lines at each of the muscle sarcomere
- I band = contains only thin filament
- A band = darker region in middle of the sarcomere, contains thick + thin filaments
- M-line = down middle of A band
- THIN FILAMENTS = thin structures coming out of Z disk into the half of the sarcomere on either side of it, composed of actin and some other proteins
- bipolar THICK FILAMENT = in middle in the A band, main component of it is myosin
what cause muscle contraction
interaction of myosin in thick filament w actin in thin filament that causes contraction
what happens to myofibrils if we supply them calcium and atp exogenously
freely contract up until they are really small
what is the basis of the sliding filament theory (1954)
when muscle contracts…
- width of A band (thus thick filaments) doesnt change
- width of I band decreases
- thin filaments ‘slide past’ thick filaments
why do muscles contract
- bc cross bridges (fine structures going between thick and thin filaments) from myosin (formed by part of the myosin molecule), attach onto actin + pull on the actin filament to generate contraction
- cross bridges stick out towards actin (thin filaments)
describe a myosin molecule
1) made up of 2 proteins
2) 2 identical heavy chains wrap around each other to form a coiled coil, then heavy chain diverges to form a globular head (2 motor domains - 1 formed by each heavy chain)
3) 2 heads and 1 tail and it’s a single heavy chain forming this structure
4) 2 more proteins (light chain) wrap around the ‘neck’ region (between heads + tail)
4) globular head is what forms the cross bridges (binds atp and actin sometimes called S1)
what do the motor domains / globular heads of myosin do
bind to nucleotide and actin, imp for generating force
- have ATPase (enzymatic) and actin binding properties
what does the tail of myosin do
assembles to form thick filament
how long is the myosin
a) head
b) tail
a) 16nm
b) 155nm
how do the light chains wrap around the neck and how does this lead to a change in the molecules orientation?
- motor region (head) binds the 2 light chains (theres an essential + a regulatory light chain)
- light chains bind onto the alpha helix which binds ATP
- ATP is in the nucleotide binding site
- acts as a lever bc top part of molecule binds to actin and hydrolyses atp and loses phosphate, this part changes its orientation
each myosin molecule (nano-machines) contains
- 2 heavy chains (-200kDa each)
- 4 light chains (2 essential and 2 regulatory)
what does the tail of myosin form
COILED COIL
formed by the 2 heavy chains (alpha helices) wrapping around each other
it dimerises the myosin
coiled coil tails self assemble into
thick filaments
- the motor domains project out on either side
where are cross bridges in relation to the thick filament
- either side
- BUT none in the centre (central bare zone)
- bc in centre of the thick filaments myosin molecules pack antiparallel
- towards edges they start to pack parallel
why is the polarity of cross bridges important for contraction
- cross bridges either side are mirror image so opposite polarity to each other
- for contraction you want to pull thin filaments in towards the middle SO cross bridges on either side pull into the middle
explain how muscle sarcomeres are v precisely built
- want every sarcomere to be the same
- always same length
- thick filaments = 1.6 µm long
- thick filament in middle has same no of myosin molecules in it (294)
Thick filaments in skeletal and cardiac muscle contain exactly how many myosin molecules
294
- in all vertebrates (ie spiders)
why are sarcomeres (thick filament) built precisely
so when contract and have lots of them in a series they all generate same amnt of force on shortening (wouldn’t work if they were different sizes as would all generate diff force = unorganised force generation)
how often do we get the ring / crown of cross bridges along the thick filament (sarcomere)
every 14.3nm (triple helix with 14.3 nm repeat)
describe the cross-bridge cycle with actin
1) driven by atp
2) atp binds to head + is hydrolysed to adp and phosphate
3) some released phosphate = motor domain can undergo rotation to pull on the actin filament
Tails self assemble into thick filament
describe actin
globular, monomeric protein (G-actin) + has to polymerise to form a filament (filament formed in regimented way)
G-actin assembles helical filaments (F-actin)
what is actin filament made up of
lots of monomers that have polymerised to form it
what can we think of the actin filament as
1) long pitch helix (w dotted lines at top)
2 long pitch helices - pitch 72 nm + 1 sub-unit every 5.5 nm
2) shallow genetic helix
2 strands which repeat as go along
which filaments are actin in
thin filaments
describe the structure of filamentous actin (F-actin)
- formed by the polymerisation of actin monomers (G-actin / globular actin)
- 2 long pitch helices - pitch 72 nm
- has a fast growing end (‘barbed’ or ‘plus’ end)
- and a slow growing end (‘pointed’ or ‘minus’ end)
- beaded substructure and crossovers
what does each end of the filamentous actin do and what do they confer
- fast growing = grows quickly
- slow growing = grows slowly and sub-units tend to fall off
- give polarity needed bc to polymerise we have to add monomers to one end
- myosin molecules recognise the polarity
- when myosin is binding it knows which way round the actin is and only walks in 1 direction (towards barbed end)
- so relative polarity of the actin + myosin is imp for generating force
where are thin filaments
anchored in the Z-disc (its fast growing end is in the z disc)
what happens to thin filaments in the z disc
crosslinked by alpha-actinin protein
capped (capping protein at fast growing end)
what does the capping protein at fast growing end do
helps stop any further polymerisation or depolymerisation in the filament so keeps it stable
what prevents further polymerisation or depolymerisation in the slow growing end
tropomodulin (protein)
- sits on end of thin filament at middle of sarcomere
what do the capping protein and tropomodulin ensure
stable actin filament where rate of polymerisation is v low + it stays the same length
so actin filament stable once assembled into a muscle sarcomere
outline the ATPase cycle
1) myosin crossbridges bound to ATP (not bound to actin)
2) cross bridges hydrolyse ATP to ADP + Pi
3) myosin head undergoes conformational change and binds strongly to actin
4) when muscle activated cross bridge binds to actin + releases Pi
5) power stroke, lever (head) of myosin changes direction
6) ADP released from myosin head and go into brief rigor state w no ATP
7) ATP from solution rebinds, cross bridge detaches and myosin not bound to actin
ATPase cycle: why does the power stroke happen
when Pi is released, nucleotide site communicates this to rest of the molecule + causing conformational change in the cross bridge
ATPase cycle: why is ADP released following the power stroke
because myosin goes into the filament, changing the lever pulls the actin filaments in a particular direction so quickly lose ADP
ATPase cycle: what do myosin cross bridges do at rest
1) not attached onto actin
2) sit back on thick filament 3) bind atp + slowly hydrolyse it to adp and Pi
ATPase cycle: what is present in the nucleotide binding pocket
Pi and adp
describe the basic properties of myosin
- protein enzyme that uses chemical energy to do work
- an ATPase: hydrolyses ATP to ADP + Pi
- self-assembles to make thick filaments
- binds to + pulls on actin in thin filaments
how does actin speed up myosins ATPase activity
1) resting muscle - myosin by itself when not attached to actin hydrolyses atp v slowly
2) when binds to actin it speeds up the process of Pi release
3) makes ATPase cycle 50-100x faster
what size movement does each cross bridge make as go through power stroke
10nm
describe the ATPase cycle in 4 stages
1) actin binding (ADP + Pi release)
2) power stroke
3) actin release (ATP binding + hydrolysis)
4) recovery stroke
how does polarity work in a sarcomere
cross bridges + actin on either side have opposite polarity + are mirror images
so cross bridges (move towards barbed end) pull the actin towards the middle of the sarcomere / thick filament
explain where the sliding filament theory comes from in the sarcomere as the muscle contracts
- filaments themselves don’t change length but they slide past each over
- sliding driven by myosin pulling actin filaments into middle of the sarcomere
- actin pulled into centre of sarcomere = size of a band increases but it doesn’t actually increase
I band (actin filaments) decreases bc overlap between thick + thin filament has increased
myofibrils contract until they are tiny but what happens if overcontraction occurs
thin filaments go through into other side of sarcomere
thick filaments are trapped in between them
thin filaments have wrong polarity for myosin and force goes down
force of contraction depends on
sarcomere length
number of myosin cross bridges (therefore force generated) that interact with actin depends on
amount of overlap between thick + thin filaments
where highly overlapped, produce max force as all cross bridges are available
a change in overlap between filaments confers
change in amount of force
what is the force at long sarcomere lengths (ie relaxed - stretched out)
0
thick + thin filaments no longer overlap
what would be the effect of changing a sarcomere length by 2um
still generate same amnt of force bc overlap between cross bridges and actin is the same (bc of centre region w no cross bridges)
how are sarcomeres arranged / connected, what does this mean
IN SERIES
- small movements in each one = ‘summed’ along the fibre
- adds up to large length changes at ends
- bc each cross bridge makes 10nm (+ there are lots of cross bridges)
- each sarcomere shortens by 10% = sums over entire length of myofibril (large no of sarcomeres in muscle as muscles much longer than 2.5um sarcomeres)
1) biceps…
2) triceps…
1) shorten = flex forearm
2) shorten = extend forearm
why is muscle shortening important
moves joints
other than ATP, what else must be added to myofibrils for contraction
Ca2+
force of contraction depends on
Ca2+ conc
- sigmoidal r/ship between % force and Ca2+
what proteins regulate contraction, where are they
Ca2+ sensitive proteins: troponin + tropomyosin
- in thin filament
how is tropomyosin organised
end to end
wraps round thin filament
head to tail polymers
2 long strands following the long pitched helices
what does tropomyosin do
in relaxed muscle position of tropomyosin blocks myosin binding sites on actin
there is 1 tropomyosin per how many actin subunits
7
for every tropomyosin there is
so what is the ratio of actin to tropomyosin to troponin
a molecule of troponin
7:1:1
what are the 3 subunits of troponin and what do they do
1) TnC = binds Ca2+
2) TnI = inhibitory, binds to TnC, TnT
bind to actin to hold actin-tropomyosin complex in place, prevents myosin binding in relaxed muscle
3) TnT = binds to tropomyosin to help position it on actin
describe the structure of tropomyosin
- coiled coil
describe the position of tropomyosin at low Ca2+
- inhibitory part of TnI binds to actin/tropomyosin
- tropomyosin lays across actin binding sites for myosin so theyre blocked
describe how the position of tropomyosin at high Ca2+ enables contraction
- Ca2+ binds to TnC
- then TnI binds to TnC
- tropomyosin moves across actin + myosin binding sites exposed
- myosin can bind + generates movement
so Ca2+ influx = ON switch for contraction
how does Ca2+ get into the sarcoplasm
- AP at motor neurones
- depolarisation of muscle membrane at neuromuscular junction
- inside of t-tubules join to outside of muscle cell (they are invaginations of the plasma membrane)
- t tubules lie next to terminal cisternae of sarcoplasmic reticulum (SR)
- when t-tubules are depolarised they communicate this to SR
- SR releases all Ca2+ into sarcoplasm
explain how depolarisation of the muscle fibre membrane (sarcolemma) begins
1) AP in nerve produces AP in muscle nerve (presynaptic) terminal
2) ACh receptors in muscle membrane detect ACh released from vesicles
what are the terminal cisternae
- ends of SR
- filled w calcium + calsequestrin (calcium binding protein)
- so much of the Ca2+ in SR is bound by calsequestrin
how do we get communication between the t-tubules and sr
ryanodine receptor (Ca2+ channel) on SR directly coupled to + interact w dihydropyridine receptors (voltage gated sensor, L-type Ca2+ channel) on t-tubules
how does the interaction between ryanodine receptor and dihydropyridine receptors work
when dihydropyridine senses depolarisation it activates / initiates a change in structure of ryanodine receptor
small channel in ryanodine receptor releases Ca2+ as soon as it senses the depolarisation
released Ca2+ activates other RyRs
where are dihydropyridine receptors (DHPRs) found
tetrads in the t-tubular membrane at SR-TT junctions
what is the gap between TT and SR
12nm
foot proteins between the 2 contain cytoplasmic domain of the ryanodine receptors (RyR) (4 subunits in each ‘foot’)
+
Ca2+ / K+ channel
what happens to Ca2+ once muscle contraction has ended
- RyR time dependent inactivation (short lived)
- Ca2+ spike reach 10um
- so Ca2+ pumped out of sarcoplasm and back into SR
- via ATP driven Ca2+ pump in SR
- to relax the muscle
what is a muscle ‘twitch’
the fast AP in nerve and muscle followed by Ca2+ transient increase then slower contraction
its the rise and drop in force w small delay caused by a single AP + single nerve impulse arriving
Ca2+ conc in resting muscle sarcomere, what is this low Ca2+ maintained by
<100nM
maintained by ATP-driven pumps in SR membrane
how do we
a) generate greater force of muscle contraction
b) regulate contraction
a) send nerve APs quickly in sequence
b) by speed of nerve impulses arriving at the muscle
how does sending nerve APs quickly in sequence increase force of contraction
- 2nd AP before all Ca2+ pumped out = summation of tension
- rapid APs = tension fused = tetanus
- more quickly send nerve impulses = more can summate the twitches
- send v quickly big contraction (max force output for unchanged muscle length)
how else can we increase tension / force and how
recruit more motor units
- each bundle of muscle fibres has motor unit responsible for activating them
- several motor units in a muscle
- if use multiple we generate more force as using more fibres
- larger motor neurones innervate larger motor units
what are the 3 sources of ATP
1) creatine phosphate
2) glycolysis
3) oxidative phosphorylation
why do we need to regenerate ATP
only have enough free atp in muscles (4mM ATP) for 1 sec of contraction
explain how creatine phosphate regenerates ATP
- quickest way (
- gives enough ATP for 4 secs
- creatine phosphate + ADP + H+ -> ATP + creatine
- catalysed by creatine kinase
explain how glycolysis regenerates ATP
- converts glucose -> pyruvate in sarcoplasm
- anaerobic
- produce 2 ATP per 1 glucose
explain how oxidative phosphorylation regenerates ATP
- phosphorylation of ADP to ATP in inner mitochondrial membrane
- requires O2
which types of fast muscles do we have, how do they differ
type I = slow (higher oxidative metabolism, resistant to fatigue)
type II = fast (higher glycogen + anaerobic glycolysis)
differ in types of myosin + other regulatory proteins they have (myosin isoforms vary between fibre types)
what happens when ATP runs out, explain this
rigor mortis
1) after death ATP levels fall and Ca2+ levels rise
2) so all myosin crossbridges bind strongly to actin
3) muscle stiffens 3-4 hours after death
4) muscle relaxes again after 24 hours due to proteolysis
what is titin
- largest known protein (3.7Mda in weight, up to 38,138 amino acids long = 1um) over 10x bigger than myosin
- 3rd most abundant muscle protein
where does a titin molecule run
from z-disc to middle of m-line / muscle sarcomere
what are the functions of titin
- regulates thick filament length + keeps them centred in middle of sarcomere during contraction (bc titin in the i band is elastic)
- organises z and m lines
- generates resting tension
what is nebulin
- big protein (0.8Mda)
- in the thin filament, wraps round it
- 2 molecules per thin filament that span its entire length
what are the functions of nebulin
- regulate thin filament length
- so get precisely regimented muscle (titin regulates thick + nebulin thin)
explain longitudinal transmission of force (along muscle fibre)
- myosin crossbridges +/or titin carry tension from 1 sarcomere to next
- titin helps do this
explain lateral transmission of force (across muscle fibre)
- around each of the z disks + the proteins in it there are connecting cytoskeletal proteins that go from the z disk to the plasma membrane + attach out into the extracellular basal lamina / layer of connective tissue
- if strip myofibrillar / sarcomeric proteins = left with cytoskeletal network of proteins called a COSTAMERE (links z-disk to sarcolemma)
where does muscle insert at its ends
tendon (lots of connective tissue)
- fibres are 100um diameter + up to 30cm long
- thin filaments attach to sub-membranous network
- connections through membrane to dense basal lamina
- incd area for force transmission to the ends from this tapered arrangement (tapering makes big SA for muscle to exert its force on)
