4 Molecular mechanisms of muscle contraction Flashcards
pennate muscle
feathered fascicles arrangement (fibre bundles): uni/bi/multipennate
fusiform muscle
spindle shaped, tapered at both ends with circular cross section, parallel fibres
tendon - muscle - tendon
= bicep
parallel muscle
fascicles lie parallel to long axis of muscle (often have aponeurosis)
= rectus abdominus (abs) is 2 parallel muscles with linea alba in between
convergent muscle
broad attachment from which fascicles converge to a single tendon
= pectoralis major
circular muscle
surround a body opening or orifice, constricting it when contracted
= oesophagus
aponuerosis =
a sheet of pearly white fibrous tissue that takes the place of a tendon in flat muscles having a wide area of attachment
tendon vs ligament
tendon = muscle to bone
ligament = bone to bone
three types of muscle
- skeletal
- cardiac
- smooth
skeletal muscle
striated, multinucleated, voluntary, non-branching, attached to skeleton
cardiac muscle
striated, single nucleus, involuntary, branched, heart muscle
smooth muscle
non-striated, single nucleus, involuntary, tapered, forms walls of organs
what does striated mean
tissue that features repeating functional units called sarcomeres = striped appearance
attaching bone to muscle =
tendon
muscle belly structure: Epimysium
sheath of fibrous elastic tissue surrounding an entire muscle
muscle belly structure: Perimysium
sheath of connective tissue surrounding a fascicle
muscle belly structure: Endomysium
surrounds individual muscle fibres / between fibres
muscle belly structure: fascicle
contain numerous muscle cells which each contain hundred to thousands of myofibrils (chains of thousands of sarcomeres)
muscle belly structure: sarcolemma
the plasma membrane of the individual muscle cell or muscle fibre
within sarcolemma = sarcoplasm, sarcoplasmic reticulum and myofibrils
muscle belly structure: myofibril
thousands of myofibrils in every muscle cell
contain actin and myosin filaments
list structures of muscle belly macroscopic -> microscopic
epimysium - perimysium - fasicle - endomysium - sarcolemma - muscle fibre - myofibril - sarcomere
myocyte
muscle cell or muscle fibre (same thing)
basic definition of sarcomere
smallest contractile unit of striated muscle
sarcomere boundaries
segment between two neighbouring Z-lines
structure of sarcomere = I-band
I for ISOTROPIC
I for thIn
= portion of sarcomere with thin filaments
= actIn or acTHIN
not superimposed with thick filaments
structure of sarcomere = A-band
A for ANISOTROPIC
A for fAt
= portion of sarcomere with thick filaments
= myosin length (which will overlap with the myosin
structure of sarcomere = H-zone
zone of the thick filaments that is not superimposed by the thin filaments
= the length of myosin not overlapping the actin
structure of sarcomere = Z-line
anchoring part for the actin filaments, marks boundary of individual sarcomere
structure of sarcomere = M-line
M for middle (in middle of myosin)
titin
aka connectin
= giant protein which extends from Z line to myosin
actin
= thin filaments, are the major component of the I-band and extend into the A-band
myosin
= thick filaments, are bipolar and extend throughout the A-band, cross-linked at the centre by the M-band
myosin structure
2 heavy chains
2 light chains
has length and heads which bind to the actin
troponin
protein to which ca++ binds to
changes shape and moves tropomyosin
tropomyosin
blocks binding sites on actin when bound, is moved out of the way by troponin when ca2+ binds to troponin
AP arrives at NMJ…
ca++ released from sarcoplasmic reticulum (SR) which then goes to bind to troponin
attachment of myosin to actin
myosin head with ADP + Pi
power stroke
myosin head bends, pulling along the actin filament, ADP + Pi are released
stage 1 of the cross-bridge theory
DETACHMENT
ATP bind to myosin head
stage 2 of the cross-bridge theory
HYDROLYSIS of ATP
myosin head attached to ADP + Pi
ready to bind to actin
stage 3 of the cross-bridge theory
CROSS-BRIDGE
myosin head binds to the actin with ADP + Pi still attached
stage 4 of the cross-bridge theory
POWER STROKE
release of the ADP + Pi causes myosin head to move and so move the thin filament
does A band change length
no!
A band stays the sAme
Isotonic contraction
cause muscle to change length as it contracts and thus move
Isometric contraction
occurs when no change in length of contracting muscle
= carrying an object in front of you as the weight of the object is pulling your arms down but your muscles are contracting to hold the object at the same level
concentric contraction
muscle shortens as it contracts - occur frequently in daily and sporting activities
eccentric contraction
muscle lengthens as it contracts - less common - usually involves control or deceleration of a movement being initiated by the eccentric muscles agonist
This type of contraction puts lots of strain through muscle - commonly involved in muscle injuries
twitch definition
muscle contraction is based on the twitch of a fibre similar to an AP because a twitch is an all or nothing event
mechanical response of an individual muscle fibre, an individual motor unit, or a whole muscle to a single AP
muscle contraction is based on the twitch of a fibre similar to an AP because
a twitch is an all or nothing event
phases of the twitch
- latent
- contraction
- relaxation
latent phase of the Twitch
the delay of a few milliseconds between an action potential and the start of a contraction and reflects the time for excitation-contraction coupling
contraction phase of the Twitch
Contraction phase starts at the end of the latent period and ends when the muscle tension peaks (tension = force expressed in grams)
during this time cytosolic calcium levels are increasing as released calcium exceeds uptake
relaxation phase of the Twitch
Relaxation phase is the time between peak tension and the end of the contraction when the tension returns to zero
during this time cytosolic calcium is decreasing as reuptake exceeds release
Type I muscle fibre
= SLOW TWITCH
Red in color due to high concentrations of myoglobin
Very resistant to fatigue
Contains large amounts of mitochondria
Contracts slowly
Produces a low amount of power when contracted
Used in aerobic activities such as long distance running
Type II a muscle fibre
= FAST TWITCH A
Red in color due to high concentrations of myoglobin
Resistant to fatigue (but not as much as Type I fibers)
Contains large amounts of mitochondria
Contracts relatively quickly
Produces a moderate amount of power when contracted
Used in long-term anaerobic activities such as swimming (activities lasting less than 30 minutes)
Type II b muscle fibre
= FAST TWITCH B
White in color due to low myoglobin concentrations
Fatigue very easily
Contains low amounts of mitochondria
Contracts very quickly
Produces a high amount of power when contracted
Used in short-term anaerobic activities such as sprinting and lifting heavy weights (activities lasting less than a minute)
Type II b muscle fibre
= FAST TWITCH B
White in color due to low myoglobin concentrations
Fatigue very easily
Contains low amounts of mitochondria
Contracts very quickly
Produces a high amount of power when contracted
Used in short-term anaerobic activities such as sprinting and lifting heavy weights (activities lasting less than a minute)
muscle disorders
- Injury or overuse – strain, sprain, cramps,
tendinitis - Genetic disorder – muscular dystrophy
- Inflammation – myositis, polymyalgia rheumatic
- Parkinson’s disease, Myasthenia gravis, Multiple
sclerosis
Force of Muscle Contraction depends on…
- Number of action potentials per second
- Number of motor units recruited
- Amount of overlap between thick & thin filaments
Optimum muscle length = greatest force of contraction
number of active cross bridges is the greatest
= actin and myosin overlap fully
muscle becomes shorter than the optimum length = decreasing length of sarcomere
- first the thin filaments at opposite ends of the sarcomere first begin to overlap one another and interfere with each other’s movements
- even shorter then the thick filaments come into contact with the Z lines
= tension reduces
muscle becomes longer than the optimum length = increasing length of sarcomere
number of active cross bridges decreases because the overlap between the actin and myosin fibres decrease
= tension reduces
incomplete/ unfused tetanus
tension can oscillate around an average level
complete/ fused tetanus
maximum number of crossbridges to cycle = at this point the tension plateau smoothes out
maximum tetanic tension
when the muscle is at maximum sustained tension
summation of APs cause increase in levels of ca2+ as not all is taken back into SR before it is released again
when the freq of stimulation is so high that Ca2+ levels rise to peak levels, summation results in the level of tension reaching a plateau called tetanus
larger motor units are required
to generate larger forces
