structure and regulation of skeletal muscle Flashcards
what is a muscle?
bundles of contractile fibres surrounded by a plasma membrane
what is the endomysium?
layer of connective tissue around muscle fibres
how are muscle fibres organised?
bundled into fascicles, surrounded by perimysium. around this, there is a tough coat called the epimysium
why is the A band called this?
it is anisotropic to light, polarised light cannot penetrate it
why is the I band called this?
it is isotropic to light, light can pass through at any direction
where is the Z-line and what is it?
located in the middle of the I band, the sarcomere spans between Z-lines
what is the sarcomere?
the minimum functional unit of striated muscle
where are thick myosin filaments located?
A band
what is the M band?
location of attachment between myosin filaments
what does the thickness of the I band depend on?
how stretched the muscle is
what happens to the I band as the muscle contracts?
I band becomes thinner as it is pulled into the A band
where are thin filaments attached?
Z line
what is the structure of thick myofilaments?
protein complexes of myosin II. consists of heavy chain and light chain. the heavy chain has a globular head and a-helical structure. it is intertwined to form a tail region and hinge region. the globular head is stabilised by alkali light chain. head forms cross bridge
what is the role of the regulatory light chain on thick myofilaments?
modulates enzyme activity
what is the structure of thin filaments?
a complex of actin and troponin/tropomyosin. the actin monomers form filamentous actin. within the complex there is tropomyosin in complex with troponin, this is where calcium binds. each actin monomer has a myosin binding site
what is the role of the myofilament titin?
stabilises myosin filament position
how is titin positioned?
attaches at the M line and extends to the next Z line. it is bound to thick filaments, and acts as an adjustable molecular spring.
what is meant by passive elasticity of muscle?
as the muscle is stretched, passive force increases
what is the neuromuscular junction?
a specialised synapse of the motor neurone
what occurs at the end plate?
branching at synaptic terminals. invaginations to increase the surface area of the end plate membrane. there are nicotinic acetylcholine receptors at the border of the invaginations
what are transverse tubules and what is their role?
invagination of the sarcolemma. carries action potential into depth of myocyte to speed up conduction of APs
what are the transverse tubules filled with?
extracellular fluid containing calcium and other physiological ions
what is the sarcoplasmic reticulum?
an intracellular calcium store
which three membranes constitute the triad?
sarcoplasmic reticulum, T-tubule, terminal cisternae
what are ryanodine receptors coupled to in skeletal muscle?
dihydropyridine receptors on the t-tubule
what event causes RYR to open?
when there is an AP propagating down the t-tubule, this is coupled to a conformational change in the dihydropyridine receptor
how does calcium regulate contraction?
in low calcium concentrations, myosin binding sites are sterically blocked, preventing myosin from binding to actin. when calcium rises, it binds to troponin, causing a conformational change which moves tropomyosin away from myosin binding site. cross-bridge cycling can occur, allowing force to be generated
outline the cross-bridge cycle
1 - ATP binds to myosin head
2 - ATP is hydrolysed, myosin head returns to resting position
3- cross bridge forms at a new position on actin
4 - P is released, conformational change results in power stroke
5- ADP is released, power stroke occurs when force is generated
on what does speed of contraction rely on?
myosin ATPase rate
what is meant by motor unit?
a single motor neuron controls and innervates many muscle fibres
how is more force generated in a muscle?
recruitment of more motor units
outline excitation-contraction coupling
1- end plate potential triggers an AP in the muscle fibre
2- AP propagates down T-tubules
3- depolarisation of T-tubules is sensed by DHPRs coupled to RYR
3- RYR open, calcium is released and cross bridge cycling is initiated
4- calcium is pumped back into the SR by SERCA, terminating cross bridge cycling
what are the differences between type I and II fibres?
- speed of myosin ATPase (type I -slow, type II - fast)
- type II fibres have a more developed SR
- type I fibres use smaller motor units
what are the characteristics of type I muscle fibres?
high aerobic endurance, produces ATP aerobically, used for low intensity exercise and daily activities
what are the characteristics of type II fibres?
poor aerobic endurance, fatigue quickly and produce ATP anaerobically
what are the characteristics of type IIa fibres?
produce more force, fatigue faster. used for short, high intensity endurance events
what are type IIx fibres used for?
short, explosive sprints
what factors are determinants of fibre type?
genetic factors
training factors
ageing
