structure and regulation of skeletal muscle

Question 1

what is a muscle?

Answer 1

bundles of contractile fibres surrounded by a plasma membrane

Question 2

what is the endomysium?

Answer 2

layer of connective tissue around muscle fibres

Question 3

how are muscle fibres organised?

Answer 3

bundled into fascicles, surrounded by perimysium. around this, there is a tough coat called the epimysium

Question 4

why is the A band called this?

Answer 4

it is anisotropic to light, polarised light cannot penetrate it

Question 5

why is the I band called this?

Answer 5

it is isotropic to light, light can pass through at any direction

Question 6

where is the Z-line and what is it?

Answer 6

located in the middle of the I band, the sarcomere spans between Z-lines

Question 7

what is the sarcomere?

Answer 7

the minimum functional unit of striated muscle

Question 8

where are thick myosin filaments located?

Answer 8

A band

Question 9

what is the M band?

Answer 9

location of attachment between myosin filaments

Question 10

what does the thickness of the I band depend on?

Answer 10

how stretched the muscle is

Question 11

what happens to the I band as the muscle contracts?

Answer 11

I band becomes thinner as it is pulled into the A band

Question 12

where are thin filaments attached?

Answer 12

Z line

Question 13

what is the structure of thick myofilaments?

Answer 13

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

Question 14

what is the role of the regulatory light chain on thick myofilaments?

Answer 14

modulates enzyme activity

Question 15

what is the structure of thin filaments?

Answer 15

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

Question 16

what is the role of the myofilament titin?

Answer 16

stabilises myosin filament position

Question 17

how is titin positioned?

Answer 17

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.

Question 18

what is meant by passive elasticity of muscle?

Answer 18

as the muscle is stretched, passive force increases

Question 19

what is the neuromuscular junction?

Answer 19

a specialised synapse of the motor neurone

Question 20

what occurs at the end plate?

Answer 20

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

Question 21

what are transverse tubules and what is their role?

Answer 21

invagination of the sarcolemma. carries action potential into depth of myocyte to speed up conduction of APs

Question 22

what are the transverse tubules filled with?

Answer 22

extracellular fluid containing calcium and other physiological ions

Question 23

what is the sarcoplasmic reticulum?

Answer 23

an intracellular calcium store

Question 24

which three membranes constitute the triad?

Answer 24

sarcoplasmic reticulum, T-tubule, terminal cisternae

Question 25

what are ryanodine receptors coupled to in skeletal muscle?

Answer 25

dihydropyridine receptors on the t-tubule

Question 26

what event causes RYR to open?

Answer 26

when there is an AP propagating down the t-tubule, this is coupled to a conformational change in the dihydropyridine receptor

Question 27

how does calcium regulate contraction?

Answer 27

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

Question 28

outline the cross-bridge cycle

Answer 28

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

Question 29

on what does speed of contraction rely on?

Answer 29

myosin ATPase rate

Question 30

what is meant by motor unit?

Answer 30

a single motor neuron controls and innervates many muscle fibres

Question 31

how is more force generated in a muscle?

Answer 31

recruitment of more motor units

Question 32

outline excitation-contraction coupling

Answer 32

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

Question 33

what are the differences between type I and II fibres?

Answer 33

• 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

Question 34

what are the characteristics of type I muscle fibres?

Answer 34

high aerobic endurance, produces ATP aerobically, used for low intensity exercise and daily activities

Question 35

what are the characteristics of type II fibres?

Answer 35

poor aerobic endurance, fatigue quickly and produce ATP anaerobically

Question 36

what are the characteristics of type IIa fibres?

Answer 36

produce more force, fatigue faster. used for short, high intensity endurance events

Question 37

what are type IIx fibres used for?

Answer 37

short, explosive sprints

Question 38

what factors are determinants of fibre type?

Answer 38

genetic factors
training factors
ageing