Skeletal muscle 2

Question 1

List the derivations of fascia/membranes from most superficial to deep within the muscle

Answer 1

Epimysium (surrounds entire muscle), perimysium (surrounds fascicles), endomysium (surrounds the muscle cell), sarcolemma (cell membrane).

Question 2

Features of skeletal muscle fibres

Answer 2

Lack cellular connections to other myofibrils, striated, multinucleate, voluntary control.

Question 3

What is the function of the sarcoplasmic reticulum

Answer 3

Intracellular store for Ca2+. Must be close to T-tubules.

Question 4

Function of T tubles

Answer 4

Invaginations of the sarcolemma where AP is conducted

Question 5

A-band

Answer 5

Contains both thick and thin filaments

Question 6

I-Band

Answer 6

Contains thin filaments only

Question 7

Z-line

Answer 7

In middle of I band. Between two Z lines is the length of a sarcomere.

Question 8

H band

Answer 8

Region in the centre of the A band which contains only thick filaments

Question 9

M-line

Answer 9

Centre of H zone

Question 10

What is the working unit of the skeletal muscle

Answer 10

The sarcomere. One between each Z disc

Question 11

Which part of the sarcomere diminishes in length during contraction

Answer 11

The I band

Question 12

The three protiens in the muscle fibre

Answer 12

Myosin (thick filaments), actin (thin filiaments), titan (anchor actin to z disc)

Question 13

Describe the thin filament

Answer 13

G actin molecules in two strings arranged in alpha helix formation called f-actin. Strands of tropomyosin wrap around the f-actin strands blocking the myosin binding sites. Troponin molecules on the tropomyosin bind to Ca2+ which changes their confirmation and moves the tropomyosin away from the binding sites allowing myosin actin binding.

Question 14

Describe the thick filaments

Answer 14

100s of myosin molecules with their heads arranged in opposite direction orientated away from central M line.

Question 15

Describe the events of cross bridge cycling

Answer 15

1) In the absence of ATP myosin is tightly bound to myosin binding site on actin (rigor state). The cross bridge is at a 45 degree angle relative to the thick filament.
2) ATP binds to the nucleotide binding site which release the bind.
3) ATP is hydrolysed to ADP and Pi which causes the cross bridge to move to a 90 degree angle relative to the thick filament and weakly binds to myosin binding site.
4) Release of the Pi causes the power stroke whereby the myosin head moves the actin strand towards the M line recreating the 45 degree angle.
5) ADP is released an rigour state occurs until new ATP binds.

Question 16

Describe the role of Ca2+ in muscle contraction

Answer 16

Ca2+ is released by the SR following AP. 4 Ca2+ molecules bind to Troponin C inducing a conformational change in it which allows tropomyosin to move away from the myosin binding sites

Question 17

Describe the intracellular regulation of Ca2+

Answer 17

Myoplasmic levels maintained low by SERCA pumps in the SR.
In the T-tubules there are lots of DHPR channels. These but up against the RYR receptors in the terminal cisternae.
Depolarisation of the TT causes the DHPR channels to ‘flip’ which in turn opens the RYR channels allowing Ca2+ to influx into the myoplasm.

Question 18

What are the components of a triad

Answer 18

Two terminal cisternae and a T tubule.

Terminal cisternae are enlargements of the SR.