Skeletal Muscle Structure and Funtion II

Question 1

What is the triad in skeletal muscle composed of?

Answer 1

Sarcoplasmic reticulum
Transverse tubule
Sarcoplasmic reticulum

Question 2

How is calcium released from the sarcoplasmic reticulum?

Answer 2

A muscle action potential propagates across the surface of a muscle cell and moves down the transverse (T-tubule) tubule where the voltage gated DHP (L-type Ca2+ channels) receptor is activated which uncouples the Ryanodine channel in the SR releasing Ca2+

Question 3

How does cross bridge formation occur?

Answer 3

Calcium from the SR binds to troponin changing its’ shape and shifting tropomyosin exposing the myosin binding site on the actin filament, myosin then binds to the actin filament

Question 4

What are the steps in cross bridge cycling?

Answer 4

1. ) ATP binds myosin head causing dissociation of A-M complex
2. ) ATP is hydrolyzed, causing myosin heads to return to their resting conformation
3. ) A cross-bridge forms and the myosin head binds to a new position on actin
4. ) Pi is released, myosin heads change conformation, resulting in the power stroke. The filaments slide past each other
5. ) ADP released (A-M still present)

Question 5

What is the function of SERCA?

Answer 5

Pumps calcium back into the SR

Question 6

Which isoforms of SERCA are found in different muscle types?

Answer 6

SERCA IA: Fast twitch
SERCA 2A: Slow twitch/Cardiac
SERCA 2B: Smooth muscle

Question 7

What does phospholamban (PLB) do?

Answer 7

Inhibits SERCA

Question 8

How is PLB deactivated?

Answer 8

Via beta-adrenergic stimulation it is phosphorylated by PKA causing PLB SERCA dissociation

Question 9

What is the main dependent of myocyte relaxation time?

Answer 9

SR Ca-pump (SERCA)

Question 10

What is the function of calsequestrin and calreticulin?

Answer 10

Uptakes calcium to regulate Ca2+ levels and make sure that levels are not too high in SR so that Ca2+ can more easily be pumped in against less of a conc. gradient

Question 11

What is the process when calcium binds troponin?

Answer 11

Calcium binds Tn-C causing conformational change that causes Tn-I and Tn-T to move and push Tm away

Question 12

What are the minor mechanisms by which Ca2+ is removed form the cytoplasm?

Answer 12

Na-Ca (NCX) exchanger and plasma membrane Ca2+ (PMCA)

Question 13

What is muscle plasticity?

Answer 13

The ability of a muscle to change

Question 14

What occurs in patients with polio?

Answer 14

Muscle atrophy and switching from FT to ST muscle fibers in the muscles that are left

Question 15

What is the sequence of fiber type transition?

Answer 15

IIX-> IIA-> I

Question 16

Which muscle fibers stain darkest?

Answer 16

Slow twitch

Question 17

How can skeletal muscle fibers be transformed?

Answer 17

By chronic low frequency (2 Hz) electrical stimulation

Question 18

What happens if you switch a motor unit originally innervating a ST muscle to a FT muscle and vice versa?

Answer 18

The newly stimulated muscle fiber will transform into the type of fiber originally stimulated by the motor unit

Question 19

With disuse atrophy, what is the order of affected muscle fiber types?

Answer 19

Type I-> IIA-> IIX (least affected)

Question 20

In cardiomyopathy why is the trapezius stimulated with 2 Hz of electricity over a period of time?

Answer 20

To produced slow twitch fibers that are fatigue resistant