Skeletal muscle innervation

Question 1

What is the skeletal muscle fibre Em?

Answer 1

-90mV

Question 2

Where are T tubule invaginations in skeletal muscle?

Answer 2

Boundary between A and I bands

Question 3

What neurones innervate skeletal muscle?

Answer 3

Somatic motor neurones

Question 4

What is the function/advantage of T-tubules?

Answer 4

Spread the impulse so it can interact with the cells more rapidly

Question 5

What does the T-tubule membrane lie adjacent to?

Answer 5

Pair of terminal SR cisternae forming a triad

Question 6

Describe ECC in skeletal muscle?

Answer 6

Depolarisation T tubule membrane opens L type channel

LTCC adjacent (mechanically coupled) to RyR in SR membrane

Conformational change in LTCC transmitted to RyR, opens and release Ca2+ from SR

Question 7

Is influx of Ca2+ (CICR) required for opening of RyR channel ?

Answer 7

No, contributes but not essential

Question 8

How are LTCC and RyR spatially related?

Answer 8

Mechanically coupled - contact between SR and T-tubule

Question 9

What does Ca2+ do after release from SR?

Answer 9

Bind to troponin C, causing conformational change

(TnI moves away from actin and TnT displaces tropomyosin)

Reveals binding sites for myosin on actin - disinhibition by Ca2+

Question 10

Are all Ca2+ binding sites on TpC saturated in skeletal muscle?

Answer 10

Yes

Question 11

How do T tubules compare in cardiac muscle to skeletal muscle?

Answer 11

Bigger and wider

Question 12

Describre the process of cross bridge cycling?

Answer 12

ATP binds myosin head -myosin disengages

ATP hydrolysed by myosin head which causes pivot

Head forms cross bridge with actin monomer further along sarcomere

Pi release from myosin changes conformation, head swivels - power stroke

ADP released and cycle complete.

Question 13

What triggers relaxation?

Answer 13

Ca2+ detaches from TpnC and goes into SR via SERCA or out of cell (Ca2+ ATPase or NCX)

Question 14

What happens in rigor mortis?

Answer 14

Muscles in attached state but no more ATP to trigger disengagement so skeletal muscle stiff

Question 15

3 ways to regulate active tension?

Answer 15

Selective recruitment of greater/smaller number of motor units

Changes in frequency of stimulation

Starting length of relaxed muscle

Question 16

What does Ca2+ bind to inside SR so more calcium can move in?

Answer 16

Calsequestrin and calreticulin

Question 17

What is the relationship between active tension and length in skeletal muscle?

Answer 17

Biphasic

Question 18

What effect does increasing/decreasing sarcomere length from normal have on tension, how come?

Answer 18

Fewer cross bridges formed, tension decreases, force generated decreases

Question 19

What is isometric contraction?

Answer 19

Tension generated by fibre without changing length

Question 20

What is isotonic contraction?

Answer 20

Muscle contracts against constant load and shortens

Question 21

What produces more tension isometric or isotonic contraction?

Answer 21

Isometric

Question 22

What is meant by graded muscle response?

Answer 22

Normal contraction modified by nervous input to produce varying amounts of force

Question 23

What is meant by frequency summation?

Answer 23

Increasing stimulation frequency

Less time between APs for Ca2+ uptake by SERCA to initiate relaxation

2nd depol reaches muscle when muscle not relaxed

Tension generated is combination of tension present in muscle and second contraction so summation of force generated

Question 24

What is meant by tetanus?

Answer 24

No relaxtion, not enough time to recapture Ca2+ so continuous contraction (but depol doesn’t generate any more tension)

Question 25

Describe why twitches are separate at low AP frequency?

Answer 25

2nd depol occurs when muscle has relaxed and recovered (i.e. first twitch has ended)

Question 26

What is a motor unit?

Answer 26

Alpha nerve and all the muscle fibres that it innervates

Question 27

What do smaller motor units allow?

Answer 27

Finer control

Question 28

What is meant by larger motor unit?

Answer 28

Single motor neurone supplies large number of fibres in muscle

Question 29

How can activation of different types of motor units cause differences in tension?

Answer 29

Smaller motor units activate to generate smaller degree of tension

When more strength needed, larger motor units enlisted to activate larger muscle fibres

Question 30

What is meant by recruitment?

Answer 30

Increasing activation or motor units to increase contraction (i.e. activate larger motor units)

Question 31

What does SERCA stand for?

Answer 31

Sarco/endoplasmic reticulum Calcium ATPase