Skeletal muscle innervation Flashcards
What is the skeletal muscle fibre Em?
-90mV
Where are T tubule invaginations in skeletal muscle?
Boundary between A and I bands
What neurones innervate skeletal muscle?
Somatic motor neurones
What is the function/advantage of T-tubules?
Spread the impulse so it can interact with the cells more rapidly
What does the T-tubule membrane lie adjacent to?
Pair of terminal SR cisternae forming a triad
Describe ECC in skeletal muscle?
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
Is influx of Ca2+ (CICR) required for opening of RyR channel ?
No, contributes but not essential
How are LTCC and RyR spatially related?
Mechanically coupled - contact between SR and T-tubule
What does Ca2+ do after release from SR?
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+
Are all Ca2+ binding sites on TpC saturated in skeletal muscle?
Yes
How do T tubules compare in cardiac muscle to skeletal muscle?
Bigger and wider
Describre the process of cross bridge cycling?
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.
What triggers relaxation?
Ca2+ detaches from TpnC and goes into SR via SERCA or out of cell (Ca2+ ATPase or NCX)
What happens in rigor mortis?
Muscles in attached state but no more ATP to trigger disengagement so skeletal muscle stiff
3 ways to regulate active tension?
Selective recruitment of greater/smaller number of motor units
Changes in frequency of stimulation
Starting length of relaxed muscle
What does Ca2+ bind to inside SR so more calcium can move in?
Calsequestrin and calreticulin
What is the relationship between active tension and length in skeletal muscle?
Biphasic
What effect does increasing/decreasing sarcomere length from normal have on tension, how come?
Fewer cross bridges formed, tension decreases, force generated decreases
What is isometric contraction?
Tension generated by fibre without changing length
What is isotonic contraction?
Muscle contracts against constant load and shortens
What produces more tension isometric or isotonic contraction?
Isometric
What is meant by graded muscle response?
Normal contraction modified by nervous input to produce varying amounts of force
What is meant by frequency summation?
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
What is meant by tetanus?
No relaxtion, not enough time to recapture Ca2+ so continuous contraction (but depol doesn’t generate any more tension)
Describe why twitches are separate at low AP frequency?
2nd depol occurs when muscle has relaxed and recovered (i.e. first twitch has ended)
What is a motor unit?
Alpha nerve and all the muscle fibres that it innervates
What do smaller motor units allow?
Finer control
What is meant by larger motor unit?
Single motor neurone supplies large number of fibres in muscle
How can activation of different types of motor units cause differences in tension?
Smaller motor units activate to generate smaller degree of tension
When more strength needed, larger motor units enlisted to activate larger muscle fibres
What is meant by recruitment?
Increasing activation or motor units to increase contraction (i.e. activate larger motor units)
What does SERCA stand for?
Sarco/endoplasmic reticulum Calcium ATPase
