L25 - skeletal muscle Flashcards
what is skeletal muscle?
- voluntary, conscious
- attached to bone
key characteristics of skeletal muscle
- made of fibres
- involved in posture and movement
what are tendons?
bundles of collagen fibres that attach muscle to bone
what is skeletal muscle made of?
bundles of multinucleate muscle cells
what is a muscle fibre?
single muscle cell
what is a myofibril?
each muscle cell formed from bundles of actin and myosin filaments which are organised into myofibrils
describe arrangement of actin filaments around a myosin filament
6 actin around 1 myosin
what is the sarcoplasmic reticulum?
- like endoplasmic reticulum
- acts as a Ca2+ store
structure of skeletal muscle
striated pattern
Z-DISC
- borders of sarcomeres
- smallest functional units
- core made of actin
- alpha actions bind and cross like the ends of F-actin from sarcomeres at z line
- alpha actinin-2 in all muscle fibres
what is the thin filament made up of?
actin, tropomyosin, troponin
M-Band
myomesin
what is the thick filament made up of?
myosin and titin
A band
- dark band
- myosin
- centre of sarcomere
I band
- lighter band
- only contains thin filaments
- actin
M line/disc
proteins that link central regions of the thick filaments
H zone
light area at end of thin filaments
describe thick filaments
- 15nm in diameter
- lots of motor protein myosin
- myosin. made of heavy and light chains
describe myosin
- tail made of 2 chains
- globular hear which projects at an angle
- half of the heads orientated to left, other to right
- creates area in middle called M-region
describe thin filaments
- made of filamentous F-actin strands
- which are made of a string of globular G-actin subunits
- contain tropomyosin: blocks active sites
- filaments attached at z disc
describe tropomyosin
- blocks active sites
- bound by calcium binding protein troponin
what is the interaction between actin and myosin filaments regulated by?
troponins
troponin I
inhibitory
troponin C
calcium binding
troponin T
tropomyosin binding
sliding filament theory
- contraction causes force
- myofilaments stay same length but it causes more overlap
what does an increase in calcium initiate?
contraction
cross bridge cycling 3 stages
- resting muscle
- activation of contraction
- terminating cross bridge
describe stage one of cross bridge cycling: resting muscle
- myosin molecule bound to ADP + Pi
- tropomyosin covers myosin binding sites on acting filaments so they can’t bind
describe stage 2 of cross bridge cycling: activation of contraction
- muscle stimulated
- calcium levels in cytosol increase
- calcium binds to troponin
- so uncovers myosin binding sites on actin
- myosin molecule + ADP binds to actin
- cross bridge formation causes release of ADP and Pi and movement of the cross bridge
describe stage 3 of cross bridge cycling: terminating cross bridge
- ATP binds to myosin
- breaks actin-myosin cross bridge
- ATP converted to ADP and Pi
- so myosin returns to energised position
how are skeletal muscle contractions neurogenic?
- contractions need synaptic input from motor neurones
- a single motor neurone can innervate muscle fibres so they all contract at same time
describe the neuromuscular junction
- axon terminal and the motor endplate = NMJ
myelin sheath that surrounds the motor neurone ends near the surface of the muscle fibre
what do the axon terminals in the NMJ contain?
vesicles with ACh
motor end plate of NMJ
area of muscle fibre directly under the axon terminal
describe how NMJ works
- A.P at motor neurone depolarised axon terminal
- so voltage dependent calcium channels open
- causes release of ACh vesicles through synapse
- ACh diffuses to motor end plate
- activates NICOTINIC ACh receptors (ionotropic receptors)
- motor end plates is depol
- this is end plate potential
fast twich fibres in skeletal muscle
- glycolytic
- explosive power
slow twich fibres in skeletal muscle
- aerobic
- fatigue resistant
what is the speed of contraction determined by?
the myosin heavy chain isoform expressed by the fibre
is there a gene for speed?
- alpha actinin 3 is expressed in a subset of fast twitch muscles
- genetic variant encoding premature stop codon results in truncated form of this actinin
- absence of variant correlates with athletic performance
myopathies (genetic skeletal muscle diseases)
- present at birth
- develops at infancy
- non progressive
- non degenrative
- defect in actin
dystrophies
- birth or later
- progessive
- degenerative
- defects in memb / proteins
