Summary so far…
Generation of membrane potentials
z Action potentials
z
z
z
z
Neuromuscular transmission
z
z
z
Generation (channels & ions involved)
The triggers (EPSP’s & IPSP’s)
Transfers α-motoneuron action potential to
skeletal muscle fibre
Site: Motor end plate
Excitation-Contraction coupling
z
Transduction of muscle action potential to
intracellular signal (elevated calcium)
1
Structural proteins
Titin
Germann & Stanfield, fig 13.5
z
Many structural proteins
z Maintain sarcomere/myofibril structure
z Provide framework for myofilament arrangement
z
Titin
z Z-line to M-line (1μm)
z Template for myosin assembly
z Stability at long sarcomere lengths
z
Helps prevent sarcomere popping & Z-line streaming
2
Structural proteins:
Dystrophin
z
Dystrophin
z Very large protein
z Anchor for contractile
apparatus to surface
membrane
z In turn, anchor to
basement membrane
via other proteins
z
Absence or defect
z Muscle wasting
disorders, or
z Muscular dystrophies
z
z
Duchenne
Becker dystrophy
Jones, Round & Haan, fig 3.5
z
z
Other structural proteins
z Dystroglycan, Sarcoglycan, Laminin
Associated signalling proteins
z Syntropin (binds signalling molecules)
z NOS – nitric oxide synthase
3
Contractile proteins: Myosin
Germann & Stanfield, fig 13.5
(Thick Filament)
z
z
z
z
z
z
Single molecule
2 identical chains (MHC)
z MW: 200,000 kD
4 light chains (MLC)
z MW: 20,000 kD
z Regulatory (RLC) & Essential (ELC)
S1 Head segment
z ATPase activity
z Actin binding site
S2 Tail segment
z Flexible hinge regions
z Combines with other tails
Thick filament
z 300 myosin molecules
z Protruding Head segments
z 10% central Bare Zone
z Can contact 6 actin filaments
Jones, Round & Haan, fig 1.2
4
Contractile proteins: Actin
Germann & Stanfield, fig 13.4
(Thin Filament)
z
z
z
G-actin (globular protein)
z MW: 42,000 kD
z Each has myosin binding site
F-actin (polymer of G-actin)
z α-helical polymer
z Thin filament proper
Thin filament
z Connected through Z-line by
α-actinin
z Square array in I-band
z Hexagonal array in A-band
Surrounded by regulatory proteins
z Tropomyosin
z Troponin complex
z
Tn-C, Tn-T, Tn-I
5
Regulatory proteins
Germann & Stanfield, fig 13.9
z
Tropomyosin
z Spans 7 G-actin monomers
Troponin complex
z 1 per tropomyosin
z Controls 7 G-actin monomers
z
Provides “Steric Blocking” model
z
At rest
z myosin binding sites blocked by tropomyosin
Ca2+ present
z Ca2+ binds to Tn-C
z Conformational alteration in Tn-complex
z Moves tropomyosin
z Exposes myosin binding sites
z
z
6
Force Production
Muscle tension is produced only when:
1. Myosin releases energy obtained via ATP hydrolysis, and
2. Actin & Myosin exist in a bound state
z
z
Forms the basis of the Sliding Filament Theory
Most commonly described by the Cross-Bridge Cycle, which considers
the interactions between:
z Myofilament contractile proteins (Actin & Myosin)
z Intracellular [Ca2+]
z ATP hydrolysis
z
Four major steps
1. REST: myosin is in an energised state following ATP hydrolysis
2. BOUND: Actin & Myosin bind at specific sites (requires ↑[Ca2+]i)
3. POWER STROKE: energy release and “cell shortening” → Rigor
4. DETACHMENT: Actin & Myosin detach following ATP binding
7
Cross bridge cycle
z
At rest myosin is energised
z
z
z
z
z
z
z
z
ATP has been cleaved to ADP, Pi & energy
Energy is stored within the myosin protein
Ca2+ binds to troponin-C
Myosin binding site on actin becomes exposed
Myosin becomes bound to actin
Pi is released from myosin upon binding with actin
Myosin releases energy through the power stoke
Actin is dragged toward the interior of the sarcomere
z
Appearance provides the Sliding Filament Theory
Sarcomere length decreases, translating to
z whole muscle shortening (CONTRACTION)
z
8
Cross bridge cycle…cont
ADP is released from myosin during power stroke
z After power stroke myosin binds new ATP molecule
z Actin and myosin detach
z ATP hydrolysis occurs to re-energised myosin
z Myosin head returns to resting position
z If [Ca2+] is maintained:
Crossbridge cycle continues
z However [Ca2+] is rapidly
pumped back into the SR
by the SR/ER Ca2+ ATPase
(SERCA).
∆L=10nm/cycle, but:
many cycles & sarcomers
z
9
Cross bridge cycle
Germann & Stanfield, fig 13.7
10
Muscle Contraction
(summary)
1.
2.
3.
4.
5.
6.
7.
Germann & Stanfield, fig 13.8
NMJ A.P. transmission
z
Via ACh release
A.P. propogation
z
In sarcolemma
z
Down T-tubules
E-C-coupling
z
Induces SR Ca2+ release
Myosin binding sites
exposed
Cross Bridge Cycle
z
Force production
Relaxation via:
z
Active Ca2+ removal
Return to resting state
11