Muscle Structure Flashcards
In 1929, Lohmann discovered ATP in muscle extracts, and it was soon accepted
that the energy for contraction was provided by _____ ___________, not from Lactic Acid
the conversion of ATP to ADP
+ Pi
Skeletal Muscle
o Cells are cylindrical and between in____length
1 mm – 4 cm
______ cells initially provide myoblasts for muscle growth and ensure
restoration of muscle cells following injury
o Satellite cells maintain their own population by self-renewal, thus are stem cells
Satellite
Cardiac muscle
o Contain
1-3 centrally located nuclei
o Oxidative (~40% cell volume is mitochondria)
o Cells electrically coupled
o AP triggers synchronised Ca2+ release from SR (CICR)
o EC spaces contain collagen
Cardiac muscle Intercalated discs at intercellular junctions, consisting of:
§ Nexus or gap junctions
§ Fascia adherens or intermediate junctions
§ Macula adherens of desmosomes (“spot welds”)
Cardiac Cells have a less extensive SR; diad or a triad system?
diad
1993 Calcium sparks
• Proved that the cell uses spatially defined sub-microscope
signals, rather than the cytoplasm behaving like a single
chemical pool
• Current Ca via ____served to trigger SR release
• This takes the form of ‘calcium sparks’ which summate to make
the whole calcium transient
• The amplitude and number (or frequency of firing) of calcium
sparks determines the ____?
DHPRs
calcium transient amplitude
§ Cardiac Calcium buffered in SR by ______, which takes 35-40 calcium
ions per calsequestrin molecule
calsequestrin
SR membrane contains:
- Calcium release channels (RyRs) abundant in junctional SR
* SERCA (2Ca2:1ATP) regulated by phospholamban
• When SR Ca2+ load is high:
o Increased Ca2+ available for release
o Enhanced fraction of Ca2+ released for any given ICa
trigger (“gain” of EC coupling)
Smooth Muscle
o Two types distinguished:
(i) multi-unit cell: lack electrical coupling, neutrally regulated (as with
skeletal muscle)
§ (ii) single-unit: electrically coupled; syncytial (as with cardiac muscle)
Smooth Muscle
o Found in the walls of hollow organs
o Cells are small what length and diameter?
o Cells contain a single, centrally-located nucleus
o Cells contain both actin and myosin but not in the strict proportions of striated
muscle
o There is no T-tubular system
o There is less extensive SR – rather a limited network of narrow tubules near the
plasmalemmal membrane
100-400 micrometres in length
2-5 um in diameter
Smooth Muscle
o Found in the walls of hollow organs
o Cells are small what length and diameter?
100-400 micrometres in length
2-5 um in diameter
Smooth Muscle
o Cells contain ____nucleus
o Cells contain both actin and myosin but?
a single, centrally-located
not in the strict proportions of striated
muscle
Smooth Muscle
There is _____T-tubular system
o There is less extensive ___– rather a limited network of narrow tubules near the
plasmalemmal membrane
no
SR
Smooth Muscle
Individual myocytes are mechanically connected by desmosome-like
attachments and electrically or ionically-connected by?
o Isolated muscle cells can shorten to about 20% of their resting length, i.e. much
____than cardiac or skeletal muscle
gap junctions
more
- Sarcomere
o Lengths
Lengths of thick (1.6 um) and thin (1.0 um) filaments remain constant (to
within ± 1%) during contraction
o As sarcomere length changes, the length of the A band remains constant while
that of the I band varies
Thick Filament consisting of some 300 ________ arranged
Myosin molecules:
Myosin molecules:Consists of 2 heads on a single tail consisting of a double stranded alpha coil
§ The tail region can be enzymatically cleaved (using naturally occurring
digestive enzymes) into two moieties: ___ meromyosin and ___
meromyosin
§ Sub-fragment 1 (the myosin ‘head’) can be further cleaved from the
HMM segment using papin
Use of detergents and denaturing agents allows dissolution of the head
to reveal both ‘heavy’ and ‘light’ chains: the myosin heavy chains
(MHC) and four myosin light chains (MLC)
light, heavy
Myosins are ____-dependent molecular motors that use the energy of
ATP hydrolysis to ?
actin
produce movement
Myosins are implicated in a variety of functions
• Cell migration and adhesion • Intracellular transport & localisation or organelles and macromolecules • Signal transduction • Tumor suppression
The cross-bridge
Consists of the two _____ ______which appear to behave more or less
independently of one another
o Function is conferred by both the heavy chains (ATPase activity) and the two
pairs of regulatory light chains (which can undergo phosphorylation)
‘globular’ heads
The cross-bridge Has two functional properties:
- Is an ATPase, Physiological levels of Mg2+ normally keep the rate of ATP
hydrolysis low
§ It can bind to actin
Thin filaments
o Thin filaments made up of 3 protein molecules:
§ Actin
§ Tropomyosin
§ Troponin (Tn-T, Tn-C & Tn-I)
Tropomyosin
inhibits the actin-activated ATPase activity of myosin
Troponin
A globular protein consisting of 3 polypeptides
o Troponin T
o Troponin C
o Troponin I
Troponin Binds to one end of the _______ molecule (via TnT) in a 1:1
stoichiometry such that each thin filament contains 50 troponin
molecules
tropomyosin
_______ is completely absent in smooth muscle
Troponin
Tethering proteins
Filaments which hold myofibrils at optimal distances allowing speed and power
in muscle contraction
Titin
s passive elasticity to sarcomere
Largest protein currently known
Keeps the thick filament centered during activation
Acts as a molecular ‘ruler’ that dictates the lenth of the thick filament
Also involved in myocyte signalling
Tethering protein
o C-protein
§ Structural role, anchors titin to MHC tail
Tethering protein
M-protein and myomesin
§ Connects thick filaments at M line
§ Binds to titin
Tethering protein
Nebulin
Z-disk to end of thin filament; acts as a molecular ruler for actin
Forms long non-elastic filaments extending either side of Z disk and
along the thin filaments
Each nebulin filament is as long as its adjacent actin filament and may
regulate the number of actin monomers that form the thin filament
Tethering protein
alpha-Actinin
§ F-actin crosslink; anchors actin at barbed end to Z-disk
Stabilisation of thin filaments
The ends of a thin filament are associated with two actin-capping
proteins,_______ and ______ that provide stability
CapZ and tropomodulin
CapZ
Is present in the Z-disk of skeletal muscle where it helps
prevent actin filaments from depolymerising at their (+)
end
Tropomodulin
Lies at the opposite end of the thin filament and protects
the (-) end from depolymerising
- The filament lattice
o In cross section, the thick filaments lie in a ____ array.
The ____ filaments lie at the trigonal points
of the hexagonal array of ____filaments.
Hence there are twitce as many ___filaments as ___filaments
hexagonal thin thick thin thick
A fundamental observation: A plot of the inverse square of i_______ (r) as a function of sarcomere length (l) yields a straight line, i.e.
1/r^2 ∝ l.
This implies that
interfilament separation
the product lr^2 (which is proportional to volume) is
constant, independent of sarcomere length. This experimental
observation demonstrates that muscle contraction takes place
isovolumetrically
