Skeletal Muscle: Activation, Contraction & Relaxation

Question 1

Steps in the Activation of Contraction (of Striated Muscle)

Answer 1

The arrival of an AP down a TT leads to Ca efflux, via the RyR1 receptor from
the SR to the myoplasm
A rise in cytosolic calcium triggers cross bridge cycling
Activation of key calcium-dependent signaling pathways
§ Mitochondrial respiration

Question 2

RyR1 is modulated by triadin, junction and _______ that together
form the calcium-releasing unit

Answer 2

calsequestrin

Question 3

Cross bridge cycling
• Ca diffuses from the ___ (where its conc
is high) to the ______ (where conc is low)
• Calcium binds to troponin-C
o In particular, to the low affinity cation binding sites (the
high affinity binding sites are largely occupied by Mg2+)
o Saturation of ______ with Ca induces movement of
the tropomyosin molecule (which is attached via
Troponin-T) further into the actin “groove”, thereby
releasing the stearic inhibition which otherwise prevents
actin from gaining access to ______.

Answer 3

SR
myofibrils
troponin-C
myosin

Question 4

Troponin Complex

Answer 4

Complex of three globular proteins located at regular intervals along the
tropomyosin filaments
o Troponin T attaches the troponin complex to tropomyosin
o Troponin I, along with tropomyosin, inhibits the interaction of actin and myosin
by covering the myosin-binding site on actin
o Troponin C is a calcium binding protein

Question 5

The Intracellular Calcium – Force (Dose response) Relation
Not possible to assess [Ca2+]i accurately with an intact muscle fibre, so use
methods for skinning the muscle fibres:

Answer 5

Mechanical microdissection (peeling) of the sarcolemma
§ Chemical ‘solubilising’ of the sarcolemma using lipid solvents
(glycerol, EGTA, Triton)
§ The skinned fibre can then be placed in an organ path

Question 6

The Intracellular Calcium – Force (Dose response) Relation

Answer 6

Sigmoidal shape typical of biological dose-response relations (slopey S shape)

Question 7

Cross bridge cycling process

Answer 7

1. Cross bridge formation: release of Pi
2. Power Stroke; ADP is released,
3. ATP binds myosin causing detachment of myosin from actin; cross bridge breaks,
4. ATP hydrolysis, cocking of myoin head.

Question 8

ATP commonly cannot be regenerated (by CrP) as rapidly as it is
hydrolysed. Thus there is a buildup of ADP and Pi. The latter has an inhibitory
effect on XB formation. In addition, if the rate of regeneration of CrP (via
oxidative phosphorylation) lags the rate of hydrolysis of ATP, then there is an
accumulation of ____. A decline of pH inhibits both ________________. Finally, if the rate of oxidative phosphorylation lags the rate of
energy demand and anaerobic glycolytic processes dominate, then pH falls even
further as lactate accumulates

Answer 8

[H+].
Ca release from the SR and
XB formation

Question 9

Relaxation
The SR membrane contains Ca pumps which can be isolated and studied in
vitro. The SERCa pump:

Answer 9

(ii) Sequester (uptake) Ca against a concentration gradient of some 10-
fold
Operates through the expenditure of metabolic energy with a
stoichiometry of 1 ATP:2 Ca translocated from cytoplasm to terminal
cisternae

Question 10

The SR Ca pump:in cardiac muscle is associated (in a 1:1 stoiciometry) with
________, an integral part of the SR membrane. Mg-dependent
phosphorylation of phospholamban (via both cyclin AMP-dependent
protein kinase and calcium-calmodulin dependent phosphorylation)
causes an increased rate of ATP hydrolysis and calcium uptake by
releasing the inhibitory action of phospholamban on the SERCA}

Answer 10

phospholamban ( ihbits SERCA)

Question 11

The lumen of the SR of striated muscle contains the glycoprotein
_______, 1 mole of which can bind up to 43 moles of calcium, with
low affinity, thereby lowering the trans-membrane gradient of calcium
concentration.

Answer 11

calsequestrin

Question 12

• Regulation of SERCA

o Regulated by

Answer 12

phospholamban and sarcolipin

Question 13

Phospholamban:

Answer 13

and to a lesser extend in slow twitch skeletal, smooth and atrial muscle.

§ PLB inhibits SERCA activity at low cytosolic calcium
§ Release of Ca from SR and/or phosphorylation (by PKA & CaMKII)
relieve inhibition of SERCA

Question 14

Sarcolipin

newly identified regulator of SERCA in skeletal muscle

Answer 14

Binds to SERCA in the presence of high cytosolic Ca
§ Has a role in mediated muscle-based thermiogenesis
§ When SLN binds to SERCA, it allows ATP hydrolysis to occur, but SR
Ca transport is decreased due to “slippage” of calcium back into the
cytosol
§ SLN therefore causes more ATP to be hydrolysed by SERCA to
transport calcium back into the SR
§ SLN is an important regulator of metabolism and thermogenesis