$A$ sarcomere consists of

$A$: All muscles follow the 'All or None' principle.
$R$: All muscles contract either fully or not at all depending upon the availability of a threshold stimulus.

In which of the following functions are white muscles not used?

Select the correct statements regarding the mechanism of muscle contraction.
$A.$ It is initiated by a signal sent by $\text{CNS}$ via a motor neuron.
$B.$ Neurotransmitter generates an action potential in the sarcolemma.
$C.$ Increased $Ca^{++}$ level leads to the binding of calcium with troponin on actin filaments.
$D.$ Masking of the active site of actin prevents contraction; unmasking leads to contraction.
$E.$ Utilising the energy from $\text{ATP}$ hydrolysis to form a cross-bridge.

Label the different components of the actin filament in the diagram given below.

Choose the correct option for the following:
Biceps $\quad$ Iris $\quad$ Heart