  We investigate the local fluctuations of filamentous actin (F-actin), with
focus on the skeletal thin filament, using single-particle optical trapping
interferometry. This experimental technique allows us to detect the Brownian
motion of a tracer bead immersed in a complex fluid with nanometric resolution
at the microsecond time-scale. The mean square displacement, loss modulus, and
velocity autocorrelation function (VAF) of the trapped microprobes in the fluid
follow power-law behaviors, whose exponents can be determined in the
short-time/high-frequency regime along several decades. We obtain 7/8
subdiffusive power-law exponents for polystyrene depleted microtracers at low
optical trapping forces. Microrheologically, the elastic modulus of these
suspensions is observed to be constant up to the limit of high frequencies,
confirming the origin of this subdiffusive exponent on the local longitudinal
fluctuations of the polymers. Deviations from this value are measured and
discussed in relation to the characteristic lengths scales of these F-actin
networks and probes' properties, and also in connection with the different
power-law exponents detected in the VAFs. Finally, we observe that the thin
filament, composed by tropomyosin (Tm) and troponin (Tn) coupled to F-actin in
the presence of Ca$^{2+}$, returns exponent values less dispersed than F-actin
alone, which we interpret as a micro-measurement of the filament stabilization.
