At resting conditions chain fibres are often kinked in the absence of fusimotor stimulation or external stretch.
Static fusimotor stimulation causes both primary and secondary endings to be stretched and increase discharge.
Stretching the chain fibres overall results in transmission of stretch to the primaries and, more so, to the secondaries which increase their discharge much more.
When stretch is maintained there is no further mechanical change, however there is a reduction in afferent discharge due to electrical adaptation.
Chain fibres bias the discharge of primary and secondary endings, disrupt length sensitivity of the primaries and ENHANCE length sensitivity of secondaries.
Primary ending (group Ia) and secondary ending (group II) afferent discharge frequency during stretch alone (black lines) and during stretch superimposed on tonic static gamma fusimotor stimulation (red lines) at 75Hz.