Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:…NMJ is regulated by factors intrinsic to the motor neuron, including proteins that regulate intracellular signaling, as well as retrograde signals that convey postsynaptic activity levels to modulate neuronal properties (Collins and DiAntonio, 2007; Harris and Littleton, 2015; Menon et al., 2013).
Category: