The　experimental　data　of　interspike　intervals　were　recorded　in　a　neural　pacemaker,　and　the　presence　of　period-adding　bifurcation　cascade　was　demonstrated.　By　detecting　unstable　periodic　orbits　from　the　chaotic　interspike　intervals　located　in　between　period-n　bursting　and　period-(n　+　1)　bursting,　it　is　found　that　the　unstable　period-　n　orbit　can　be　detected　from　the　chaotic　interspike　intervals　near　the　period-　n　bursting,　and　the　unstable　period-(　n　+　I)　orbit　exists　in　the　chaotic　interspike　intervals　near　period-(　n　+　1)　bursting.　Moreover,　the　unstable　period-　n　orbit　also　can　be　detected.　For　such　a　phenomenon　the　theoretical　analyses　are　presented　from　the　point　of　view　of　the　nonlinear　dynamics　by　way　of　the　pancreatic　beta-cell　model　suggested　by　Sherman.　The　direct　causes　of　emergence　of　the　phenomenon　are　shown　to　be　the　intermittency　of　type　I　and　that　of　type　M　and　they　are　induced　by　the　saddle-node　bifurcation　and　period-doubling　bifurcation,　respectively.　Consequently,　the　mechanism　for　identification　of　chaotic　firing　interval　is　revealed.　At　the　same　time,　the　universality　of　the　phenomenon　is　elucidated　to　a　certain　extent.　Furthermore,　a　novel　method　is　put　forward　for　identifying　the　chaotic　firing　interval.