AFAIK, no multiple Interface inheritance in FreePascal.
2 possible Methods come to mind:
1) "Chain" them
INeuronReceptor inherits from INeuronSignal
INeuron inherits from INeuronReceptor
2) Don't bother. A Class can inherit multiple interfaces.
Type MyClass = Class(TInterfacedObject, IInterface1, IInterface2 .... )