How the wiring of IR emitter detector pairs work
It appears that each emitter is an IRLED and that they're all emitting all the time. 

The detectors, on the other hand, work individually and each one only turns ON when it's  hit with IR. Since their V1's are connected to +5V, when they're OFF no current will pass through them and the voltage across their 10k ohm pulldown resistors will be close to zero volts (GND).

On the other hand, when they're turned ON by IR, current will pass through them and the pulldown, with the result that a voltage will be developed across the pulldown which will be sent to the Arduino, letting the Arduino know that a sensor has picked up an IR signal.

The sensors appear to be used to detect IR reflected back from the emitter.