The pins & needles is basically a massive histamine reaction.
In my case it feels literally like someone poking every inch of your body with a needle repeatedly.
Often times this is accompanied by massive itching & burning to the point of being painful.
Though this may be because of the doses involved.
Another point that lends credence to the histamine idea is that oftentimes one turns red after a shot.
This redness oftentimes moves from the injection site up the head & then down to the feet.
The redness is accompanied by the pins & needles in the affected areas.
Another point of note is that the pins & needles is dose dependent.
If your using a regular insulin syringe (1cc) your dosage is limited & as such it might not be overwhelming.
Now if your using say a 3cc barrel & as such can dose higher then your much more likely to experience the pins.
As as far as I recall morphine's solubility is approx 65mg per ML.
I don't have the time to double check that figure at the moment but it's close enough.
Once you move onto the larger barrel size & the increased dosages that are available as a result,
chances are you'll really get the pins & needles.
Personally I'm not to keen on the full pins & needles effect.
As such I take hydroxyzine or a similar anti-histamine prior to my shot.
If I time it right I can reduce the histamine reaction (pins & needles) to a pleasant sensation.
As opposed to being intense to the point of being painful that I would experience otherwise.
Or if I take the anti-histamine to early it sometimes completely eliminates the pins & needles entirely.
If I want a wake up call then I might skip the anti-histamine as the pins & needles definitely gets my attention.

Otherwise I'll take my anti-histamine & go.
Considering that the anti-histamines reduce/remove the pins & needles I would assume it's mostly a histamine reaction.
Hope that helps answer your question.
Note: Obviously this all applies to IV usage, your not going to be getting a rush &/or pins / needles with other ROA's.