It's been a long time since I looked into it, and Lupron makes me forget.
But, in short, Caverject causes the erection IN LIEU of normal stimulus from
the nerve bundle.

Hello Bob Anthony,
   Not sure if I can help:
I lost 1 set of nerves.  Viagra did not work for 2 years......
I was told to use it or lose it theory:
I bought a pump.  I understand that is not the same as specific blood flow
into the cavernous tissue.
I used that and it did produce plump soft.......you know.....
I asked the Dr. for help.
I had a show and tell visit.  He showed me prostaglandin, and injection.  I
understand that mimics the bodies response and provides blood in the
cavernous tissue.....
I learned how to use it, and it was amazing, but at differing times of day
could be painful,
At night you were left with a swollen member, and much pain.
In the a.m. you could get good results and walk them off...........Like your
trip back from caverjet injection...
So, I used the injections prescribed, did morning rather than evening, and I
had good luck.
This is all do to injection.....I waited for a long time, and tried viagra,
and got nasty headache, and no results......
I kept a log of shots, and such since they only wanted you to use the meds
every 3 days or more....
I did 36 if not more of those injections, some great some miserable....
I started to notice a little fire........hum,  1.5 years later.....not much
but worth remembering.
The fire got a bit brighter about 1 3/4 years.....hum....It was summer, I
had viagra, and said what the hey.
I tried it, and what a response......scared the hell out of my wife!
I gave the pump to another survivor, did not get any more injections, and
used Viagra....
I found out now that 1/3 of a 100 mgs pill is all that is needed.
This is 6 years later by the way.....
That is my story, I hope it helps, and realize, walking, sexual stimulation,
pumps, injections, pills, are all avenues to regain what once was.
We are compromised, and such, and need to feel ok to try other avenues to
attain what we have lost.
John Loomis

Bob Anthony,
I 've done a lot of reading about ICI (Intra-Cavernosal Injection), and
have used it hundreds of times. They will work with no input from the
nerves. They act directly on the smooth muscle controlling the blood
vessels. Once you get the hang of doing the injection, you will hardly
feel it. Anxiety is most of the problem at first. Getting the dose
right requires some experimentation. I think they helped me get my
natural erection back. The dosage reqirement dropped as that happened.
Plus the sex is super. I can easily have sex twice with the same
erection using injections. IMHO they are massively underutilized by the
PCa community. Ask your doc for some gauge #31  5mm needles. They are
much better than the harpoons I started with. You may be at the start
of a very happy part of your life!
Steve U