The technique has yet to be tested on humans, but if it works could ease one of the biggest problems for people with catastrophic damage to their spinal cord, they said.
Damage above the fourth cervical vertebra in the neck invariably interrupts breathing. The patient has to be put on a ventilator, and this carries a recurrent danger of respiratory infection.
Under normal circumstances, the rate and depth of breathing are controlled automatically by nerve cells in the brain stem.
To do this, these neurons in turn control specialised motor cells located from the third to the sixth cervical vertebrae.
These cells develop farther down into the so-called phrenic nerve, which causes the muscles of the diaphram to relax and contract and the lungs to fill and empty.
A team led by Jerry Silver, a professor at Case Western Reserve University School of Medicine in Ohio, report on an experimental way of bridging damage in the key cervical area.
They injured rats' spinal cords at the second cervical level to paralyse one-half of the diaphragm, and grafted in a tiny section of peripheral nerve from the rodents' limbs, attaching it both before and after the damage site.
The idea was to use the peripheral nerve as a bridge on which the precursor cells for the phrenic nerve could grow.
The technique had been tried before but was hampered by molecules which build up at the site of spinal cord injuries and inhibit regrowth of nerves.
To get around this, the researchers injected an enzyme called chondroitinase ABC at both ends of the graft to degrade the inhibitors and open up avenues in the scar tissue through which the nerves could -- hopefully -- grow.
After three months, tests showed that between 80 and 100 percent of breathing function had been restored, a performance that was maintained at the six-month mark.
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