Pattern of irregular dorsal root discharge in the spinal cat

Neonatal Mice Spinal Cord Interneurons Send Axons through the Dorsal Roots

Spontaneous interneuron activity plays a critical role in developing neuronal networks. Discharges conducted antidromically along the dorsal root (DR) precede those from the ventral root’s (VR) motoneurons. This work studied whether spinal interneurons project axons into the neonate’s dorsal roots. Experiments were carried out in postnatal Swiss-Webster mice. We utilized a staining technique and found that interneurons in the spinal cord’s dorsal horn send axons through the dorsal roots. In vitro electrophysiological recordings showed antidromic action potentials (dorsal root reflex; DRR) produced by depolarizing the primary afferent terminals. These reflexes appeared by stimulating the adjacent dorsal roots. We found that bicuculline reduced the DRR evoked by L5 dorsal root stimulation when recording from the L4 dorsal root. Simultaneously, the monosynaptic reflex (MR) in the L5 ventral root was not affected; nevertheless, a long-lasting after-discharge appeared. The addition of 2-amino-5 phosphonovaleric acid (AP5), an NMDA receptor antagonist, abolished the MR without changing the after-discharge. The absence of DRR and MR facilitated single action potentials in the dorsal and ventral roots that persisted even in low Ca2+ concentrations. The results suggest that firing interneurons could send their axons through the dorsal roots. These interneurons could activate motoneurons producing individual spikes recorded in the ventral roots. Identifying these interneurons and the persistence of their neuronal connectivity in adulthood remains to be established.

Which afferents induce and transmit dorsal root reflex in rats?

The dorsal root reflex (DRR) in single fibers, recorded in the severed L5 dorsal root central stump, initiated spontaneously or by electric stimulation (St) of sciatic nerve, was studied in anesthetized rats. The excitation of group II afferents were the major inputs to induce DRRs which were transmitted mainly in fibers with conduction velocity below 20 m/s, and its central delay was 3-5 ms. The firing frequency of both spontaneous and evoked DRRs were increased two to four times after spinalization (T13). These results suggested that the DRR may play an important role in the modulation of sensory afferent inputs regulated by supraspinal structures.

Can antidromic stimulation of rat muscle afferents modulate the sensitivity of muscle spindles?

The sensitivity of muscle spindles in gastrocnemius-soleus (GS) muscles following dorsal root (DR) antidromic stimulation (ASt) was investigated in anesthetized rats, in which the hindleg was totally denervated except the GS muscle, ventral roots from L3 to S1 and DRs from L4 to L6, which were severed after laminectomy. The peripheral end of a teased DR filament with single unit tonic discharges from a constantly stretched muscle spindle were recorded and antidromically stimulated after identification. In response to ASt with various frequencies, two different types of ending were found: (1) type I, its tonic discharge was not altered following ASt at 100 Hz for 10 s, while type II was significantly inhibited even at a frequency of 50 Hz; (2) at 300 Hz, both types of endings were inhibited, however, type I recovered in less than 4 s, while type II took more than 15 s to return to control level, and a silent period (1-5 s) was found in type II but not in type I; (3) the effective minimum inhibitory frequency for type I and type II was 200 and 50 Hz, respectively. These results suggest that type I and type II endings may correspond to the primary and the secondary endings of the muscle spindle whose sensitivity may be modulated antidromically by the dorsal root reflex.