The delay and blockage of sensory impulses in the dorsal root ganglion*

Abstract

The aim of the present series of experiments is to try to clarify these controversial issues and to determine whether the dorsal root ganglion modified in any way the passage of sensory impulses. Besides suffering a delay under normal conditions, the sensory impulses are found to be blocked in their passage through the dorsal root ganglion during the early relatively refractory period of the afferent fibres. METHODIn most of the experiments the 9th or 10th dorsal root ganglion of the frog (Hyla aurea, Rana cate8biana and R. pipiens) was used. These ganglia were dissected intact and in continuity with the roots and the trunk. The roots were severed adjacent to the spinal cord. The trunk was either cut prior to its entry into the sciatic plexus, or followed into one or several of the branches of the plexus, e.g. (1) posterior femoral cutaneous nerve, (2) lateral crural cutaneous nerve, (3) tibial nerve, and (4) peroneal nerve (see Ecker, 1881). The fibres of the posterior femoral cutaneous nerve have been found to go mainly to the 10th dorsal root, while those of the lateral crural cutaneous nerve go mainly to the 9th. The preparations were always scrupulously cleaned to remove connective tissue and blood clots. Every effort was made to avoid injury ofthe preparation.In some experiments requiring a small number of nerve fibres the 11th dorsal root ganglion was used. This ganglion is very small, lying on the lateral surface of the columella. As a considerable segment of its nerve trunk is buried in the coccygeal muscle and as its roots are thin and penetrate * Part of the work was carried out in the Department of Physiology, University of Otago Medical School, Dunedin, New Zealand, and has been reported as short communications (J. cell. comp. Physiol. 38, 131-135, 1951).

The Mode of Impulse Conduction Through the Spinal Ganglion

“…6 b, A) should indicate that the centripetal conduction from the bifurcation to the dorsal root was blocked partly. Conse quently, the safety factor of the conduction from the bifurcation to the dorsal root might be smaller than that from the peripheral nerve to the bifurcation, as was argued by Dun (1955), but it should be larger than that from the dorsal root to the bifurcation.…”

“…3 The presence of the M spike in the intracellular record shows that the impulse from the peripheral nerve arrives at the bifurcation, but it does not immediately indicate that the same impulse further conducts to the dorsal root. However, Svaetichin (1951) observed that the impulse, elicited by stimulating a single fiber in the peripheral nerve, conducted to the dorsal root even when the block was noticed in the extracellular potential of the ganglion cell.…”

Facilitation of individual gamma‐motoneurones by the discharge of single slowly adapting type 1 mechanoreceptors in cats.

“…The latter measurement frequently gave conduction velocities well in excess of the known maximum value of A a/f, axons in the cat. The discrepancy was presumably a consequence of the delay that can occur in transmission of an action potential into the dorsal root ganglion cell body (Dunn, 1955). Figure 5A shows the distribution of central delays of y-efferent facilitation by SAl afferents.…”

Effects of nerve impulses on threshold of frog sciatic nerve fibres.

“…As noted, regions of low conduction safety have been associated with branches (Grossman et al 1973;Dun, 1955;Ito & Takahashi, 1960;Yau, 1976), expansions and constrictions (Goldstein & Rall, 1976;Spira, Varom & Parnas, 1976) and other axonal specializations such as nodal distribution and demyelination (Waxman, Pappas & Bennett, 1972) or profound variation in the local density of Na+ channels (Zeevi, 1972;Ritchie & Rogart, 1977). Such features may present no obstacle to conduction at low rates of discharge, yet they may become completely impassable as the activity rate rises (Krnjeci6 & Miledi, 1959), suggesting that the resulting growth in depression may be responsible for the conduction block.…”

“…Recent direct work from several laboratories (Smith & Hatt, 1976;Parnas, 1972; Grossman, Spira & Parnas, 1973;Yau, 1976;Van Essen, 1973) supports earlier observations and suggestions that not all branches of an axon are invaded by each spike (Adrian, 1920(Adrian, , 1921Lucas, 1917;Barron & Matthews, 1935;Krnjevic & Miledi, 1959;Howland, Lettvin, McCulloch,.Pitts & Wall, 1955, Raymond & Lettvin, 1969Chung, Raymond & Lettvin, 1970), and that bifurcations constitute regions of low conduction safety (Dun, 1955;Ito & Takahashi, 1960;Lloyd & McIntyre, 1950;Wall, Lettvin, McCulloch & Pitts, 1956). Probability for invasion of a branch is neither constant nor independent of activity (Raymond, 1969;Grossman et al 1973).…”