Monosynaptic activation of different portions of the motor neuron membrane

Abstract

Using isolated frog spinal cords, treated with pentobarbital to silence internuncial discharge, intracellular recordings from motor neurons reveal differences in dendritically initiated EPSP evoked by dorsal root stimulation and somatically initiated EPSP evoked by lateral column stimulation. Under these conditions, dorsal root EPSP never reached motor neuron threshold whereas threshold was easily reached by lateral column EPSP. EPSP's initiated by dorsal root volleys were slower in their time course and smaller in their amplitude than those initiated by lateral column volleys. EPSP's initiated by lateral column volleys reduced the amplitude of antidromic spikes, while those produced by dorsal root stimulation did not. Lateral column induced EPSP was also capable of reducing the amplitude of orthodromic spikes. Some observations on duration of transmitter action are reported. It is concluded that dendritic excitation following a dorsal root volley influences the level of polarization of the cell body by electrotonic propagation of the resulting EPSP.

Pharmacological characterization of D‐aminophosphonovaleric acid antagonism of amino acid and synaptically evoked excitations on frog motoneurones in vitro: an intracellular study

“…Interestingly, there was an apparent saturation in the D-APV antagonism of glutamate responses at about 60% of control ones; these results thus indicate that glutamate is probably a mixed agonist acting partly on NMDA receptors and partly on APV-insensitive quisqualate receptors (Watkins & Evans, 1981;Mayer & Westbrook, 1984). In analogy to a recent intracellular study on rat hippocampal neurones in vitro (Crunelli et al, 1983) (Shapovalov & Shiriaev, 1980) and even unable to generate a motoneuronal action potential (Eccles, 1946;Fadiga & Brookhart, 1960 (Thomson et al, 1985). From the present results the possibility exists that the natural transmitter of the monosynaptic e.p.s.p.…”

“…Their corresponding percentage actual conductance increases were 22 ± 8, 3 ± 1 and 39 ± 24, respectively. These cells, (Schwindt, 1976 Shapovalov & Shiriaev, 1980) and inability to evoke orthodromic action potentials (Eccles, 1946;Fadiga & Brookhart, 1960). Some of these features are illustrated in Figure 4 in which (a) shows several superimposed polysynaptic e.p.s.ps near threshold for action (Colquhoun, 1971 Figure 4a).…”

Electrotonic coupling between neurones in the rat mesencephalic nucleus

“…Thirdly, since no appreciable change of amplitude was observed when the resting potential was modified artificially through the intracellular electrode, this potential does not seem to have the same dependency on the membrane potential that chemical synapses have in the central nervous system, where EEPSP is close to 0 mV (Coombs, Eccles & Fatt, 1955;Eccles, 1964). The absence of dendrites and the fact that the few synapses present occur directly on their soma excludes geometrical factors (Fadiga & Brookhart, 1960;Terzuolo & Llina's, 1966;Smith, Wuerker & Frank, 1967) as responsible for the apparent insensitivity of these SLDs to membrane potential variation.…”

The slow potentials of thoracic respiratory motoneurones and their relation to breathing

“…On the other hand Fadiga & Brookhart (1960) have described such a depression of the antidromic spike of frog motoneurones during synaptic activation by fibres of the lateral funiculus. These considerations raise the possibility that in the mammal the segmental and central tracts may also differ in their site of termination on motoneurones but more quantitative studies are clearly required to settle this important issue.…”