Dual mode of junctional transmission at synapses between single primary afferent fibres and motoneurones in the amphibian

1. The isolated hemisected frog spinal cord has been used to examine the effects of changes in ionic composition of perfusing medium on the intracellularly recorded e.p.s.p.s produced in single motoneurones by direct stimulation of individual dorsal root fibres through a separate intracellular micro‐electrode. 2. The monosynaptic single‐fibre e.p.s.p.s usually reveal two distinct components, early and late. The early component is resistant to replacement of external Ca 2+ by Mn 2+ whereas the later is reversibly abolished. It is concluded that the junction between primary afferent fibre and motoneurone provides joint electrical and chemical transmission. 3. The average amplitude of the unitary electrical e.p.s.p. varies at different junctions from 25 to 430 μV, mean 124 ± 17 μV ( n = 50). It is relatively stable and changes its amplitude only with changes in the height of presynaptic spike. The individual amplitudes observed in a given cell usually have a normal distribution suggesting that any variability in electrical response is entirely due to noise. 4. The average amplitude of the single‐fibre chemically mediated e.p.s.p. varies from less than 20 μV to 1·7 mV, mean 222 ± 33 μV ( n = 71). A positive correlation was found between the amplitudes of chemical and electrical e.p.s.p.s recorded from different motoneurones. 5. Chemical e.p.s.p.s evoked by consecutive impulses in a single presynaptic fibre show statistical fluctuations in amplitude. The fluctuations occur in quantal steps in a manner described by binomial or Poisson statistics. Only in a few cases the deviation from stochastic distribution can be attributed to variable invasion of nerve impulses into the terminal region. 6. The amplitude of the unit e.p.s.p. varies between 33 and 104 μV, mean 66·4 ± 4·3 μV ( n = 19). The mean quantum content ( m ) varies from less than 1 to more than 10. The m is reversibly reduced by Ca 2+ lack and by Mn 2+ until the chemically mediated response fluctuates according to a Poisson distribution with the unit e.p.s.p. equivalent to the single quantum of transmitter. 7. When paired or repetitive stimuli are applied to the presynaptic fibre the facilitation of the chemically mediated unitary e.p.s.p. can be observed as well as the post‐tetanic potentiation. The amplitude of the electrical e.p.s.p. remains unchanged during the period of post‐tetanic potentiation, suggesting that the latter is attributable to some change restricted to the specific presynaptic mechanism responsible for the transmitter release but not to changes in presynaptic spike height.