Abstract
Intracellular recording was made from the anterior digastric motoneurons of immature (6 days after birth) and mature Wistar albino rats, to study the mechanisms underlying the postnatal decrease in the nuclear delay of the anterior digastric motoneuron pool observed in the jaw-opening reflex. When the motoneurons were orthodromically activated by supramaximal electrical stimulation of the spinal trigeminal nucleus, they responded with monosynaptic EPSPs and spike potentials. No significant differences in the latency of the EPSP from the positive peak of the simultaneously recorded presynaptic terminal potentials, i. e., the synaptic delay, was found between the immature and mature motoneurons. Their values were about 0.4 ms on the average and did not exceed 0.5 ms. The latency of the spike potentials after EPSP onset, on the contrary, was significantly threefold longer in the immature motoneurons than in the mature ones: 5.92 ms and 2.13 ms on the average, respectively, in the immature and mature motoneurons. This resulted from the considerably slow rising phase of the immature EPSPs.
Passing constant current pulses into the soma through the intracellular electrodes revealed the passive electrical properties of the motoneuronal membrane, namely the input resistance and the membrane time constant. The mean values were 38.5 M. Ω and 1.92 ms in the immature motoneurons and 9.9 M. Ω and 1.03 ms in the mature ones. Both of them were significantly different between the immature and mature animals. Histological observations demonstrated that the size of the motoneurons was 16.3. μ18.1pm and 29.9. μ37.8pm on the average, respectively, in the immature and mature rats, showing an area of the soma surface in the mature motoneurons of about 4 times larger. The specific membrane resistance calculated from these values was not different between the immature and mature motoneurons. The specific membrane capacitance of the immature motoneurons, however, was nearly twice that of the mature ones.
It is concluded that the speed of information transmission in the anterior digastric motoneuron pool is increased by a steeper slope of EPSP rising phase in the more developed motoneurons. The decrease of the specific motoneuronal membrane capacitance is suggested to play a role in this mechanism.
