Electrical properties of motoneurons in the spinal cord of rat embryos.

Electrical properties of immature motoneurons were studied in vitro using isolated segments of spinal cords of rat embryos aged 14-21 days of gestation. Stable resting potentials and evoked synaptic potentials were recorded for more than 9 hr, indicating that motoneurons remain viable for many hours. Motoneurons are electrically excitable at 14 days of gestation and from the onset of excitability the action potentials are Na+-dependent but slow rising long-duration Ca2+-dependent action potentials can be evoked if K+ conductance is reduced. Thus, during embryonic development the regenerative potential inward current is Na+-and Ca2+-dependent. During motoneurons' differentiation there are some changes in their electrical properties: resting membrane potential increases, input resistance decreases, input capacitance increases, threshold for action potential decreases, and maximum rate of rise of action potential increases. Afferent motoneuron contacts are formed at 16-18 days of gestation when excitatory synaptic potentials can first be evoked in response to dorsal root stimulation. The changes in input capacitance and threshold for action potential occur at the onset of functional afferent motoneuron contacts, but it is not known whether these changes are autonomous or are influenced by the newly formed sensory inputs.