Effects of chronic partial deafferentiation on the electrical properties of lumbar alpha-motoneurones in the cat.

Membrane properties of cat spinal alpha-motoneurones were compared in cats following acute and chronic low (L5) spinal section and dorsal rhizotomy (L7--S1) and in cats with intact spinal cord in order to investigate effects of chronic partial deafferentation. The most significant change observed was a decrease in electrotonic length of the chronically deafferented neurones. Calculations showed that this decrease was related to a 15--20% reduction in length of an equivalent cylinder used to represent a motoneurone. Using a compartmental model, calculations showed that the peak voltage produced by a given synaptic input would be increased by 6--36% by chronic section. Such an increase is not sufficient to explain reported increases in EPSP peak amplitudes. Neither peak amplitude (and underlying conductance change) nor duration of the afterhyperpolarizaton were affected by the acute or chronic sections. No obvious changes in the delayed depolarization were observed. The properties of the repetitive discharge induced by intracellular current injection was not altered by chronic section.