Physiological adaptations in the rhythmic firing properties of alpha-motoneurones to changes in physical activity and aging
Our previous research has revealed that hindlimb α-motoneurones respond to increases and decreases in the activity of the organism, as well as aging, by alterations in biophysical properties measured in anesthetized rats using intracellular recording techniques. Properties that change include resting membrane potential (RMP), voltage threshold (Vth), afterhyperpolarization (AHP) amplitude, rates of antidromic spike development, and rheobase. Rhythmic fi ring properties that also show changes include minimum and maximum fi ring frequencies, frequency/current slopes, and the degree of late adaptation. Thus, generally speaking, “excitability” of motoneurones increases and decreases in various models of increased and decreased chronic activity, respectively, and decreases during aging, with implications for motor unit recruitment during voluntary movement. Single-cell modelling of ionic currents has suggested that these chronic adaptations most likely involve changes in the transient sodium and DR potassium ion channels, and perhaps channels associated with the AHP as well as leak currents. Results from this research program have implications for treating the neuromuscular system compromised by disease, trauma, age, and even “chronic sedentarism”.