Subthreshold activation of spinal motoneurones in the stretch reflex: experimental data and modeling

Physiological consequences of doublet discharges on motoneuronal firing and motor unit force

The double discharges are observed at the onset of contractions of mammalian motor units (MUs), especially during their recruitment to strong or fast movements. Doublets lead to MU force increase and improve ability of muscles to maintain high force during prolonged contractions. In this review we discuss an ability to produce doublets by fast and slow motoneurons (MNs), their influence on the course of action potential afterhyperpolarization (AHP) as well as its role in modulation of the initial stage of the firing pattern of MNs. In conclusion, a generation of doublets is an important strategy of motor control, responsible for fitting the motoneuronal firing rate to the optimal for MUs at the start of their contraction, necessary for increment of muscle force.

A diverse pattern of the spike threshold changes in feline gastrocnemius-soleus motoneurons during stretch reflex activation

The aim of the study was to define the spike threshold changes in motoneurons during naturally evoked activation. Spike activity was evoked in the gastrocnemius-soleus motoneurons of decerebrate cats by controlled stretches of the homonymous muscles. The spike thresholds were defined by using the first derivative of the membrane potential and statistical boundaries of its change. The evoked firing was analyzed in terms of the spike thresholds and the membrane potential trajectories between spikes, which were dependent on the intensity of the firing. The main inference of the study is the absence of a single-valued dependence between the spike rates and thresholds; the last ones can both increase and decrease with a rise in the firing rate; moreover, opposite directions in the threshold changes are often observed at various phases of the stretch-evoked firing. The dependency of the spike thresholds on firing rates was checked in 42 of the total set of 57 motoneurons. In a group of cells (n = 27) showing statistically significant correlation (P < 0.05), this parameter was negative in 18 and positive in 9 motoneurons. The obtained results indicate that the threshold-crossing models with a single-valued dependency of the spike thresholds on the firing rates are not completely suitable to analyze the spike generation processes in the motor unit records.