A, during voltage clamp, a brief (1.5 s) period of excitatory, synaptic input was delivered to the cell via activation of muscle spindle Ia afferents. When the cell was held hyperpolarized, this produced an effective synaptic current with a sharp onset and offset matching input duration. At a depolarized holding potential at approximately spike voltage threshold in unclamped conditions, a repetition of the very same input had a much different result. Current during the input was much larger (amplification) and a prolonged tail current was generated, both due to activation of the dendritic (PIC). As this input does not act via either NMDA receptors or metabotropic glutamate receptors, the amplification can be attributed to the PIC (Lee & Heckman, 2000). Inward (depolarizing) currents are downward, the usual voltage clamp convention. B, the interaction between amplification of effective synaptic current and level of descending neuromodulatory input (labelled as weak, medium and strong). In the weak state, there is no significant PIC and synaptic current steadily decreases as holding potential is slowly depolarized. At medium and high levels, activation of the PIC causes a large increase in current, at a voltage level where spikes would be generated in unclamped conditions. As the cell is depolarized further, current sharply declines. This saturation is likely to be due both to loss of driving force in depolarized dendritic regions and to activation of dendritic voltage-sensitive outward currents. Inward (depolarizing) currents are downward. Based on data from Lee & Heckman (2000).

The PIC is manifest as a strong upward deflection in current during voltage clamp of a cat medial gastrocnemius (MG) motoneurone (in normal voltage clamp converntion, inward current is negative, but the sign of the current here has been reversed to match Figs 2 and 4). For both blue traces, the ankle joint angle was such that the antagonists to MG were short; for both the red traces, the antagonist was stretched inducing reciprocal Ia inhibition and greatly reducing the amplitude of the PIC. The thick traces indicate the first pair of trials, the thin traces a repeat to show that the PIC could be turned on and off consistently. Data from Hyngstrom et al. (2007).

The transformation is from average synaptic current to the whole motor pool (x-axis) to total force generated by the entire muscle (y-axis). The simulations are based on the extensive data for the cat medial gastrocnemius motor pool and muscle. The right vertical dashed line indicates the input level produced by activation of many sensory and descending systems. The associated horizontal dashed line indicates that this ‘maximal’ activation generates less than 40% of maximum force. The lower horizontal dashed line and its associated vertical line indicate the maximum input output levels for postural activity in this motor pool and muscle. Simulations based on studies by Heckman & Binder (1991).

The left panel shows the effective synaptic current generated in a cat ankle extensor motoneurone (medial gastrocnemius) during voltage clamp by a slow stretch of the Achilles tendon. The middle panel shows the firing response of the same cell to the same stretch during unclamped conditions. The right panel shows the firing pattern of a human motor unit during a linearly rising volitional command that produced a linearly rising force (biceps muscle; data courtesy of Carol Mottram, at the Rehabilitation Institute of Chicago). Note in all three examples, there is an initial steep increase (surge) followed by a saturation that, during firing, produced rate limitation.