Figure 2.

Imagine an alpha-motoneuron receiving excitation from two sources, central and reflex (the middle insert). A: If its membrane is depolarized above the threshold, it would generate action potentials (AP) at the highest possible frequency; B and C: If the membrane is depolarized below the threshold, it would generate potentials at a frequency defined by the reflex input (length-sensitive), lower in B compared to C. D: The dependence of active muscle force (F) on muscle length (L). The muscle starts generating force at a length corresponding to the threshold of the tonic stretch reflex (λ). It reaches an equilibrium point (EP) when its force is equal in magnitude to the external load.