This is the 2D companion to the 3D fiber-bundle gamma-loop model: the same fusimotor equations, but read out the way a neurophysiology rig actually presents them — as live signal traces and a state-space trajectory, not a rendered anatomical scene.
bag2/chain (static) stretch: S = (L − 1) + k_gs·γ_static
bag1 (dynamic) gain: G = 1 + k_gd·γ_dynamic
Ia rate ≈ base + a·max(0,S) + b·G·max(0, dL/dt)
II rate ≈ base + c·max(0,S) (static component only)
- Top strip-chart — six scrolling channels (muscle length, stretch velocity, γ-static, γ-dynamic, Ia rate, II rate), each auto-scaled to its own band, exactly like a multi-channel EMG/afferent recorder.
- Bottom phase portrait — plots the spindle's static stretch S (x-axis) against stretch velocity dL/dt (y-axis) as a fading trajectory. The dot's colour encodes instantaneous Ia rate (blue → yellow → red) and the ring around it pulses with II rate, so the whole dynamical state of the sensory ending is visible as one point moving through a 2D plane rather than a 3D fiber geometry.
- Because S and dL/dt are literally the two inputs to the Ia/II rate equations above, this phase plane is a mathematically exact, not merely illustrative, state-space view of the same fusimotor model.
Alpha–gamma coactivation: the CNS fires gamma motor neurons alongside alpha motor neurons during voluntary movement. Without it, a shortening muscle would slacken its own spindles and the Ia afferent would fall silent — watch the phase-plane trajectory collapse toward the S-axis origin during a contraction with coactivation off, versus staying displaced to the right with it on.