Sensory Ia fibre from the spindleα motor neuron to the quadricepsGrey matter (cord ~6× life size)Spindle, hammer; scope tracesThe knee jerk (ruler: 1 tick = 5 cm on the leg; the shank is drawn shortened, and the L3–L4 cord slice ~6× life size). Not to scale: each cable stands for ~0.8 m of nerve. Tapping the patellar tendon stretches the quadriceps and its muscle spindles; their Ia fibres fire and the volley runs at ~70 m/s into the dorsal root, reaching the cord in ~11 ms. There it makes one synapse (0.7 ms) directly onto α motor neurons in the ventral horn, and, through an inhibitory interneuron, relaxes the hamstrings. Motor axons conduct at ~60 m/s back to the muscle, so the EMG starts ~25 ms after the tap (lower trace) and the kick follows. A tap below threshold gives a volley but no kick. Everything is shown 25× slower.
1膝跳反射的突触数（单突触）synapse in the knee jerk (monosynaptic)~70 m/sIa 感觉纤维传导速度Ia sensory fibre conduction~60 m/sα 运动纤维传导速度α motor fibre conduction~0.7 ms一个突触的延搁delay at one synapse~20–25 ms叩击到股四头肌肌电出现tap to quadriceps EMGL2–L4参与的脊髓节段spinal segments involved