Flexor muscles of the hamstrings relax to prevent antagonizing the quadriceps muscles A tap on the patellar ligament excites nerve endings of muscle spindles in the quadriceps femoris Stretch signals travel to the spinal cord via a primary afferent neuron. Efferent signals in the motor nerve fiber stimulate the quadriceps femoris to contract. 4 The alpha motor neuron to the hamstrings muscles is inhibited. A primary afferent nerve fiber stimulates an alpha motor neuron in the spinal cord. A branch of the afferent nerve fiber stimulates an inhibitory motor neuron in the spinal cord.