 When a similar current is passed through the hamstring, the opposing muscle to the quadriceps, an inhibitory postsynaptic potential (IPSP) is produced in the quadriceps motor neuron.Although a single EPSP was not enough to fire an action potential in the motor neuron, the sum of several EPSPs from multiple sensory neurons synapsing onto the motor neuron can cause the motor neuron to fire, thus contracting the quadriceps.On the other hand, IPSPs could subtract from this sum of EPSPs, preventing the motor neuron from firing.