In the early 1950s, Eccles and his colleagues performed the research that would lead to his receiving the Nobel Prize. To study synapses in the peripheral nervous system, Eccles and colleagues used the stretch reflex as a model, which is easily studied because it consists of only two neurons: a sensory neuron (the muscle spindle fibre) and the motor neuron. The sensory neuron synapses onto the motor neuron in the spinal cord. When a current is passed into the sensory neuron in the quadriceps, the motor neuron innervating the quadriceps produced a small excitatory postsynaptic potential (EPSP). 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.