Synaptic potential
When the sensory neuron fires an action potential, a voltage change is seen in the motor neuron. This is called the synaptic potential.
By itself, this single synaptic potential was not sufficient to generate an action potential in the motor neuron.
The motor neuron trace is a singular flat horizontal line with a low synaptic potential peak following the spike on top. The sensory neruon trace has a tall, sharp action potential spike which emerges from a group of smaller peaks on the bottom. One of the lines extends below the horizontal line.