What is the difference between a 2-neuron reflex and a 3-neuron reflex?
A 2-neuron reflex requires a sensory and a motor neuron. The sensory neuron is stimulated with a stretch, for instance the tendon in the knee that provides the kicking reflex. Once this sensory neuron has been stimulated, a signal is sent from the tendon to the spinal cord connecting to a motor neuron for the same muscle. This motor neuron then causes the muscle to stretch, kicking our leg out.
Most sensory neurons do not go directly to the brain, but rather synapse in the spinal cord. This is what allows the reflex to occur so quickly. It would take more time and energy for each signal to be sent all the way to the brain. The spinal cord is a shortcut.
A 3-neuron reflex requires a sensory, motor and interneuron. The interneuron is involved with withdrawing, or stopping an action. For instance in class we described the funny image of a dog. If you scratch a dog on the back he may left up his leg to scratch that side. If you scratch a dog on the other side, he will repeat on that side. But- if you scratch a dog on each side at the same time he will unlikely fall over. This interneuron jumps in putting the action on hold. This same 3-neuron reflex is seen when we ourselves find danger, like touching a hot stove. You will feel the pain and then quickly withdraw as quickly as you feel the pain.
Two neuron: sensory, motor
Three neuron: sensory, motor, interneuron