The role of an inter neuron is to connect a sensory neuron to a motor neuron.

Inter neurons are one of the most common neurons in the body. In fact over 97% of human neurons are inter neurons.

Observe the image to the right. This is a typical inter neuron, however you will notice that it is un-myelinated. Some inter neurons with small axons are left un-myelinated.

However most inter neurons are myelinated.

Interneuron

When an interneuron is added to a pathway, the opportunity for co-ordination and integration increase.

Observe the animation to the left. The signal is sent from one sensory neuron too an interneuron. However this particular interneuron is capable of split the signal too two different neurons. These neurons are still considered to have one axon.  

Interneurons are extremely useful for co-coordinating two or more different muscles. If you try to pick up a weight, your biceps will contract while your triceps stretches. This is the work of interneurons. When a muscle is stimulated to contract, such as in a flex response, inter-neurons in the spinal cord carry messages, inhibiting the contraction of the opposing muscle.

This is shown in the animation to the right. As one muscle is signalled to relax, the other is signalled to contract.

Imagine you are walking and you step on a pin, if you simply lift your foot you will loose your balance and fall. So when you step on a pin your back foot will brace, preparing to take your full weight. Something as simple as this can use multiple neurons and muscles, all working together.