The Inter-neuron (also known as the local circuit neuron, relay neuron or the association neuron) is the neuron which connects the afferent and the efferent neurons in the neural pathways.

The cells that makes up the nervous system is the Inter-neuron (circuit neuron), sensory neuron and motor neuron.

A neuron is called a inter-neuron because that specific neuron takes impulse from one neuron to a next neuron. For example your sensory neuron sends a impulse that you had felt a hot object. It goes through the spine to a inter-neuron to a motor neuron (this processes is called a reflex). Then the motor neuron tells your muscles in your hand to move

a relay neuron is the neuron that picks up the message from the sensory neuron and delivers it to the motor neuron in the spinal cord or the brain

A relay neuron is the neuron that picks up the message from the sensory neuron and delivers it to the motor neuron in the spinal cord then to the brain.