I generally see it written that there are three types of neurons, classified by projection: (1) sensory neurons, (2) interneurons, and (3) motor neurons. Now, in the CNS, I don’t think there would be sensory or motor neurons, seeing as they connect the PNS to the CNS. Does that mean, then, that all the neurons in the CNS are interneurons? I remember my biology teacher telling me not all neural pathways involve interneurons, so that suggests to me that the CNS consists of other neurons as well. What neurons would these be, if they are existent?
Sensory neurons are invariably considered to be located in the periphery. Interneurons and motor neurons are part of the CNS.
There are various classifications possible to categorize neurons.
- Sensory neurons (feed information to the central nervous system (CNS));
- Interneurons (are stimulated by sensory neurons and/or other interneurons);
- Motor neurons (feed information from the CNS to the periphery).
Sensory neurons are invariably considered to be part of the peripheral nervous system. Their cell bodies are located in the periphery. Their projections may feed directly into the brain. For example, olfactory sensory neurons directly project onto the olfactory bulb in the brain.
Motor neurons include the motor neurons controlling muscles. Their cell bodies are located in the spinal cord, which is the CNS, but not the brain. Here the terminology becomes confusing, as this functional classification uses overlapping terminology with the projectional classification. That's why I, personally, prefer to talk about efferent fibers (away from CNS) and afferent fibers (to the CNS). Especially because motor neuron implies something motoric. In case of neurons driving glandular secretion this is counter intuitive. Efferent and afferent fibers are essentially the same names for motor neurons and sensory neurons. One way or another, motor neurons controlling muscles have their cell bodies in the spinal cord and hence are part of the CNS.
Interneurons are invariably located only within the CNS. Interneuron is also a confusing term, because often they are implicitly considered to be inhibitory. Often they are, but they don't have to be. In the classification scheme discussed, most CNS neurons can indeed be classified as interneurons. For example in the neocortex, 70–80% of the neurons are excitatory pyramidal cells, while the remaining 20–30% are inhibitory interneurons (Markram, 2004). The spinal cord also contains interneurons.
In all, most interneurons are located in the CNS, but not all CNS neurons are interneurons, because motor neurons are part of the CNS too. I agree with 243's answer that the classification scheme is most appropriate for the relatively simple neuronal circuitry characteristic of the spinal cord.
For example, interneurons account for 20% of neurons in neocortex (1)(2). I think that the classification, mortar-, sensory- and inter-nerons, comes from the simplest neuron circuits like spinal reflex.
In human brain, their thoughts and experiences modify such simple neuron circuits, suggesting existence of other neurons affecting the simple circuits. It should be noted that many neurons in CNS spontaneously fire(3). If interneurons spontaineously fire, your leg spontaineously move unconsciously. Some other neurons secrete hormones in brain, called neurosecretory cells cells(4).