The Spinal interneuron is a neuron found in the spinal cord of vertebrates.
Spinal interneurons are found in the spinal cord, and like all interneurons, trasmit a signals between afferent and efferent neurons. These interneurons are vital to sensorimotor integration and motor coordination. Consequently this network of interneurons has been implicated in regaining balance and associated movements of limbs. In order to conserve balance throughout a wide range of concerted movements, these neurons must be continuosly reconfigured. This stems from the need to use the same muscle tissue to achieve several different funcitons. The network of these cells has been shown to grow in complexity with overall complexity of the vertebrate in question. However it is also likely that systems oberved in less complex species are also present in more complex species, such as humans. This feature of the spinal internueronal network allows for research relevant to the human CNS to be conducted on animals.
Neuronal Type: Local interneuron
The cell body of the spinal interneurons is located, as the name implies, in the CNS or more specifically the spine. The axons on spinal interneurons project in ascending and/or descending directions, indicating that information must travel in both directions along these nerves. Certain projections are common among many species of vertebrates, such as Renshaw cells, but there is much variety between species in this regard as can be seen by the many studies on this topic. From studies done in the red-eared turtle  a rough estimation of 10^4-10^6 of these neurons are present in humans.
Neurotransmitter: GABA or glycine  Unique molecular markers: combinations of HD factors 
These neurons get input from supraspinal neurons as well as motor neurons
Similar to inputs. Imprecise mapping of inputs and outputs, perhaps due to dynamic reconfiguration.
Tonically at rest, no unique patterns for specific movements.
This neuron is likely confined to control of locomotion.
1 Nissen UV, et al. (2008) Organization of Projection-Specific Interneurons in the Spinal Cord of the Red-Eared Turtle. Brain Behav Evol, 72:179-191. 2 Misiaszek JE. (2006) Neural control of walking balance: if falling then react, else continue. Exerc Sport Sci Rev, 34(3):128-34. PMID: 16829740 3 Firgon A. (2009) Reconfiguration of the Spinal Interneuronal Network During Locomotion in Vertebrates. J Neurophysiol, 101:2201-3. 4 Sapir T, et al. (2004) Pax6 and Engrailed 1 Regulate Two Distinct Aspects of Renshaw Cell Development. J of Neuroscience, 24(5):1255-64. 5 http://en.wikipedia.org/wiki/Interneuron 6 Helms AW; Johnson JE. (2003) Specification of dorsal spinal cord interneurons. Curr Op in Neurobiol, 13:42-49. 7 Prut Y; Perlmutter SI. (2003) Firing Properties of Spinal Interneurons during Voluntary Movement. I. State-Dependent Regularity of Firing. J Neuroscience, 23(29):9600-9610.