Abstract
The spinal cord is engaged in all forms of motor performance but its functions are far from understood. Because network connectivity defines function, we explored the connectivity for muscular, tendon and tactile sensory inputs among a wide population of spinal interneurons in the lower cervical segments. Using low noise intracellular whole cell recordings in the decerebrated, nonanesthetized cat in vivo, we could define mono-, di-, trisynaptic inputs as well as the weights of each input. Whereas each neuron had a highly specific input, and each indirect input could moreover be explained by inputs in other recorded neurons, we unexpectedly also found the input connectivity of the spinal interneuron population to form a continuum. Our data hence contrasts with the currently widespread notion of distinct classes of interneurons. We argue that this suggested diversified physiological connectivity, which likely requires a major component of circuitry learning, implies a more flexible functionality.

Highlights
In vivo whole cell, intracellular recording of spinal interneurons.
Patterns of input from Ia, Ib and cutaneous afferents is highly diversified.
Learning appears to be a defining factor of spinal interneuron connectivity.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Email addresses: fredrik.bengtsson{at}med.lu.se (Fredrik Bengtsson), Philipp.Stratmann{at}dlr.de (Philipp Stratmann), florian.roehrbein{at}informatik.tu-chemnitz.de (Florian Röhrbein), knoll{at}in.tum.de (Alois Knoll), Alin.Albu-Schaeffer{at}dlr.de (Alin Albu-Schäffer), henrik.jorntell{at}med.lu.se (Henrik Jörntell)