Spatial characterization of the motor neuron columns supplying the rat forelimb

Rats can generate a rich array of forepaw and forelimb movements that are similar, although not as complex, to those produced by human and non-human primates. When reaching for food for instance, rats display skilled movements of the forelimb and the paw, therefore, making them attractive models to validate strategies aimed at the recovery of fine motor control. Surprisingly however, few anatomical studies have been performed on the central control of forelimb movements in the rat. The current series of experiments examined the details of the segmental arrangement of motor neurons that supply the rat forelimb. The distribution of motor end plates across the rat forelimb was first visualized by means of acetylcholinesterase histochemistry, and this information was used to create a motor end plate map of the forelimb muscles. This map was subsequently used as a guide for multiple injections of retrograde tracers along the motor end plate regions of 11 forelimb muscles. The entire cervical region of the spinal cord was subsequently analyzed under epifluorescence. This tract-tracing analysis confirmed that motor neurons innervating the rat forelimb are arranged in columns within the cervical segments of the spinal cord. This anatomical investigation also supports the previous observation that, although discrete, some of the motor neuron columns lying in the cervical aspect of the rat spinal cord are inter-mingled. The length of these columns, and hence the overlap between them, appears to be greater than previously reported, particularly within the uppermost segments of the brachial plexus.