RED NUCLEUS ("THE RUBER")
The red nucleus is a prominent structure within the rostral midbrain and lies just dorsal to the substantia nigra. It appears to have a high iron content and is more vascular than the surrounding tissue, and in some brains is pinkish. Very little is known about the function(s) of the red nucleus in humans. Inputs to the ruber arise from motor areas of the brain and in particular the deep cerebellar nuclei (via superior cerebellar peduncle; crossed projection) and the motor cortex (corticorubral; ipsilateral projection)
The most important efferent projection of the red nucleus is to the contralateral spinal cord i.e., the RUBROSPINAL projection. Thus red nucleus neurons possess axons that cross just ventral to the nucleus and descend in the midbrain, pons and medulla (we cannot identify this pathway in our brain stem series of cross sections) to reach the spinal cord. In the spinal cord the rubrospinal tract courses within the LATERAL FUNICULUS JUST VENTRAL TO THE LATERAL CORTICOSPINAL TRACT.
The rubrospinal tract is thought to be involved in the control of both the flexor and extensor muscles, but even this is debated. This tract courses adjacent to the lateral corticospinal tract and terminates in roughly the same region (laminae) of the spinal cord gray. These two pathways are therefore thought to act somewhat in concert. This close association (rubrospinal/corticospinal) is further exemplified by the fact that the motor cortex also projects to the red nucleus. This means that the corticospinal tract is paralleled by an indirect corticospinal tract with a relay in the red nucleus, i.e., the corticorubrospinal tract. The rubrospinal projection is also, of course, influenced by the motor information coming out of the cerebellum, as well as from the motor cortex.
As far as I know there are no clinical case studies involving a lesion limited to the red nucleus. You already know that many of the fibers of the brachium conjunctivum run through and around the ruber on their way to VA and VL of the thalamus (have they crossed yet?). Therefore a lesion in the ruber will not only destroy rubrospinal neurons but also cerebellothalamic axons destined for VA and VL. Since the fibers in the brachium conjunctivum have already crossed, their interruption will result in a contralateral motor deficit (remember VA, VL to ipsi motor cortex and then the crossed corticospinal system). A contralateral motor deficit would also result from a lesion of the rubrospinal neurons, whose axons innervate the contralateral spinal cord gray. Thus it is difficult to know just what particular motor deficit(s) are associated with damage to the ruber versus damage to the cerebellothalamic fibers. It has been reported that large lesions of the midbrain tegmentum involving the red nucleus (called the tegmental syndrome) result in hemichorea, which is a tremor or involuntary movement of the contralateral limbs. Since we dont know that this problem is due to involvement of only the rubrospinal system, I feel it is best to consider a lesion of the ruber to result in MOTOR PROBLEMS OF THE CONTRALATERAL LIMBS for our problem solving exercises.