Experiments were performed on decerebrate cats and on intact chloralose anesthetized cats to analyze longitudinal sensorimotor coordination among spinocranial and craniospinal reflexes. Stimulation of cutaneous afferent nerves in all forelimbs and hind limbs elicited reflex responses from all cranial motor outflows. Impulses entered the spinal cord, partially crossed from one side of the cord to the other at the level of entry, ascended the cord bilaterally, but predominantly on the ipsilateral side, reached the bulbar level without any further intermediate crossing or recrossing of the spinal cord and crossed and recrossed at the bulbar level to provide bilaterally equivalent cranial motor effects. This is, therefore, a spinobulbocranial reflex system. The spinal ascending conduction is uniform, at about 60 m/sec, and in other ways is similar to the ascending limb of the spinobulbospinal reflexes. Upper cervical dorsal root stimulation yielded similar responses as well as an additional earlier reflex response. This relatively direct pathway bypasses the bulbar reticular formation. Afferent stimulation of the trigeminal nerve yielded bilateral responses from cranial and spinal motor outflows at all levels. Decussation was abundant within the bulbar reticular formation, but after entering the spinal cord the impulses did not cross again. This is, therefore, a craniobulbospinal reflex system. The final descending conduction is uniform at about 35 m/sec, and in other ways is similar to the descending limb of the spinobulbospinal reflex system. The craniobulbospinal reflexes are conveyed via fast-conducting (alpha) motor fibers to flexor muscles. Trigeminal stimulation yielded additional earlier reflex responses in cranial and upper cervical segments. This relatively direct pathway bypasses the bulbar reticular formation. Just as there are spinobulbospinal recurrent reflexes which go to the bulbar reticular fomation to be reflected back down upon spinal motor mechanisms, there are craniobulbocranial recurrent reflexes which descend to the bulbar reticular formation and re-ascend the brainstem to influence cranial motor outflow. There are thus four reflex systems engaged in relay through the medulla oblongata: spinobulbospinal, spinobulbocranial, craniobulbocranial and craniobulbospinal. Each of these four systems interacts within the medial bulbar reticular formation with each of the others. These systems may be instrumentally involved in the coordination of somatic and visceral and interrelated longitudinal reflexes, including those which extend their influence across the boundary between head and body.
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