Abstract
1. Relationships of propriospinal (PS) and spino-bulbo-spinal (SBS) reflexes to peripheral input and output systems were studied in cats. The significance of cutaneous and muscular afferents as well as efferent connections with intrafusal and extrafusal motor units and the involvement of flexor and extensor muscles were investigated.
2. A comparison of afferent stimulation of the two reflex systems shows that SBS reflexes respond at a lower threshold of cutaneous nerve stimulation than PS reflexes. Conversely, PS reflexes are readily elicited by muscular afferents whereas SBS reflexes have a higher threshold and are inconsistent. Conduction velocities of cutaneous afferents responsible for SBS reflexes were found to be as high as50-80m/sec. It was possible to elicit SBS reflexes by mechanical stimulation of the skin, ipsilaterally and contralaterally.
3. A comparison of the muscular responses of these two reflex systems shows that the propriospinal reflexes involve contraction of both flexor and extensor muscles of trunk and extremities bilaterally, whereas spino-bulbo-spinal reflexes involve flexor muscles if excited by cutaneous afferents, and extensor muscles if excited by extensor muscle afferents. Both reflex systems are served exclusively by alpha motoneurons and identical units may respond to the two reflex events in rapid succession.
4. The excitability of PS and SBS reflexes may be increased or decreased by neck proprioceptors as well as by stretch receptors of limb muscles. While both reflex mechanisms are similarly modified by proprioceptive influences, they are differentially affected by Nembutal anesthesia and asphyxia, i. e., SBS reflexes are more vulnerable as compared to PS reflexes.
