Anatomical basis of specific connections between sensory axons and motor neurons in the brachial spinal cord of the bullfrog.
The anatomical basis for the specificity of the monosynaptic stretch reflex has been studied in the brachial spinal cord of bullfrogs. Sensory axons from the triceps brachii muscle innervate the corresponding triceps motoneurons but do not innervate two types of unrelated motoneurons (subscapularis and pectoralis) (Lichtman, J.W., and E. Frank (1984) J. Neurosci. 4: 1745-1753). Retrograde labeling of these three types of motoneurons with horseradish peroxidase (HRP) demonstrated that their cell bodies had overlapping distributions in the lateral motor column, and their dendrites all occupied the same region of the dorsal horn. In addition, triceps sensory axons aborized extensively in the dorsal horn throughout the brachial spinal cord, with no obvious predilection for the region of the triceps motoneurons. Thus, the physiological specificity of these sensory-motor connections was not apparent from the anatomical location of the sensory or motor neurons. However, by injecting single pairs of related or unrelated sensory and motor cells with HRP, we found that related pairs formed anatomical contacts with each other more frequently than unrelated sensory-motor pairs did. These observations suggest that the specificity of these connections is most likely the result of local interactions between sensory and motor processes.
D000818:Animals; D001369:Axons / Q000502:physiology / Q000648:ultrastructure; D001917:Brachial Plexus / Q000502:physiology / Q000648:ultrastructure; D008564:Membrane Potentials; D009046:Motor Neurons / Q000502:physiology / Q000648:ultrastructure*; D009132:Muscles / Q000294:innervation; D009469:Neuromuscular Junction / Q000502:physiology / Q000648:ultrastructure; D009475:Neurons, Afferent / Q000502:physiology / Q000648:ultrastructure*; D011892:Rana catesbeiana; D013116:Spinal Cord / Q000502:physiology / Q000648:ultrastructure*; D013569:Synapses / Q000502:physiology / Q000648:ultrastructure*