A comparison of homonymous and heteronymous connectivity in the spinal monosynaptic reflex arc of the cat

Multi-unit spike triggered averaging was used to determine functional connectivity between spindle afferent fibers from the medial gastrocnemius muscle and the motoneurons innervating the medial (homonymous connections) and the lateral gastrocnemius-soleus muscle (heteronymous connections). As many as 288 possible connections between 24 motoneurons and 12 afferent fibers were studied in single, acute experiments. The influences of morphological and topographical factors, as well as of motoneuron species on functional connectivity were analysed. The probability that a motoneuron would receive functional connections from a given population of afferent fibers was related to its size and its proximity to the spinal entry level of the afferent fibers. The faster the axonal conduction velocity of the motoneuron (i.e. the larger the motoneuron) and the closer its location to the entry zone of the afferent fibers, the higher was its probability of receiving functional connections. The greater the conduction velocity (i.e. diameter) of a stretch receptor afferent fiber, the higher was its probability of making functional connections with motoneurons. These relationships were qualitatively similar for homonymous and heteronymous connections. 58% (233/399) of the Ia and group II afferents (combined) had functional connections with homonymous motoneurons, 32% (75/234) with heteronymous motoneurons. However, homonymous and heteronymous motoneurons of similar sizes were equally likely to receive functional connections when located at the same craniocaudal level. Differences in the locations and mean sizes of homonymous and heteronymous motoneurons however, cannot account completely for the observed overall differences in homonymous and heteronymous connectivity.