Propriospinal fiber connections of the cervical motor nuclei in the cat: A light and electron microscope study

Propriospinal fiber connections of the cervical motor nuclei were studied in the cat with the Nauta and the Golgi methods. Synaptic boutons belonging to this fiber system were observed and identified under the electron microscope. Lamina IX of the cervical cord receives a large number of fibers from the short ascending and descending propriospinal tracts but only a very restricted number from the long ascending propriospinal tract. Observations with the Golgi method show that these fibers are collaterals derived from axons ascending or descending in the ventrolateral cord. They are of three types of terminal patterns: Fibers terminating in a confined area of lamina IX with repeated branchings, fibers terminating in a wide area of both laminae IX and VIII with short terminal fibers, and simple, unbranched fibers giving off short terminal fibers to cells of laminae IX and VII. In the cervical motor nuclei, it was found that synaptic boutons containing round vesicles usually form asymmetrical synapses with wide synaptic clefts while those containing flattened vesicles form symmetrical synapses with narrow clefts. There are some boutons which contain round synaptic vesicles and make symmetrical synapses. However, they are invariably associated with wide synaptic clefts. In lamina IX of the cervical cord many axodendritic and axosomatic boutons were found to be degenerate after section of the proprispinal fibers. They were filled with either round or flattened vesicles. A few large boutons, filled with round synaptic vesicles and associated with subsynaptic cisternae, were also seen to be degenerate.