Ultrastructure of interneurons within motor nuclei of the thoracic region of the spinal cord of the adult cat

Alpha (> 40 μm) and gamma (< 30 μm) motoneurons in inspiratory motor nuclei of the thoracic spinal cord of the adult cat were labelled retrogradely by the intramuscular injection of HRP. Small (< 30 μm) unlabelled neurons within 200-300 μm of labelled motoneurons were analysed qualitatively and quantitatively with both the light and electron microscope. Most of these small unlabelled neurons had inconspicuous nucleoli, wrinkled nuclear membranes, low numbers of nuclear pores, and Nissl bodies which were either small or had the form of an amorphous perinuclear band. Such Nissl bodies were composed primarily of aggregates of polyribosomes within which short fragments of granular endoplasmic reticulum were distributed. Alpha and gamma motoneurons in contrast had prominent nucleoli, smooth-contoured nuclei, more nuclear pores and large, discrete Nissl bodies. Such Nissl bodies were composed primarily of several lamellae of granular endoplasmic reticulum with linear arrays of polyribosomes arranged between individual cisternae. Alpha motoneurons had most synaptic terminals on their cell bodies, gamma motoneurons had least and small unlabelled neurons had intermediate values. Synaptic terminals of the S-, F-T- and C-type were observed on alpha motoneurons, whereas only S- and F-types were observed on gamma motoneurons and small unlabelled neurons. Since they were unlabelled and differed morphologically from both alpha and gamma motoneurons, but were similar to small interneurons described elsewhere in the spinal cord and brain, it is suggested that the small unlabelled neurons located in the external intercostal and levator costae motor pools are interneurons. The functional significance of some of the morphological features which distinguish interneurons from motoneurons is discussed.