Quantitative analysis of cervical musculature in rats: histochemical composition and motor pool organization. II. Deep dorsal muscles.

In this paper we characterize the architecture, histochemistry, spindle composition, and motor pool organization of rat dorsal neck muscles: splenius (SP), biventer cervicis (BC), and complexus (COM). We also consider whether individual rat neck muscles are innervated by more than one population of motor neurons as they are in turtles and cats and whether in these muscles motor neuron size scales with muscle fiber type. Dorsal neck muscles are divided by tendinous inscriptions into quasiserially arranged compartments. Each compartment is supplied by a separate peripheral nerve branch from C1-C5 dorsal rami; thus each dorsal neck muscle is innervated from multiple segments. Our histochemical data indicate that slow-twitch oxidative (SO) fibers are always the least numerous group, and fast fibers are split almost equally between fast oxidative glycolytic (FOG) and fast glycolytic (FG) types. There are significant cross-muscle differences in the relative frequency of each fiber type. Topographic analyses reveal a mosaic pattern of fiber types with no clear compartmentalization, and muscle spindle distribution is essentially homogeneous. Motor neurons supplying rat dorsal neck muscles are restricted to the ventral third of the ipsilateral ventral horn from lower medulla to C4. The SP motor pool is located dorsally in the ventral horn, and BC and COM motor pools form overlapping columns in the tip. There is little evidence that these muscles are supplied by more than one population of motor neurons, as reported in turtles and cats. To help assess these differing results, in two cats we infiltrated the C2 nerve to BC and COM; our results were similar to those previously reported in cats including the presence of contralateral motor neurons. Thus disparate results between rat and cat experiments may represent species differences in neck muscle innervation. In five experiments we made direct comparisons between the sizes of motor neurons that supply muscles with significantly different histochemical compositions. In each case there was a difference in mean soma size in the direction predicted by the muscles' histochemical profiles, thus suggesting that in these muscles motor neuron size may scale with muscle fiber type.