The selectivity of fusimotor innervation in muscle spindles of the rat studied by light microscopy.

Six muscle spindles and three muscle spindle poles from four rat soleus muscles have been sectioned serially at 1 micron intervals to trace the motor innervation by light microscopy. Forty myelinated axons had 92 endings on the intrafusal muscle fibres. 67.5% of these axons supplied a single type of muscle fibre only, 22.5% to dynamic bag1 (Db1) fibres, 15% to static bag2 (Sb2) fibres and 30% to chain fibres. The rest supplied more than one fibre type, 5% supplying the Db1 and one chain fibre, 20% supplying the Sb2 and chain fibres, but 7.5% (three axons) supplied all the fibre types together. Apart from these three axons all the fusimotor axons would be expected to have a clear dynamic or static action on the Group Ia discharge. Whilst for the cat entirely non-specific distribution does not exist, or at least is very rare, since only a small proportion of the rat fusimotor axons were in this category we conclude that fusimotor distribution in rats and cats is essentially similar.