1. A method of isolation of individual afferent fibers from muscle has yielded a representative sample of the fibers which comprise groups 1 (12 to 20 µ) and II (4 to 12 µ) of the afferent fiber diameter distribution of muscle nerves in cat.
2. Afferent fibers from muscle stretch receptors account for groups I and II of the afferent diameter spectrum of muscle nerves to soleus and medial gastrocnemius. Fibers from tendon organs are largely confined to the diameter range above 12 µ. This fiber group, which has a simple one-peak diameter distribution, is termed group IB. Fibers from muscle spindles show a bimodal diameter distribution and account for the remainder of fibers in the 12 to 20 µ group (termed IA) and substantially all of group II (4 to 12 µ).
3. No significant difference has been found in the receptor characteristics of the large (group IA) and intermediate sized (group II) spindle afferent fibers other than a slightly higher threshold of the latter to steady external stretch.