Classification of longissimus lumborum muscle spindle afferents in the anaesthetized cat

Recordings have been made from 127 single muscle spindle afferents from the longissimus lumborum muscles of anaesthetized cats. They have been characterized by their responses to passive muscle stretch and the effects of succinylcholine (SCh) and by their sensitivity to vibration. The use of SCh permitted the assessment for each afferent of the influence of bag(1) (b(1)) and bag(2) (b(2)) intrafusal muscle fibres. From this, on the assumption that all afferents were affected by chain (c) fibres, they were classified in four groups: b(1)b(2)c (41.9%), b(2)c (51.4%), b(1)c (1.3%) and c (5.4%). All the afferents with b(1) influence were able to respond one to one to vibration at frequencies above 100 Hz and were considered to belong to primary endings. On the basis of the vibration test, 64% of the b(2)c type afferents appeared to be primaries and 36% secondaries. Of the units classified as primaries, 41% were designated as b(2)c and would not therefore be able to respond to dynamic fusimotor activity. The significance of this relatively high proportion of b(2)c-type spindle primary afferents is discussed in relation to the specialized postural function of the back muscles.