Tullidora (Karwinskia humboldtiana) toxin mainly affects fast conducting axons

Cats were given a single oral dose of ether extracts from tullidora (Karwinskia humboldtiana) fruit which contains an identified neurotoxin. Acute experiments were performed 4-7 weeks after toxin administration when flaccid limb paralysis was evident. Normal cats were used as controls. The medial gastrocnemius, the soleus and the sural nerves were electrically stimulated and the unitary potentials evoked by the stimuli were extracellularly recorded from spinal root filaments to measure the conduction velocity of single fibres. In control cats, the average conduction velocity (CV) was greater in medial gastrocnemius motor fibres than in the afferent ones of the same nerve and the soleus motor axons, whereas in the sural nerve CV was less than in the aforementioned cases. The CV values and the proportion of fast conducting fibres (greater than 80 m/s) in each nerve were directly related (r = 0.99). In treated cats, CV diminished in all the nerves studied, but the conduction velocity was further reduced in the faster fibres. Consequently, the motor division of the medial gastrocnemius nerve, normally composed of a high proportion (57%) of fast fibres, was more affected by tullidora and the sural nerve, which has the lowest proportion (0.7%) of these type of fibres, was the less affected. Our findings suggest that the preferential involvement of motor nerves in the experimental tullidora (buckthorn) neuropathy, as well as the preservation of somatic sensation in quadriplegic children accidentally poisoned with tullidora, are related to the distribution of axonal diameters in peripheral nerves.