Two types of cat static fusimotor neurones under separate central control?

The effect on the fusimotor system of an injection of the gamma-aminobutyric acid (GABA) antagonist picrotoxin into the reticular part of the substantia nigra (SNR) was studied in muscle spindle primary and secondary endings of pretibial flexors in cats anaesthetized with ketamine. Changes in fusimotor action after drug administration were deduced from changes in bias and sensitivity to sinusoidal stretching of the receptor-bearing muscles when compared to the deefferented preparation. Intranigral injection of picrotoxin (2 micrograms) removed a tonic static fusimotor action from flexor muscle spindle primary endings without affecting the secondary endings in the same muscles. It is concluded that static gamma motoneurones of cat flexors are of more than one functional type, and that the central nervous control of the static fusimotor input to cat flexor muscle spindles has selective access to static bag2 fibres and nuclear chain fibres.