A curious form of coupling was noted between the group Ia and II discharges recorded from single spindles in decerebrated cats with intact ventral roots. (Anaesthesia before decerebration was with pentobarbitone sodium 45 mg kg^-1 intraperitoneally, maintained I.V.; all animals were killed with an overdose of anaesthetic.) In cross-correlograms during fusimotor activity Ia spikes were distributed symmetrically with respect to group II interspike intervals. No known mechanism could explain this – stimulation of single γ-axons entrains Ia but not group II afferents in cats.

The mechanism for coupling was investigated by correlating records from the nerve supplying the last spindle in tenuissimus muscles with intrafusal contractions seen in the dissected spindle in cats anaesthetised with barbiturates as above. γ-Motoneurones were activated through group II spinal pathways by stretching other hindlimb muscles exposed by the dissection (see Gladden et al. 1998). Coupling depends on the action of a γs-motoneurone (γsT), which tenses the secondary ending. The ending can then be entrained by a second γs-motoneurone (γsC) tonically active at frequencies even as low as 10 Hz. In the example illustrated, γsC innervated the static bag2 fibre in the opposite pole to the secondary ending. γsC spikes were followed by II and Ia spikes at mean latencies of 17.5 and 21.5 ms, respectively. If a coupling γs-motoneurone innervates chain fibres as well as a static bag2 fibre, group Ia spikes precede group II spikes because of the well-known Ia driving effect of chain contractions. Appropriately for an enabling role, γsT maintained a steady mean frequency of 11 Hz, increasing to 12 Hz only during intense reflex activation, unlike γsC and other γ-motoneurones that were driven to much higher frequencies. The conduction velocities of γsT and γsC, estimated by spike triggered averaging, were 32 and 16 m s^-1.Preliminary indications are that coupling is part of an orderly recruitment sequence. Tonic activity of type-γsT motoneurones first adjusts mechanical conditions, then type-γsC motoneurones couple the afferents. Coupling may continue for many minutes but can be masked by tonic activity of additional γ-motoneurones. Thus the rationale for siting two endings in the one receptor is not simply structural economy or developmental accident, but allows γ-motoneurones to play off group Ia and II afferents against each other.

Figure 1. A, neurogram when group Ia and II afferents were coupled with identity of spikes indicated. B, cross-correlograms normalised for spike rate show that the afferents were coupled to γsC spikes, but not to γsT spikes. C, the afferents were thus coupled. Bin size: 0.5 ms.

Gladden, M.H., Jankowska, E. & Czarkowska-Bauch, J. (1998). J. Physiol. 512, 507-520. abstract