Pinna stimulation has long been known to evoke reflex changes of discharge in γ efferents to the hind limb and hence changes in muscle spindle discharges. The present experiments were made in the rat to determine the involvement of the static and dynamic fusimotor systems in the pinna reflex.
We first observed fusimotor activity indirectly, by recording spindle responses to various length changes with and without concurrent pinna stimulation. Afferent responses were clearly influenced by static fusimotion during the reflex; evidence for dynamic fusimotion was sought but not found. Ipsilateral and contralateral stimuli appeared equally effective in evoking this static fusimotion.
The magnitudes of afferent responses differed markedly between muscles. Observations were made simultaneously on activities recorded directly from γ efferents to peroneal and soleus muscles. Several γ efferents were spontaneously active in each nerve; pinna stimulation usually enhanced their activities and aroused also several others, previously quiescent. The frequencies of discharge in γ efferents to peroneus were usually higher than in those to soleus. This must in part be responsible for differences in afferent responses. The spontaneously active and activated fibres together formed about one quarter to one third of the total γ motoneurone pool and had conduction velocities restricted to the lower end of the conduction velocity spectrum for γ fibres.
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Andrew, B.L., Leslie, G.C. & Part, N.J. Some observations on responses evoked by pinna stimulation in efferent and afferent fibres innervating hind limb muscle spindles of the rat. Exp Brain Res 34, 263–272 (1979). https://doi.org/10.1007/BF00235672
- Fusimotor efferents
- Pinna stimulation