Cutaneous Reflex Control of Fusimotor Neurones

Abstract

The fusimotor (γ) system receives a wide variety of inputs from peripheral, spinal and supraspinal structures (for review, see Hulliger, 1984). Although many previous investigations, in animal preparations, have shown that cutaneous afferents produce potent reflex effects on hindlimb γ-motoneurones, no consistent pattern of response has emerged and a functional interpretation of such sensorimotor control has been difficult to achieve (Hunt, 1951; Eldred & Hagbarth, 1954; Hunt & Paintal, 1958; Voorhoeve & van Kanten, 1962; Grillner, Hongo & Lund, 1969; Catley & Pascoe, 1978; Bessou, Joffroy & Pagés, 1981; Johansson & Sojka, 1985; Davey & Ellaway, 1989; Murphy & Hammond, 1991, 1992). The reported responses have ranged from a “simple” flexor reflex pattern (i.e. flexor excitation, extensor inhibition; Hunt, 1951) to those that appeared individualised for a given γ-motoneurone (Johansson & Sojka, 1985). Various factors may have contributed to the diversity of effects in these studies including differences in preparation, afferent input and γ sampling. In addition, the presence of two functionally distinct types of fusimotor neurone, static and dynamic (Matthews, 1962), is likely to have contributed since direct studies of the reflex behaviour of the γ system have largely involved recordings from unclassified units. In the present article we review results from a decerebrate cat preparation in which cutaneous reflex responses to electrical stimulation have been investigated in the resting state and during locomotion.