Sensorimotor transformation in cat nociceptive withdrawal reflex system

Abstract

The withdrawal reflex system of higher vertebrates has been extensively used as a model for spinal sensorimotor integration, nociceptive processing and plasticity. In the rat, the nociceptive withdrawal reflex system appears to have a modular organization. Each reflex module controls a single muscle or a few synergistic muscles, and its cutaneous receptive field corresponds to the skin area withdrawn upon contraction of the effector muscle(s) when the limb is in the standing position. This organization principle is at odds with the 'flexion reflex' concept postulated from cat studies. To assess the generality of the modular organization principle we have therefore re-examined the cutaneous input to the withdrawal reflex system of the cat. The cutaneous receptive fields of hindlimb and forelimb muscles were mapped using calibrated noxious pinch stimulation and electromyographic recording technique in barbiturate anaesthetized animals. The investigated muscles had specific cutaneous receptive fields that appeared to correspond to the area of the skin withdrawn upon contraction of the muscle when the limb is in the standing position. The spatial organization of receptive fields in the cat was similar to that in the rat. However, differences in gain properties of reflexes to some anatomically equivalent muscles in the two species were observed, possibly reflecting adaptations to the biomechanics characteristic of the digitigrade and plantigrade stance in cats and rats, respectively. Implications of the findings for the generality of the modular organization of the withdrawal reflex system and for its adaptive properties are discussed.

Fatigue-Sensitive Afferents Inhibit Extensor but Not Flexor Motoneurons in Humans

“…In acute spinal cats, however, a reversed pattern of flexor inhibition or extensor excitation was observed under some conditions, indicating that there are also "private" paths apart from the flexion reflex (Holmqvist and Lundberg, 1961). Additionally, in animals (Levinsson et al, 1999) as well as the human lower limb (Hagbarth, 1960;Andersen et al, 1999), actions evoked by specific cutaneous afferents depended on the location of stimulation. Our results suggest that such alternative paths do not dominate during fatiguing contractions in humans.…”

Common principles of sensory encoding in spinal reflex modules and cerebellar climbing fibres

“…For a given muscle, the extent of the receptive field corresponded to the skin area withdrawn upon contraction of the muscle itself when the limb was in a standing position. The receptive field focus coincided with the patch of skin located where the withdrawal had its maximal amplitude (Levinsson et al 1999). …”

“…Hence, the individual muscle in the adult animal (see also Holmberg & Schouenborg, 1996) has a highly characteristic cutaneous receptive field that amounts to a sensory encoding of the action of that muscle (Schouenborg & Weng, 1994). While withdrawal reflexes have been thoroughly investigated in the rat hindlimb (Schouenborg & Kalliomäki, 1990) and recently also in the cat hindlimb (Levinsson et al 1999), data on the cat forelimb are insufficient for a meaningful comparison with the extensive pool of forelimb-related climbing fibre receptive fields. Therefore, in order to allow an evaluation of functional relatedness between climbing fibres and withdrawal reflexes, we report here an extension of the pilot investigation of withdrawal reflex modules in the cat forelimb (Levinsson et al 1999) and carry out novel analyses of data derived from databases obtained in two previous studies (Ekerot et al 1991a;Jörntell et al 1996).…”

“…While withdrawal reflexes have been thoroughly investigated in the rat hindlimb (Schouenborg & Kalliomäki, 1990) and recently also in the cat hindlimb (Levinsson et al 1999), data on the cat forelimb are insufficient for a meaningful comparison with the extensive pool of forelimb-related climbing fibre receptive fields. Therefore, in order to allow an evaluation of functional relatedness between climbing fibres and withdrawal reflexes, we report here an extension of the pilot investigation of withdrawal reflex modules in the cat forelimb (Levinsson et al 1999) and carry out novel analyses of data derived from databases obtained in two previous studies (Ekerot et al 1991a;Jörntell et al 1996). Does the receptive field of a given climbing fibre subclass correspond to the cutaneous receptive field of a withdrawal reflex module and therefore, by analogy, to the skin area affected by the action of a particular single muscle?…”

“…The evoked muscle twitches resulted in discrete limb movements. To approximate withdrawal effects on the skin resulting from contractions of each muscle studied, the paw was held against a supporting surface during muscle stimulation, mimicking the action of the muscle when the limb is in a standing position (Levinsson et al 1999).…”