Quantitative analysis of dorsal horn cell receptive fields following limited deafferentation

Abstract

1. To test the hypothesis that subtotal deafferentation of dorsal horn cells can stimulate plastic changes in their receptive fields (RFs), diffuse deafferentation of the cat hindlimb dorsal horn was produced by transection of L7 or L6 and L7 dorsal roots. The following single-unit cutaneous low-threshold mechanoreceptor RF properties were compared between operated and control dorsal horns: 1) distance of RF center from tips of toes, 2) RF length-width ratio; and 3) RF area. 2. In both L7 and L6-L7 rhizotomized animals there was an increased incidence of silent electrode tracks in the most deafferented portion of the hindlimb map (the foot and toe representation). In the rhizotomized L6-L7 animals, there was also an increased incidence of symmetrically placed tracks in deafferented and control dorsal horns, in which cell RFs had no mirror-symmetrical components. In addition, cells in the lateral half of the L6 and L7 dorsal horns exhibited a proximal shift in the location of their RFs. In the rhizotomized L7 animals there was a distal shift of RFs in the L5 segment at long survival times. RFs had lower length-width ratios in L5 and L6 at short survival times and in L6 at long survival times. 3. In intact preparations, dorsal horn cells normally respond to inputs via single or small numbers of low-threshold cutaneous mechanoreceptors. Because these rhizotomies do not remove all inputs from any given area of skin, the deafferentations would produce only patchy loss of input from individual receptors. Therefore observed changes cannot be accounted for entirely by loss of afferent input, suggesting that some reorganization of dorsal horn cell RFs occurred. We conclude that the threshold stimulus for plastic change is less than total deafferentation of dorsal horn cells. At least some of the mechanisms underlying these changes may be active in normal animals in the maintenance of the somatotopic map or in conditioning.

The effects of neonatal median nerve injury on the responsiveness of Tactile neurones within the cuneate nucleus of the cat

“…It has also been reported in several studies on the spinal dorsal horn (e.g. Basbaum & Wall, 1979; Devor & Wall, 1981; Lisney, 1983; Koerber & Brown, 1995) but not in others (Brown, Brown, Fyffe & Pubols, 1983; Brown, Fyffe, Noble & Rowe, 1984; Wilson, 1987). In the present experiments, even if such somatotopic reorganization occurred in the short term within the cuneate nucleus in response to the neonatal median nerve injury, the reorganization may have been reversed during the 12–24 months that elapsed before our electrophysiological analysis.…”

“…However, the clinical assessments are often based on simple detection tasks and may not reveal more subtle discriminative disturbances that reflect impaired transmission and signalling, and which might result either from the incomplete anatomical regeneration of the peripheral nerve, or from some form of functional reorganization or ‘plasticity’ within the central pathways that has been reported to follow partial deafferentation brought about by nerve injury or local anaesthetic blockade (e.g. Dostrovsky, Millar & Wall, 1976; Lisney, 1983; Merzenich, Kaas, Wall, Nelson & Sur, 1983; Calford & Tweedale, 1988, 1990; Wall, Huerta & Kaas, 1992; Nicolelis, Lin, Woodward & Chapin, 1993; Pettit & Schwark, 1993; Koerber & Brown, 1995). The reorganization described involves an alteration in body maps at the levels of the dorsal horn, the dorsal column nuclei, thalamus and cortex, in which there is an expansion in the central representation of body regions around the zone of deafferentation into areas of the map that previously represented the deafferented body part.…”

Temporary Nerve Block at Selected Digits Revealed Hand Motor Deficits in Grasping Tasks

“…Although none of these studies investigated the whole hand grasp control (i.e., only two-digit pinch or three-digit grasp was employed), their findings revealed an impaired sensorimotor integration process following somatosensory feedback deprivation at peripheral levels. At corticospinal levels, neurophysiological studies suggested that cutaneous feedback deprivation decreases the activation of motor neurons in several parts of the central nervous system (Rossini et al, 1996; Rossi et al, 1998), such as the dorsal horn of the spinal cord (Koerber and Brown, 1995), thalamus (Nicolelis et al, 1993; Rasmusson, 1996), and the cerebral cortex (Wall et al, 1986). However, most previous studies focused on the effect of local afferent abnormalities on adaptive peripheral behavioral and central processing changes, and none of them addressed the following questions: (1) how the disturbed and intact sensory input integrate and interact with each other to modulate the motor program execution; and (2) whether the motor coordination based on motor output variability presents between affected and non-affected elements (e.g., digits) and becomes interfered by the local sensory deficiency.…”

Spatial convergence and divergence between cutaneous afferent axons and dorsal horn cells are not constant

“…Rostrocaudal (RC) and mediolateral (ML) recording locations were expressed in normalized form, as fractional segmental and mediolateral locations (e.g., RC ϭ L 7.24 , ML ϭ 0.36, for a site 0.24ϫ the distance from the rostral to the caudal end of L 7 , 0.36ϫ the distance from the medial to the lateral edge of the dorsal horn). The recording locations and RFs of the single units were added to our database of 551 cells (Koerber et al, 1993;Koerber and Brown, 1995;Wang et al, 1997;Brown et al, 1998) for purposes of modeling somatotopy. This was accomplished by recalculating the descriptive model of the lamina III-IV cells including the new cell data.…”