Nicole Tremblay

Cholinergic Modulation of Synaptic Plasticity in Sensory Neocortex

Activity-dependent synaptic plasticity occurs in neocortex devoted to each sensory system at all postnatal ages. The process is crucial for the refinement of topographic cortical representations and the development of stimulus selectivity in the neonate, and although the plasticity is more subtle in the adult cortex, it probably forms the basis of dynamic changes in sensory representations and may account for some forms of memory. A question of central importance is whether the fundamental mechanisms of neuronal plasticity are the same at different ages and in different cortical areas.

Differences between the tactile sensitivity on the anterior torso of normal individuals and those having suffered complete transection of the spinal cord.

Using the method of limits and a magnitude estimation procedure, the sense of touch was examined at multiple sites on the anterior torso of normal subjects. Their performance was compared with the performance of individuals having experienced a functionally complete spinal cord transection more than 6 months prior to the tests. Near the insentient regions of the spinal cord-injured patients there was a zone wherein the threshold for light touch was elevated and variable. Within this same transition zone, estimates of the magnitude of a brushing stimulus increased as a linear function of distance from the border for approximately 12 cm away from insentient skin. Throughout the rest of the thorax, spinal cord-injured patients displayed touch thresholds 67% higher than normals and, at the same test sites, spinal cord-injured patients offered estimates of the intensity of the brushing stimulus that averaged 62% higher than normal subjects. The greater intensity of the sensations experienced by spinal cord-injured patients with even very weak stimuli and the smaller range within which they were able to scale stimulus intensity, produced a situation wherein the patients made frequent errors of judgement even on skin regions far from the body parts affected by the lesion. These observations support the hypothesis that spinal cord lesions interrupt tonic modulatory mechanisms having global influences on the sense of touch. This loss produces an elevation of the touch threshold and a reduction of the normal dynamic range of tactile sensory perception for all skin surfaces on the anterior torso.

Differences in the sensations of warmth on the anterior torso of normal and spinal-cord transected individuals.

The threshold to warming was measured at 10 sites on the anterior torso between the umbilicus and the clavicle of normal and spinal-cord transected individuals. In normal individuals, thresholds were higher on the thorax than on the abdomen. Men had higher and more variable thresholds than women. Magnitude estimations of supra-threshold stimuli showed that men offer verbal estimates of warmth that are about half of the size of the estimates given by women to the same stimuli. The psychometric function shows that in women, the sensation of warmth grows more rapidly than in men after starting from a higher initial value. After spinal-cord injury, thresholds for detection of warming were elevated. This effect was most noticeable within 8 cm of the anesthetic zone, but farther away, thresholds were still elevated but uniform as a function of distance, being about 30% higher than in normal individuals. After spinal-cord injury, the psychometric functions show that small stimuli elicit relatively large sensations and that these sensations grow more slowly with increasing skin temperatures than for normal individuals. Thus, for small warm stimuli spinal-cord-injured patients (both men and women) have a response similar to normal women but the slope of the psychometric function is flat, being similar to the slope observed for normal men.

Differences in the sensations of cold on the anterior torso of control and spinal-cord-transected individuals

Application of cold stimuli to the sentient portion of the anterior torso of 12 spinal-cord-transected individuals (patients) and to comparable sites of 11 control participants showed that thresholds are lower for women than for men and that the difference between the sexes is maintained following spinal-cord transection. Patients of both sexes were more sensitive to cold stimuli than were controls. Estimates of stimulus intensity showed that participants reliably distinguished the 3 cool stimuli but that control women offered significantly larger estimates than control men. Spinal-cord transection produced an increase in the intensity of the sensations in women and a reduction in men. The changes in sensory perception that follow spinal-cord injury extend throughout the somatosensory system and involve all modalities. These changes cannot be explained as a simple release from inhibition.

Neuromodulatory Substances, Somatosensory Cortical Neuronal Responsiveness and Long-term Changes in Neuronal Excitability

Models of the somatosensory system require that the patterns of neuronal activity which relay sensory information along systems of neurons between the skin surface and somatosensory cortex reproduce the spatial and temporal characteristics of the cutaneous message with a fidelity sufficient to account for the sensory discriminations which that information elicits (Mountcastle, 1975). Furthermore these models presume that identical stimuli produce nearly identical spatiotemporal patterns and that any differences among the neural patterns evoked by identical stimuli determine the limits of an the animal’s ability to detect or discriminate among those stimuli. The reproducibility of the behavioral data and of their neural correlates imply that the functional characteristics of neurons in the somatosensory pathway are highly reproducible over time and that faithful central representations of sensory events are dependent upon their reproducibility.