There is dual tactile innervation of the human hairy skin: in addition to fast-conducting myelinated afferent fibers, there is a system of slow-conducting unmyelinated (C) afferents that respond to light touch. In a unique patient lacking large myelinated afferents, we found that activation of C tactile (CT) afferents produced a faint sensation of pleasant touch. Functional magnetic resonance imaging (fMRI) analysis during CT stimulation showed activation of the insular region, but not of somatosensory areas S1 and S2. These findings identify CT as a system for limbic touch that may underlie emotional, hormonal and affiliative responses to caress-like, skin-to-skin contact between individuals.
Monzée, Joël, Yves Lamarre, and Allan M. Smith. The effects of digital anesthesia on force control using a precision grip. J Neurophysiol 89: 672-683, 2003; 10.1152/jn.00434.2001. A total of 20 right-handed subjects were asked to perform a grasp-lift-and-hold task using a precision grip. The grasped object was a one-degree-offreedom manipuladum consisting of a vertically mounted linear motor capable of generating resistive forces to simulate a range of object weights. In the initial study, seven subjects (6 women, 1 man; ages 24 -56 yr) were first asked to lift and hold the object stationary for 4 s. The object presented a metal tab with two different surface textures and offered one of four resistive forces (0.5, 1.0, 1.5, and 2.0 N). The lifts were performed both with and without visual feedback. Next, the subjects were asked to perform the same grasping sequence again after ring block anesthesia of the thumb and index finger with mepivacaine. The objective was to determine the degree to which an internal model obtained through prior familiarity might compensate for the loss of cutaneous sensation. In agreement with previous studies, it was found that all subjects applied significantly greater grip force after digital anesthesia, and the coordination between grip and load forces was disrupted. It appears from these data, that the internal model alone is insufficient to completely compensate for the loss of cutaneous sensation. Moreover, the results suggest that the internal model must have either continuous tonic excitation from cutaneous receptors or at least frequent intermittent reiteration to function optimally. A subsequent study performed with 10 additional subjects (9 women, 1 man; ages 24 -49 yr) indicated that with unimpaired cutaneous feedback, the grasping and lifting forces were applied together with negligible forces and torques in other directions. In contrast, after digital anesthesia, significant additional linear and torsional forces appeared, particularly in the horizontal and frontal planes. These torques were thought to arise partially from the application of excessive grip force and partially from a misalignment of the two grasping fingers. These torques were further increased by an imbalance in the pressure exerted by the two opposing fingers. Vision of the grasping hand did not significantly correct the finger misalignment after digital anesthesia. Taken together, these results suggest that mechanoreceptors in the fingertips signal the source and direction of pressure applied to the skin. The nervous system uses this information to adjust the fingers and direct the pinch forces optimally for grasping and object manipulation.
In addition to A-beta fibres the human hairy skin has unmyelinated (C) fibres responsive to light touch. Previous functional magnetic resonance imaging (fMRI) studies in a subject with a neuronopathy who specifically lacks A-beta afferents indicated that tactile C afferents (CT) activate insular cortex, whereas no response was seen in somatosensory areas 1 and 2. Psychophysical tests suggested that CT afferents give rise to an inconsistent perception of weak and pleasant touch. By examining two neuronopathy subjects as well as control subjects we have now demonstrated that CT stimulation can elicit a sympathetic skin response. Further, the neuronopathy subjects' ability to localize stimuli which activate CT afferents was very poor but above chance level. The findings support the interpretation that the CT system is well suited to underpin affective rather than discriminative functions of tactile sensations.
Sustained oscillations of 13-18 Hz were observed in local field potentials (LFPs) in the cerebellar cortex of a behaving monkey. These oscillations, which appeared to be generated in the granular cell layer, were particularly prominent in the paramedian lobule. The oscillatory activity decreased during drowsiness or extreme arousal and occurred most often when the animal was immobile but alert. In a task requiring the animal to move the arm approximately 1 s after an auditory cue, the oscillations stopped some 150-200 ms after the cue, resumed 200-300 ms later, and stopped again 50-100 ms before movement onset. This modulation pattern was observed with consistency only when the animal responded reliably to the auditory cue. The results suggest that the cerebellum could be involved in some higher level of integration particularly during complex sensorimotor behavior.
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