Thin-fibre signalling in humans: Cortical processing of sensory afference and autonomic efference
Year: 2010
Type: dissertation
Abstract: Thin nerve fibres innervate the entire human body and mediate sensations such as pain,\ntemperature and visceral sensory input. Moreover, a special class of unmyelinated afferents\nresponsive to light touch has recently been found in humans: C-tactile (CT) fibres. In the\nefferent side, C-fibres are the path for signalling in the autonomic nervous system, controlling\nthe internal milieu of the body. There is growing evidence that C-fibres form the basis for\nmonitoring and regulating the physical status of the body. This thesis focuses on central\nprojections of mild thin-fibre input and their integration with autonomic reactions. Brain\nactivity was studied with functional magnetic resonance imaging (fMRI).\nThe first paper examined cortical activation of selective CT-stimulation by soft tactile\nstimulation in two rare patients lacking Aβ fibres. The results confirmed previous findings\nbased on one of these patients, showing that CT stimulation activates the insular cortex. In\naddition, CT stimulation deactivated somatosensory cortices.\nThe second paper further investigated cortical effects of CT stimulation in healthy controls by\ncomparing rapid vibration (predominantly activating Aβ fibres) and soft brush stroking\n(combined Aβ and CT activation) on the skin. The ventromedial prefrontal cortex was\nsignificantly more activated by brushing than by vibration, an area previously implied in\ncoding for the expected emotional value of an event.\nThe third paper focused on the role of CT fibres and autonomic function. We again studied\nthe two Aβ deafferented patients to examine whether CT stimulation could evoke an\nautonomic response. We also examined their ability to localise the CT stimulations to the\ncorrect limb. Capacity for localisation of the stimulus was poor but above chance. Despite\nproducing only a vague percept in the patients, the CT stimulus gave rise to a skin\nsympathetic reaction which was indexed by a galvanic skin response.\nThe fourth paper studied the cortical mechanisms behind a restricted autonomic response\nelicited by a perceptually weak C-fibre input in healthy subjects. We used low-intensity rectal\ndistension while recording autonomic variables and cortical responses. Rectal distension\nactivated insular cortex. Central activation specifically related to the skin sympathetic\nresponse was, in addition to the brainstem, limited to the right inferior frontal gyrus (IFG).\nThe CT evoked insular activation and the Aβ-denervated patients’ poor ability to localise a\nCT stimulation support the concept that these fibres underpin affective rather than\ndiscriminative aspects of touch. The rectal distension study indicated that insular activation\nvia low-threshold mechanovisceral thin fibres predominantly reflects afferent processing\nwhereas IFG and the brainstem may be important in the generation of autonomic responses.\nFurther, the studies suggest that stimulus perception is a prerequisite for cutaneous autonomic\nresponses to both CT and visceral thin fibre stimuli. These findings set the stage for future\nstudies of thin nerve fibre function, including neural mechanisms of hedonic processing as\nwell as pathophysiological studies of conditions such as irritable bowel syndrome, which may\ntease out putative contributions from afferent input, cognitive processing and autonomic\nconsequences.
Author: Karin Rylander
Language: English
Subfield: Cognitive Neuroscience
Field: Neuroscience
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W1608288709