Autonomic dysreflexia : somatosympathetic and viscerosympathetic vasoconstrictor responses to innocuous and noxious sensory stimulation below lesion in human spinal cord injury

Autonomic dysreflexia is a dangerous elevation in blood pressure in people with spinal cord injury (SCI), produced by a spinally-mediated reflex activation of sympathetic vasoconstrictor neurones supplying skeletal muscle and the gut. Current dogma states that, apart from visceral inputs - such as those originating from a distended bladder or impacted colon - autonomic dysreflexia is triggered by noxious inputs below the lesion. However, while selective stimulation of small-diameter afferents in muscle or skin evokes a sustained increase in muscle sympathetic nerve activity and blood pressure, and a transient increase in skin sympathetic nerve activity and decrease in skin blood flow in able-bodied subjects, such noxious inputs have no effects on blood pressure and skin blood flow in SCI individuals. Conversely, weak electrical stimulation over the abdominal wall, which in able-bodied subjects is not painful and activates large-diameter cutaneous afferents, causes a marked increase in blood pressure in SCI but not in able-bodied subjects. Moreover, vibration of the penis in spinal-injured men, which is not noxious, caused marked vasoconstriction and increases in blood pressure, similar to those produced by non-noxious distension of the bladder during urodynamics procedures. This suggests that activation of large-diameter somatic afferents, not small-diameter afferents, triggers the increases in vasoconstrictor drive that lead to autonomic dysreflexia, arguing against current dogma on the importance of noxious inputs in triggering autonomic dysreflexia

Microneurography from the posterior tibial nerve : a novel method of recording activity from the foot in freely standing humans

The posterior tibial nerve, located behind the medial malleolus of the ankle, supplies the intrinsic muscles of the foot and most of the skin of the sole. We describe a novel approach for recording from this nerve via a percutaneously inserted tungsten microelectrode, and provide examples of recordings from presumed muscle spindle endings recorded in freely-standing human subjects. The fact that the angular excursions of the ankle joint are small as the foot is loaded during the transition from the seated position to standing means that one can obtain stable recordings of neural traffic in unloaded, loaded and freely standing conditions. We conclude that this novel approach will allow studies that will increase our understanding of the roles of muscle and cutaneous afferents in the foot in the control of upright posture

Firing properties of muscle spindles supplying the intrinsic foot muscles of humans in unloaded and free-standing conditions

We recently developed an approach to record from muscle spindles in the intrinsic muscles of the foot in free-standing humans by inserting a tungsten microelectrode into the posterior tibial nerve behind the medial malleolus of the ankle. Here, we characterize the behaviour of muscle spindles in the small muscles of the foot in (i) seated subjects with the leg horizontal and foot naturally plantarflexed and (ii) in standing subjects. In the first study recordings were made from 26 muscle spindle afferents located within flexor digiti minimi brevis (n=4), abductor digiti minimi (n=3), quadratus plantae (n=3), plantar interossei (n=4), flexor digitorum brevis (n=3), dorsal interossei (n=2) and lumbricals (n=2), with one each supplying abductor hallucis, adductor hallucis and flexor hallucis brevis. The identity of another two muscle afferents was unknown. The majority of the units were silent at rest; only 7 (27%) being spontaneously active. Because of the anatomic constraints of the foot, some spindles supplying muscles acting on the toes responded to movements of one or more digits. In the second study 12 muscle spindle afferents were examined during standing. The ongoing discharge of 8 spindle afferents covaried with changes in the centre of pressure during postural sway. We conclude that the majority of spindle endings in the small muscles of the foot are silent at rest, which may allow them to encode changes in conformation of the foot when it is loaded during standing. Moreover, these muscle spindle afferents can provide useful proprioceptive information during standing and postural sway

Effects of tonic muscle pain on fusimotor control of human muscle spindles during isometric ankle dorsiflexion

Studies on anaesthetised animals have revealed that nociceptors can excite fusimotor neurones and thereby change the sensitivity of muscle spindles to stretch; such nociceptive reflexes have been suggested to underlie the mechanisms that lead to chronic musculoskeletal pain syndromes. However, the validity of the "vicious cycle" hypothesis in humans has yielded contrasting results to that found in animals. Given that spindle firing rates are much lower in humans than in animals, it is possible that some of the discrepancies between human experimental data and those obtained in animals could be explained by differences in background fusimotor drive when the leg muscles are relaxed. We examined the effects of tonic muscle pain during voluntary contractions of the ankle dorsiflexors. Unitary recordings were obtained from 10 fusimotor-driven muscle spindle afferents (6 primary, 4 secondary) supplying the ankle dorsiflexors via a microelectrode inserted percutaneously into the common peroneal nerve. A series of one-minute weak contractions was performed at rest and during one hour of muscle pain induced by intramuscular infusion of 5% hypertonic saline into the tibialis anterior muscle. We did not observe any statistically significant increases in muscle spindle firing rates of six afferents followed during tonic muscle pain, though discharge variability increased slightly. Furthermore, a participant's capacity to maintain a constant level of force, while relying on proprioceptive feedback in the absence of visual feedback, was not compromised during pain. We conclude that nociceptive inputs from contracting muscle do not excite fusimotor neurones during voluntary isometric contractions in humans

The impact of childhood trauma on psychosocial functioning and physical health in a non-clinical community sample of young adults

Objective: Given the fundamental emotional, social and physical development that occurs during the early years of life, childhood experiences are formative in shaping a person’s life trajectory. Childhood trauma is a prevalent, multifaceted issue with well-documented long-term adverse health effects in clinical populations however; the impact of childhood trauma in the community is less clear. To address this, this study investigated how childhood trauma may impact physical and psychological health, sleep quality and autonomic function in a non-clinical community sample of adults. Method: Participants completed questionnaires, an in-laboratory autonomic assessment (including stress reactivity to mental and physical stressors) and overnight autonomic and sleep monitoring. Overall childhood trauma and its subtypes (e.g. physical abuse, emotional neglect) were defined using the Childhood Trauma Questionnaire. Results: We identified 22 childhood trauma cases (total score > 36) and, of the 89 non-childhood trauma cases, some individuals also experienced significant levels of trauma in one or more of the childhood trauma subtypes. Childhood trauma and some trauma subtypes were significantly correlated with a myriad of negative physiological and physical health outcomes including elevated psychological distress, increased sleep disturbances, reduced emotional wellbeing and lower perceived social support. Autonomic dysregulation was found in those with high levels of childhood trauma, which was reflected in an increased stress response to laboratory tasks. Notably, the experience of physical abuse in childhood was significantly associated with alterations in nocturnal heart rate and heart rate variability. Conclusion: Together, these results highlight that childhood trauma can have lasting detrimental consequences on an individual’s emotional and physical health, sleep quality and stress reactivity. © The Royal Australian and New Zealand College of Psychiatrists 2019