Effects of postural threat on spinal stretch reflexes: evidence for increased muscle spindle sensitivity?

Abstract

Standing balance is often threatened in everyday life. These threats typically involve scenarios in which either the likelihood or the consequence of falling is higher than normal. When cats are placed in these scenarios they respond by increasing the sensitivity of muscle spindles imbedded in the leg muscles, presumably to increase balance-relevant afferent information available to the nervous system. At present, it is unknown whether humans also respond to such postural threats by altering muscle spindle sensitivity. Here we present two studies that probed the effects of postural threat on spinal stretch reflexes. In study 1 we manipulated the threat associated with an increased consequence of a fall by having subjects stand at the edge of an elevated surface (3.2 m). In study 2 we manipulated the threat by increasing the likelihood of a fall by occasionally tilting the support surface on which subjects stood. In both scenarios we used Hoffmann (H) and tendon stretch (T) reflexes to probe the spinal stretch reflex circuit of the soleus muscle. We observed increased T-reflex amplitudes and unchanged H-reflex amplitudes in both threat scenarios. These results suggest that the synaptic state of the spinal stretch reflex is unaffected by postural threat and that therefore the muscle spindles activated in the T-reflexes must be more sensitive in the threatening conditions. We propose that this increase in sensitivity may function to satisfy the conflicting needs to restrict movement with threat, while maintaining a certain amount of sensory information related to postural control.

Influence of virtual height exposure on postural reactions to support surface translations

“…After each block, participants completed questionnaires (in approximately 3 min) to assess their perceived anxiety (sum of 10-item, 9-point Likert scale; max = 90), fear (0% = not fearful, 100% = extremely fearful), and stability (0% = not stable, 100% = completely stable) [14]. In the High Condition only, subjects were also asked to estimate the height at which they stood, in order to calculate a ratio of overestimation (RoO, estimated height/actual virtual height of 3.2 m) which is known to be increased with fear [4,15]. All questionnaires were projected in the virtual environment, and verbal responses to each question were manually recorded.…”

“…Under threatening conditions, the amplitude and frequency of sway during static balance is altered, and may be associated with concomitant changes in tonic muscle activity of lower leg muscles [3], increased stiffness [3], and increased proprioceptive and vestibular reflex gains [4,5]. The effects of threat on dynamic balance reactions are less clear.…”

“…Virtual height has been shown to induce emotional and postural changes similar to those experienced in real settings [11], and has been used to study the effects of threat on static balance and gait [11,12], but not, to our knowledge, during dynamic balance reactions. After controlling for potential confounding effects of leaning and expectation by using bi-directional threat and perturbation, we hypothesized that threat-related increases in stiffness and sensory gain [3][4][5] would influence dynamic postural responses in terms of (a) larger and earlier peak center of pressure (COP) displacements [8]; (b) reduced center of mass (COM) and leg segment displacements [6,9]; and (c) larger and earlier balance correcting EMG responses, particularly in the arm [8,9].…”

Effects of Aquatic Pole Walking on the Reduction of Spastic Hypertonia in a Patient with Hemiplegia: A Case Study

“…Fear of falling is one of the factors of "postural threat, " which has been reported to increase the excitability of the spinal stretch reflex [12]. Enhanced sympathetic and fusimotor drive during postural threat are thought to increase muscle spindle sensitivity [13]. Therefore, an unsteady gait on the ground may have led to spastic EMG activities through postural threat.…”

Cognitively and socially induced stress affects postural control

“…Standing on an elevated surface also induced increases in fear of falling, sympathetic arousal, and stress response (Cleworth et al 2012;Horslen et al 2014). More recent studies focused on the underlying mechanisms of this adaptive response and showed that postural threat is likely to induce changes in two of the sensory channels involved in postural control, proprioceptive (Davis et al 2011;Horslen et al 2013), and vestibular (Horslen et al 2014;Lim et al 2017).…”