Crystal nucleation and growth of spherulites demonstrated by coral skeletons and phase-field simulations.

Spherulites are radial distributions of acicular crystals, common in biogenic, geologic, and synthetic systems, yet exactly how spherulitic crystals nucleate and grow is still poorly understood. To investigate these processes in more detail, we chose scleractinian corals as a model system, because they are well known to form their skeletons from aragonite (CaCO3) spherulites, and because a comparative study of crystal structures across coral species has not been performed previously. We observed that all 12 diverse coral species analyzed here exhibit plumose spherulites in their skeletons, with well-defined centers of calcification (CoCs), and crystalline fibers radiating from them. In 7 of the 12 species, we observed a skeletal structural motif not observed previously: randomly oriented, equant crystals, which we termed "sprinkles". In Acropora pharaonis, these sprinkles are localized at the CoCs, while in 6 other species, sprinkles are either layered at the growth front (GF) of the spherulites, or randomly distributed. At the nano- and micro-scale, coral skeletons fill space as much as single crystals of aragonite. Based on these observations, we tentatively propose a spherulite formation mechanism in which growth front nucleation (GFN) of randomly oriented sprinkles, competition for space, and coarsening produce spherulites, rather than the previously assumed slightly misoriented nucleations termed "non-crystallographic branching". Phase-field simulations support this mechanism, and, using a minimal set of thermodynamic parameters, are able to reproduce all of the microstructural variation observed experimentally in all of the investigated coral skeletons. Beyond coral skeletons, other spherulitic systems, from aspirin to semicrystalline polymers and chocolate, may also form according to the mechanism for spherulite formation proposed here. STATEMENT OF SIGNIFICANCE: Understanding the fundamental mechanisms of spherulite nucleation and growth has broad ranging applications in the fields of metallurgy, polymers, food science, and pharmaceutical production. Using the skeletons of reef-building corals as a model system for investigating these processes, we propose a new spherulite growth mechanism that can not only explain the micro-structural diversity observed in distantly related coral species, but may point to a universal growth mechanism in a wide range of biologically and technologically relevant spherulitic materials systems.

Changes in the cutaneous silent period by paired stimulation.

OBJECTIVE: The cutaneous silent period (CSP) corresponds to the inhibition of motor neuronal activity that is induced by electrical cutaneous stimulation. This motor neuronal inhibition might be useful as a therapeutic strategy for modulating the excitability of motor neurons. Therefore, we investigated the CSP changes that can be observed using the paired-stimulation method. METHODS: Fifteen healthy adults were recruited. The digital cutaneous nerve of the right index finger was stimulated, and the CSP was recorded at the right thenar muscle. During the stimulation, contraction of the opposing right thumb and third finger was maintained at 20% of maximal voluntary contraction. A single stimulation was applied at the right index finger, and the duration and latency of the CSP (CSP1) was recorded. Paired electrical stimulations were then delivered with 60-, 80-, 100-, 120-, 140-, 160-, 180-, and 200-ms interstimulus intervals (ISI), and the latency and duration of a second CSP (CSP2) was measured and compared with that for the single stimulation. RESULTS: The CSP2 onset latencies were delayed in the 60-, 80-, and 100-ms ISI when compared to CSP1. CSP2 durations were shorter in the 60-, 80-, and 100-ms ISI. No significant differences in the latencies and durations between CSP1 and CSP2 were observed for ISI durations greater than 120 ms. CONCLUSIONS: We found that repetitive electrical stimulation changed the latency and duration of the CSP. These results suggest that the refractory period of the spinal inhibitory circuit in CSP is less than 100 ms.

Differences in cervical musculoskeletal impairment between episodic and chronic tension-type headache.

BACKGROUND: Tension-type headache (TTH) is a headache in which musculoskeletal impairments of the craniocervical region may play an important role in its pathogenesis. We investigated the presence of myofascial, postural and mechanical abnormalities in patients with frequent episodic and chronic tension-type headache (ETTH and CTTH, respectively). METHODS: The study population consisted of 36 patients with ETTH, 23 with CTTH and 42 control subjects. Myofascial trigger points (MTrPs) were identified in the upper trapezius, sternocleidomastoid, temporalis and suboccipital muscles. Sagittal C7-tragus angle was measured to evaluate flexor head posture (FHP), and neck mobility was assessed using an inclinometer. RESULTS: Only active MTrPs were significantly different between the ETTH and CTTH groups (p < .001). Patients with CTTH showed a larger sagittal C7-tragus angle (p = .011), that is, greater FHP and restricted neck mobility for both rotations compared to controls (p < .001). Although active MTrPs were correlated with the frequency and duration of headache, no correlations were observed for FHP or neck mobility. CONCLUSION: Active MTrPs in the craniocervical region contribute to triggering or maintenance of TTH and posture or neck mobility may be a result of chronic headache.