Muscle thixotropy as a tool in the study of proprioception

When a muscle relaxes after a contraction, cross-bridges between actin and myosin in sarcomeres detach, but about 1 % spontaneously form new, non-force-generating attachments. These bridges give muscle its thixotropic property. They remain in place for long periods if the muscle is left undisturbed and give the muscle a passive stiffness in response to a stretch. They are detached by stretch, but reform at the new length. If the muscle is then shortened, the presence of these bridges prevents muscle fibres from shortening and they fall slack. So, resting muscle can be in one of two states, where it presents in response to a stretch with a high stiffness, if no slack is present, or with a compliant response in the presence of slack. Intrafusal fibres of muscle spindles show thixotropic behaviour. For spindles, after a conditioning contraction, they are left stretch sensitive, with a high level of background discharge. Alternatively, if after the contraction the muscle is shortened, intrafusal fibres fall slack, leaving spindles with a low level of background activity and insensitivity to stretch. Muscle spindles are receptors involved in the senses of human limb position and movement. The technique of muscle conditioning can be used to help understand the contribution of muscle spindles to these senses and how the brain interprets signals arising in spindles. When, in a two-arm position-matching task, elbow muscles of the two arms are deliberately conditioned in opposite ways, the blindfolded subject makes large position errors of which they are unaware. The evidence suggests that the brain is concerned with the difference signal coming from the antagonists acting at the elbow and with the overall difference in signal from the two arms. Another way of measuring position sense is to use a single arm and indicate its perceived position with a pointer. Here, there is no access to a signal from the other limb, and position sense relies on referral to a central map of the body, the postural schema.     

Mirror neuron activity during contagious yawning—an fMRI study

Yawning is contagious. However, little research has been done to elucidate the neuronal representation of this phenomenon. Our study objective was to test the hypothesis that the human mirror neuron system (MNS) is activated by visually perceived yawning. We used functional magnetic resonance imaging to assess brain activity during contagious yawning (CY). Signal-dependent changes in blood oxygen levels were compared when subjects viewed videotapes of yawning faces as opposed to faces with a neutral expression. In response to yawning, subjects showed unilateral activation of their Brodmann’s area 9 (BA 9) portion of the right inferior frontal gyrus, a region of the MNS. In this way, two individuals could share physiological and associated emotional states based on perceived motor patterns. This is one component of empathy (motor empathy) that underlies the development of cognitive empathy. The BA 9 is reportedly active in tasks requiring mentalizing abilities. Our results emphasize the connection between the MNS and higher cognitive empathic functions, including mentalizing. We conclude that CY is based on a functional substrate of empathy.     

Association of midregional proadrenomedullin with coronary artery stenoses, soft atherosclerotic plaques and coronary artery calcium

Midregional proadrenomedullin (MR-proADM) is elevated in patients with heart failure and myocardial infarction. The aim of this study was to evaluate the association of MR-proADM with the grade of coronary artery stenosis, presence of coronary artery soft plaques and coronary artery calcification score (CACS), determined by 64-multislice computed tomography (MSCT) in patients without known prior cardiovascular disease. This retrospective study included 107 patients undergoing MSCT for confirmation (or exclusion) of coronary artery disease. MR-proADM levels were measured in all patients. The assessment of coronary artery stenoses, CACS and soft coronary plaques was made by MSCT using known criteria. The MR-proADM [median (25th–75th percentiles)] level was 0.33 (0.21–0.43)?nmol/l. The MR-proADM level was 0.28 (0.22–0.40)?nmol/l in patients with coronary stenoses ≥50% (n?=?23) versus 0.33 (0.27–0.40)?nmol/l in patients with coronary stenoses <50% (n?=?83, P?=?0.59), 0.33 (0.26–0.40)?nmol/l in patients with soft plaques (n?=?56) versus 0.33 (0.25–0.41)?nmol/l in patients without soft plaques (n?=?50, P?=?0.73) and 0.33 (0.25–0.39)?nmol/l in patients with CACS?<200 (n?=?81) versus 0.32 (0.26–0.44)?nmol/l in patients with CACS?≥200 (n?=?26, P?=?0.77). In multivariate analysis, the MR-proADM level was a significant correlate of coronary artery stenoses [odds ratio (OR)?=?0.93; 95% confidence interval (CI) 0.86–0.99; P?=?0.026] and soft plaques (OR?=?0.94; 95% CI 0.90–0.99; P?=?0.015) but not of CACS (OR?=?0.98; 95% CI 0.93–1.03; P?=?0.36). A decreased MR-proADM level is an independent correlate of the presence of coronary artery disease and of soft atherosclerotic plaques. Patients with decreased MR-proADM levels may need invasive examinations to diagnose more severe forms of coronary artery disease.