Artificial SA-I and RA-I afferents for tactile sensing of ridges and gratings

For robot touch to reach the capabilities of human touch, artificial tactile sensors may require transduction principles like those of natural tactile afferents. Here we propose that a biomimetic tactile sensor (the TacTip) could provide suitable artificial analogues of the tactile skin dynamics, afferent responses and population encoding. Our three-dimensionally printed sensor skin is based on the physiology of the dermal-epidermal interface with an underlying mesh of biomimetic intermediate ridges and dermal papillae, comprising inner pins tipped with markers. Slowly adapting SA-I activity is modelled by marker displacements and rapidly adapting RA-I activity by marker speeds. We test the biological plausibility of these artificial population codes with three classic experiments used for natural touch: (1a) responses to normal pressure to test adaptation of single afferents and spatial modulation across the population; (1b) responses to bars, edges and gratings to compare with measurements from monkey primary afferents; and (2) discrimination of grating orientation to compare with human perceptual performance. Our results show a match between artificial and natural touch at single afferent, population and perceptual levels. As expected, natural skin is more sensitive, which raises a challenge to fabricate a biomimetic fingertip that demonstrates human sensitivity using the transduction principles of human touch.

Control of Ca2+ Influx and Calmodulin Activation by SK-Channels in Dendritic Spines

The key trigger for Hebbian synaptic plasticity is influx of Ca²⁺ into postsynaptic dendritic spines. The magnitude of [Ca²⁺] increase caused by NMDA-receptor (NMDAR) and voltage-gated Ca²⁺ -channel (VGCC) activation is thought to determine both the amplitude and direction of synaptic plasticity by differential activation of Ca²⁺ -sensitive enzymes such as calmodulin. Ca²⁺ influx is negatively regulated by Ca²⁺ -activated K⁺ channels (SK-channels) which are in turn inhibited by neuromodulators such as acetylcholine. However, the precise mechanisms by which SK-channels control the induction of synaptic plasticity remain unclear. Using a 3-dimensional model of Ca²⁺ and calmodulin dynamics within an idealised, but biophysically-plausible, dendritic spine, we show that SK-channels regulate calmodulin activation specifically during neuron-firing patterns associated with induction of spike timing-dependent plasticity. SK-channel activation and the subsequent reduction in Ca²⁺ influx through NMDARs and L-type VGCCs results in an order of magnitude decrease in calmodulin (CaM) activation, providing a mechanism for the effective gating of synaptic plasticity induction. This provides a common mechanism for the regulation of synaptic plasticity by neuromodulators.

Hebbian or associative plasticity is triggered by postsynaptic Ca²⁺ influx which activates calmodulin and CaMKII. The influx of Ca²⁺ through voltage-dependent NMDA receptors and Ca²⁺ channels is regulated by Ca²⁺ -activated K⁺ channels (SK-channels) providing negative feedback regulation of postsynaptic [Ca²⁺]. Using 3-dimensional modeling of Ca²⁺ and calmodulin dynamics within dendritic spines we show that the non-linear relationship between Ca²⁺ influx and calmodulin activation endows SK-channels with the ability to “gate” calmodulin activation and therefore the induction of Hebbian synaptic plasticity. Since SK-channels are inhibited by several neuromodulator receptors including acetylcholine and noradrenaline, the gating of synaptic plasticity by SK-channels could represent a common mechanism by which neuromodulators control the induction of synaptic plasticity.

A man with a history of skin lesions and seizures

A 38-year-old man presented with painful growths around his toenails. Since their first occurrence at age three, several had been surgically removed. He had had similar growths around his fingernails and multiple facial lesions since age seven.

Laser pointers: toys, nuisances, or significant eye hazards?

BACKGROUND

Laser pointers have been used inappropriately to harass and "dazzle" victims. Reports of retinal damage caused by pointers have also been circulated in the popular press.

METHODS

Information on pointer abuse was collected from the literature and through discussions with specialists.

RESULTS

Few, if any, documented cases of permanent retinal damage caused by laser pointers could be found. For actual damage to occur, viewing, times need to exceed approximately 10 seconds. Exposures of this duration would require the person being lased to cooperate by holding fixation on the laser beam.

CONCLUSIONS

Although the risk of permanent retinal damage from a laser-pointer beam is minimal, other risks include dazzle, annoyance, and concern that a weapon-aiming device rather than a pointer is generating the laser beam.

Does it help to undiagnose angina?

We reported recently the results of re-evaluating a series of 64 patients still experiencing pain originally diagnosed as angina but with normal coronary arteriograms. A musculoskeletal cause for the chest pain was established in 38 and an oesophageal cause in 17 of these patients. We here report these patients' self-assessment of their pain 2 years after this diagnostic reclassification. It is clear that the effects of a diagnosis of angina are not easily rescinded even when the non-cardiac cause of the chest pain is identified.