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This study involved the corneal effects of the laser beam in infrared at 1573 nm. Investigations were directed toward verifying the limit values by defining the thresholds of corneal damage in experiments carried out on the rabbit, and using biochemical techniques, to specify a damage threshold at the cellular level and a better understanding of the cellular steps of the damaging process. Corneal damage thresholds were determined for exposures to 3 ns single pulses using clinical observations and histology. The ED50 obtained with a corneal beam diameter of 400 μm was 26.6 J cm−2. The corresponding radiant exposure, calculated with the 1 mm aperture diameter recommended by standards, was 4.3 J cm−2. This value was still higher than the present 1 J cm−2 exposure limit. In vitro experiments were also carried out on primary keratocytes and HT1080 epithelial cell line. An expanded beam diameter of 3.5 mm was delivered on plates for study of cell proliferation, senescence, and cytotoxicity. The laser beam was e...

Most laser safety documents advise the laser user that caution must be used in the evaluation of exposure to repetitively pulsed radiation since they are only limited data on multiple pulse exposure criteria. The empirical multiple pulse formula is based on some data indicating that there is generally a cumulative effect in multiple-pulse exposures. This effect is a reduction in the threshold energy per pulse relative to the single pulse threshold. The best fit to experimental data predicts that the threshold energy per pulse decreases in proportion to the fourth root of the number of pulses n. This model has been reported to be in agreement for exposures ranging from picoseconds to seconds. However, the very limited data reported in the sub-nanosecond domain of time were obtained with a low pulse ratio frequency. No data exists in the litterature concerning the effect on the retina of ultrashort pulses delivered with a very high repetition rate. Also, it has been hypothesized that one cannot extrapolate from longer pulses width because the ultrashort pulse (single or multiple) induce effects which may involve fundamentally different mechanisms of damage. Thus, some laser safety guidelines do not specifically provide exposure limits for a single-pulse duration less than 1 ns because of a lack of biological data1. If exposure limits are given, the limit values for these exposure times has been derived by maintening the irradiances applicable to nanosecond pulses2. With the goal in mind to obtain adequate data base for single and multiple ultrashort pulses, an experimental study has been directed toward determining the retinal damage thresholds induced by picosecond pulses emited in the visible spectrum.Most laser safety documents advise the laser user that caution must be used in the evaluation of exposure to repetitively pulsed radiation since they are only limited data on multiple pulse exposure criteria. The empirical multiple pulse formula is based on some data indicating that there is generally a cumulative effect in multiple-pulse exposures. This effect is a reduction in the threshold energy per pulse relative to the single pulse threshold. The best fit to experimental data predicts that the threshold energy per pulse decreases in proportion to the fourth root of the number of pulses n. This model has been reported to be in agreement for exposures ranging from picoseconds to seconds. However, the very limited data reported in the sub-nanosecond domain of time were obtained with a low pulse ratio frequency. No data exists in the litterature concerning the effect on the retina of ultrashort pulses delivered with a very high repetition rate. Also, it has been hypothesized that one cannot extrapolate ...

With reference to the concept that contraction of a muscle can be inhibited by contraction of its antagonists, the excitability of a bifunctional muscle, the soleus (plantarflexor and adductor) was studied during voluntary isometric contractions of the abductors in man. For this purpose, the soleus monosynaptic stretch reflex (MSR) was solicited by means of quick-release dorsiflexion movements which also induce inactivation of the tibialis anterior (unloading reflex). When different initial torques of dorsiflexion were developed (reference conditions) the amplitude of the soleus MSR was found to be related linearly to the maximal value of angular acceleration. When an abduction torque was superimposed (combined efforts), the soleus response was always depressed, but no significant reinforcement of this inhibition was observed when the abduction torque increased. From present results, and from classical data concerning reciprocal inhibition between soleus and tibialis anterior, it is concluded that the MSR of a bifunctional muscle is depressed when the antagonists of either one of its functions are voluntarily activated.

The activation of antagonistic muscles is studied during isometric combined contractions of bifunctional muscles. It is established that the co-contraction mechanism observed in such situations limits unwanted movements. Furthermore, when a co-contraction appears between two biantagonistic muscles, the phenomenon underlies a conflicting situation and, consequently, the muscular patterns encountered are in most cases the reverse of those classically described. In these circumstances the inhibition usually present between antagonistic muscles could be masked, due to convergence on the motoneurones of both inhibitory and excitatory discharges coming from homonymous and antagonistic muscles of a given bifunctional muscle.