Abstract: The equine eye was treated as a general lens system and calculations were done to determine image position in relation to the retina for objects at a distance of infinity, 100 m, and 1 m. The retina is 19.1 mm behind the posterior surface of the lens; therefore, the image appears 14.6 mm posterior to the retina at infinity and at 100 m, and 16.3 mm at 1-m distance on a horizontal axis. The animals studied were hyperopic. It is evident that the horse must move its head or eye, or both, for optimal visual acuity. At the same time, some objects in the total field of vision are imperceptible or indistinct.

Abstract: OBJECTIVE: To investigate whether training for show jumping that is commenced early after birth affects the characteristics of equine locomotory muscle. ANIMALS: 19 Dutch Warmblood horses. PROCEDURES: Horses were assigned to a trained or not trained (control) group. After weaning, training (free jumping [2 d/wk] that was alternated with a 20-minute period of exercise in a mechanical rotating walker [3 d/wk]) was started and continued until horses were 3 years old. Fiber type composition (determined from myosin heavy chain [MyHC] content), fiber area, diffusion index (area supplied by 1 capillary), citrate synthase activity, and Na(+),K(+)-ATPase content were assessed in gluteus medius muscle specimens collected at 0.5, 1, 2, and 3 years. RESULTS: Developmental changes included an increase in MyHC fiber type IIa and a decrease in type IIad; increases in fiber area, diffusion index, and citrate synthase activity; and a decrease in Na(+),K(+)-ATPase content. The MyHC fiber type I and type IId were detected in high and low proportions, respectively. Training increased Na(+),K(+)-ATPase content, but did not affect other variables. CONCLUSIONS AND CLINICAL RELEVANCE: In horses, show jumping training at an early age resulted in increased Na(+),K(+)-ATPase content of the deep portions of the gluteus medius muscle. The lack of training effects on the other muscle characteristics can partly be explained by the fact that an appropriate (aerobic) fiber type composition was already established at training commencement. These data also suggested that the developmental changes in equine muscle represent sufficient adaptation to meet the demands of this specific training.

Abstract: OBJECTIVE: To determine effects of microcurrent electrical tissue stimulation (METS) on equine tenocytes cultured from the superficial digital flexor tendon (SDFT). SAMPLE POPULATION: SDFTs were collected from 20 horses at slaughter. PROCEDURE: Tenocytes were isolated following outgrowth from explants and grown in 48-well plates. Four methods of delivering current to the tenocytes with a METS device were tested. Once the optimal method was selected, current consisting of 0 (negative control), 0.05, 0.1, 0.5, 1.0, or 1.5 mA was applied to cells (8 wells/current intensity) once daily for 8 minutes. Cells were treated for 1, 2, or 3 days. Cell proliferation, DNA content, protein content, and apoptosis rate were determined. RESULTS: Application of microcurrent of moderate intensity increased cell proliferation and DNA content, with greater increases with multiple versus single application. Application of microcurrent of moderate intensity once or twice increased protein content, but application 3 times decreased protein content. Application of current a single time did not significantly alter apoptosis rate; however, application twice or 3 times resulted in significant increases in apoptosis rate, and there were significant linear (second order) correlations between current intensity and apoptosis rate when current was applied twice or 3 times. CONCLUSIONS AND CLINICAL RELEVANCE: Results of the present study indicate that microcurrent affects the behavior of equine tenocytes in culture, but that effects may be negative or positive depending on current intensity and number of applications. Therefore, results are far from conclusive with respect to the suitability of using METS to promote tendon healing in horses.

Abstract: OBJECTIVE: To compare nociceptive withdrawal reflexes (NWRs) evoked from the distal aspect of the left forelimb and hind limb in conscious standing horses and to investigate NWR recruitment for graded electrical stimulation intensities. ANIMALS: 20 adult horses. PROCEDURE: Surface electromyographic (EMG) activity evoked by transcutaneous electrical stimulation of the digital palmar (or plantar) nerve was recorded from the common digital extensor and cranial tibial muscles. Stimuli consisted of 25-millisecond train-of-5 constant current pulses. Current intensity was gradually increased until NWR threshold intensity was reached. The EMG signal was analyzed for quantification of the NWR. Behavioral responses accompanying the reflex were scored (scale, 0 to 5). The NWR recruitment curves were determined at 0.9, 1.1, 1.2, and 1.3 times the NWR threshold intensity. RESULTS: The NWR threshold was significantly higher for the hind limb (median value, 6.6 mA; range, 3 to 10 mA) than the forelimb (median, 3 mA; range, 1.7 to 5.5 mA). The NWR of the hind limb had a significantly longer latency (median, 122.8 milliseconds; range, 106 to 172 milliseconds), compared with the forelimb (median, 98 milliseconds; range, 86 to 137 milliseconds), and it was associated with significantly stronger behavioral reactions. Gradual increase of NWR amplitude was evident at increasing stimulation intensities and supported by the behavioral observations. CONCLUSIONS AND CLINICAL RELEVANCE: We documented NWRs evoked from the forelimb and hind limb and their recruitment with stimuli of increasing intensity in horses. These results provide a basis for use of NWRs in studies on nociceptive modulation in horses.