Visual adaptations in a diurnal rodent, Octodon degus.

The degu (Octodon degus) is a diurnal rodent, native to Chile. Basic features of vision and visual organization in this species were examined in a series of anatomical, electrophysiological and behavioral experiments. The lens of the degu eye selectively absorbs short-wavelength light and shows a progressive increase in optical density as a function of age. Electroretinograms recorded using a flicker-photometric procedure reveal three spectral mechanisms: a rod with peak sensitivity of about 500 nm and two types of cone having respective spectral peaks of about 362 nm and 507 nm. Opsin antibody labeling was used to determine the retinal distributions of the three receptor types. A total of about one-third of the approximately 9 million photoreceptors of the degu retina are cones with the two types (507 nm/362 nm) represented in a ratio of about 13:1. The contributions to vision of all three receptor types were examined in a series of behavioral experiments. A consistent feature of both the electrophysiological and behavioral results is that relatively high levels of light adaptation are required to effect the full transition from rod-based to cone-based vision. In behavioral tests degus were shown to be able to make color discriminations between ultraviolet and visible lights.

Cone-based vision of rats for ultraviolet and visible lights.

Rats (Rattus norvegicus) have two classes of cone, one containing an ultraviolet (UV)-sensitive photopigment and the other housing a pigment maximally sensitive in the middle (M) wavelengths of the visible spectrum. The manner in which signals from these two cone types contribute to rat vision was investigated through recordings of a gross electrical potential (the electroretinogram, ERG) and behavioral discrimination tests. Spectral sensitivity functions obtained from both types of measurement indicate clear contributions from each of the cone classes, but there is a marked enhancement of the relative sensitivity to UV light in the behavioral index; for instance, under some photopic test conditions, rats are approximately equally sensitive to middle-wavelength and UV lights. In adaptation tests, thresholds for UV and M lights were found to be differentially elevated in the presence of chromatic adapting backgrounds, thus providing the possibility that signals from the two cones could be used by the rat visual system to support color discriminations. Evidence of dichromatic color vision in the rat was subsequently obtained from tests of wavelength discrimination.

The biomechanical interdependence between the anterior cruciate ligament replacement graft and the medial meniscus.

To establish a quantitative biomechanical relationship between the anterior cruciate ligament graft and the medial meniscus, 10 human cadaveric knees were examined using the robotic/universal force-moment sensor testing system. In response to a combined 134-N anterior and 200-N axial compressive tibial load, the resulting kinematics of the knee and the in situ forces in the anterior cruciate ligament, the anterior cruciate ligament graft, and the medial meniscus were measured. Anterior tibial translation significantly increased after anterior cruciate ligament transection, between 6.8 +/- 2.3 mm at full extension and 12.6 +/- 3.3 mm at 30 degrees of flexion. Consequently, the resultant forces on the medial meniscus, ranging from 52 +/- 30 N to 63 +/- 51 N between full extension and 90 degrees of knee flexion in the intact knee, were doubled as a result of anterior cruciate ligament deficiency. However, after anterior cruciate ligament reconstruction, anterior tibial translations were restored to the levels of the intact knee, and thus the forces on the medial meniscus were restored as well. Likewise, the in situ forces in the anterior cruciate ligament replacement graft increased between 33% and 50% after medial meniscectomy.

Injury and repair of ligaments and tendons.

In this chapter, biomechanical methods used to analyze healing and repair of ligaments and tendons are initially described such that the tensile properties of these soft tissues as well as their contribution to joint motion can be determined. The focus then turns to the important mechanical and biological factors that improve the healing process of ligaments. The biomechanics of surgical reconstruction of the anterior cruciate ligament and the key surgical parameters that affect the performance of the replacement grafts are subsequently reviewed. Finally, injury mechanisms and the biomechanical analysis of various treatment techniques for various types of tendon injuries are described.

Tissue engineering of ligament and tendon healing.

Ligaments and tendons are bands of dense connective tissue that mediate normal joint movement and stability. Injury to these structures may result in significant joint dysfunction because they either heal by production of inferior matrix or do not heal at all. The process of ligament and tendon healing is complex and the roles of cellular and biochemical mediators continue to be elucidated. The expression of growth factors and growth factor receptors is modulated after injury, and cells from healing tissues are responsive to growth factors. Tissue engineering offers the potential to improve the quality of ligaments and tendons during the healing process. The concept is based on the manipulation of cellular and biochemical mediators to affect protein synthesis and improve tissue remodeling. Recently, novel techniques such as application of growth factors, gene transfer techniques, and cell therapy have shown promise and may become effective biologic therapies in the future. Many groups have been successful in introducing marker and therapeutic genes into ligaments and tendons. Cell therapy involves the introduction of mesenchymal progenitor cells as a pluripotent cell source into the healing environment. The combination of cell therapy with growth factor application via gene transfer offers new avenues to improve ligament and tendon healing.

Photopigments of dogs and foxes and their implications for canid vision.

Electroretinogram (ERG) flicker photometry was used to examine the photopigment complements of representatives of four genera of Canid: domestic dog (Canis familiaris), Island gray fox (Urocyon littoralis), red fox (Vulpes vulpes), and Arctic fox (Alopex lagopus). These four genera share a common cone pigment complement; each has one cone pigment with peak sensitivity of about 555 nm and a second cone pigment with peak at 430-435 nm. These pigment measurements accord well with the conclusions of an earlier investigation of color vision in the dog, and this fact allows some predictions about color vision in the wild canids. An additional set of measurements place the peak of the dog rod pigment at about 508 nm.

Attempts to suppress tinnitus with transcutaneous electrical stimulation.

Various electrical stimuli were tested for their ability to suppress or relieve severe tinnitus. Stimulation was applied transdermally by electrodes placed on the preauricular and postauricular regions and on the two mastoids. Of the 50 patients tested, only 14 (28%) obtained relief that met the criterion of a reduction in the tinnitus by 40% or more. When relief was obtained, it usually extended for several hours into the poststimulation period. There was only one positive response (2%) in the placebo trial, which was administered to all patients. It was concluded that transdermal electrical stimulation such as that used in this research is not a practical therapeutic procedure for the relief of tinnitus.

Reduction of acoustically-induced auditory impairment by inhalation of carbogen gas. I. Permanent noise-induced cochlear damage.

The possible beneficial effects of carbogen gas (95% O2/5% CO2) inhalation on noise-induced permanent cochlear impairment were investigated. Groups of guinea pigs were exposed to daily 3 1/2-hour presentations of 120 dB of broad-band noise in an atmosphere of either carbogen gas or normal air. Other groups received carbogen or normal air with a low intensity (50 dB) acoustic exposure. After a 30-day stabilization period each ear was examined electrophysiologically and histologically. Measures included a.c. cochlear potentials from 100 Hz through 20 kHz as well as counts of missing inner hair cells (IHCs) and outer hair cells (OHCs). Statistically significant improvement in the 1 microV isopotential function of the a.c. cochlear potential over all test frequencies and significant reduction in the number of missing IHCs and OHCs was found to result from inhalation of carbogen gas. Blood gas analysis demonstrated a rapid and marked elevation in arterial PCO2 and PO2 with a corresponding drop in pH to result from inhalation of carbogen. The vasodilating effects of increased arterial carbon dioxide tension are discussed in terms of current speculations regarding the role of vasoconstriction in noise-induced hearing loss.

A reaction-time analysis of electrocochlear stimulation in cats.

The effects of pulse width and interstimulus interval of electrocochlear stimulation on behavioral reaction times were investigated in cats. It was determined that square wave biphasic stimuli with pulse widths of 0.5 msec. elicited faster reaction times than did pulse widths of 0.2 msec. On the other hand, stimuli of 1.0 msec pulse widths showed no appreciable difference in reaction times when compared to 0.5 msec pulse widths. The effects of interstimulus interval (ISI) on reaction times are also discussed. An interstimulus interval of 2 msec demonstrated no marked difference in reaction time when compared to the same stimulus with an ISI of 4 msec.