Role of the trochlear nerve in eye abduction and frontal vision of the red-eared slider turtle (Trachemys scripta elegans).

Horizontal head rotation evokes significant responses from trochlear motoneurons of turtle that suggests they have a functional role in abduction of the eyes like that in frontal-eyed mammals. The finding is unexpected given that the turtle is generally considered lateral-eyed and assumed to have eye movements instead like that of lateral-eyed mammals, in which innervation of the superior oblique muscle by the trochlear nerve (nIV) produces intorsion, elevation, and adduction (not abduction). Using an isolated turtle head preparation with the brain removed, glass suction electrodes were used to stimulate nIV with trains of current pulses. Eyes were monitored via an infrared camera with the head placed in a gimble to quantify eye rotations and their directions. Stimulations of nIV evoked intorsion, elevation, and abduction. Dissection of the superior oblique muscle identified lines of action and a location of insertion on the eye, which supported kinematics evoked by nIV stimulation. Eye positions in alert behaving turtles with their head extended were compared with that when their heads were retracted in the carapace. When the head was retracted, there was a reduction in interpupillary distance and an increase in binocular overlap. Occlusion of peripheral fields by the carapace forces the turtle to a more frontal-eyed state, perhaps the reason for the action of abduction by the superior oblique muscle. These findings support why trochlear motoneurons in turtle respond in the same way as abducens motoneurons to horizontal rotations, an unusual characteristic of vestibulo-ocular physiology in comparison with other mammalian lateral-eyed species.

Parasympathetic control of the pupillary light response in the red-eared slider turtle (Pseudemys scripta elegans).

OBJECTIVE: We investigated effects of both vecuronium bromide, a nicotinic cholinergic antagonist, and atropine, a muscarinic cholinergic antagonist, on the pupil of the turtle to determine whether responses to light are controlled by parasympathetic innervations acting on the iris. ANIMAL STUDIED: Three red-eared slider turtles, Pseudemys scripta elegans. PROCEDURE: Turtles were secured to immobilize their head movements and then inserted into a light-integrating sphere. Both pupils were monitored through small apertures by digital video cameras. Pupil diameters were measured manually with a digital caliper. During each trial, drugs (0.4%) were topically applied, four times at 15 min intervals, to the corneas of each eye. One eye was randomly selected for treatment of the drug while the other, treated with saline (0.9% NaCl), was used as control. Pupil sizes under adaptation to light were tracked after drug or saline applications. RESULTS: Mean pupillary diameters of eyes treated with vercuronium bromide increased by 28%, reaching peak size in 90 min. Onset of response occurred 20 min after drug application and then increased at a rate having a time constant of 26 min. Recovery began at 120 min after initial application. Atropine had no effect on pupil size. No systemic side effects by drugs were observed in turtles. CONCLUSIONS: Although atropine does not cause mydriasis, vecuronium bromide does. These results suggest that the parasympathetic system in turtles acts through acetylcholine onto nicotinic receptors to stimulate pupillary light constriction.

Balanced interactions in ganglion-cell receptive fields.

Receptive fields of retinal ganglion cells in turtle have excitatory and inhibitory components that are balanced along the dimensions of wavelength, functional ON and OFF responses, and spatial assignments of center and surround. These components were analyzed by spectral light adaptations and by the glutamate agonist, 2-amino-4-phosphonobutyric acid (APB). Extracellular recordings to stationary and moving spots of light were used to map changes in receptive fields. ON spike counts minus OFF spike counts, derived from flashed stationary light spots, quantified functional shifts by calculating normalized mean response modulations. The data show that receptive fields are not static, but rather are dynamic arrangements which depend on linked, antagonistic balances among the three dimensions of wavelength, ON and OFF response functions, and center/surround areas.

The pupillary response to light in the turtle.

When intense adapting lights are turned off, the pupil of the turtle, Pseudemys scripta elegans, enlarges. The recovery functions for pupillary dilation have different time constants that are defined by red- and green-sensitive cones and rods as they are affected by prior light adaptation and time in the dark. Pupillary area related to dilation responds over at least a three- to four-fold range. Following white-light adaptation, the course of pupil dilation in the dark shows a three-legged curve of differing time constants. With spectral-light adaptations, the contributions of separate classes of photoreceptors can be isolated. Red- and green-sensitive cones contribute shorter time constants of 3.31 and 3.65 min to prior white-light adaptation--4.81 and 4.18 min to prior spectral-light adaptations. Rods contribute a much longer time constant of 6.69 min to prior white-light adaptation--7.60 min to prior spectral-light adaptation. The ratios are in keeping with the flash sensitivities of photoreceptors in this same animal, as well as with psychophysical visual threshold mechanisms of color sensitivity.