The authors thank Mrs. Paulette McKenna and Lisa Pezzino in this study for secretarial support, and Mr. Phil Auerbach for technical support. The authors also thank Drs. Michael Ariel and Michael S. Jones (Saint Louis University School of Medicine) for introducing us to the in vitro isolated head preparation. Funding for support of this collaboration was provided by the Department of Biology (Robert S. Chase Fund), the Academic Research Committee, and the Neuroscience Program at Lafayette College. Lastly, this work is dedicated to Mr. Phil Auerbach, who passed away 28 September 2016; he decommissioned a scanning electron microscope and recognized the usefulness of its 5-axis stage for use in this protocol. His friendship and resourcefulness will be missed greatly.

NOTE: Red-eared slider turtles, both male and female, were purchased from a vendor. Turtles were housed in a warm animal suite containing two 60-gallon tubs equipped with brick islands for sunning under 250-W infrared lights. The environment was maintained on a 14/10-h light/dark cycle with the water temperature at 22 &#176;C. Lights were turned on at 6:00 am and turned off at 8:00 pm. The tanks equipped with filtering systems were cleaned weekly, and turtles were fed ad libitum every other day. The care of red-...

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Critical Steps:
The critical steps within this protocol are the following: 1) the dissection and the care taken to maintain the viability of the transected nerves; 2) the matching of sizes by the suction electrodes to cranial nerves to provide consistent responses; and 3) the placement of the head in the gimbal to provide adequate calibration of the rotations of the eye.
Troubleshooting:
The dissection can be challenging, but after completing i...

Rationale for Using Red-eared Slider Turtles in Electrophysiological Experiments:
Red-eared slider turtles (Trachemys scripta elegans), are considered one of world&#39;s worst invasive species1 and can indicate that an ecosystem is in trouble. The reason why red-eared slider turtles are so successful is poorly understood but it may in part be due to their tolerant physiology and possession of nervous tissues that can survive under hypoxic conditions2,3,4. Using them for experimentation does not threaten their numbers and with minimal efforts, electrophysiological preparations can remain viable over extended durations, as long as 18 hours5,6. The benefit is similar to the advantage of using invertebrate animals such as crayfish7, which also have the ability to withstand low levels of oxygen8.
Techniques for Measuring Eye Movements:
Approaches to measure eye movements in frontal-eyed animals using non-human primates have been well developed9. The eye rotates in the orbit around three axes: horizontal, vertical, and torsional. The magnetic search coil method is generally considered the most reliable for measuring rotations, but is invasive, requiring small coils to be inserted into the scleras of animals10,11. Video-based systems also can measure rotations and have the advantage of being non-invasive. The development of better cameras along with innovative image processing have enhanced their functionality making video-based systems an attractive alternative to consider12,13,14.
The techniques developed for measuring eye movements in nonmammals have been much less significant. Measures are either low resolution or describe only some of the rotations15,16,17,18. The lack of development can be partially blamed on the difficulty in training nonmammals to follow visual targets. Although eye movements have been well studied in red-eared slider turtles19,20,21,22,23,24,25,26,27,28,29,30, because of the challenge in training animals to track targets, the precise kinematics of their eye movements is poorly understood.
Red-eared slider turtles are generally considered lateral-eyed vertebrates, but because they can fully retract their heads into their shell31, significant occlusion of the lateral visual fields by the carapace occurs32. The result is that their visual line of sight is forced toward the front, making them behave more like frontal-eyed mammals. Therefore, their use as a model for developing approaches for measuring eye movements also offers a unique evolutionary perspective.
The protocol described in this work uses an in vitro isolated head preparation to identify the kinematics of the eye movements in red-eared slider turtles. Brains are dissected from the skulls leaving the cranial nerves intact. Heads are placed into a gimbal to calibrate eye movements and evoke responses by electrical stimulation of the cranial nerves innervating the eye muscles. Measures of rotations by the eyes are done by a video-based system, using software algorithms, which track the dark pupil and the markings of the iris. The preparation provides the opportunity to measure kinematics of both extraocular (i.e., horizontal, vertical, and torsional rotations)32 and intraocular (i.e., pupil changes)33 movements.
Model System for Analysis of Efferent Neural Pathways:
More generally, the approach provides investigators the chance to study how efferent neural signals generate eye movements when muscles start from their relaxed states and in the absence of integrated sensory information processed by the brain32,33. Therefore, the eye kinematics can be examined in a model system in which they are solely processed by the efferent neural pathway leaving the brain and synapsing onto the muscles.

<table>
	<tbody>
		<tr>
			<td>Red-eared slider turtles</td>
			<td>Kons Scientific</td>
			<td>Trachemys scripta elegans</td>
			<td>Large size (carapace length 15-20 cm)</td>
		</tr>
		<tr>
			<td>Sodium chloride</td>
			<td>Sigma-Aldrich Co. LLC.</td>
			<td>S5886</td>
			<td></td>
		</tr>
		<tr>
			<td>Potassium chloride</td>
			<td>Sigma-Aldrich Co. LLC.</td>
			<td>P5405</td>
			<td></td>
		</tr>
		<tr>
			<td>Magnesium choride</td>
			<td>Sigma-Aldrich Co. LLC.</td>
			<td>M7304</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium bicarbonate</td>
			<td>Sigma-Aldrich Co. LLC.</td>
			<td>S5761</td>
			<td></td>
		</tr>
		<tr>
			<td>Dextrose</td>
			<td>Sigma-Aldrich Co. LLC.</td>
			<td>C5767</td>
			<td></td>
		</tr>
		<tr>
			<td>Concentrated hydrochloric acid</td>
			<td>Sigma-Aldrich Co. LLC.</td>
			<td>H7020</td>
			<td></td>
		</tr>
		<tr>
			<td>Calcium chloride</td>
			<td>Sigma-Aldrich Co. LLC.</td>
			<td>C7902</td>
			<td></td>
		</tr>
		<tr>
			<td>pH meter</td>
			<td>Oakton</td>
			<td>pH 6+</td>
			<td></td>
		</tr>
		<tr>
			<td>Suction stimulation electrode</td>
			<td>A-M Systems</td>
			<td>573000</td>
			<td>Bipolar suction electrode. Note that 573000 has been replaced with 573050.</td>
		</tr>
		<tr>
			<td>Capillary glass</td>
			<td>A-M systems</td>
			<td>626000</td>
			<td>Single-barrel borosilicate capillary glass without microfilament, length 10 cm, outside diameter 1.0 mm, inner diameter 0.50 mm</td>
		</tr>
		<tr>
			<td>Alternative suction stimulation electrode</td>
			<td>A-M Systems</td>
			<td>573050</td>
			<td>Bipolar suction electrode. Requires larger diameter capillary glass: 627000, outside diameter 1.2 mm, inner diameter 0.68 mm</td>
		</tr>
		<tr>
			<td>Stereoscope</td>
			<td>Lieca</td>
			<td>GZ7</td>
			<td>Magnification range, 10x &#8211; 70x</td>
		</tr>
		<tr>
			<td>Fiber optic light source</td>
			<td>Amscope</td>
			<td>HL250-A</td>
			<td>150W Fiber optical microscope illuminator light box</td>
		</tr>
		<tr>
			<td>Rongeurs</td>
			<td>Carolina Biological Supply Company</td>
			<td>625654</td>
			<td>stainless steel, straight spring, 5.25&#34;</td>
		</tr>
		<tr>
			<td>Blunt dissection probe</td>
			<td>Carolina Biological Supply Company</td>
			<td>627405</td>
			<td>Huber mall probe, double-ended probe and seeker, 6&#34;</td>
		</tr>
		<tr>
			<td>Microscissors</td>
			<td>Carolina Biological Supply Company</td>
			<td>623555</td>
			<td>Iris microdissecting scissors, stainless steel, 0.5&#34; blades, 4.75&#34; long</td>
		</tr>
		<tr>
			<td>Fine forceps</td>
			<td>Sigma-Aldrich Co. LLC.</td>
			<td>F6521</td>
			<td>Jewelers forceps, dumont No. 5, inox alloy, 4.25&#34;</td>
		</tr>
		<tr>
			<td>Curved forceps</td>
			<td>Sigma-Aldrich Co. LLC.</td>
			<td>Z168696</td>
			<td>Medium tip, curved forceps, stainless steel, 4&#34;</td>
		</tr>
		<tr>
			<td>Scalpel handle</td>
			<td>Sigma-Aldrich Co. LLC.</td>
			<td>S2896</td>
			<td>Scalpel handles, No. 3, stainless steel</td>
		</tr>
		<tr>
			<td>Scalpel blade</td>
			<td>Sigma-Aldrich Co. LLC.</td>
			<td>S2771</td>
			<td>Scalpel blades, No. 11, steel</td>
		</tr>
		<tr>
			<td>Guillotine</td>
			<td>Harvard Apparatus</td>
			<td>73-1918</td>
			<td>Kleine guillotine type 7575</td>
		</tr>
		<tr>
			<td>Spatula</td>
			<td>Sigma</td>
			<td>Z648299</td>
			<td>Micro spoon and spatula weighing set. Use small spatula: 5.9&#8221; long x 0.07&#8221; diameter handle with square end: 0.17&#8221; x 1.3&#8221; long, other end round: 0.17&#8221; x 1.27&#8221; long</td>
		</tr>
		<tr>
			<td>Hook</td>
			<td>Autozone</td>
			<td>98069</td>
			<td>SureBilt hook and pick set. Use grinder to dull sharp points of hook to prevent injury to animals mouth.</td>
		</tr>
		<tr>
			<td>95/5% O2/CO2</td>
			<td>Airgas, Inc.</td>
			<td>X02OX95C2003102</td>
			<td>5% Carbon dioxide balance oxygen certified standard gas mixture, size 200 Cylinder, CGA-296</td>
		</tr>
		<tr>
			<td>Regulator</td>
			<td>Airgas, Inc.</td>
			<td>Y11244D296-AG</td>
			<td>Single stage brass 0-100 psi analytical cylinder regulator CGA-296 with needle outlet. Use brass adjustable airline pipe valve to go from 3/8&#34;, inner diameter, vinyl airline tubing connected to regulator to a 3/16&#34;, inner diameter, airline connection going to airstone or glass pasteur pipette.</td>
		</tr>
		<tr>
			<td>Adjustable airline pipe valve</td>
			<td>Doctors Foster and Smith</td>
			<td>CD-12061</td>
			<td>Brass valve</td>
		</tr>
		<tr>
			<td>Rigid table</td>
			<td>Unknown</td>
			<td>Unknown</td>
			<td>Auto-clave door laid on top of a sturdy table. Nine 5&#34; diameter tennis balls isolate vibrations from the top surface of the table.</td>
		</tr>
		<tr>
			<td>5&#34; tennis ball</td>
			<td>Petco Animal Supplies, Inc.</td>
			<td>712868</td>
			<td>Petco Jumbo Pet Tennis Ball: balls are unsliced and held within an integrated frame on the underside part of the autoclave door.</td>
		</tr>
		<tr>
			<td>Alternative vibration isolation table</td>
			<td>Newport Corporation</td>
			<td>INT1-36-6-N</td>
			<td>Rigid vibration control system, integrity 1: Surface dimensions, 3&#39; x 6&#39;</td>
		</tr>
		<tr>
			<td>Gimbal</td>
			<td>ISI, International Scientific Instruments, Inc.</td>
			<td>Stage from SUPER III-A Scanning EM</td>
			<td>5-axis eucentric stage: X, Y, and Z linear movements, &#177;20 mm, 0.1 mm precision; Rotations, vertical, &#177;10&#176;, and horizontal, &#177;12.5&#176;, with 1.25&#176; precision. Note: from decommission instrument.</td>
		</tr>
		<tr>
			<td>Chuck for gimbal</td>
			<td>Unknown</td>
			<td>Unknown</td>
			<td>Chuck from an old microtome of unknown manufacture was machined to fit the shaft of the specimen holder of the Scanning EM stage</td>
		</tr>
		<tr>
			<td>Alternative gimbal</td>
			<td>ThorLabs, Inc.</td>
			<td>GN2/M with MBT602/M</td>
			<td>Dual-axis goniometer (GN2/M) mounted on 3-axis microblock stage with thumbscrew adjusters (MBT602/M): design a chuck to hold turtle head with eye at 12.7 mm above top surface of goniometer (distance to point of rotation)</td>
		</tr>
		<tr>
			<td>Video-based eye tracking system</td>
			<td>Arrington Research, Inc.</td>
			<td>ViewPoint EyeTracker, PC-60</td>
			<td>Tracking method: Infrared video by dark pupil; Black and white camera (Item BC02): 30 Hz, 640 x 480; System requirements: Windows 2000, XP, 7, 8, 8.1, 10; Visual range: Horizontal +/- 44&#176;; vertical +/- 20&#176;; Accuracy ~0.5&#176;; Spatial resolution ~0.15&#176;; Pupil size resolution ~0.03 mm; Eye data: X, Y position of gaze, pupil height and width, torsion, delta time, total time, and regions of interest (ROI); Real-time communication (Item 0022): 4-Channel AnalogOut with eight TTL input channels to mark codes into the data file</td>
		</tr>
		<tr>
			<td>Multi-position magnetic base</td>
			<td>Harbor Freight Tools</td>
			<td>Pittsburg, item #5645</td>
			<td>Magnetic holder reaches up to 12&#34; and produces 45 lbs. of magnetic pull. Use to position camera. Machine thread holes onto the end of the rod to mount cameras.</td>
		</tr>
		<tr>
			<td>Micromanipulator</td>
			<td>Kopf</td>
			<td>900</td>
			<td>5 axis manipulation for mount of suction electrode: X, Y, Z linear travel, 2 axis of rotation</td>
		</tr>
		<tr>
			<td>Dissection scope on boom</td>
			<td>Lieca</td>
			<td>GZ6</td>
			<td>Magnification range, 6.7x &#8211; 40x</td>
		</tr>
		<tr>
			<td>Nerve/muscle stimulator</td>
			<td>Astro-Med Grass Telefactor</td>
			<td>Grass S88</td>
			<td>Dual pulse voltage stimulator: two output channels that can be operated independently or synchronized to generate non-isolated constant voltage pulses (10 mv to 150 V). Pulses can be single (10 &#956;sec to 10 sec), repetitive (0.01 Hz to 1 KHz), and trains (1 ms to 10 s) and synchronized with TTL inputs and output. Send TTL outputs via the output channels of a DB25 connector to the TTL input channels of the ViewPoint EyeTracker. Note: Astro-Med Grass Telefactor is no longer in business.</td>
		</tr>
		<tr>
			<td>Current isolation device</td>
			<td>Astro-Med Grass Telefactor</td>
			<td>PSIU6</td>
			<td>Current stimulus isolation unit: enables safe delivery of constant currents by the S88 to the preparation. The PSIU6 connects by a BNC cable to one of the output channels of the S88. Multiplier switches on the PSIU6 allow the S88 to generate a wide array of current amplitudes ranging from 0.1 &#181;A to 15 mA.</td>
		</tr>
		<tr>
			<td>Alternative nerve/muscle stimulator with isolation</td>
			<td>A-M Systems</td>
			<td>2100</td>
			<td>Isolated Pulse Stimulator: Unit has built-in isolator to produce constant currents.</td>
		</tr>
	</tbody>
</table>

ocular kinematics measured by in vitro stimulation of the cranial nerves in the turtle

After animals are euthanized, their tissues begin to die. Turtles offer an advantage because of a longer survival time of their tissues, especially when compared to warm-blooded vertebrates. Because of this, in vitro experiments in turtles can be performed for extended periods of time to investigate the neural signals and control of their target actions. Using an isolated head preparation, we measured the kinematics of eye movements in turtles, and their modulation by electrical signals carried by cranial nerves. After the brain was removed from the skull, leaving the cranial nerves intact, the dissected head was placed in a gimbal to calibrate eye movements. Glass electrodes were attached to cranial nerves (oculomotor, trochlear, and abducens) and stimulated with currents to evoke eye movements. We monitored eye movements with an infrared video tracking system and quantified rotations of the eyes. Current pulses with a range of amplitudes, frequencies, and train durations were used to observe effects on responses. Because the preparation is separated from the brain, the efferent pathway going to muscle targets can be examined in isolation to investigate neural signaling in the absence of centrally processed sensory information.

Figure 1 shows stills of images taken from a video describing the dissection. Images provide typical locations of the nerves prior to cutting from the brain.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/56864/56864fig1.jpg" /> 
Figure 1: Stills of images captured from the video of the dissection to show locations of the optic nerve (nII), oculomotor nerve (nIII), trochlear...

Watch this Scientific Journal Video about Ocular Kinematics Measured by In Vitro Stimulation of the Cranial Nerves in the Turtle at JoVE.com

Ocular Kinematics Measured by In Vitro Stimulation of the Cranial Nerves in the Turtle

This protocol describes how to use an in vitro isolated turtle head preparation to measure the kinematics of their eye movements. After removal of the brain from the cranium, cranial nerves can be stimulated with currents to quantify rotations of the eye and changes in pupil sizes.

Ocular Kinematics Measured by In Vitro Stimulation of the Cranial Nerves in the Turtle

After animals are euthanized, their tissues begin to die. Turtles offer an advantage because of a longer survival time of their tissues, especially when ...

The overall goal of this electrophysiological protocol is tho provide the ability to measure the kinematics of eye movements of the red-eared slider turtle using an in vitro isolated head preparation. This method can help answer key questions in the ocular-motor field, such as how vision evolved in turtles. The main advantage of this technique is that it does not require training animals to track targets and uses a gimbal to calibrate their eye movements.Demonstrating this technique is Steven Nesbit, an undergraduate student in my laboratory. Begin with the turtle head in a dissection dish. Have enough turtle ringer solution on hand to irrigate the tissue.Maintain the tissue at four degrees Celsius, by placing ice around the outside of the dish. Place a blunt dissection probe through the mouth to provide easier handling of the head. Then, while working under a dissection microscope, cut the joint connecting the dentary bone to the cranium with the scalpel.Next, use rongeurs to pull the lower jaw away from the cranium. Then, pull off the skin and muscles from their attachments at the dorsal and lateral regions of the cranium. After identifying the vertebral column at the cavum end of the cranium, bend the vertebral column ventrally to expose the spinal cord, and use micro scissors to the snip the spinal cord.Use rongeurs to remove the vertebral column and other tissue from the cranium by pulling caudally. Then, make two superficial cuts on the dorsal cranium starting at the foramen magnum and carefully pull off the dorsal cranium. Use micro scissors to remove the meninges and expose the rest of the brain.Remove enough meninges to allow identification of the olfactory bulbs in the anterior cranial cavity. Continue to irrigate the brain with turtle ringer solution as necessary. Next, used curved forceps to gently pull the cerebrum caudally and produce slight tension on the cranial nerves.Carefully cut away and remove the olfactory bulbs and cerebrum with curved forceps. Then use micro scissors to gentle push the midbrain toward the midline and expose the cranial nerves. Cranial nerve three can be seen in front of cranial nerve four.The diameter of nerve four is slightly less than nerve three. Then cut the left and right ride of the optic nerve, or cranial nerve two. Cut nerves three and four where they attach to the midbrain.Repeat this on the other side. Tilt the brainstem to one side and observe cranial nerve six, the abducens nerve, emerging from the ventral surface near the junction of the pons and medulla. Cut cranial nerve six on both the left and right side.Remove the remaining parts of the brainstem from the head with fine forceps and micro scissors. Once the cranium is empty, examine the cranial cavity floor and identify nerves three, four, and six. Lastly, remove the upper and lower eyelids with fine forceps and micro scissors.Place the turtle head into the gimbal chuck. Then use a small bubble level to check that the dorsal surface of the head is parallel to the horizon. Position one of the eyes at the center of the gimbal's horizontal and vertical rotations.Angle an infrared camera 45 degrees above the line of sight of the eye. The infrared LED should be at the 11 o'clock position, when looking at the camera lens, to center the LED along the optical access of the eye. Adjust the distance of the camera from the eye so that the camera view is maximally filled by the eyeball.Ensure that the corners of the eye are at the edges of the horizontal view. Focus the camera to obtain a clear image of the eye. Then use the three degrees of linear adjustment provided with the gimbal to fine position the eye at the center of the camera view.Detect the duct pupil by setting the threshold and contrast appropriately using the program provided with the video based eye tracking system. Using the mouse, click on Video menu, and under Mode, select High Precision to capture images at a sampling rate of 30 hertz, giving resolution of 640 pixels by 480 lines. Also, under Video, open Pupil Type, and select Dark Pupil, and Ellipse, rotated ellipse, for pupil segmentation method.In the EyeCamera window, click on the pupil search area adjustment icon. Use the mouse to drag out a rectangle that limits an area around the pupil, avoiding dark areas that could be confused with the pupil. Then, in the Controls window, confirm that boxes for AutoImage, and Positive Lock Threshold Tracking are checked.Click on AutoThreshold to optimize the density of scanning, which will show as green dots over the dark pupil. Calibrate the video display of the video based eye tracking program to the rotations of the gimbal, and calibrate torsional rotation as described in the text. Place a ruler in the same focal plane as the pupil and record the width of the full camera view.To position the electrodes, first insert a pin reference electrode into the connective or muscle tissue remaining on the head. Match the size of the nerve to be stimulated to a fire polished capillary glass tip, and place the glass tip onto the suction electrode. Fill the suction electrode with ringer solution, and adjust the volume within the syringe to about half of its capacity.Ensure that the cranial nerves are clearly visible. Then, while looking through a boom mounted dissection scope, use the micro-manipulator to carefully move the glass tip of the electrode to a position above the cut end of the selected nerve and below the surface of the ringer solution that fills the cranium. Pull back on the plunger of the syringe.The vacuum will draw the nerve into the end of the capillary tip. Connect the suction electrode to the current isolation device using a cable. Connect the lead from the pin reference electrode to the ground connection of the isolation device.Use the dials and switches on the stimulator and isolation device to input the stimulation parameters. Use a range of currents from one to 100 mircoamps, with frequency of 10 to 400 hertz. Use one or two millisecond pulses in trains lasting 100, 500, 1000 milliseconds.To visualize the timing of the current applications and their influence on eye movements, click on PenPlots menu. Select X Gaze Position, Y Gaze Position, Torsion, and Pupil Width to show real time raw data plots x and y eye positions, torsion, and pupil width. Also, select the Seconds and Markers from the PenPlots menu to show the timing plot with tick marks, which appear at one second intervals.To help keep track of the type of currents applied, click on the Windows menu and select DataPad. The KeyPad, DataMarker window will appear. Click on the letter or number to identify parameters of the current stimulations being delivered to the nerve, and analyze data as described in the protocol text.This trace shows the mean pupil diameter from six stimulations in one preparation, the dashed lines show standard deviation. The rectangular waveform on the x axis denotes onset and offset of a 100 hertz train of one millisecond pulses, with an amplitude of 50 micro amps. This sketch shows the orientation of the iris line in the eye prior to stimulation.This image shows a still frame from a representative trial before stimulation. This image shows the eye during stimulation. Here, the eye is seen after stimulation.While attempting this procedure it's important to take your time during the dissection to match the size of the suction electrode to each cranial nerve, and to place the head appropriately in the gimbal. Following these procedure, other methods, like testing different temperatures, or applying pharmacological agents, including analgesics, can be performed to answer additional questions, like, how these factors influence eye movements and nervous tissue survivability.

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Research

JoVE Journal

Neuroscience

8.3K Views.  Lafayette College. The overall goal of this electrophysiological protocol is tho provide the ability to measure the kinematics of eye movements of the red-eared slider turtle using an in vitro isolated head preparation. This method can help answer key questions in the ocular-motor field, such as how vision evolved in turtles. The main advantage of this technique is that it does not require training animals to track targets and uses a gimbal to calibrate their eye movements.Demonstrating this technique is...