Hartmans Results

It appears that each tricholith socket is innervated by one stretch- or positionsensitive mechanoceptor neuron. Using ablation experiments, Walthall and Hartman showed that all the mechanoreceptor neurons of the tricholiths in the medial row of the animal's right cercus provide additive excitatory inputs to the (larger) right ipsilateral positional interneuron (RIPI). Tricholiths in the lateral row on the right cercus provide additive excitatory inputs to the left (smaller) contralateral positional interneuron (LCPI). Similarly, the medial row of tricholiths on the left cercus drive the (larger) left ipsilateral positional interneuron (LIPI), and the lateral row of tricholiths on the left cercus drive the (smaller) right contralateral positional interneuron (RCPI). It is well known that the larger spikes recorded extracellularly from a nerve bundle arise in the nerve bers with the lar ger diameters. Thus, it was easy to separate out the spikes visually from the RCPI and the RIPI, etc. on the oscilloscope.

Figure 2.6-4 summarizes the results reported by Walthall and Hartman (1981). All four VNC positional interneurons were found to have approximately cosine(2. ) response patterns (number of spikes in the rst 10 s after tilt) in terms of pitch/ya w angle, given a xed rate of tilt and maximum tilt angle. For example, when the animal was oriented so its head pointed to 45°, and was tilted so its head went down and its right side came up, the RCPI, driven from the lateral row of tricholiths on

Coolant Driven Right Angle Heads

FIGURE 2.6-3 Schematic dorsal view of the tricholith system "wiring." Each separate tricholith sensory neuron (from a row of tricholiths) sends an axon forward to the cell body of the giant afferent interneuron in the sixth abdominal ganglion. There are only four giant afferent interneurons: The right ipsilateral positional interneuron (RIPI) is driven by the right medial row of tricholiths. The right contralateral positional interneuron (RCPI) is driven by the left lateral row of tricholiths. Similarly, the LIPI is driven by the left medial row of tricholiths, and the LCPI is driven by the right lateral row of tricholiths. The four PI axons travel uninterrupted to the brain in the ventral nerve cord.

FIGURE 2.6-3 Schematic dorsal view of the tricholith system "wiring." Each separate tricholith sensory neuron (from a row of tricholiths) sends an axon forward to the cell body of the giant afferent interneuron in the sixth abdominal ganglion. There are only four giant afferent interneurons: The right ipsilateral positional interneuron (RIPI) is driven by the right medial row of tricholiths. The right contralateral positional interneuron (RCPI) is driven by the left lateral row of tricholiths. Similarly, the LIPI is driven by the left medial row of tricholiths, and the LCPI is driven by the right lateral row of tricholiths. The four PI axons travel uninterrupted to the brain in the ventral nerve cord.

the left cercus, red at its maximum rate. From inspection of the polar plots obtained experimentally that describe the VNC interneuron ring rates in response to v arious . and $, it is possible to model the RCPI directional sensitivity by fRCPI = 300(0.5){1 + cos[2(. - 45°)]}, -45° = . = 135°, 0 elsewhere 2.6-1

for a $ = -45° net tilt. The 300 is the number of spikes counted in 10 s following initiation of tilt. Note that. is the direction to which the animal's head points, and $ is the tilt angle of the platform. Similarly, for the same animal head orientation, the LIPI's response to the excitation from the medial row of tricholiths on the left cercus can be approximated by

FIGURE 2.6-4 Polar plot of the responses of the four positional interneurons recorded from the ventral nerve cord while the animal was tilted 30° in various directions. Spikes were counted in the rst 10 s follo wing a tilt. In Walthal and Hartman's notation, PIC stands for positional interneuron driven from tricholiths on the contralateral cercus, PII is for positional interneuron driven from tricholiths on the ipsilateral cercus. Thus, the dotted line at the upper right of the polar plot is the RCPI response to various tilt vectors, the solid line in the upper left of the polar plot is the LCPI responses, the solid line in the lower right is the RIPI responses, and the dotted line in the lower left is the LIPI responses to tilt. The little cockroach drawings with tilt arrows clarify the exact degree of roll and pitch each direction around the polar plot represents. There are three major observations one can make from this gure: The contralateral PIs have the stronger response, the nearly circular plots suggest a cosinusoidal directional response (see text for discussion), and two PIs are ring for e very tilt direction except 45°, 135°, 225°, and 315°. (From Walthal, W.W. and Hartman, H.B., J. Comp. Physiol. A, 142: 359, 1981. With permission.)

FIGURE 2.6-4 Polar plot of the responses of the four positional interneurons recorded from the ventral nerve cord while the animal was tilted 30° in various directions. Spikes were counted in the rst 10 s follo wing a tilt. In Walthal and Hartman's notation, PIC stands for positional interneuron driven from tricholiths on the contralateral cercus, PII is for positional interneuron driven from tricholiths on the ipsilateral cercus. Thus, the dotted line at the upper right of the polar plot is the RCPI response to various tilt vectors, the solid line in the upper left of the polar plot is the LCPI responses, the solid line in the lower right is the RIPI responses, and the dotted line in the lower left is the LIPI responses to tilt. The little cockroach drawings with tilt arrows clarify the exact degree of roll and pitch each direction around the polar plot represents. There are three major observations one can make from this gure: The contralateral PIs have the stronger response, the nearly circular plots suggest a cosinusoidal directional response (see text for discussion), and two PIs are ring for e very tilt direction except 45°, 135°, 225°, and 315°. (From Walthal, W.W. and Hartman, H.B., J. Comp. Physiol. A, 142: 359, 1981. With permission.)

fLIPI = 140(0.5){1 + cos[2(. - 225°)]}, 135° = . = 315°, 0 elsewhere 2.6-2

for the head tipped up and right-side down. Note that the RCPI does not re for this stimulus.

Next considered are responses from tricholiths on the right cercus. The RIPI is driven from the medial row of tricholiths on the right cercus. The RIPI 10-s spike count for a ^ = 45° tilt is given by fRIPI = 140(0.5){1 + cos[2(. - 135°)]}, 45° = .= 225°, 0 elsewhere 2.6-3

The animal's head points to 135°, and the maximum response occurs for tilt such that the head and the right side come up. The LCPI interneuron is driven from the lateral row of tricholiths on the right cercus. The 10-s spike count is approximated by fLCPI = 300(0.5){1 + cos[2(. - 315)]}, 225° = . = 405°, 0 elsewhere. 2.6-4

The LCPI responds only to tilts that make the head and the right side go down. The RIPI does not re when the LCPI res, and vice v ersa.

Figures 2.6-2 and 2.6-3 show that if the cockroach head angle is . = 90°, a standard tilt of the 0° point down will produce pure roll, right side up, and the RIPI and the RCPI neurons will both re.

In summary, any tilting of an Arenivaga that causes the tricholith balls to swing away from the centerline of a cercus will stimulate the corresponding sensilla sensory neurons and cause the targeted VNC interneuron to re. Thus, in the example above, where . = 90°, the left lateral row and the right medial row of tricholith sensillae are stimulated because the corresponding tricholiths bend away from the center lines of the cerci.

Figure 2.6-5A and B summarize the dynamics of the tilt responses of an RCPI and an LIPI interneuron given different tilt maxima ^ and rates of tilt, d^/dt. Walthall and Hartman (1981) showed that the four positional interneurons had a strong derivative component in their respective responses. That is, their instantaneous ring frequency was proportional not only to the tilt angle, but also to d^/dt. When d^/dt . 0, the peak ring rate decayed slo wly toward zero. Walthall and Hartman stated, "it took many minutes during a maintained 15° displacement [tilt] for the PIC unit [LCPI] to cease ring. " Thus, it would appear that an Arenivaga must move to have its CNS advised of its orientation relative to the gravity vector.

FIGURE 2.6-5 (A) Mean instantaneous frequency of the RCPI unit with . = 45°for various degrees of tilt (^). Plots illustrate rate sensitivity and accommodation of the unit. Solid curves, tilt at 21°/s; dashed curves, tilt at 7°/s. (B). Mean instantaneous frequency of the LIPI unit with . = 225° for various degrees of tilt (^). Plots illustrate rate sensitivity and accommodation of the unit. Solid curves, tilt at 21°/s; dashed curves, tilt at 7°/s. (From Walthal, W.W. and Hartman, H.B., J. Comp. Physiol. A, 142: 359, 1981, slightly modi ed. With permission.)

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