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INTRODUCTION |
The detection and regulation of forces acting on
the body and legs are now considered integral components in the control
of posture and locomotion in many animals (Prochazka
1996
). In walking, for example, increase in loading after leg
contact is signaled by receptors that reflexly adjust the activities of
muscles which generate support and propulsion (Bässler et
al. 1991
; Pearson and Collins 1993
). The
subsequent decrease in loading at the end of a step is necessary for
the initiation of leg lifting in swing (Whelan et al.
1995
) and for normal coordination of leg movements (Bässler 1987
). The activities of some groups of
limb muscles are also strongly correlated with leg unloading during
postural perturbations, and decrements in load may be important factors in determining responses to leg slipping (Jacobs and Macpherson 1996
; Mcilroy and Maki 1994
). However, the
specific mechanisms or receptors detecting unloading of a leg have not
been identified in many systems (Zill 1993
).
We studied the responses of the tibial campaniform sensilla of
the cockroach, Periplaneta americana, which detect
forces acting on the legs through strains in the exoskeleton
(Schnorbus 1971
), to determine the specific parameters
that the receptors can encode during walking. The responses and
locomotor functions of these sense organs have also been incorporated
into models and control systems based on insect walking (Schmitz
et al. 1995
). Campaniform sensilla of insect legs are known to
respond directionally to forces that bend the exoskeleton
(Delcomyn 1991
; Hofmann and Bässler 1986
), such as those that occur during leg loading. The
directional sensitivity of an individual receptor is correlated with
the orientation of its ovoid cuticular cap (Spinola and Chapman
1975
). The cap, which is embedded in the exoskeleton, is the
site of termination of the sensory dendrite and is thought to be the
locus of mechanoelectric transduction (French 1992
). The
tibial campaniform sensilla are unique in that they form two subgroups
(proximal and distal sensilla) with mutually perpendicular cap
orientations (Fig. 1B).
Each subgroup exhibits discrete responses when forces are applied to
the distal tibia with joint movement resisted (Schnorbus
1971
). For example, the proximal sensilla respond to bending in
the direction of joint extension, whereas the distal sensilla discharge
to forced flexions (Zill and Moran 1981
). We applied
controlled rates and levels of bending to the leg and found that the
tibial campaniform sensilla, as a whole, respond not only to increasing
levels of force but also exhibit discrete responses to decrements of
force. These responses are consistent with the demonstrated directional
sensitivities of the receptors and do not represent
ON-OFF or bidirectional responses
(Dickinson 1992
). Instead these new results suggest that, for a given direction of force application, different receptors signal loading or unloading according to the orientation of the cuticular cap.

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Fig. 1.
Preparation and identification of unit discharges to applied
forces. A: forces were applied to the tibia with joint
movement resisted (Pin) via a probe driven by a piezoelectric (PE)
crystal. The applied forces were monitored with strain gauges attached
to the probe and the activities of the campaniform sensilla were
recorded extracellularly (Sens). B: drawing of cuticular
caps of tibial campaniform sensilla (after Schnorbus
1971 ). The receptors are located in 2 subgroups (proximal and
distal sensilla), which differ in their directional sensitivity
according to their cap orientation. C: bending to forced
extension (down on the Force trace) elicited discharges to different
units during force increases and decreases (asterisk).
D: individual receptors were identified by indenting
their cuticular caps (Indent Caps in A) with another
probe (arrow), which produced a discharge of equivalent amplitude.
E and F: ablating individual receptors
(hollow arrow in E) could selectively eliminate recorded
responses to force decreases (hollow asterisk in F).
G: bending to forced extension (Force) was followed
rapidly by indentation of the cuticular cap (arrow) of a distal
sensillum. H: coincident cap indentation (arrow) and
decline of force application (asterisk) produced a summation in the
discharge of the distal sensillum.
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METHODS |
Adult cockroaches, P. americana (n = 21), were anesthetized with carbon dioxide, and the nerves innervating
the left metathoracic leg were cut in the thorax. Animals were then
restrained with small staples on a resin-coated platform. Pairs of fine
(50 µm) wires were implanted in the femoral segment of the denervated leg adjacent to the main leg nerve (nerve 5) or its major branch (nerve
5r8) (Nijenhuis and Dresden 1956
). These electrodes
recorded the activities of both proximal and distal sensilla, as the
axons of receptors of both subgroups travel in the same nerve branches (Schnorbus 1971
). Typically, the action potentials of
one distal and one or two proximal sensilla were evident in
extracellular recordings (n = 17/21 experiments)
(Spinola and Chapman 1975
). The tarsus (foot) and tibial
sensory spines were then severed. The femorotibial joint was
immobilized by gluing a pin adjacent to the proximal end of the tibia
with the joint at an angle of 90° or in a position of full extension.
Forces were applied to the distal tibia as ramp and hold stimuli via a
probe that was driven by a piezoelectric crystal. The levels and rates
of applied force were monitored through a pair of strain gauges
attached to the probe (Fig. 1A). In a typical sequence, we
applied 16 different rates, each repeated twice, within a single
series, and up to three series were repeated in each experiment. In
addition, individual campaniform sensilla were identified by indenting
their cuticular caps with a fine-etched tungsten wire attached to
another piezoelectric crystal (Fig. 1, A and
D) (Dickinson 1992
; Spinola and
Chapman 1975
). All signals were stored on tape for subsequent
transcription and data analysis.
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RESULTS |
Forces applied to the tibia as ramp and hold
stimuli elicited discharges both during the rising and the falling ramp
phases (n = 16/17 experiments in which responses of
both proximal and distal sensilla were recorded) (Figs. 1,
2, and
3). In all recordings, the amplitudes
of the discharges during the two phases were sufficiently distinct to
clearly indicate that different units were active during increasing
versus decreasing levels of load. We performed a number of controls to
confirm that these responses originated from the tibial campaniform
sensilla and to identify the individual receptors from which the
discharges were derived. After testing responses to bending (Fig.
1C), we mechanically stimulated individual receptors by
indenting their cuticular caps with a separate probe (Fig.
1D) and were able to elicit discharges of equivalent
amplitude in extracellular recordings to those seen during declining
levels of bending force. In most experiments the sensillum was then
ablated by increasing the level of indentation until the probe
penetrated the cap (Fig. 1E). These ablations could
eliminate the entire response to decreasing force levels in a single
direction while leaving the discharges to force increases intact (Fig.
1F). Furthermore, in three preparations, we were able to
simultaneously apply leg bending and cap indentation, which could show
a summation during the responses to declining forces (Fig. 1,
G and H). Thus the discharges we recorded to
bending were clearly derived from the tibial campaniform sensilla.

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Fig. 2.
Responses to ramp stimuli applied in different directions.
A and C: bending forces applied in the
direction of joint extension (down on the Force trace) elicited
discharges of proximal sensilla to force increases and distal sensilla
to force decreases. B and D: bending the
tibia in the opposite direction (forced flexion, up on the force trace)
caused excitation of distal sensilla during force application and
proximal sensilla during force declines. E and
F: discharges to decreasing forces occurred during the
declining ramp and were not due to rebounds in the opposite direction
of bending (0 level = no bending force applied) or oscillations in
the probe. G: tibial bending applied as repeated
increases and decreases without a hold phase approximated the magnitude
and time course of ground reaction forces that have been recorded
during walking. Bending in the direction of joint extension elicited
alternating bursts of activity from the tibial campaniform sensilla,
even though bending in direction of forced flexion had not occurred.
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Fig. 3.
Encoding of rate of force increase and decrease. A:
bending forces were applied as ramps of varying rate but identical
amplitude. A series of force extensions (down on Force trace) elicits
discharges in proximal and distal sensilla during respective force
increases and decreases that varied in frequency with the ramp rate.
B: plot of maximum discharge frequencies of a proximal
sensilla during the increasing ramp phase of forced extension
( , R2 = 0.935, slope = 7.6, y-intercept = 109.3, P < 0.01) and the decreasing phase of forced flexion ( ,
R2 = 0.964, slope = 7.9, y-intercept = 68.6, P < 0.01)
during bending tests in a single preparation. Both discharges to
increasing and decreasing forces showed rate sensitivities with similar
slopes. C: plot as in B for a distal
sensillum for tests of rate sensitivities to force increases
( , R2 = 0.782, slope = 5.68, y-intercept = 12.8, P < 0.01) and decreases ( , R2 = 0.893, slope = 3.85, y-intercept = 18.1, P < 0.01).
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The responses of individual afferents to decreasing force levels
depended on the direction of force application and the orientation of
the receptor's cuticular cap. The distal sensilla fired to decrements
in the level of forced extension (Fig. 2, A, C,
E, and G), and the proximal sensilla were active
during decreases in forced flexion (Fig. 2, B, D,
and F). This general pattern occurred stably over time (Fig.
2, A and B) and at a variety of levels of
displacement (Fig. 2, C and D) in repetitive
bending tests. Discharges to force decrements were initiated during and not after the declining phase of the ramp stimulus (Fig. 2,
E and F) and were therefore not the result of
resonance in our apparatus or the consequence of inadvertent
application of forces in the opposite direction. We also tested
responses to bending by using patterns that approximated the durations
and magnitudes of ground reaction forces that have been measured during
cockroach walking (Full and Tu 1991
). Forces applied in
the direction of extension as simple, rapid triangle functions, without
a substantial hold phase, elicited reciprocal discharges in proximal
and distal campaniform sensilla in the same pattern, with consistent
bursts during force decrements (Fig. 2G).
Do these discharges simply signal the occurrence of declining force
levels, or do they also encode the rate of force decrease? To address
this question, we applied ramp and hold stimuli at varying rates of
rise and decline (Fig. 3). Firing of proximal sensilla during decreases
from forced flexions consisted of intense bursts of activity in the
range of 50-200 Hz that showed strong correlation with the velocity of
force changes. In contrast, distal sensilla discharged at much lower
frequencies with fewer action potentials to ramps declining from forced
extensions. At low amplitudes of bending, the distal sensilla fired
single spikes and could thus only indicate the occurrence of force
decreases. Change in bending levels of higher magnitude, however, could
elicit bursts that showed distinct modulation of firing frequency
according to the rate of force decline (Fig. 3A). To compare
the sensitivities to rate of change in applied force, we plotted the
maximum firing frequencies during phasic discharges of single
campaniform sensilla. These plots show a dependence on force velocity
as a power function in both proximal and distal sensilla during both
increasing and decreasing ramps (Fig. 3, B and
C). However, similar results were also seen in other
experiments (n = 3) in which forces were applied in
both directions, and sensitivities to rate of change were observed in
all experiments in which sensilla discharged with multiple spikes.
Further experiments are necessary to assess these different sensitivities quantitatively and to characterize the effect of force
amplitude on afferent firing.
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DISCUSSION |
This study has shown that the tibial group of campaniform sensilla
can actively signal both increases and decreases in bending forces.
These findings are important in understanding how forces are encoded by
these receptors and in evaluating their function as inputs to the
cockroach walking control system. Responses to decreasing levels of
force have not been previously explicitly reported and studied, but
they are evident in some earlier published recordings of campaniform
sensilla in the cockroach (Spinola and Chapman 1975
;
Zill and Moran 1981
) and stick insect (Delcomyn 1991
). However, the potential information content of these
discharges was not systematically examined. Similar responses to
declining levels of sinusoidally applied bending were also recently
reported for the locust tibial campaniform sensilla (Newland and
Emptage 1996
), but the individual receptors and the orientation
of their cuticular caps were not identified. It was therefore unclear
whether those responses implied a bidirectionality in unit discharges. Discharges to decreasing levels of leg loading were demonstrated in
groups of cuticular force receptors in other arthropods
(Marchand et al. 1995
) and may be present in the walking
systems of a number of animals.
The mechanisms underlying the generation of discharges to decreasing
forces are at present undetermined. Our finding that the orientation of
the cuticular cap predicts the directionality of the tibial sensilla to
declining forces suggests the parsimonious hypothesis that the cap
provides a final common mechanism of transduction for both force
increases and decreases. The generation of these diverse responses
could then depend on the specific temporal and spatial distribution of
strains within the exoskeleton, which can also show viscoelastic
properties that could contribute to responses to decreasing forces
(Blickhan and Barth 1985
). The current findings are in
clear contrast to the bidirectional responses obtained from campaniform
sensilla of dipteran wings (Dickinson 1992
), which
differ from leg sensilla in the shape of the cuticular cap and sensory
dendrite. Thus the morphological and mechanical properties responsible
for the discharges to declining forces remain a subject for further investigation.
What are the potential advantages in actively signaling decreases in
load in a walking system? First, the information that forces are
declining is prerequisite for the initiation of leg lifting in swing
(Bässler 1987
; Whelan et al. 1995
)
and also enhances the placement of other legs in support
(Bässler et al. 1991
). These functions could be
accomplished by monitoring the declining frequency of a receptor that
simply encodes the level of load. However, such a signal could be
compromised by processes such as adaptation or hysteresis, which are
present in campaniform sensilla and common in many sensory systems
(French 1992
; Zill and Moran 1981
).
Furthermore, the force that a leg exerts must drop substantially when
the foot or point of contact slips on the substrate. The system would
therefore be providing an active signal to allow for the initiation of
the necessary rapid compensatory reactions (Jacobs and
Macpherson 1996
). The selective responses of the tibial
sensilla to both decreases and increases in load provide a mechanism
for anticipating the need for further support and for initiating rapid
responses to maintain stable postures. Experiments are planned to test
these hypotheses in the cockroach. However, other biological and
control systems may utilize signals of force decrements to similar advantage.
We thank G. Nelson for many helpful discussions and E.-A. Seyfarth
and two anonymous reviewers for helpful comments on the manuscript.
This work was supported by Office of Naval Research URISP Grant
N00014-96-1-0694.
Address for reprint requests: A. L. Ridgel, Dept. of Anatomy, Cell
and Neurobiology, Marshall University School of Medicine, Huntington,
WV 25704.
The costs of publication of this article were defrayed in part by the
payment of page charges. The article must therefore be hereby marked
"advertisement" in accordance with 18 U.S.C. Section
1734 solely to indicate this fact.