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First published online January 8, 2007
Journal of Experimental Biology 210, 238-260 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02654
The kinematics of multifunctionality: comparisons of biting and swallowing in Aplysia californica
1 Department of Biomedical Engineering, Case Western Reserve University,
Cleveland, OH 44106, USA
2 Department of Electrical Engineering and Computer Science, Case Western
Reserve University, Cleveland, OH 44106, USA
3 Department of Biology, Case Western Reserve University, Cleveland, OH
44106, USA
4 Department of Neurosciences, Case Western Reserve University, Cleveland,
OH 44106, USA
Author for correspondence (e-mail:
hjc{at}case.edu)
Accepted 7 November 2006
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Summary |
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 TOP Summary IntroductionMaterials and methodsResultsDiscussionReferences |
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Key words: feeding behavior, biomechanics, kinematics, mollusk, muscular hydrostat
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Introduction |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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), for
swimming, for stepping, for paddling or for walking
(Earhart and Stein, 2000). In
crustaceans, the digestive system can process food by grinding, squeezing or
filtering it (Harris-Warrick et al.,
1992). In many vertebrates, the same limbs are used for both
forward and backward locomotion
(Ashley-Ross and Lauder, 1997;
Ting et al., 1999). The human
hand can be used for a remarkable range of tasks, ranging from rapid ballistic
movements (as in boxing), to skilled manipulations (as in unscrewing a jar) to
a full exercise of its many degrees of freedom (as in piano playing). A
hallmark of complex motor systems is the multifunctionality of the periphery,
i.e. the ability of the same peripheral structure to execute qualitatively
different behaviors (Kelso,
1995).
What are the mechanisms of multifunctionality? Previous work suggests that
the many degrees of freedom of the periphery and reorganizing neural
architectures contribute to multifunctionality. Altering the timing or phasing
of degrees of freedom makes it possible to rapidly switch among different
coordinated movements. For example, changing the timing of monoarticular knee
extensor activation contributes to forward swimming as opposed to backpaddling
in the turtle (Earhart and Stein,
2000). Changing the activation of unit pattern generators for
different joints may be crucial for multi-limbed locomotion in a wide variety
of animals, including stick insect and cat
(Büschges, 2005).
Studies of neural control in both invertebrates and vertebrates suggest
that neural circuitry can control qualitatively different behaviors by
reorganizing, i.e. changing connections, or functionally including or
excluding neurons, thus rapidly generating different motor synergies
(Morton and Chiel, 1994). For
example, studies of hypoglossal motor neurons and premotor neurons controlling
tongue musculature during breathing, coughing and swallowing in vertebrates
such as rats and cats demonstrated that there are shared and unique patterns
of activation of motor neurons during these different behaviors. These
observations support the hypothesis that many of the premotor neurons are
multifunctional, contributing to the generation of several behaviors (Gestrau
et al., 2005). Recently, Berkowitz described spinal interneurons in the turtle
whose axon terminal arborizations extend to the ventral horn of the spinal
cord, and are rhythmically active in multiple forms of fictive scratching.
These observations also suggest that shared interneuronal circuitry is
responsible for different motor outputs
(Berkowitz, 2005).
Comparing forces and movements underlying similar but qualitatively
distinct behaviors is an approach to understanding multifunctionality. For
example, a study of movements and EMG associated with forward and backward
pedaling in humans demonstrated that the activity of muscles whose
biomechanical functions were common in both behaviors were unchanged. In
contrast, the activity of muscles whose biomechanical function was different
in each behavior was significantly altered by pedaling direction
(Ting et al., 1999).
Multifunctionality has primarily been analyzed in musculoskeletal systems,
because it is technically feasible to monitor limb movements and forces during
behavior (Biewener, 2002). It
has been difficult to analyze multifunctionality in soft-tissue structures.
But understanding multifunctionality in soft-tissue structures is likely to be
important for deriving general principles, because soft-tissue structures such
as tongues, trunks or tentacles [collectively known as muscular hydrostats
(Kier and Smith, 1985;
van Leeuwen et al., 2000)]
have fewer constraints on their degrees of freedom, and generate complex
behaviors.
To study multifunctionality in a soft-tissue structure whose nervous system
is tractable to detailed experimental analysis, we have focused on
qualitatively different feeding responses in the marine mollusk Aplysia
californica. In Aplysia, a `bite' is an attempt to grasp food
(Kufpermann, 1974). As a consequence, bites are associated with large
amplitude protractions of the grasper (i.e. the radula/odontophore) past an
animal's jaws. If an animal fails to grasp food, it rapidly generates another
bite. In contrast, the function of `swallows' is to convey food that has been
successfully grasped into an animal's buccal cavity. As a consequence,
swallows are associated with large amplitude retractions of the grasper
towards the animal's esophagus
(Kupfermann, 1974;
Neustadter et al., 2002a;
Neustadter et al., 2002b).
During the protraction phase of a swallow, the grasper must be moved towards
the jaws to grasp more food to ingest, but must not be protracted so far
forward that it pushes food out of the buccal cavity. As a consequence,
swallows are associated with small amplitude protractions of the grasper
(Kupfermann, 1974).
The present study describes how changes in deployment of degrees of freedom
of a soft tissue periphery can generate qualitatively different behaviors.
Using magnetic resonance (MR) imaging and a three-dimensional kinematic model,
we compare muscle movements during biting and swallowing. Our results support
the hypothesis that the posterior part of the I1/I3/jaw complex
(Fig. 1), previously described
as a `retractor' (Howells,
1942), may have a context-dependent function and contribute to the
larger amplitude protraction observed during biting. The data also suggest
that differences in the position of the musculature at the onset of biting are
correlated with larger amplitude protractions, and that changes in the closing
and retraction of the grasper contribute to the larger amplitude retraction
movements observed during swallowing. These changes have important
implications for the neural control of multifunctionality.
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Materials and methods |
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;
Neustadter et al., 2002b;
Neustadter and Chiel, 2004).
We will therefore only briefly summarize the MR imaging technique and the
kinematic modeling, and focus primarily on aspects of the materials and
methods that differ from the previous studies.
Magnetic resonance imaging
By continuously scanning and interleaving orthogonal images, it was
possible to create a rapidly updated reference frame intrinsic to a moving
animal. Using this approach, we obtained relatively parallax-free mid-sagittal
images of intact, behaving animals. Data were acquired using echo planar
imaging with standard two-dimensional Fourier transform reconstruction. The
Elscint 2T-Prestige whole-body MRI system was used, with a 15 mT
m-1 maximum gradient strength and 30 mT m-1
ms-1 maximum slew rate, allowing 64 encodings with a 1 mm pixel
resolution to be acquired in 155 ms. The resolution was 1 mmx1 mm pixels
using a total acquisition matrix of 64x128. This spatial resolution is
adequate for the buccal masses that were imaged, whose size was on the order 3
cmx3 cmx3 cm. The time between repeated acquisitions of the main
(mid-sagittal) image was 310 ms, and the time between repeated acquisitions of
each orthogonal image (i.e. axial or coronal) was 620 ms.
Animals and feeding stimuli
The animals used in these studies [Aplysia californica (Cooper)
obtained from Marinus, Inc., Garden Grove, CA, USA] were the largest that
would fit in the holding capsule, and ranged in mass from 400 to 580 g.
Analyzable bites were harder to obtain than swallows. As animals swallow a
narrow seaweed strip, a seaweed-flavored noodle, or a thin polyethylene tube,
they generally do not move their heads and ingest at a regular rate. The low
variability in inter-response intervals as animals swallow a narrow seaweed
strip is quantified elsewhere (Weiss et
al., 1986). In contrast, if an animal does not succeed in grasping
food after a bite, it usually moves its head and body in an attempt to better
position the radula to grasp food.
We found two ways of reliably inducing bites. First, when animals were
initially presented with seaweed-flavored noodles (see
Neustadter et al., 2002a),
they frequently would bite at the noodle at least once or twice before they
succeeded in grasping it to swallow it. Second, we constructed a coil of wire
connected, via a string and a pulley, to a short length of tubing at
the front of the capsule into which the animal was placed for imaging. A small
piece of seaweed was placed into the short tube. When a switch was closed,
allowing current to flow through the coil, the coil rapidly rotated to align
with the MRI's magnetic field, pulling strongly on the string and rapidly
retracting the small piece of seaweed held in the tube. Thus, animals were
presented with a food stimulus that could be rapidly withdrawn before the
animal succeeded in grasping it. Out of the approximately 12 bites that were
relatively parallax-free, we chose to analyze four bites, one from an animal
whose swallowing responses were previously analyzed [7725
(Neustadter et al., 2002a)],
and three from a second animal. The first and fourth bites were made in
response to a seaweed-flavored noodle. The second and third bites were made in
response to a rapidly withdrawn piece of seaweed.
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The kinematic model consists of a model of the radula/odontophore, whose
three-dimensional shape is constructed based on parameters extracted from the
mid-sagittal MR image, kinematic properties of isolated radula/odontophores,
and the assumption that all structures change shape isovolumetrically. It
includes a model of the surrounding I3 musculature and an iterative algorithm
that positions the I3 model muscles so as to best fit the mid-sagittal MR
image of the buccal mass (Neustadter et
al., 2002b). Details of the construction of components of the
model have been previously described
(Neustadter et al., 2002b). We
examined the symmetric differences between the coronal MR images and
corresponding cross-sections of the model at the peak protraction of biting,
and found that they fell within the error tolerances of the results obtained
for swallowing [i.e. less than 15%; see
fig. 10 in Neustadter et al.
(Neustadter et al.,
2002b)].
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Measurements from MR images
To extract specific measurements of muscle lengths, and of parameters for
the kinematic model, MR images were imported into Paint Shop Pro (version 7.0,
JASC Software, Eden Prairie, MN, USA), and the following kinematic measures
were drawn on each image in different layers: (1) jaw line, (2) radular stalk
outline and radular stalk angle, (3) lateral groove (the borders of the
I1/I3/jaw complex dorsally and ventrally, (4) odontophore angle (the angle of
the anterior edge of the I6 muscle), (5) an outline of the odontophore,
excluding the base of the radular stalk if it protruded below the odontophore,
and (6) an outline of the entire buccal mass including the jaw musculature,
the odontophore and the radular stalk, but excluding the pharyngeal tissue
[fig. 4
(Neustadter et al., 2002a)].
As was done previously, the length of the I2 muscle was estimated from the
posterior portion of the buccal mass outline bounded dorsally and ventrally by
the location of the lateral groove, which is the anatomical border of the I2
muscle (Neustadter et al.,
2002a). Previous studies showed that these measures were accurate
within 5% (Neustadter et al.,
2002a).
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; Neustadter et al.,
2002b), although the quantitative values of some measures
referenced to the jaw line have changed slightly. New views of the front of
the feeding apparatus are provided based on the new jaw line measurement
(Fig. 1D), which match in
vivo behavior, and can be compared to the views published previously
using the less accurate jaw line measurement [see
fig. 11C in Neustadter et al.
(Neustadter et al., 2002b)].
All averaged swallowing data shown in this paper are based on the new jaw
measurements.
Visualizing fiber directions in the I1/I3/jaw complex
To visualize the fiber directions in the I1/I3/jaw complex, buccal masses
were fixed in 10% v/v formalin in isotonic MgCl2, pH 7.5
(Drushel et al., 1998).
Hematoxylin (Sigma, St Louis, MO, USA) was mixed with distilled water to
create a saturated solution, and then oxidized with sodium iodate (Sigma) to
create hematein (a brownish dye), which was applied to the outer surface of
the I1/I3/jaw complex. The thin I1 tissue was then dissected away, and the
thick bands of the underlying I3 muscle were stained using Fast Green (Sigma).
Fibers were then photographed at low power through a stereo dissecting
microscope.
Measurement of jaw circumference during biting
To test the hypothesis that the anterior portion of the I1/I3/jaw complex
might exert forces differently than its posterior portion, we measured the
most anterior portion of the jaw cartilage during biting. During a significant
portion of the biting cycle, it is possible to see the anterior margin of the
I1/I3/jaw complex. To measure the circumference of the anterior I1/I3/jaw
complex, we placed a digital video camera (ZR10, Canon Inc., Tokyo, Japan)
immediately above an animal's mouth, while inducing it to make strong bites by
stroking its anterior tentacles and lips with seaweed, and simultaneously
applying drops of seaweed extract to its lips. Three animals were used for
these experiments. We analyzed four bites from one animal that provided the
clearest images and showed minimal movement of the plane of the jaws during
bites. During the initial phases of protraction, the jaw line is partially
obscured by the anterior tentacles, which close prior to protraction [see
fig. 2A, first two frames in
Morton and Chiel (Morton and Chiel,
1993a); fig. 10C,D
in Hurwitz et al. (Hurwitz et al.,
1996)], so we measured the jaw cartilage circumference from just
before the peak of protraction through jaw closing, during which the jaws are
completely visible. Canvas 9.0 (ACD Systems, Miami, FL, USA) was used to trace
and measure the circumference of the jaws from digital video images.
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Data analysis
The feeding cycle was normalized on the basis of the definitions of feeding
cycle components from our previous work
(Drushel et al., 1997;
Drushel et al., 1998;
Neustadter et al., 2002a).
From the onset of protraction to its peak is designated as
t4. Peak protraction to peak retraction is designated as
t1. For swallows, the time from peak retraction to the
loss of the shape in which the base of the elongated radula/odontophore
extends ventrally along the antero-posterior axis of the buccal mass (termed
the 
shape) is designated as t2. Cycle times for
swallows were normalized to the sum of these three periods,
t4+t1+t2. In
biting, the t4 period was also observed. However, as we
report below, the shape is not observed during bites, although the
ventral protrusion of the radular stalk and the posterior rotation of the
odontophore (components of the movements that give rise to the shape
in swallowing) are observed during the retraction phase of biting. Thus, in
biting, the t1 and t2 periods blend
into one another and are referred to as t1. As a
consequence, cycle times for bites were normalized to the sum of the periods
t4+t1.
To directly compare biting and swallowing on the same scale, we computed our standard reference length, the radular stalk width (RSW). For the first animal (first bite), it was 61 pixels, and for the second animal, it was 59 pixels (second through fourth bite). We found that using these values made no qualitative, and small quantitative differences in the data. Moreover, in the previous study, the RSW for both animals studied was 61. Thus, we chose to report lengths in mm rather than in units of RSW.
After normalizing and averaging, data were smoothed using cubic spline
interpolation. Functions for the standard deviation of the data were
constructed (Neustadter et al.,
2002a; Neustadter et al.,
2002b): interpolation functions for each individual normalized
data set were subtracted from the interpolation function of the averaged
normalized data set. These differences were squared, summed and divided by the
number of samples minus 1 (i.e. by 4-1=3). The square root of the resulting
function was taken, creating a standard deviation function. The normalized,
averaged data function was plotted, with the standard deviation function added
to or subtracted from it. This indicates the dispersion around each point in
the averaged function. Inferences about significant changes in kinematic
variables during the swallowing or biting cycle were drawn only if two points
on the averaged curve differed by more than two standard deviations. This is a
conservative measure of statistical significance, because the appropriate
statistic is a difference larger than two standard errors of the mean,
obtained by dividing the standard deviation functions by the square root of
N, or by 2 (for N=4). All numerical values are reported as
mean ± standard deviation (s.d.). Statistical significance of numerical
differences was determined using Student's t-test.
As adjuncts to the text, we provide digital movies (in Quick Time format) of one MRI sequence of biting (Movie 1 in supplementary material), showing the second bite analyzed in this paper in sagittal, coronal and axial views, as well as three-dimensional views of this sequence generated by the kinematic model (Movies 2-4 in supplementary material).
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First, the larger amplitude protractions in biting as compared to swallowing could be due to differences in the positions of the muscles at the onset of each behavior, i.e. at the onset of protraction. We therefore compared the initial positions of the musculature in biting and swallowing.
Second, the grasper opens more widely near the peak of protraction during biting than it does during swallowing, which could be due to the position of the grasper as a whole, or of structures within the grasper. We therefore compared the position of the grasper and changes in its shape and internal structures prior to and at the peak of protraction.
Third, a kinetic model of the buccal mass predicted that the protractor
muscle I2 (Fig. 1A,
Fig. 2) might become too short
in biting to fully protract the radula/odontophore. The kinetic model also
predicted that the posterior portion of the I1/I3/jaw muscle (Figs
1,
2) could change function and
contribute to protraction (Sutton et al.,
2004b). We therefore compared the lengths of I2 and the different
regions of the I1/I3/jaw complex in biting and swallowing, and predicted the
upper bounds on the net force that the posterior portion of the I1/I3/jaw
complex could exert.
Fourth, during swallowing, closure of the grasper and retraction are the power phase, in which the grasper exerts maximum force against seaweed that it is attempting to ingest. We therefore compared closure and retraction of the grasper during biting and swallowing.
Overview of biting versus swallowing kinematics
Bites had a similar overall temporal structure to swallows. The duration of
the bites that we analyzed was comparable to the duration of the swallows that
we studied previously. Bites had a duration of 5.9±0.9 s, whereas
swallows had a duration of 6.4±0.4 s [mean ± s.d.; swallow
durations are for swallows previously reported
(Neustadter et al., 2002a;
Neustadter et al., 2002b)].
Although the percentage of the cycle devoted to protraction was larger in
biting than in swallowing (43±6% for biting versus
37±8% for swallowing), the difference was not statistically
significant.
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Furthermore, near the peak of protraction, the I2 muscle (the thin muscle that
wraps around the posterior of the buccal mass, and acts to protract the
odontophore [(Hurwitz et al.,
1996); Fig. 1A] was
shorter in biting than in swallowing [compare
Fig. 3, frames 8 and 9, of this
paper with fig. 5,frames 9 and
10 in Neustadter et al. (Neustadter et
al., 2002a)].
During retraction, the position of the radular stalk and odontophore
differed between biting and swallowing. During swallowing, the base of the
radular stalk extended beyond the base of the odontophore for about a third of
the entire swallowing cycle [frames 13-20 out of the 22 frames of the swallow
shown in fig. 5 in Neustadter
et al. (Neustadter et al.,
2002a)]. In contrast, during biting, the base of the radular stalk
extended beyond the base of the odontophore for only about one-sixth of the
entire biting cycle (frames 13-15 out of the 18 frames of
Fig. 3). By the time the
odontophore had fully rotated posteriorly, the radular stalk protruded only
slightly out of the base of the odontophore, so that the characteristic
shape seen at the peak retraction of the swallowing
[fig. 5, frames 18 and 19, in
Neustadter et al. (Neustadter et al.,
2002a)] was not observed in biting (frame 15 of
Fig. 3 of this paper).
A three-dimensional model of a bite
(Fig. 4), based on the
kinematic model previously described
(Neustadter et al., 2002b),
clearly illustrates these differences. During the initial transition phase, a
larger fraction of the volume of the odontophore (yellow mesh) lies within the
lumen of the I1/I3/jaw complex (blue mesh) than during the transition phase of
swallowing [compare column labeled Transition in
Fig. 4, with the identical
column in fig. 11 of
Neustadter et al. (Neustadter et al.,
2002b)]. Near the peak of protraction, the anterior tip of the
odontophore penetrates through the widely opened jaws, and the anterior tip of
the radular stalk is closer to the anterior surface of the odontophore than it
is in swallowing. The hemispherical region posterior to the I1/I3 musculature,
which represents the attachment and posterior extent of the I2 muscle, is
smaller in biting than in swallowing (left arrows in top frame of side view of
Protraction; Fig. 4). The
three-dimensional model reconstruction also clearly shows that the posterior
section of the I1/I3 musculature is posterior to the widest portion of the
radula/odontophore, and has a narrower diameter than it did in transition
[compare column labeled Protraction in Fig.
4, with the identical column in
fig. 11 of Neustadter et al.
(Neustadter et al., 2002b)].
Finally, the extension of the base of the radular stalk beyond the base of the
odontophore is smaller than in swallowing [compare column labeled Retraction
in Fig. 4, with the identical
column in fig. 11 of
Neustadter et al. (Neustadter et al.,
2002b)].
Initial positions in biting versus swallowing
At the onset of biting or swallowing movements, the buccal mass is not
generally at rest; rather, it is in a position that we refer to as
`transition'. This is consistent with earlier observations in a semi-intact
preparation. In that preparation, prior to the onset of rhythmic feeding-like
behaviors, the entire system underwent activation that prepared it to generate
feeding responses (referred to as a `cocking phase')
(Weiss et al., 1986).
The initial position of the radula and odontophore within the buccal mass
at the time of protraction onset differed from its initial position in
swallowing. In biting, the initial length of the I2 protractor muscle was
significantly shorter [Fig. 5E,
arrow 1; note that the error bars for biting (black) do not overlap those for
swallowing (gray)]. The antero-posterior length of the ventral surface of the
I1/I3/jaw complex was significantly shorter in biting
(Fig. 6C, arrow 1). The
dorso-ventral length of I3 at the lateral groove was significantly shorter
(Fig. 6E, arrow 1), and the
length of the I1/I3/jaw complex at the jaw line was significantly longer
(Fig. 6G, arrow 1). The
odontophore protraction into the lumen of the I1/I3/complex allows the
posterior tissue of the I1/I3/complex to shorten behind the grasper
(dorso-laterally at the lateral groove), and lengthen anterior to it
(dorso-laterally at the jaw). Moreover, the model suggests that the
medio-lateral width is expanded at the lateral groove [top black lines in
Fig. 7A-D; compare the top
lines in fig. 15A-D of Neustadter et al.
(Neustadter et al., 2002b)],
but of similar width at the jaws [bottom broken lines in
Fig. 7A-D; compare the bottom
lines in fig. 15A-D of Neustadter et al.
(Neustadter et al., 2002b)].
Finally, the initial position of the tip of the odontophore is closer to the
jaw line (Fig. 8C, arrow
1).
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Odontophore shape near peak protraction in biting versus swallowing
During biting, the grasper is protracted further anteriorly than in
swallowing, inducing it to pass into the lumen of the I1/I3/jaw complex. In
turn, it is possible that the forces within the I1/I3/jaw complex could deform
the grasper. In response, the internal forces of the grasper might alter its
shape to allow it to open and then shut prior to the peak of protraction. We
therefore compared the shape of the odontophore near the peak of protraction
in biting and swallowing. The antero-posterior length of the odontophore was
significantly shorter near the peak protraction of biting as compared to
swallowing (Fig. 9A, arrow 1).
The odontophore was significantly shorter dorso-ventrally near and after the
peak of protraction in biting compared to swallowing
(Fig. 9C, arrow 1), but its
medio-lateral width was not significantly different
(Fig. 9E). These results
suggest that the overall shape of the odontophore is compressed at the peak
protraction of biting in comparison to its shape during swallowing.
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I2 and I1/I3/jaw complex lengths near peak protraction in biting versus swallowing
Do the in vivo kinematics support the hypothesis that I2's ability
to protract may be greatly reduced, and that the posterior region of the
I1/I3/complex could assist protraction? At the peak of protraction in biting,
the length of the I2 muscle is significantly shorter than it is at the peak
protraction of swallowing (Fig.
5E, arrow 2). At the peak protraction of biting, I2 is
15.9±1.5 mm long, whereas at the peak protraction of swallowing I2 is
23.3±1.8 mm long (P<0.005, N=4). The I2 muscle
remains at or near its shortest length for a longer fraction of the total
biting cycle than it does during the swallowing cycle
(Fig. 5E). This suggests that
the protractor muscle I2 is more strongly contracted during biting, consistent
with the stronger activation that it receives during biting
[fig. 13A,B in Hurwitz et al.
(Hurwitz et al., 1996)].
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Analysis of I2's length/tension and force velocity properties suggests that
I2 will become weak at the peak protraction of biting. Assuming that the
length of I2 at the end of the biting cycle is close to the resting length of
the I2 muscle, it will be equal to 0.86lmto [where
lmto is defined as the optimal muscle and tendon length of
I2 (Yu et al., 1999); note
that if the longer transition length for swallowing rather than biting is used
for these calculations, it will strengthen the conclusions presented]. From
the actual lengths measured from the MR images, the minimum length reached by
I2 prior to the peak protraction of biting is
0.46±0.02lmto (mean ± s.d., N=4),
which is significantly shorter than the minimum length reach by I2 prior to
the peak protraction of swallowing (0.66±0.03lmto;
P<0.0001). The active forces at the minimum length that I2 reaches
in the protraction of biting become close to zero
[fig. 2C in Yu et al.
(Yu et al., 1999)]. The
ability of I2 to exert force is further reduced by its force/velocity
properties. Within 200 ms ofreaching its shortest length during the
protraction of biting, the I2 shortens at a velocity of 0.18±0.09
lmto s-1, which will reduce I2's force
to about 40% of the maximum it could exert isometrically
[fig. 2D in Yu et al.
(Yu et al., 1999)]. A kinetic
model of the odontophore, I3 and I2 muscle has demonstrated that the
mechanical advantage of I2 drops precipitously as it shortens
(Sutton et al., 2004b).
Finally, the ability of I2 to protract the radula/odontophore at the
displacement associated with biting is also antagonized by both passive and
active forces in the hinge, i.e. the interdigitation of the I2 muscle, the
I1/I3/jaw complex and I4 (Fig.
2B1), which is stretched at the peak protraction of biting
(Sutton et al., 2004a). Taken
together, these data strongly suggest that other factors must contribute to
the peak protraction of biting.
A kinetic model of the buccal mass predicted that the posterior part of the
I1/I3/jaw complex could contribute to protraction during the peak protraction
of biting (Sutton et al.,
2004b). The in vivo kinematic data are consistent with
this hypothesis. Prior to peak protraction, the posterior portion of the
I1/I3/jaw complex becomes significantly shorter dorso-ventrally at the lateral
groove in biting than it does in swallowing
(Fig. 6E, arrow 2). At the peak
protraction of biting, the length of the I3 muscle at the lateral groove is
13.2±0.8 mm, whereas at the peak protraction of swallowing it is
16.3±1.1 mm (P<0.003, N=4). This constriction is
likely to be an active pinching down, because the length decreases
significantly below the rest length of the muscle at the lateral groove
(compare the length in Fig. 6E,
arrow 1). The posterior part of the I1/I3/jaw complex also shortens
medio-laterally at the lateral groove (Fig.
7A-D, top thick lines, prior to t1 line
marking end of protraction).
Other changes in the lengths of the I1/I3/jaw complex reflect how much further forward the grasper is protracted through the lumen of the jaws in biting than in swallowing. The antero-posterior length on the dorsal surface is shorter (Fig. 6A, arrow 1). The muscle expands dorso-ventrally (Fig. 6G, arrow 2) and medio-laterally at the jaws (Fig. 7A-D, bottom broken lines). The changes in antero-posterior length could be due both to the expansion of the entire lumen of the I1/I3/jaw complex as the grasper moves into it, and to active contraction of the I1 muscle.
Differential contractile forces in the I1/I3/jaw complex
The lengths of I2 and of the I1/I3/jaw complex prior to and at the peak of
protraction support the hypothesis that the posterior section of the I1/I3/jaw
complex could contribute to the protraction phase of biting. However, if the
force exerted by the anterior portion of the I1/I3/jaw complex were stronger
than the force exerted by the posterior region, the net force exerted by the
I1/I3/jaw complex would not generate protraction. We therefore examined the
anatomy of the I1/I3/jaw complex in vitro and in vivo to
determine whether there might be a difference in the forces exerted by these
parts of the I1/I3/jaw complex.
The fiber directions of the I1/I3/jaw complex were visualized in formaldehyde-fixed buccal masses (Fig. 12A; lines indicate fiber directions). Contraction of the I1 muscle may shorten the antero-posterior length of the entire I1/I3/jaw complex, whereas contractions of the I3 muscle bands may constrict the entire lumen of the I1/I3/jaw complex. External fiber directions do not distinguish the anterior or posterior regions of the I1/I3/jaw complex.
The internal anatomy of the I1/I3/jaw complex does suggest differences between the anterior and posterior regions (Fig. 12C). Anteriorly, the medial surfaces of the I3 muscle bands are covered with cartilage; posteriorly, no cartilage is present. Our anatomical studies have shown that the dorsal and ventral connections of the jaw cartilages are flexible, consisting of muscle and connective tissue. Moreover, at rest, the jaw cartilage has folds in it (Fig. 12C). In freshly dissected tissue, it is possible to manually stretch these folds in a dorso-ventral direction without encountering a significant resisting force until they pull taut, after which the cartilage rigidly resists further expansion (H.J.C., unpublished observations). In contrast, the posterior region of the I3 bands generates a steadily increasing resistive force to dorso-ventral expansion (presumably due to passive forces within the I3 muscle bands).
External observations of the jaws during biting suggest that the jaw cartilage does not fully expand until near the peak of protraction. We measured (1) the circumference of the jaws as they closed (Fig. 12G), (2) their circumference at the time that wrinkles appeared (indicating that the cartilage was not tightly stretched; Fig. 12F), and (3) their circumference at the peak of protraction (Fig. 12D). The ratio of the circumference at the time wrinkles were observed to the circumference at jaw closure (i.e. (2)/(1), Fig. 12F/Fig. 12G) was 1.5±0.3, and this expansion in circumference is significant (P<0.05, N=4 bites from one animal). In contrast, the ratio of the circumference of the jaws at the peak of protraction relative to the circumference at the time wrinkles were observed (i.e. (3)/(2). Fig. 12D/Fig. 12F) was 0.97±0.1 (N=4 bites from one animal). The circumference of the jaws at the peak of protraction was not significantly greater than the circumference of the jaws at the onset of appearance of folds, suggesting that the anterior cartilages were not stretched further at the peak of protraction.
A previous kinetic model suggested that the posterior portion of the I3
muscle could act to protract the odontophore when it was posterior to its
midline, but the I3 muscle was represented as a torus, and the odontophore as
a sphere (Sutton et al.,
2004b). To estimate the net forces exerted by the I1/I3/jaw
complex based on its shape and the shape of the odontophore in vivo,
we used the meshes generated by the kinematic model
(Fig. 13). The four plots in
Fig. 13 indicate the total
force produced by the I3 muscle at differential contraction ratios of 0.0,
0.3, 0.6 and 1.0 (from top to bottom). If the magnitude of the forces in the
anterior portion of the I1/I3/jaw complex were 30% or less than the forces in
the posterior portion of the I1/I3/jaw complex, it was possible for the
posterior portion of the I1/I3/jaw complex to exert significant protractive
forces on the odontophore prior to the peak of protraction
(Fig. 13, top two lines).
These results are an upper bound on the forces that the I3 muscle could exert,
because they do not account for the length/tension or force/velocity
properties of the I3 muscle, which are likely to reduce the forces that I3
could generate.
Peak retraction in biting versus swallowing
Retraction is stronger in swallowing than in biting, because the animal
exerts force against food to be ingested during the retraction phase of
swallowing (Hurwitz and Susswein,
1992). The halves of the radula may close together more tightly
during swallowing than during biting. Shortly after peak protraction, the
dorso-ventral length of the odontophore is significantly greater during
swallowing than in biting (Fig.
9C, arrow 1), and remains greater during the first half of
retraction. Prior to the peak of retraction, the antero-posterior length of
the odontophore is significantly greater during swallowing than in biting
(Fig. 9A, halfway between arrow
1 and 2). Both of these differences are consistent with tighter closure, which
would expand the musculature of the odontophore in these dimensions, since the
medio-lateral width is not changed significantly
(Fig. 9E). Consistent with a
tighter closure of the odontophore is a larger protrusion of the base of the
radular stalk beyond the base of the odontophore
(Fig. 10C, arrow 2), and a
significantly greater increase in the estimated length of the I7 muscle
(Fig. 11, between arrows 1 and
2), reflecting the greater movement of the radular stalk out of the halves of
the I4 muscle.
The external forces acting on the odontophore during swallowing, which are
absent during biting (because the animal has not yet grasped anything), may
also contribute to the change in shape of the odontophore. In
Fig. 9C, the dorso-ventral
length of the odontophore is shorter during biting than in swallowing during
the first half of retraction. During this part of retraction in both biting
and swallowing, the odontophore is rotated into the lumen of the I1/I3/jaw
complex. As a consequence, during swallowing, the dorso-ventral dimension of
the odontophore is directly in line with any opposing force due to seaweed
[see Fig. 9D, top right
outline, which corresponds to the onset of protraction; compare with
fig. 12F, frame 18, in
Neustadter et al. (Neustadter et al.,
2002a)]; encountering an antagonistic force in this direction
could induce expansion in this dimension during swallowing. Similarly, during
late retraction in both biting and swallowing, the odontophore has rotated out
of the lumen of the I1/I3/jaw complex. As a consequence, during swallowing,
the antero-posterior dimension of the odontophore is directly in line with any
opposing force due to seaweed [see Fig.
9B, bottom left outline, which corresponds to the peak of
retraction; compare with fig.
12F, frame 18, in Neustadter et al.
(Neustadter et al., 2002a)],
and a force in this direction could expand the antero-posterior dimension of
the odontophore during swallowing. Since an external force is absent during
biting, and the odontophore does not close as tightly, the odontophore does
not expand as much in either the dorso-ventral or the antero-posterior
dimension during biting as it does during swallowing.
In addition to tighter closure of the odontophore halves, the overall
retraction of the odontophore is greater during swallowing. The I2 muscle is
longer at the peak retraction of swallowing
(Fig. 5E, arrow 3), the entire
odontophore moves more posteriorly (Fig.
8C, arrow 3) in swallowing than in biting, and rotates back
further from the jaw line in swallowing than in biting
(Fig. 8A, arrow 1). In
conjunction with the tighter closure and the larger amplitude protrusion of
the radular stalk beyond the base of the odontophore, the entire structure
shows the shape in swallowing, which is not observed during
biting.
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Discussion |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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;
Morton and Chiel, 1993a). The
data presented in this paper confirm these observations. Furthermore, indirect
evidence suggested that the retraction phase was larger in amplitude in
swallowing than in biting [(Kupfermann,
1974); fig. 5A,B in
Morton and Chiel (Morton and Chiel,
1993a), showing more intense buccal nerve 2 activity, i.e.
activation of the I1/I3/jaw muscle complex, during the retraction phase of
swallowing than in biting]. The data presented in this paper directly
demonstrate the larger amplitude of retraction movements and the tighter
closing of the radula/odontophore in swallowing as compared to biting.
Three novel phenomena were also observed. First, the radula/odontophore begins in a more anterior initial position at the onset of biting than at the onset of swallowing. Second, prior to and at the peak of protraction of biting, the length of the I2 muscle becomes short, and the posterior part of the I1/I3/jaw complex also shortens significantly. These observations provide kinematic support for the hypothesis that the posterior part of the I1/I3/jaw complex could contribute to the peak protraction of biting. Third, the radular stalk moves close to the surface of the radula/odontophore at the peak protraction of biting, which does not occur during the peak protraction of swallowing.
Context-dependent role of I1/I3/jaw complex in biting protractions
Several lines of evidence suggest that the posterior part of the I1/I3/jaw
complex may play a context-dependent role, i.e. the direction of the forces
that it exerts may reverse with mechanical context. The I1/I3/jaw complex was
inferred to act as a retractor from anatomical studies
(Howells, 1942), and was shown
to induce retraction when stimulated
(Morton and Chiel, 1993a).
However, near the peak protraction of biting, the I1/I3/jaw complex may also
play a role in protraction. First, data from the present study demonstrate
that I2 becomes short near the peak protraction of biting
(Fig. 5E, arrow 2) and previous
work suggests that when it is short, I2 becomes weak both because of its
intrinsic length/tension and force/velocity properties
[fig. 2 in Yu et al.
(Yu et al., 1999)], and
because I2 loses mechanical advantage, i.e. it loses the ability to convert
its internal forces into forces on the odontophore
[fig. 4 in Sutton et al.
(Sutton et al., 2004b)].
Second, as I2 protracts the odotontophore, I2 stretches the `hinge' that
connects the base of the odontophore to the surrounding muscles of the buccal
mass, generating passive and active forces that antagonize the forces in I2
(Sutton et al., 2004a). Third,
the position of the posterior portion of the I1/I3/jaw complex relative to the
odontophore (Fig. 13B,C,
middle column), and the significant shortening of the posterior parts of the
I1/I3/jaw complex in the dorso-ventral
(Fig. 6E, arrow 2) and
medio-lateral (Fig. 7A-D, top
thick line) dimensions could allow the posterior part of the I1/I3/complex to
exert protractive forces near the peak of protraction in biting. Fourth, the
internal anatomy of the I1/I3/jaw complex and external views of the jaw
cartilage suggest that forces in the anterior portion of the I1/I3/jaw complex
could be lower than those in the posterior portion prior to the peak of
protraction (Fig. 12C-G).
Fifth, calculation of the upper limits of the forces that the posterior
I1/I3/jaw complex could exert on the radula/odontophore relative to the
anterior part suggest that the net force in the I1/I3/jaw complex could
protract the radula/odontophore near the peak protraction of biting
(Fig. 13A).
|
)],
neuron B10 is active prior to other I1/I3/jaw complex neurons
[fig. 6 in Morton and Chiel
(Morton and Chiel, 1993b)],
and neuron B10 innervates the posterior portion of the I1/I3/jaw complex
(Church and Lloyd, 1994).
Second, during biting, neural activation of the I2 muscle stops prior to the
peak of protraction, and activity in the third largest unit innervating the
I1/I3/jaw complex (presumably B10) begins before I2 has shut off. The
different pattern of activity in biting and swallowing is shown schematically
in Fig. 14A-C [based on
fig. 11A,B in Hurwitz et al.
(Hurwitz et al., 1996)]. If
the I1/I3/jaw complex acted as an antagonist to the I2 muscle, it is hard to
understand these observations: how could the large protractions at the peak of
biting be produced by turning off activation of the sole protractor muscle and
activating its antagonist? In contrast, if the posterior part of the I1/I3/jaw
complex assists the I2 muscle, as predicted by our kinetic model
(Sutton et al., 2004b) and by
the kinematics shown in this paper, the neural control could exploit the
context dependent biomechanical properties of the posterior part of the
I1/I3/jaw complex. A third line of evidence is provided by the construction of
a biologically inspired gripper device consisting of concentric rings of
muscle-like actuators, similar to the bands of the I3 muscle, that control a
central grasper. When the central grasper was posterior to the midline of the
rings, ring contraction induced a retraction of the grasper; when the central
grasper was anterior to the midline of the rings, ring contraction induced a
protraction of the grasper (Mangan et al.,
2005).
Could neuromodulation of the I2 protractor muscle alone account for the
larger amplitude protraction observed during biting? Analysis of the kinetic
model suggested that if a neuromodulator increased the maximum contractile
force of I2 by a factor of three, I2 might be able to act as sole protractor
without assistance from the posterior region of the I1/I3/jaw complex
[fig. 5 in Sutton et al.
(Sutton et al., 2004b)].
Indeed, I2 is subject to neuromodulation
(Hurwitz et al., 2000). Motor
neurons for I2, in addition to using acetylcholine as a conventional
transmitter, are also immunoreactive to myomodulin, which can act as an
intrinsic neuromodulator to increase both muscle contraction amplitude and
relaxation rate. Furthermore, an extrinsic neuromodulator, serotonin, can also
increase muscle contraction amplitude and relaxation rate. Preliminary studies
show that, at the I2 lengths observed during the peak of biting, physiological
concentrations of serotonin can increase the maximum isometric force by at
most a factor of two (Sutton et al.,
2005). Moreover, serotonin also increases I2's rate of relaxation
(as does myomodulin) (Hurwitz et al.,
2000). Since I2 is turned off before the peak of protraction,
neuromodulation may actually decrease the amount of force that I2 can exert at
the peak protraction of biting. Thus, neuromodulation alone cannot fully
account for the large amplitude protractions at the peak of biting, though it
may contribute.
Several additional experiments should be done to test the hypothesis that
the posterior region of the I1/I3/jaw complex contributes to the peak
protraction of biting. Lesions of the posterior portion of the I1/I3/jaw
complex (i.e. the region posterior to the jaw cartilage,
Fig. 12C), or of its
innervation, should reduce the magnitude and/or duration of the peak
protraction of biting in intact, behaving animals, whereas lesions of the
anterior portion of the I1/I3/jaw complex, or its innervation, should not
significantly affect the peak protraction of biting. Similarly, stimulation of
the posterior region of the I1/I3/jaw complex in vitro should
generate protractive forces if the radula/odontophore is moved into the
position corresponding to the peak protraction of biting, whereas the same
stimulation should generate retraction if the radula/odontophore is not as
strongly protracted. Preliminary results support these hypotheses
(Tan et al., 2006).
If the function of the I1/I3/jaw complex depends on its mechanical context,
components of the feeding cycle cannot be defined by recording activity of
identified motor neurons without regard to changes in their biomechanical
function (e.g. Church and Lloyd,
1994; Murphy,
2001; Elliott and Susswein,
2002). Activity of motor neurons innervating I1/I3 may not
invariably represent a `retraction' phase, and this could alter the
classification of motor phases, as well as the interpretation of the
functional significance of the synaptic effects of higher order interneurons.
A parallel example is provided by recent work showing that the B8 motor neuron
may act as a radular closer during small amplitude swallows, but contribute to
both radular closure and retraction during larger amplitude swallows
(Ye et al., 2006a).
Understanding the neural and mechanical mechanisms of context dependence in
Aplysia may clarify neural control of context-dependence in mammalian
and human systems (Murray et al.,
1995).
Neuromuscular control of biting versus swallowing
By combining previously published data on the neural and muscular activity
during biting (Cropper et al.,
1990a; Cropper et al.,
1990b; Morton and Chiel,
1993a; Hurwitz et al.,
1996; Evans et al.,
1996) (D. W. Morton and H.J.C., unpublished observations) with the
kinematic data presented in this paper, it is possible to suggest hypotheses
about the sequence of muscle activations that may underlie the biting cycle in
contrast to the swallowing cycle (Figs
2 and
14). Because we have
previously provided a detailed description of the neural activity during the
swallowing cycle (Neustadter et al.,
2002b), we will focus on the differences between biting and
swallowing.
What differences in neural activity could account for the initial more
anterior position of the entire buccal mass prior to the onset of biting as
compared to swallowing? One possible mechanism could be differential
activation of the extrinsic muscles that suspend the buccal mass within the
head, and can move the entire structure anteriorly
(Chiel et al., 1986). Indeed,
in vivo recordings of the activity of extrinsic muscle E1, which
inserts on the dorsal surface of the buccal mass at the lateral groove (see
Fig. 12C) and also inserts
into the anterior lip tissue (Chiel et
al., 1986) (Fig.
1), demonstrate that E1 is strongly activated during the cocking
phase, prior to biting, and is more strongly activated prior to a bite than to
a swallow [fig. 20 in Chiel et al. (Chiel
et al., 1986)]. Once the cocking phase is over, the weaker
retraction movement associated with a bite may leave the musculature in a more
protracted mode than after a swallow. Indeed, examining all of the kinematic
measures that were different from swallowing at the onset of biting shows that
they remain different at the end of the biting cycle, strongly supporting this
hypothesis (Fig. 5E,
Fig. 6C,E,G and
Fig. 8C).
What neural activity generates the larger amplitude protraction observed
during biting than during swallowing? Activity in the I2 protractor muscle is
more intense and prolonged during biting than during swallowing
[Fig. 14A,B, I2 schematic
traces (Hurwitz et al.,
1996)], which would induce a larger amplitude protraction. By
pulling posteriorly on the I1/I3/jaw complex, and by translating and rotating
the odontophore anteriorly, the I2 is acting not only to protract the
odontophore, but also to open the jaws. As discussed above, activation of the
motor neurons for the posterior of the I1/I3/jaw complex (i.e. B10) could also
allow the posterior part of this muscle complex to aid protraction
[Fig. 14A,B, BN2 schematic
trace, smallest amplitude unit (Sutton et
al., 2004b)].
What changes in neural activity could account for the change in timing of
activation of the I2 muscle and the I1/I3/jaw complex in biting as opposed to
swallowing? The end of activity in the I2 muscle overlaps the onset of
activity in BN2 during biting, but I2 activity ends before BN2 activity begins
during swallowing (Fig. 14A,B,
I2 and BN2 schematic traces; Fig.
14C). A striking feature is observed at the onset of large unit
activity in swallowing that is not observed during biting: there is a burst of
large units on BN3 that is absent during biting, and these large units
correspond to activity in the B4/B5 neurons
[fig. 8A,C in Warman and Chiel
(Warman and Chiel, 1995)]
(Fig. 14A,B, BN3 schematic
traces; Fig. 14C). Since
multiaction neurons B4/B5 strongly inhibit many of the motor neurons for the
I1/I3/jaw complex (Gardner,
1993), it is possible that their increased activity in swallowing
may play a role in delaying the onset of activity in the motor neuronal pool
for the I1/I3/jaw muscle complex that projects through BN2
(Fig. 14C). Indeed, recent
studies have shown that increased activity in the B4/B5 neurons is associated
with larger delays in the onset of activity on BN2 in swallowing and rejection
(Ye et al., 2006a;
Ye et al., 2006b).
What changes in neural activity could account for the opening of the
grasper during biting as opposed to swallowing? Several lines of evidence
suggest that the I4 muscle may play a context dependent role during the peak
protraction of biting. First, a large upward movement of the radular stalk may
be induced by an early burst of activity in the I7 muscle [labeled 1a in
fig. 4A of Evans et al.
(Evans et al., 1996)], which
is absent in swallowing (Fig.
14A,B, I7 traces; also note greater shortening of I7 prior to peak
protraction in biting, Fig.
11, arrow 1). Second, as the radular stalk moves so that it is
near the top of the odontophore (Fig.
10F), contraction of the I4 muscle could act to raise the stalk
further [rather than pushing the stalk downwards, as I4 does at the peak of
retraction; see fig. 19B,C in Neustadter et al.
(Neustadter et al., 2002b)].
Third, prior to the peak of opening, large unit activity on the radular nerve
is consistently observed (Fig.
14A, RN trace), so that I4 could actively contract to enhance the
upward movement of the radular stalk.
What changes in neural activity could account for the closing of the
grasper during biting as opposed to swallowing? If I4 has a context-dependent
role, then the critical step for allowing the grasper to close would be
pulling the radular stalk downwards within the I4, so that the I4 could then
act to push the radular stalk further downwards. Interestingly, this could
explain the second burst that was recorded in the I7-10 muscles [labeled 2, in
fig. 4A,B of Evans et al.
(Evans et al., 1996)]. Since
the recordings were performed on I10, rather than directly on I7, it is
possible that this burst of activation was primarily addressed to the I8-I10
muscles, which would tend to pull the base of the radular stalk towards the
base of the I4 muscle. The same neural activity might have little effect on
the I7 muscle, because I7 is already short. In addition, the early activation
of the I5 muscle in biting might also act to pull the radular stalk down
relative to the surface of the I4 muscle
(Orekhova et al., 2001). In
this new configuration, the intense activation of the I4 muscle will act to
push the radular stalk downwards and strongly close the grasper halves.
How is retraction initiated during biting? The initiation of retraction in
biting, as well as in larger amplitude (Type B) swallows, is likely to be due
to activation of the `hinge', the fibers connecting the dorsal base of the
grasper with the I2 muscle and the I1/I3/jaw complex. The hinge is likely to
exert significant active and passive retractive forces when it is strongly
stretched, as it is at the peak of biting or of large amplitude (Type B)
swallows (Sutton et al.,
2004a; Sutton et al.,
2004b; Ye et al.,
2006a). This suggests that motor neuron B7, which controls the
hinge (Ye et al., 2006a), is
likely to be intensely active during the retraction phase of biting. Once
retraction is initiated, closure of the radular halves will induce the grasper
to elongate, and net forces in the I1/I3/jaw complex will become retractive
(Fig. 13)
(Novakovic et al., 2006).
What changes in neural activity could account for the larger amplitude
retraction in swallowing than in biting? A possible neural basis for this
difference is an increase in intensity of motor neuronal activity on BN2,
especially amongst the second largest and largest extracellular units on BN2
(which are likely to correspond to motor neurons B3, B6 and B9; H. Ye and H.
J. Chiel, unpublished observations; Fig.
14A,B, schematic BN2 traces, largest units;
Fig. 14C). These motor
neurons, which innervate the medial and anterior regions of the I1/I3/jaw
complex (Church and Lloyd,
1994) are likely to generate a strong contraction of the entire
I1/I3/jaw complex, pushing the grasper further posteriorly than during biting.
In addition, longer duration and higher frequency activity in the B8 motor
neurons [reflected in the large unit activity on RN;
fig. 5A,B in Morton and Chiel
(Morton and Chiel, 1993a)],
and higher frequency and longer duration activity in the B15/B16 motor neurons
that innervate the I5 (ARC) muscle during swallowing
(Cropper et al., 1990a) may
also contribute to the stronger closure of the radular halves. The greater
protrusion of the radular stalk generates the shape that is observed
during the peak retraction of swallowing, but not of biting
(Fig. 14A,B, schematic RN and
I5 traces).
Implications for pattern generation in Aplysia
These results suggest that a single pattern generator, by changing the
phasing, intensity and duration of activation of similar pools of motor
neuron, generates the qualitatively different ingestive behaviors of biting
and swallowing. Many of the interneurons that have been suggested to play a
role in switching between ingestive and egestive behaviors, as well as
proprioceptors that can sense whether or not an animal has grasped food, may
reorganize the pattern generator for feeding
(Jing and Weiss, 2001;
Jing and Weiss, 2002;
Jing et al., 2004;
Evans and Cropper, 1998).
The kinematic results suggest that the feeding pattern generator must alter activity in several motor neuronal groups. First, activation duration of the I2 motor neurons should increase during biting to increase protraction amplitude. Second, the intensity of activity of the B4/B5 multiaction neurons should decrease during biting to allow motor neurons for the I1/I3/jaw complex to turn on before the end of activity in the I2 motor neurons (Fig. 14C). Third, I7 should be activated early in biting to induce strong opening. Finally, motor neurons for the I1/I3/jaw complex should be more intensely activated during swallowing to increase retraction amplitude.
The duration of activity in the I2 protractor muscle could be controlled in
several ways. The B31/B32 neurons, which are half of I2's motor pool for the
I2 muscle (the other motor neurons are B61 and B62), are also interneurons
whose activity triggers the initiation and the protraction phase of all
feeding responses (Hurwitz et al.,
1996). Control of the activation duration of the B31/B32 neurons
would therefore control the duration of I2's excitation. Two neurons that
could control B31/B32 activation duration are B63 and B64
(Hurwitz et al., 1997).
Activation of neuron B63 could enhance activity in the B31/B32 neurons, since
B63 is tightly coupled to the B31/B32 neurons, but has a lower threshold for
excitation, and is a synaptic target for B50
(Dembrow et al., 2003;
Dembrow et al., 2004) and
higher order interneurons (Hurwitz et al.,
2003). Increasing the excitation of B63 has been shown to initiate
and prolong activity in the B31/B32 neurons
(Hurwitz et al., 2003;
Dembrow et al., 2004). In
contrast, activity in the B31/B32 neurons can be reduced through the actions
of neuron B64, which strongly hyperpolarizes the B31/B32 neurons, and
contributes to the termination of protraction
(Hurwitz and Susswein, 1996).
Thus, neurons that hyperpolarize B64 could prolong protraction
(Jing et al., 2003).
Prolonging the duration of activity in the I2 protractor muscle alone is
not enough to generate biting versus swallowing. A recent study
(Ye et al., 2006a)
demonstrated that Aplysia can generate smaller amplitude (Type A) or
larger amplitude (Type B) swallows, and that Type B swallows are associated
with a significant increase in activation duration of the I2 muscle [figs
11A,
12A in Ye et al.
(Ye et al., 2006a)]. The key
neural difference between both types of swallows and biting is the level of
activity in the B4/B5 multiaction neurons, which show little or no activity
during biting, and more intense activity during swallowing
(Warman and Chiel, 1995), with
the most intense activity associated with the swallow that has the larger
amplitude protraction (Ye et al.,
2006a). Thus, recent in vitro studies that distinguished
biting-like and swallowing-like patterns based primarily on the duration of
activity in the I2 motor neurons (Jing et
al., 2004) may not have accurately associated these patterns with
corresponding in vivo behaviors.
Implications for multifunctionality
What implications do these results have for understanding
multifunctionality? By design, an engineered device has a clearly defined
function by which it can be evaluated. A hammer whose head breaks off as it is
being used to pound nails into a board is either worn out or poorly designed.
Each component of a multifunctional engineered device, such as a Swiss Army
knife, can be evaluated by focusing on each function. Thus, a Swiss Army knife
may function well as a knife, but its scissors may be small and cut
poorly.
How should functions of an evolved system be defined and evaluated? From an
evolutionary standpoint, the only `evaluation' that matters is whether an
animal survives long enough to leave offspring, and whether it leaves more
offspring than other animals in the population. Thus, it would be reasonable
to define `function' in terms of survival and reproduction. From this
viewpoint, the three feeding responses in Aplysia, i.e. biting,
swallowing and rejection, are all aspects of the single behavior of feeding,
since rejection clears the buccal cavity so that animals may again attempt to
ingest food (Katzoff et al.,
2006). In contrast, egg laying, although it also uses the anterior
tentacles and lips of the animals (Begnoche
et al., 1996), clearly serves a distinctive reproductive
function.
Defining function is not a purely semantic exercise. If biting, swallowing
and rejection all serve a single behavioral function, i.e. feeding, neural
control that can flexibly switch among the different behavioral responses may
confer a selective advantage on an animal. At the same time, neural control
that shuts down one set of behaviors when incompatible behaviors are to be
performed by the same peripheral structures may also confer a selective
advantage. Indeed, in Aplysia, egg-laying hormone not only induces
egg laying movements of the feeding apparatus, but also suppresses the
animal's own feeding responses (Stuart and
Strumwasser, 1980), thus preventing an Aplysia from
eating its own eggs and blocking its own reproduction. Similarly,
Clione's slow swimming movements are completely inhibited by
defensive withdrawal behaviors, during which its wings are strongly retracted
(Norekian and Satterlie,
1996). Under these conditions, neural control may directly
instantiate behavioral hierarchies
[(Tinbergen, 1951), pp.
102-104].
Alternatively, function can be defined purely by biomechanical constraints
(Stein et al., 1986). Since it
is not possible to locomote forwards and backwards simultaneously, these
behaviors may be distinct. Since an animal cannot ingest food and reject it
simultaneously, swallowing and rejection may be distinct. Similarly, since it
is not possible to attempt to grasp food (i.e. bite) and to have succeeded in
grasping the food and ingesting it (i.e. swallow) simultaneously, biting and
swallowing may be distinct. However, functional distinctions due to
biomechanics may not be strongly distinguished in neural control, if the
different movements are all components of the same overall behavior. Indeed,
previous studies have described intermediate motor patterns that combine
features of both swallowing and rejection
(Morton and Chiel, 1993a), and
have also described bite/swallows: an animal begins with a bite (i.e.
generates a large amplitude protraction) but, as it grasps food, completes the
behavior as a swallow [i.e. generates a large amplitude retraction
(Weiss et al., 1986)].
Functional differences among distinct and incompatible behaviors may affect
neural control in other systems. For example, red-eared slider turtles
(Trachemys scripta elegans) use both forward and backward swimming
and claw vibration as part of courtship
(Cagle, 1950). Thus, it is not
surprising that the detailed studies of Stein, Berkowitz, and their colleagues
have found, at the motor pattern level, blends (i.e. switches between
behaviors for several cycles, or aspects of different behaviors in successive
cycles) that lead to rapid transitions among these behaviors, and, at the
neural level, shared neural circuitry controlling these motor tasks
(Stein, 2005;
Berkowitz, 2001;
Berkowitz, 2002;
Berkowitz, 2005). Since limb
withdrawal into the carapace is a defensive withdrawal response, it is also
not surprising that neural control, and even specialized musculature, may
differ for this behavior (Callister et
al., 1992; Callister and
Peterson, 1992).
Multifunctionality is a ubiquitous feature of many biological organisms.
The ability to rapidly reconfigure a peripheral structure, flexibly adjusting
motor responses as the environment changes, may confer selective advantages on
animals. Motor control is likely to exploit the fluidity of peripheral
function during related responses that subserve a single behavior such as
feeding (Ye et al., 2006b). In
contrast, when behaviors such as feeding, reproduction or escape are in
conflict, neural control may act to suppress one function and enhance another.
These principles are likely to be relevant to the analysis of
multifunctionality in other animals and humans.
List of abbreviations
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Acknowledgments |
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Footnotes |
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* Present address: 3 Ruth Hamoavia, Apartment 3, Netanya 42756, Israel 
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References |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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