J. exp. Biol. (1979), 78, 121-136
121
With 9 figures
Printed in Great Britain
STRUCTURE AND FUNCTION OF THE LATERAL GIANT
NEURONE OF THE PRIMITIVE CRUSTACEAN
ANASPIDES TASMANIAE
BY GERALD E. SILVEY AND IAN S. WILSON
Department of Zoology, University of Tasmania,
Hobart, TAS 7001, Australia
(Received 3 April 1978)
SUMMARY
The syncarid crustacean Anaspides tasmaniae rapidlyflexesits free thoracic
and abdominal segments in response to tactile stimulation of its body.
This response decrements but recovers in slightly more than one hour.
The fast flexion is evoked by single action potentials in the lateral of two
large diameter fibres (40 /im) which lie on either side of the cord. The
lateral giant fibre is made up of fused axons of 11 neurones, one in each
of the last 5 thoracic and 6 abdominal ganglia. The soma of each neurone
lies contralateral to the axon. Its neurite crosses that of its counterpart in
the commissure and gives out dendrites into the neuropile of each hemiganglion.
The lateral giant neurone receives input from the whole body but fires in
response only to input from the fourth thoracic segment posteriorly. Both
fibres respond with tactile stimulation of only one side. Since neither
current nor action potentials spread from one fibre to the other, afferents
must synapse with both giant neurones.
The close morphological and physiological similarities of the lateral
giant neurone in Anaspides to that in the crayfish (Eucarida) suggest that
the lateral giant system arose in the ancestor common to syncarids and
eucarids, prior to the Carboniferous.
INTRODUCTION
In mountain streams and tarns of southwest Tasmania lives a syncarid crustacean,
Anaspides tasmaniae, whose family arose more than 200 million years ago in the late
Palaeozoic, 100 million years earlier than the now dominant advanced crustaceans,
the Decapoda (cf. Brooks, 1962a; and Glaessner, 1969). The animal, 3-4 cm in
length and caridoid in form, has many features that are considered primitive; unfused
free thoracic segments, biramous homonomous appendages and leaf-like epipodite
gills (cf. Siewing, 1959; Smith, 1909; and Snodgrass, 1952). Behaviourally, this
animal makes a rapid flexion of its body when touched. In the decapod crayfish
similar behaviour is known to be mediated by giant fibres (Wiersma, 1947; Wine &
Krasne, 1972). Since the Syncarida and the Eucarida, the malacostracan divisions to
which Anaspides and the crayfish respectively belong, seemingly arose from common
stock (Brooks, 19626) we examined the neural basis of the evasive response in the
122
G. E. SlLVEY AND I. S. WlLSON
primitive Anaspides in order to gain an insight into the origin of the neural organization of the decapod escape response.
Anaspides, we will show, has two giant fibres in each half of its ventral nerve cord
and we have concentrated on the lateral of the two for comparison with the much
studied lateral giant fibre of the crayfish. Since the thoracic body segments are not
fused in Anaspides as they are in the crayfish, the thorax can contribute to the flexion
that propels the animal away from a disturbance. Little attention has been given to the
structure and function of the lateral giant fibre in the thorax of the crayfish, probably
because its thoracic body segments are fused and covered with a carapace and thus
do not participate in the escape reaction. Nevertheless, the crayfish lateral giant fibre
is segmented in the thorax (Johnson, 1924) as well as in the abdomen, and one
might expect that the thoracic segments of this fibre are organized homologously to
those in the abdomen (Remler, Selverston & Kennedy, 1968). The organization of
Anaspides prompted us to investigate the function of the lateral giant neurone in the
thorax.
MATERIALS AND METHODS
min
n
Anaspides 20-35
l° 6 were collected from small pools in streams above 500 m
on Mt Wellington, near Hobart. After isolating them in small containers of stream
water they were stored at 5 °C, which is within the temperature range most commonly
found in their natural environment (0-8 °C; Reid, 1975). Our morphological and
physiological results are taken from animals without regard to the duration of their
captivity. However, the animals that were involved in the behavioural decrement
experiments were tested within 2 days of their capture.
In behavioural experiments the animals were maintained in stream water cooled
to 10-12 °C. Evasive behaviour was recorded on film with a 16 mm Bolex cine
camera at 64 frames per second. For the behavioural decrement studies each animal
was fastened to the bottom of the dish with a staple across the anterior segments of
the body, so that it could flex its abdomen and thorax but could not escape. To give
a constant intensity stimulus, a forceps was used with the gap between the tips adjusted by a set screw so that when the forceps was squeezed its tips just touched each
side of the pleurite of the fourth abdominal segment. The strength of stimulation
chosen was the setting which just provoked a rapid flexion.
For recordings from the lateral giant axon the ventral nerve cord was exposed
from above and the tissue bathed in saline composed of (g/1): NaCl, 6-3; KC1, 0-34;
CaCla^HjO, 1-035; MgQ 2 .6H 2 O, 0-213; NaHCO3, 0-2. The ionic composition of
the saline was the same as that of the blood, as determined by flame photometry
(R. Swain, personal communication). Saline flowed continuously through the experimental chamber, and was oxygenated and cooled to 6-7 °C. After fastening the animal
to the floor of the chamber by a staple over the head-thorax joint and a pin through
the abdomen-telson joint the dorsal cuticle was slit lengthwise, the underlying ventral
wall of the pericardium was severed along one side of the heart and the gonads were
removed. Minutien pins were inserted through the sternites on either side of the
ventral nerve cord to hold the body walls apart. The muscles of the body wall in the
thorax separated neatly into bilateral bundles; muscles in the abdomen, however,
Lateral giant neurone of Anaspides
123
required severance of cross anastomoses before they could be spread to reveal the
cord. The gut was sometimes pinned aside, sometimes partly removed with the ends
ligatured, and sometimes removed entirely. As far as possible the supraneural artery
was kept intact.
To record from single cells in the exposed cord, lateral giant fibres were penetrated,
under visual observation, with 20-30 Mil microelectrodes filled with 3 M-KC1.
Penetrations were usually made in a ganglion rather than a connective. Electrical
stimulation was made through the microelectrode and recorded potentials were
amplified with FET input amplifiers. To record externally from the cord a suction
electrode was coupled to an AC pre-ampUfier. Flexing movements were monitored
with a phonograph cartridge by attaching one end of a light-weight probe to the
stylus holder and placing the other end upon the dorsum of the abdomen. When the
animal flexed it deflected the probe and oscilloscope trace upwards. Signals were
displayed conventionally and recorded on moving paper film.
Tactile stimuli in physiological experiments were applied with forceps by sharp
proddings with the closed tips or by quick pinches of the cuticle. The instant of
contact with the cuticle was not monitored.
To determine the morphology of lateral giant neurones, cells were filled with cobalt
chloride (Pitman, Tweedle & Cohen, 1972). Axons of lateral giant neurones were
penetrated under visual observation with microelectrodes filled with 1 M-COC18. The
tips of these electrodes were broken off and the dye expelled using pressure from a
micrometer syringe. We could tell when a fibre was injected by the sudden onset of
an opacity around the tip of the electrode and along the fibre. After injection m situ
at just one point in any segment of the fibre the cord was removed to a dish of saline.
Cobalt was precipitated with a few drops of 10% ammonium sulphide. The cord
was fixed in Carnoy's fluid, cleared in methyl benzoate, mounted, and immediately
sketched by means of a camera lucida. To examine giant fibres in transverse section
the cord was removed from the body and fixed in alcoholic Bouin's, and embedded
in paraffin. Serial sections were cut at 8-10 /im. and stained with Mallory's Triple.
A set of serial sections prepared in the same way by Dr S. Lake and one of us (I. S. W.)
was also used.
RESULTS
We found Anaspides to be a very difficult and sensitive animal with which to work.
Its small size precluded manipulation and access to nerves. Viability following
exposure of the ventral nerve cord was highly variable and typically brief. Even in
those animals in which interneuronal activity in the cord and postsynaptic activity
in the lateral giant neurone persisted for 3-4 h the lateral giant axon would spike
for only several minutes after the cord was exposed. During intracellular recording
from the lateral giant fibre, spikes progressively and rapidly decremented in amplitude
from overshooting down to several millivolts. In chronic and direct extracellular
recordings from the cord the amplitude of giant fibre discharge was indistinguishable
from action potentials of other neurones. These features of the animal prevented us
from obtaining answers to certain questions and they will undoubtedly hinder further
productive pursuit into neural circuitry.
124
G. E. SlLVEY AND I. S. WlLSON
Behaviour
The evasive action of Anaspides, as evoked by tactile stimulation, involves a fast
flexion of every free segment of the body. Thoracic and abdominal segments bend
one upon the other as illustrated by the records of Fig. i. The records show that
Anaspides, once touched upon the side (frame i), adducts its appendages and bends
the ends of its body ventrally (frame 3) so that its rostrum nearly touches its telson
(frame 4). It adducts all its appendages but swimmerets are moved caudad, the more
anterior legs rostrad and the middle legs extended ventrally as well. The flexion and
adductions reach their maximum within about 40-50 ms (frames 3-4) and then the
animal straightens its body (frames 5-7), returns its appendages to their normal
position and uses them in swimming fashion (Wilson, Macmillan & Silvey, 1978) to
move further away from the stimulus (frame 8). The force of these bodily actions
against the water propels the animal upwards off the substrate.
The fast flexion response we attribute to lateral giant mediation. However, we
have no direct evidence that the lateral giant fired during the sequences portrayed in
Fig. 1 nor that selective firing of the lateral giant in an intact animal would lead to
the movements displayed. Below, however, we will show (Fig. 6) that tactile stimulation applied similarly to that in the experiment of Fig. 1 and to the same regions
of the body touched in the experiment of Fig. 1 provokes both lateral giant fibres to
fire. We will also show that direct excitation of the lateral giant provokes a rapid and
full flexion of the body (Fig. 5). On these grounds we assume that the behaviour, at
least the rapid flexion, displayed in Fig. 1 is mediated by the lateral giant neurone.
In the evasive response the animal also moves away from the side on which it was
touched, that is, there is a directional component in the response. In fact, the direction in the movement, namely away from the probe (frame 2 of Fig. 1), begins prior
to complete development of the flexion (frame 4). Since we were unable to distinguish
lateral giant spikes in chronic recordings we cannot specify the source of this directionality. Certainly it is due to activation of motor neurones ipsilateral to the side
touched because muscles on this side of the body contract to produce the curvature
of the body away from the probe (frame 2 of Fig. 1). Whether in fact directionality
is due in a behaving animal to the firing of only one giant, namely, that ipsilateral to
the side touched, or to the ipsilateral giant firing slightly before the contralateral giant
relative to the side touched, or to the activation of other interneurones, we are unable
to say.
Touches to parts of the body other than the sides also evoke rapid evasive responses
and these too show a directional component. A touch to the antennae or head elicits
an upward, usually forward movement containing some lateral component. These
responses are presumably due to the medial giant interneurone (cf. Fig. 3). A touch
to the telson or uropods of the abdomen leads predominantly to an upward and forward
movement. Occasionally Anaspides may move backwards; it does so by an upward
and backward somersault or twist but not by a full flexion of its abdomen as does the
crayfish (Larimer et al. 1971; Wine & Krasne, 1972).
Behavioural decrement. Evasive responses to tactile stimulation can be repeated
frequently and do not decrement to the degree that the tail flip escape reaction habituates in the crayfish (Wine, Krasne & Chen, 1975). It is not unusual in the pursuit
of an Anaspides to find that it rapidly flexes its body to each contact made with it.
Frames
Fig. i. Development of the evasive reaction of Anaspides sketched from successive frames of
cin£ films. Each sketch is separated by slightly more than 15-5 ms. With the animal on the
left a touch to an abdominal swimmeret leads to a full body flexion in about 45 ms followed
by recovery. With the one on the right a touch to a thoracic appendage evokes a similar fast
flexion. The direction of movement in each case is upward and away from the point of
contact with the probe, as shown in black. Frame 1 is the last with each animal at rest and
does not precisely indicate the moment of touch, which was beyond the optical and frequency
resolution of the camera.
3
EXB
78
126
G. E. SILVEY AND I. S. WILSON
100
100
80
80
rI
20
60
40
o o Experimental (n= 14)
• - - - • Control (n = 4)
20
— 168± 13
2
4
6
8
10
Initial test (no. of trials at I/mm)
Trials to
criterion
2
4
6
8
10
Retest (no. of trials at 1/min)
Fig. z. Behavioural decrement in Anaspides in response to tactile stimulation of the right
fourth abdominal pleurite. O—O> Experimental (n = 14). • — • , Control (n = 4). Graph
A shows that all animals responded with a fast flexion to the first two stimuli but only 70 %
of them responded on the last trials. Trials consisted of one touch of the fourth abdominal
pleurite once per minute with the tips of a forceps set to just contact the cuticle upon closure.
The experimental animals were then stimulated more frequently until they achieved criterion:
10 successive failures. At four touches per minute approximately 17 stimuli were required
to produce criterion but there was a great deal of variation among individuals. Graph B shows
that of animals retested immediately after criterion (o h) 30 % gave responses. At 1 h 80 %
of these animals responded. Thus, they recovered rapidly from their decrement. The controls
at 1 h responded as they did initially.
To determine whether the response showed behavioural decrement, several animals
were stimulated tactilely on the right fourth abdominal pleurite as described in
Materials and Methods. Tactile stimulation of this pleurite leads to firing of the
lateral giant as we shall show below (Fig. 6). However, we were not able to substantiate by chronic recordings that each stimulus which evokes an evasive response also
excited a lateral giant. Nevertheless we assumed that fast flexions were due to lateral
giant excitation. Initially we stimulated at the rate of 1 touch per min for 10 successive trials (Fig. 2 A). One group was then allowed to rest for an hour before retesting.
This group served as controls for the other animals, which were subjected to a test
of successive stimuli at 4 per second until individuals failed to respond to ten consecutive trials. This was our criterion for complete decrement. Once an animal was
decremented to criterion it was immediately retested at 1 touch per min for 10 min
and again 1 h later. These are the tests at o h and 1 h in Fig. 2 B for the experimental
animals. They are compared to a retest of the controls at 1 h.
Fig. 2A indicates that the animals do show behavioural decrement. After ten
touches at 1 per min only 70 % of the animals responded. There is no significant
difference between experimental and control animals (analysis of variance double
classification, P > 0-20). The crayfish, when stimulated to give a lateral giant mediated escape response with taps at 1 per 5 min, habituates to 40 % and some animals
fail to respond during the first two trials (Wine et al. 1975).
Lateral giant neurone of Anaspides
127
Following decrement to criterion the experimental animals retested immediately
showed a recovery to 30 % (o h, Fig. 2 B). One hour later 80 % of the experimental
animals responded to the first stimuli. At 1 h the control animals responded as they
did initially (P > 0-20). The difference between the experimental animals at o h
and the experimental animals at 1 h is significant (P < o-ooi). The experimental
animals at 1 h decreased their responses slightly more rapidly and completely than
did the controls at 1 h (P < o-ooi). Nevertheless, by the end of 1 h Anaspides nearly
recovers its ability to respond to a stimulus that decremented its evasive reaction.
The lateral giant mediated escape response of the crayfish, on the other hand, when
habituated to a criterion of ten successive failures, does not recover for more than a
day (Wine et al. 1975).
To determine the site of decrement we touched the contralateral pleurite after
each test sequence and after complete decrement (criterion), and recorded whether
the animal flexed. Only two animals failed to elicit a rapid body flexion to stimulation
of the contralateral pleurite. This means that the lateral giant to motor circuit was
still operating. Unfortunately, the test does not clearly show whether the behavioural
decrement was due to sensory adaptation or to changes between sensory and lateral
giant pathways. Nevertheless, the test does indicate that the source of behavioural
decrement is on the input side of the circuit.
In the matter of behavioural response decrement it is difficult to compare Anaspides
with the crayfish because it was impossible to give identical intensity and quality of
tactile stimulation to the small, soft-bodied Anaspides as has been used on the larger,
hard-bodied crayfish. Moreover, certain refinements in recording possible with the
crayfish and which would have given our conclusions greater certitude were not
possible with Anaspides. Nevertheless, there are differences between the two species
with respect to rates of entrance into and recovery from decrement, both those noticed
in thefieldand those measured in the laboratory, and we believe these to be expressions
of differences in circuitry in the lateral giant system of each animal.
Morphology
Cross-sections of the ventral nerve cord reveal a pair of large diameter fibres in
each connective and hemiganglion (Fig. 3). Each giant fibre in the connective between the eighth thoracic and first abdominal ganglia is about 40 /an in diameter
in a 35 mm long animal. The medial fibre is somewhat narrower posteriorly than
anteriorly. In contrast the lateral fibre is broader posteriorly than anteriorly. In their
passage through a ganglion both fibres narrow somewhat. This is apparent from
cross-sections (Fig. 3 A) and from dye injected fibres (Fig. 4). The medial fibre is
continuous through the length of the body and dye passes through it unobstructed.
The lateral fibre is discontinuous, made up of fused processes from single nerve cells.
It is formed from axons which arise from cell bodies in each of the six abdominal
ganglia and the last five thoracic ganglia. Between two segments the posterolateral
membrane of the more anterior cell forms a septum. This is largely impermeable to
cobalt ions: in stained cells, only faint indication of precipitated cobalt could be seen
on the uninjected side of the septum and along the membrane of the unfilled fibre.
The septate organization of the crayfish lateral giant is similar (Johnson, 1924;
iiViersma, 1947).
128
G. E. SILVEY AND I. S. WILSON
Th,_2
Ab 4 .,
Fig. 3. Morphology of the lateral giant intemeurone of Anaspidu. In the left side of the
figure, cross-sections of the cord sketched by camera lucida from serial sections reveal in
each connective and hemiganglion two large diameter fibres that run the length of the cord.
These are the lateral and medial giantfibres,named with respect to their position in each half
of the cord. The section through the eighth thoracic ganglion shows a septum in the lateral
giant formed by the overlap of the posterior axon (outer side) and the axon which runs anteriorly (inner side). In the right half of the figure the lateral giant fibre is diagrammed.
Somata lie contralateral to the fibre and bear dendrites which enter the neuropiles of ipsiand contralateral ganglia. Scale: 50 fun.
Lateral giant neurone of Anaspides
129
Th 6
100 nm
Fig. 4. Morphology of a pair of lateral giant neurones from the sixth thoracic ganglion. The
camera lucida sketch of cobalt sulphide filled cells shows somata near the posterolateral wall
of each hemiganglion and contralateral to the fibre. The neuritic processes of the cells cross
over in the commissure before expanding into the axon that runs anteriorly in the fifth-sixth
connective. Each neurite sends branches into both halves of the ganglion, n ^ : nerve roots.
Scale: 100 /im.
Each segment of the lateral giant fibre arises from a cell in the contralateral hemiganglion. The example shown in Fig. 4 is representative of each of the 11 segments
of the fibre. The cell body of each segment of the fibre is stituated near the posterolateral border of its hemiganglion and ventral to the level of the giant fibres. The
neurite of each lateral giant soma rises upwards, crosses beneath the ipsilateral giant
fibres and descends into the commissure. Within the commissure the neurites from
opposite hemiganglia cross over one another. Each neurite sends dendrites into both
ipsilateral and contralateral neuropiles. Within the opposite hemiganglion the neurite
ascends, broadens and forms the axon of the fibre that passes forward in the cord.
The axon that arises from the fourth thoracic ganglion passes uninterrupted into the
circumoesophageal connective before entering the brain. Thus it traverses six pairs
of ganglia: the first three thoracic and three suboesophageal. The lateral giant neurones
in the sixth and last ganglion differ from those in the other ganglia in that the dendrites which invade the neuropile contralateral to the soma are longer and not directed
laterally. Instead, one branch passes dorsally and two others posteriorly.
130
G. E. SILVEY AND I. S. WILSON
A
Fig. 5. Competency of the lateral giant fibre to elicit the rapid body flexion. Depolarizing
current was applied through a microelectrode within a lateral giant fibre. Movement of the
body was monitored (top trace) and extracellular activity recorded in the connective (bottom
trace). (A) Subthreshold stimulus. (B) Threshold stimulus evokes a giant fibre action potential
(first dot) followed by cord activity and a rapid body flexion (upward deflexion of the movement monitor about 10 ma after the giant spike). Subsequently the giant is re-excited and
fires twice (second and third dots), probably due to afference evoked from the body touching
the probe of the monitor. Scale: 20 ms.
Physiology
Output. When the lateral giant neurone spikes it excites interneurones and motor
neurones and causes a fast body flexion, as shown in Fig. 5. A just subthreshold
stimulus to the fibre (A) produces neither action potentials in the cord (bottom trace)
nor movement of the body (top trace). A slight increment in current passed through
the intracellular electrode within the fibre (B) elicits an action potential (first dot)
followed immediately by neuronal activity in the cord, and, within approximately
10 ms, a flexion of the body, as indicated by the upward deflexion of the top trace.
In addition, the thoracic and abdominal appendages are sharply adducted. Thus,
the lateral giant is competent to release the fast flexion component of the behaviour
described above. As for the crayfish (Wiersma, 1947) it is a true command fibre: a
single interneurone which by its firing releases a behavioural repertoire that in this
case is the fast body flexion.
Of further interest in Fig. 5 are the two additional spikes of the same neurone
(second and third dots). The waveform and size of these potentials are the same as
those of the lateral giant spike (first dot). They are followed by a complex change in
the movement trace which indicates further muscular contractions. We believe that
the flexion which was triggered by electrical stimulation of the lateral giant re-excited
the lateral giant orthodromically because the probe of the monitor acted as a tactile
stimulus.
Only slightly more than 40 ms separates the electrically generated spike (first dot)
from the spikes (second and third dots) generated by re-excitation. Normally a fast
flexion-recovery cycle lasts 75 ms; at least this is the briefest interval we have recorded
of successive evasive responses. In the crayfish an electrically generated spike ir\
Lateral giant neurone of Anaspides
131
Lt. Con.
I
Lt. LG
•
^
Fig. 6. Lateral giant action potentials evoked by tactile stimulation of the abdomen. Responses
to tactile stimulation of the thorax as far forward as the fourth segment are identical to these.
The left lateral giant was recorded extracellularry with a suction electrode from the fourthfifth connective in the thorax (top trace) and intracellularly in its seventh thoracic segment
(bottom trace). It fires to touches on the ipsi (A) and contra- (B) lateral fourth abdominal
pleurite. The giant fibre fired twice within less than 20 ms. The latency between stimulus
and response was not measured. Scale: 100 ms; 25 mV.
the lateral giant inhibits it from firing again for 80 ms (Roberts, 1968), which is in
the order of time required for completion of a flexion-extension cycle (Wine & Krasne,
1972). The occurrence in Anaspides of spiking following an electrically generated
spike before the animal would have extended from flexion suggests that it does not
possess a recurrent inhibitory mechanism.
The multiple firing associated with the second flexion in Fig. 5 was commonly
observed in this neurone; two further examples are shown in Fig. 6. At other times
brief bursts of up to eight spikes were recorded, with intervals between spikes ranging
from 5 to 15 ms. Multiple firing to tactile stimulation also occurs in the crayfish
lateral giant although at a higher frequency and in shorter bursts, complete within
20 ms (Wine & Krasne, 1972).
Afference and its integration. Tactile stimulation is adequate to provoke the lateral
giant to fire. Each of the records in Fig. 6 shows two spikes recorded intracellularly
in a lateral giant, and coincident firing in the cord. These firings were elicited by
touch to one of the abdominal segments. We have, however, recorded firing of the
lateral giant to tactile stimulation of the thorax and its appendages as far forward as
the fourth segment. Nevertheless, lateral giant firing is most readily elicited by touching the abdominal pleurites. The more posterior are the more sensitive. This leads
us to assume that the fast flexions (Fig. 2) are mediated by the lateral giant.
The records in Fig. 6 also show that tactile input to either side of the body, and
thus input to either side of a ganglion, is capable of exciting the same lateral giant.
This means that both giants can fire to a single touch of the abdomen or thorax.
Spikes of the lateral giant fibre are often masked in extracellular recordings with
many potentials being as large as those that appear to correlate with the spikes in
the giant fibre (Fig. 6 A). Spikes recorded intracellularly within the lateral giant
progressively decrement in amplitude as the preparation ages. Initially the intracellularly recorded spikes from the preparation in Fig. 6 were overshooting from a
resting potential of about 60 mV but had attenuated to only 25-30 mV by the time
of photographing.
Within a lateral giant a continuous bombardment of postsynaptic potentials can
fee recorded when the animal is at rest. These appear along the length of the fibre
G. E. SILVEY AND I. S. WILSON
132
Rt. Th, i
Rt. Ab. i
Fig. 7. Postsynaptic potentials in the lateral giant while the animal wag at rest. Intracellular
recordings in the giant show very low amplitude synaptic bombardment. Potentials do not
correlate on the same side of the body at different segmental levels (A) but many do on
opposite sides of the body in the same segment of the cord (B). Scale: 400 ms, 2 mV.
but are more readily recorded from the fibre near the septum within a ganglion.
There is no synchrony in these potentials at two points along the length of the fibre
(Fig. 7 A) but within contralateral fibres at the same segmental level (Fig. 7B) there
is some correlation.
Postsynaptic potentials become highly synchronous, however, in response to touch.
Tactile stimulation evokes depolarizations that arise coincidently in both fibres within
the same segment (Fig. 8). The responses shown in Fig. 8 are to tactile stimulation
rather than to visual stimulation or to vibration transmitted through the saline in
the chamber because the potentials occurred only when the points of the forceps
actually touched the animal; never when the forceps were moved about the saline
even quite close to the restrained animal. Stimulation of the head (Fig. 8 A) as well
as of the thorax (Fig. 8B) and abdomen (Fig. 8C) is effective in eliciting these
potentials. Thus, the receptive field of the lateral giant is the whole of the body and not
just the abdomen as in the crayfish (Wine & Krasne, 1972). Moreover, postsynaptic
potentials which arise to a touch of the body appear identically in each segment of
the same fibre. There is some delay, as one would expect, in the time at which potentials arise in different segments. Potentials arise progressively later in segments
further away from the site of afference.
Since spikes arise in both giants in response to input from one side of the body
and since postsynaptic potentials are coordinated in the same ganglion the question
arose of whether this is due to afferent neurones synapsing with each lateral giant
or to current spread between the giants. The crossover of the neurites in a ganglion
suggests that this is a site for current transfer as it is in the crayfish (Wiersma, 1947;
Watanabe & Grundfest, 1961). We tested this by passing current into one fibre while
recording in the other fibre at the same level on the opposite side of the body. Never
did we record a spread of current, either depolarizing or hyperpolarizing, whethe^
Lateral giant neurone of Anaspides
133
A
Lt. Th 3
Rt. Th3
>^?y***+***w *******
Fig. 8. Receptive field of the lateral giant encompasses the whole body. Touches to the head
(A), thorax (B) and abdomen ( Q provoke large amplitude postsynaptic potentials that are
synchronous within contralateral fibres at the same segmental level. Potentials were recorded
intracellularly. Scale: (A, C) 400 ms, 5 mV; (B) 200 ms, 2 mV.
passed across a bridge (Fig. 9 A) or directly through a microelectrode, despite introducing up to one microamp of current. We could cause the lateral giant to fire but
a spike so initiated never generated a spike or altered the level of polarization within
the contralateral fibre (Fig. 9B). Thus, the cells do not appear to be electrically
coupled. We also conclude that in order to produce synchronous postsynaptic potentials (Fig. 8) afferents must synapse identically with the processes of both fibres in
each hemiganglion.
DISCUSSION
In many respects the lateral giant fibre system in Anaspides is similar to that in the
crayfish. Morphologically the fibre is composed of 11 segments (cf. Johnson, 1924),
134
G. E. SILVEY AND I. S. WILSON
A,
A,
Lt. LG
Rt. LG
LI. Con. — ^ j i L
Rt. Con.
Stim: Lt. LG
_ J 'V^VV—
/-, ^ V
Rt. LG
Fig. 9. Failure of current and action potentials to spread between contralateral lateral giant
neurones. Neither depolarizing nor hyperpolarizing current ( ± 2 , 5 , 7 and ionA) passed
across a bridge into the left (AJ or right (A,) lateral giant spreads to the other cell. Similarly,
a giant fibre spike (dots under the potentials in the extracellular cord recording) evoked by
intracellular depolarization of the left giant (BO or right giant (B,) failed to excite the other
giant. The intracellular recordings in A l t , were made in the fifth thoracic segment of each
fibre. The extracellular cord recordings in Bi.i were made in the same preparation at the
same gain. This evidence suggests that the crossover between processes of the two lateral
giants in the commissure is not functional. Scale: (A) 100 ms, 50 mV; (B) 10 ms.
each arising from a soma in the last 5 thoracic ganglia and all 6 abdominal ganglia.
Each soma lies contralateral to its fibre (cf. Render et al. 1968). Behaviourally the
lateral giant mediated response consists of a rapid contraction of muscles of the body
wall (cf. Wine & Krasne, 1972). The response decrements (cf. Wine et al. 1975) and
the source of the decrement is on the input side of the circuit (cf. Krasne & Bryan,
1973). Tactile input causes the lateral giant fibre on both sides of the body to fire
and this firing alone is sufficient to elicit the fast contraction of the flexor muscles
(cf. Wiersma, 1947). The production of the flexion may be generated by multiple
firing of the lateral giant (cf. Kao, i960; Wine & Krasne, 1972).
In other respects the lateral giant systems are quite different. In Anaspides the
neurite of the soma gives off processes which invade the neuropiles of both ipsilateral
and contralateral hemiganglia. Only one process, that ipsilateral to the giant axon,
invades the crayfish neuropile (Render et al. 1968). Although the giant fibre mediated
response in both animals decrements to tactile stimulation, the decrement in Anaspides
begins much less rapidly than in the crayfish and recovers in slightly more than an
hour compared with more than a day for the crayfish (Wine et al. 1975). The neurites
of contralateral somata, despite coming close to each other where they cross over in
the commissure of a ganglion of Anaspides, are apparently not physiologically coupled;
current does not spread from one fibre to the other nor do antidromic spikes in one
fibre excite the other as they do in the crayfish (Watanabe & Grundfest, 1961;
Wiersma, 1947). This means that the mechanism by which a single touch on one
side of the body can excite both lateral giants of Anaspides is dependent on synapses
between afferents and both giants. Finally, Anaspides appears to lack a mechanism of
Lateral giant neurone of Anaspides
135
recurrent inhibition such as that in the crayfish (Roberts, 1968). The lateral giant in
Anaspides can apparently be re-excited during the course of a flexion-extension cycle
but to what effect and advantage is unknown.
In two respects the lateral giant system of Anaspides shows novel features. One is
that the lateral giant receives input from tactile receptors of the head and thorax as
well as of the abdomen, although only input from the fourth thoracic and posterior
segments was shown to stimulate the giant. Because the fast flexion developed to
rostral stimulation is different from that to abdominal and thoracic stimulation we
suspect that the input either recruits the medial giants alone or medial and lateral
giants together. The second novel feature, which is related to the first, is the flexion
of all segments of the body during an evasive response. Output from the lateral giant
is effective in recruiting thoracic muscles as well as abdominal muscles.
One significant feature of the system is the absence of electrical coupling between
giants. Tactile afferents, however, synapse separately with both lateral giants, and
thereby assure a mechanism by which both fibres can fire to input from any one side.
Nevertheless, in nature only the fibre ipsilateral to the side stimulated might fire.
If so, then, in addition to contraction of the flexor muscles of the body, certain
muscles ipsilateral to the excited giant could also contract and cause the directional
component of the evasive response.
The features described in this paper provide Anaspides with a useful means of
defence. Although it behaves somewhat cryptically, squeezing between and under
rocks and amongst roots, stems and debris, individuals walk about exposed at all
times of the day (Smith, 1909, and our observations). Also, Anaspides has a soft
and easily penetrated cuticle, and bears no chelae nor other offensive/defensive
morphological modifications (Manton, 1930). The evasive reaction seems to be its
only defence mechanism. That it can repeatedly flex to subsequent approaches of
a predator enables it to avoid continuous advances and the likelihood of being damaged
if it stood its ground. The crayfish does not give repeated flips to successive advances
(Wine et al. 1975)- But the crayfish has a relatively hard cuticle with heavily sclerotized
crushing and tearing chelae. When startled it may escape with one or more fast
flexions but advances of the predator are repelled by active application of the chelipeds
rather than further retreat.
The similarities in morphology and physiology of the lateral giant neurone in
Anaspides and the crayfish suggest that the lateral giant system in both animals is
homologous. Since the major groups to which Anaspides and the crayfish belong,
the Syncarida and Eucarida respectively, differentiated in the Carboniferous (Brooks,
1962 a, b) and considering the phylogenetic closeness of the Syncarida to the Eucarida
(Siewing, 1963), the lateral giant system probably arose in the stem line of these
two groups if not in the Cambrian stem line of the Malacostraca. The system, however, is not exactly the same in each animal. In each, it is structured and functions
slightly differently and seemingly to separate advantage for Anaspides and the crayfish.
In both animals the system has been commensurate with evolutionary survival,
namely, the ascendancy of the crayfish within relatively recent time and the persistence of Anaspides since the late Palaeozoic.
136
G. E. SILVEY AND I. S. WILSON
We are grateful to Dr Sam Lake for a set of slides of the nerve cord of Anaspides
and to Dr Roy Swain for the ion analysis of the blood that permitted derivation of
the saline for Anaspides.
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