observations on giant fibres of the nervous system of locusta migratoria · 2006-05-17 · of...

297

Observations on Giant Fibres of the Nervous Systemof Locusta migratoria

By P. M. COOK, B.Sc.(From the Department of Zoology, The University, Glasgow)

SUMMARY

There are four fibres of giant dimensions in each cord of the abdominal ventralnerve-chain of the locust. These fibres are the axons of large neurons situated in. theextreme posterior region of the last abdominal ganglion, and run uninterruptedlythrough the abdominal nerve-chain to the metathoracic ganglion. Posteriorly theseneurons synapse with preganglionic fibres which appear to be cereal nerve-fibres andanteriorly with motor pathways to the metathoracic legs.

Oscillographic observations have been correlated with those from the histologicalstudy. The possible function of these pathways has been discussed and comparisonmade with the cockroach, where a similar organization has been described in detail.

INTRODUCTION

THE method developed by Pumphrey and Rawdon-Smith (1937) torecord electrical activity from the ventral nerve-cord of a cockroach

offers a simple preparation for the study of a central synapse in an inverte-brate. It is the synapse in the last abdominal ganglion between the primaryvibration receptor afferents from the cereal nerve and ascending axons ofgiant neurons in the abdominal cord. This preparation involves a singlesynapse in each of a relatively few ascending pathways, thereby simplifyinganalysis of the records and approximating to a single fibre preparation.Lowenstein (1942) used the time of abolition of the response to 'air-puff'stimulation in this preparation as a method of bio-assay of pyrethrum extracts.Roeder et al. (1947) used the preparation for the study of the action of variousdrugs on the cockroach synapse and Roeder (1948) extended the study of theorganization of the giant fibre system both electrically and histologically.

A similar preparation is at present being used by the author for the studyof the mode of action of certain neurotoxic insecticides on Locusta migratoriamigratorioides (R. & F.). Since the literature contains little or no detaileddescription of this region of the nervous system in the locust, this histologicalstudy was undertaken, partly as a necessary background for the physiologicalwork, and partly to provide anatomical information not as yet available in thecase of the locust. This information is important for comparisons of thefunctions of the giant fibre systems in Periplaneta and Locusta.

METHODS

Adult locusts were dissected, and the nervous system was exposed by theremoval of the alimentary canal and genital organs with the nervous system[Quarterly Journal of Microscopical Science, Vol. 92, part 3, pp. 297-305, September 1.951.]

2421.19 X

298 Cook—Giant Fibres of the Nervous System of Locusta migratoria

intact in situ. The dissection was then fixed in Bouin's fluid for 24 hours.After fixation, short regions of the abdominal cord, including the last twoganglia and cereal nerves, were dissected out and embedded in paraffin wax.In other cases the ventral nervous system, excluding the cephalic, sub-oesophageal, and prothoracic ganglia, was embedded in ester wax (Steedman,1947). Serial sections were cut at 10/n and stained regressively in Heidenhain'shaematoxylin.

The method used in recording the electrical activity from the exposedventral nervous system is essentially that described by Pumphrey and Rawdon-Smith (1937), Lowenstein (1942), and Roeder (1947). It will be described indetail elsewhere.

RESULTS

The acridian central nervous system (C.N.S.) as described by Snodgrass(1935) consists anteriorly of a dorsal cephalic ganglion united by a pair ofconnectives round the oesophagus with a sub-oesophageal ganglion. This isfollowed by three thoracic and five abdominal ganglia. The present study isconcerned only with the posterior region of the C.N.S. of Locusta migratoriaas far forward as the metathoracic ganglion (fig. 1). This does not differ in anyessential details from that of Dissosteira (Snodgrass, 1935; Nesbitt, 1941) orRhomalea (Nesbitt, 1941). In Locusta adipose tissue forms a complete sheatharound each abdominal cord, leaving only the ganglia exposed. Anteriorly,paired segmental nerves, whose ganglia are situated far forward, are closelyapplied to the abdominal cords and are consequently included in the fattysheath. The various nerves entering the posterior region of the last abdominalganglion on either side are similarly ensheathed in adipose tissue and may thusappear as a single pair. Each member of this pair contains four separatebundles of nerve-fibres. One is an afferent sensory nerve from the cercus ofthat side, one innervates the genitalia, and the other two the musculature inthis region.

A transverse section of these nerves shows them to be composed of nerve-fibres all of a diameter less than 5 /n. The general appearance is one of homo-geneity, with little variation in fibre size. A transverse section of the abdominalcord between the ultimate and penultimate ganglia has a markedly differentappearance. Approximately two-thirds of the area of cross-section of each'cord is taken up by closely packed fibres running parallel to the longitudinalaxis of the cord, whilst the remaining third, ventro-lateral in position, con-tains only a few scattered distinct fibres, the remainder of the tissue showingup rather indistinctly in sections (fig. 2A). While the majority of fibres are lessthan 5/x in diameter there is a definite variation in fibre size with four fibresof diameter greater than 5/i and one of 5/4 diameter in each cord.

These four large fibres can be followed from the anterior region of the fifthabdominal ganglion right up to the posterior region of the metathoracicganglion. They pass through all the intervening ganglia without interruption(fig. 2, B, D, E). These four fibres are arranged in two groups. In each cord there

Cook—Giant Fibres of the Nervous System of Locusta migratoria 299

wing,muscle

mesothoracicganglion

metathoracic

abdominalsegment 6

abdominalseoment 7

abdominalsegment 5

remainsqemtaha

right cercus

FIG. I. Dissection of an adult male Locusta migratoria migratorioides to show the posteriorregion of the central nervous system. The locust was dissected from the dorsal side; a mediandorsal strip of body-wall, the alimentary canal, and most of the gentalia were removed. Asheet of tissue, which separates off a median ventral portion of the haemocoel in which liesthe nervous system, was carefully dissected away. The adipose tissue deposits around the

nerve-cords were also removed.


is a single median fibre of an approximate average diameter of 13/x, and aperipheral latero-dorsal group of three fibres, two of which have an averagediameter of 12/u, and the third an average diameter of S-^fi. This arrangementis maintained throughout the length of the abdominal cord. In each ganglionthe dorsal group of three remains on the periphery following the dorsal con-tours of the C.N.S. while the median fibre takes a straighter course, beingmedian in the cord but more dorsal in the ganglia. These fibres do not retaina constant diameter throughout their length but show a general tendency totaper towards their anterior ends. In addition they are constricted as they passthrough each ganglion. Table I shows average measurements for these fibres at

TABLE I. Measurements, in microns, of Giant Fibres concerned in CerealResponse on Left Side of C.N.S.

All measurements are of fixed material. The fibre measurements are the means of'horizontal' and 'vertical' dimensions.

Region of C.N.S.

Cord betw. abd. g. 5 and 4Ant. reg. abd. g. 4 .Cord betw. abd. g. 4 and 3Post. reg. abd. g. 3 .Cord betw. abd. g. 3 and zPost. reg. abd. g. 2 .Cord betw. abd. g. 2 and 1Post. reg. abd. g. 1 .Cord betw. abd. g. 1 and thor

g- 3 •Extreme post. reg. thor. g. 3

Fibres

1

I S-212-6iS-7I2-O14-7168136147II-O6-8

2

14212-615710-5136

I2-I12-6

9-457

3

126136136io-o147

I2-II2-I

946-8

4

8-98-48-7639'4

9-4I2"I

7'95'2

Cord or ganglion over-allmeasurements

Vertical(D-V)130-2

117-6293 41092293-4924

2771

92-41956

Horizontal{L-R).

134-4537-9ii3-4521-6" 3 4472-7966

352-6

92-4326-0

different levels in the ventral nerve-cord made on one series of sections. All themeasurements given are taken from fixed material; there does not, however,appear to be any gross shrinkage, since in vivo measurements for the largefibres are 12—16 fj.. These in vivo measurements were not made from transversesections, but from whole cords dissected in saline (Belar, 1929) and completelyfreed from fat tissue. The maximum diameter of the largest fibres visible wasthen measured. Attempts to prepare transverse sections of fresh material havenot proved successful.

The single median fibre in each cord is the axon of a large neuron, the nearlyspherical cell body of which is situated in the extreme posterior region of the

EXPLANATION OF FIG. zTransverse sections at various levels in the abdominal region of the central nervous

system of one locust. A. The left cord between abdominal ganglia 5 and 4. c. The left cordbetween abdominal ganglia 4 and 3. E. The left cord between abdominal ganglion 1 and themetathoracic ganglion. B. Posterior region of abdominal ganglion 4. Low magnification.D. Left dorsal region of abdominal ganglion 4 as indicated in B. F. Posterior region of abdominalganglion 1.

1, single median giant fibre; z, 3, 4, peripheral group of three giant fibres; dotted line en-closes indistinct region containing few axons. .

Quart. Journ. Micr. Set., New Series, Vol. 92

region shownin fig. D

P. M. COOK—Fio. 2


last abdominal ganglion. It has a diameter of about $O\L. Processes from thisgiant neuron are in close contact with the most median group of preganglionicfibres entering the fifth abdominal ganglion (fig. 3).

The peripheral group of three axons could not be traced back to their con-nexions with individual. cells. These three fibres could be followed quitereadily through the anterior region of the fifth abdominal ganglion; behindthis the three fibres become very closely applied to the dorsal boundary of theganglion, and finally disappear in the posterior third of the ganglion where anumber of fairly large cells is situated. Some of these cells also have processesin close contact with the most median group of preganglionic fibres, whichwould therefore appear to be those derived from the cercus.

Anteriorly the giant fibres cannot be traced farther forward than the posteriorregion of the metathoracic ganglion. The cord between the metathoracic andthe mesothoracic ganglia carries many fibres of diameter comparable withthat of the abdominal giants. Such fibres are also found in considerablenumbers in the nerves leaving the metathoracic ganglion.

In the abdominal cord the four fibres described are not the only ones ofgiant dimensions; sections anterior to the fourth abdominal ganglion showother such fibres. In the cords between ganglia 4 and 3, 3 and 2, and 2 and 1are found a number of fibres resembling the smaller giants, i.e. approximately9JU diameter; their origin is unknown (fig. 2c). One such fibre resembling insize the larger giants is found in the cord between the metathoracic and thefirst abdominal ganglia (fig. 2E).

Oscillographic recording of the impulses from the cereal nerve shows aburst of impulses coinciding with a puff of air in the region of the intact cercus(fig. 4A). Recording electrodes placed on the abdominal cord pick up a similarburst of impulses on stimulation of the cereal receptors, except that twomuch larger sizes of spikes are present (fig. 4B). These spikes are also con-siderably faster than those in the cereal nerve and are therefore consideredto belong to fibres of greater diameter than those in the cereal nerve. Electrodesplaced on the large nerve from the metathoracic ganglion innervating themetathoracic leg pick up a burst of larger slower impulses every time thereis a puff of air directed at the cerci. Removal of the cerci causes complete dis-appearance of such impulses in all positions on 'air-puff' stimulation. Elec-trical stimulation of the cereal nerve provokes a response similar to that ofnatural stimulation.

A few comparative behaviour experiments on Periplaneta and Locusta werecarried out in a hot room (300 C) . A puff of air was directed on to the cerciof both these insects by means of a glass jet attached to a length of rubbertubing. The jet was moved about near the insects before the experimentr, inorder to accustom them to its presence, and to ascertain that it did not, itself,produce a reaction. When a puff of air was then directed at the cerci of Peri-planeta it caused the insect to move away rapidly from the region of the glassjet. The distance covered in this 'evasion response' was usually only a fewinches, but the 'locomotor activity which may carry the insect several feet'


k ft cereal afferents

right cercaaFfzrents

left cell body I

50 fi

axon 1 right cell body 1

right cerca! aFFerents

B

FIG. 3. A. Transverse section through extreme posterior region of abdominal ganglion 5showing the cell bodies of the two median giant fibres, one in each cord (i.e. fibre 1). B. Trans-verse section slightly anterior to that shown in A (30 n). The axon can be seen emergingfrom the right cell body. This section has, unfortunately, been damaged in the region of

the left axon.


(Roeder, 1948) was not observed. The locusts, on the other hand, behavedvery differently. When the glass jet was placed near the cerci without dis-turbing the insect, a puff of air did not produce any response resembling the'evasion response' in the cockroach. Locusts, blinded by painting the com-pound eyes with black paint, were much more quiescent and showed noresponse at all to movements in the region of the cerci, nor did a decapitatedlocust show any response to this stimulus, despite the elimination of possibleinhibitory centres in the cephalic ganglion.

B

FIG. 4. Oscillographic recordings from (A) cereal nerve, and (B) abdominal cord, betweenganglia 4 and 3 on natural cereal stimulation. Upper time marker in each case represents

50 per second; lower action signal in each case represents duration of stimulation.

DISCUSSION

It is, perhaps, useful at this stage to compare the giant fibre system of thelocust with that of the cockroach. Periplaneta americana has been most com-monly used in insect neurophysiological investigations, due mainly to itslarge size and availability. Thus it is the only species amongst the Orthopterain which the giant fibre system has hitherto been studied histologically.Roeder (1948) describes six giant fibres in each abdominal cord ranging from20 to 45 n in diameter and 10 to 12 fibres of 5 to 20/x in diameter. The fibresare arranged in two main groups, a ventral group formed by the three largestfibres and a smaller fibre and a dorsal, similarly arranged, quartet of fibreswhich are all slightly smaller than their ventral counterparts. The largestfibres in Periplaneta are thus considerably bigger than the four large fibres inLocusta, which only reach a diameter of 12 to 15 p. Both in Periplaneta andin Locusta the giant fibres pass uninterruptedly through the abdominal ganglia


and* are considerably reduced in diameter while passingthroughthe abdominalganglia. Roeder was not able to trace connexions of these fibres with individualcell bodies, but thought that they are multicellular, and arise from groups ofcell bodies situated in the posterior peripheral region of the last abdominalganglion. This region is close to the point of entrance of the cereal nerve.While it was not possible in the cockroach to pin-point the synaptic regionbetween giants and efferents, there was no morphological evidence for thepresence of interneurons.

Not only are the giant fibres of Locusta thinner than those of Periplaneta:there are also fewer of them, viz. only five fibres of a diameter of 5 /A or over.The most medianly situated of these fibres could be traced to its connexionwith a cell body and is, in all probability, unicellular. The other three largefibres were not traced to individual cell bodies, although they disappear inthe region of large cell bodies in the peripheral part of the posterior region ofthe fifth abdominal ganglion. The possibility cannot be excluded that the cellbodies of these fibres lie in the metathoracic ganglion. This, however, doesnot seem likely since these fibres, like the median fibre, taper towards theiranterior ends.

Roeder (1948) considered it probable that one of the two quartets of fibresin Periplaneta served as a pathway for ascending impulses, the other quartetprobably being concerned with descending impulses. In Locusta, both naturaland electrical cereal stimulation evoke spikes in the abdominal cord whichare both larger and faster than those recorded in the cereal nerve. These largespikes are not uniform in size, so that it would seem probable that more thanone large fibre is involved in the response.

Prosser (1950) states that giant nerve-fibres have evolved many times, andthat whereas they are present in some species, they can be absent in closelyrelated forms. Wherever giant fibres are present they seem to function in rapidescape reactions, their efficiency depending on the fact that a single nervousunit brings about a complex fast response. Roeder (1948), in summarizing theobservations of Pumphrey and Rawdon-Smith and himself, states that'Characteristic of cockroach behaviour is the burst of evasive locomotionwhich follows a sound or gentle air movement near the insects'. This reactionis called the 'evasion response'. It is considered to be initiated by the stimula-tion of fine hair sensillae on the cerci. Impulses are carried into the sixthabdominal ganglion by the preganglionic cereal fibres, spatially summated atsynapses with the giant fibres, and carried forwards to the metathoracicganglion. Here two other types of synapse are found, one with shorter giantsrunning forward, the other with fibres innervating the metathoracic legs whichare the most powerful legs. This initiates the 'evasive locomotion' which isthen sustained by descending impulses from the head region. Decapitatedcockroaches show only a brief twitch of the legs, or at the most a few rapidsteps, after stimulation in the same manner.

The 'evasion response' as such is not strikingly evident in the locust.According to Ewing (1904) a grasshopper with the cephalic ganglion removed


reacts to all external stimuli with exaggerated movements, so that if an'evasion response' similar to that in the cockroach did exist in the locust,decapitation would be expected to exaggerate the initial reaction in thisresponse. No such increase could be observed in Locusta. The fact that thecereal 'evasion response' is absent in the locust is not altogether surprising.The locust's cerci are smaller and less prominent than in the cockroach andare to some extent shielded by the wings. On the other hand, it is highly prob-able that the compound eyes and aerodynamic sense organs (Weis-Fogh,1949) on the head, and auditory organs play a more important part in locustbehaviour. The habitats of the two genera are also very different. Whereas thecereal 'evasion response' in the cockroach would seem to be of definite survivalvalue, its value for the migratory locust is not so evident.

The functional significance of the giant fibre system in the locust is notthus very obvious although, oscillographically, the systems in Periplaneta andLocusta behave very similarly. The extra large multicellular giant fibres of thecockroach may conceivably represent a specialization, perhaps an aggregationof several neurons, tending to give an increase in fibre size with a correspond-ing increase in rate of conduction.

ACKNOWLEDGEMENTS

I wish to express my sincere thanks to Dr. O. Lowenstein for his very kindhelp, interest, and criticism in this work, also to Dr. H. F. Steedman for hishelp and advice over the choice of an embedding medium and with the photo-micrographs. This work was carried out with the aid of a grant from the Anti-Locust Research Centre (under the direction of Dr. B. P. Uvarov, F.R.S.).

REFERENCESBELAR, K., 1929. Arch. EntwMech., 118, 359.EWING, H. E.( 1904. Kansas Univ. Sci. Bull., 2, 305.LOWENSTEIN, O., 1942. Nature, 150, 760.

'NESBITT, H. H. J., 1941. Ann. ent. Soc. Amer., 34, 51.PROSSER, C. L., 1950. Comparative Animal Physiology. Philadelphia (W. B. Saunders

Company).PUMPHREY, R. J., and RAWDON-SMITH, F., 1937. Proc. Roy. Soc. B, 122, 106.ROEDER, K. D., KENNEDY, N. K., and SAMSON, E. A., 1947. J. Neurophysiol., 10, 1.

1948. J. exp. Zool., 108, 243.STEEDMAN, H. F., 1947. Quart, j . micr. Sci., 88, 123.SNODGRASS, R. E., 1935. Principles of Insect Morphology. London.WEIS-FOGH, T., 1949. Nature, 164, 873.

observations on giant fibres of the nervous system of locusta migratoria · 2006-05-17 · of...

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