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Efferent innervation to Limulus eyes
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eds), pp. 311-322, New York Academy of Sciences, New York Fourcin, A. J., Douek, E. E., Moore, B. C.J., Rosen, S. M., Walliker, J. IL, Howard, D. M., Abberton, E. and Frampton, S. (1983) in Cochlear Prostheses" An International Symposium (Parkins, C. W. and Anderson, S. W., eds), pp. 280-294, New York Academy of Sciences, New York Moore, B. C. J., Fourcin, A. J., Rosen, S. M., Walliker, J. R., Howard, D. M., Abberton, E., Douek, E. E. and Frampton, S. in lOth Anniversary Conference on Cochlear lmplants: An International Symposium (Schindler, IL A., ed.), Raven Press (in press) Tong, Y. C., Blarney, P. J., Dowell, R. C. and Clark, G. M. (1983) J. Acoust Soc. Am. 74, 73-80 Rosen, S. M., Fourcin, A. J. and Moore, B. C. J.
( 1981 ) Nature (London) 291, 150-I 52 14 Douek, E. El, Fourcin, A. J., Moore, B. C. J., Rosen, S. M., Walliker, J. R., Frampton, S. and Howard, D. M. (1983) in Cochlear Prostheses: An International Symposium (Parkins, C. W. and Anderson, S. W., eds), pp. 332-336, New York Academy of Sciences, New York 15 Walliker, J. R., Douek, E. E., Frampton, S., Abberton, E., Fourcin, A. J., Howard, D. M., Nevard, S., Rosen, S. M. and Moore, B. C. J. in lOth Anniversary Conference on Cochlear Implants: An International Symposium (Schindler, R.A., ed.), Raven Press (in press) 16 Howard, D.M. and Fourcin, A.J. (1983) ElectrorL LetL 19, 776-777 17 Abberton, E., Fourcin, A.J., Rosen, S.M., Walliker. J. 1L, Howard, D. M., Moore, B. C. J., Douek~ E. E. and Frampton, S. in lOth Anniversar3, Conference on Cochlear Implants: An
Efferent innervation to Limulus eyes B-A. Battelle The visual system of the horseshoe crab Limulus polyphemus has been an important model for studies of basic aspects of visiorL Experiments investigating phototransduction and contrast enhancement by lateral inhibition using Limulus eyes are famous. Recent studies with this animal are providing new information on still another feature of the visual system common to many species: the modulation of retinal function by efferent innervation from the brain. Retinal efferent systems have been identified in a variety of animals from invertebrates to mammals (see Ref 1 for selected references) and of these the efferent system projecting to Limulus eyes is now the most thoroughly understood Studies with Limulus have revealed that efferent innervation controls circadian changes in the sensitivity of the eye and influences the eye's integration of visual informatior~ adaptation to ambient light levels, and" renewal of photosensitive membrane Postsynaptic targets of efferent projections have been identified as has an efferent fiber neurotransmitter. The retinal efferent system in Limulus has opened the wayfor detailed studies of mechanisms underlying the central control of visual function. Organization of efferent projections to Lintuhts ventral and lateral eyes Limulus has three structurally very different types of eyes that have been important experimentally, lateral, median and ventral The locations of these eyes are illustrated in the left-hand drawing of Fig, 1. The lateral and median eyes are found on the dorsal carapace; the ventral eyes lie below the cuticle on the ventral side of the animal. Efferent fibers project to all three types of eyes but as most of the work on retinal efferents thus far has been done with ventral and lateral eyes, the present discussion will focus only on these two types. The organization of efferent projections in ventral and lateral eyes reflects the relative structural and functional complexity of the eyes.
Ventral eye The ventral eye is a very simple eye. It consists of an optic nerve that extends
anteriorly from the brain and ends immediately under the animal's ventral cuticle in a structure called the end organ. Each ventral optic nerve contains a.xons of approximately 300 photoreceptor cellsa. Generally, large clusters ofphotoreceptor cell bodies are found in the end organ and near the brain. Other photoreceptor cell bodies are scattered, either singly or in small clusters, apparently randomly along the periphery of the nerve trunk3 (Fig, 1, center and right). Ventral photoreceptor cell bodies can be very large, measuring up to 200 by 60 /.tm, and they have two lobes: a rhabdomeral lobe and an arhabdomeral lobe 4.5 (Fig, 1, fight). The rhabdomeral lobe contains all of the photosensitive membrane or rhabdorn. External rhabdom, on the surface of the rhabdomeral lobe, is formed by an elaborate folding of the plasma membrane into microvillar arrays. The internal rhabdom is formed from
International Symposium (Schindler, R.A., ed.), Raven Press (in press) 18 Simmons, F. B. and Smith, L. (1983) in Cochlear Prosthesesc An International Symposium (Parkinson, C. W. and Anderson, S. W., eds), pp. 422-423, New York Academy of Sciences, New York 19 Rosen, S.M., Fourcin, A.J., Abberton, E., Walliker, J. R., Howard, D. M., Moore, B. C. J., Douek, E. E. and Frampton, S. in lOth Anniversary Conference on Cochlear Implants: An International Symposium (Schindler, R.A., ed.), Raven Press (in press)
BHan C J. Moore is a Fellow of Wolfson College Cambridge He is also a Lecturer in Experimental Psychology at the University of Cambridg~ Downing Stree~ Cambridge CB2 3EB, UK.
invaginations of the extemal rhabdom 4. The arhabdomeral lobe, located nearest the axort, contains the nucleus of the cell and other cellular organelles but no photosensitive membrane. The bilobed organization of the ventral photoreceptor cell of Limulus can be likened to that of vertebrate cones with the rhabdomeral lobe analogous to the outer segment and the arhabdomeral lobe to the inner segment. The only neuronal elements in ventral optic nerves other than the photoreceptor cell bodies and their axons are the axons of retinal efferent fibers3. These efferent fibers are very small, measuring between 0.1 and 1.2 p.m in diameter. They have the appearance of the neurosecretory fibers found in many invertebrate preparations in that large pleomorphic dense granules accumulate in their terminal regions2.3(Fig, 3 ). These efferent fibers invaginate deeply into the photoreceptor cell bodies. The fibers are normally separated from the photoreceptor cell membrane by glia, but there are also areas of direct apposition between efferents and photoreceptors L3.4(Fig, 3), suggesting that the efferent fibers may innervate the ventral photoreceptor cells directly. In recent light and electron microscopic autoradiographic studies ~, the distribution of efferent fibers within the ventral eye and the relationship between efferent fibers and ventral photoreceptor cells were examined in detail These studies, described below, revealed that efferents project specifically to the internal rhabdom of ventral photoreceptor cells ~.~. A series of experiments investigating the neurotransmitter chemistry of cells in the Lira ulus visual system (some of which will be described in a later section) revealed that radiolabelled tyramine, a precursor for the biogenic amine octopamine, was taken up very specifically into efferent fibers 1.~. Efferent fibers were the only structures labelled in light and electron
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278 microscope autoradiographs of ventral have been corff'trmed by electron microand lateral eyes that had been incubated scopic studies in which dense granulewith radiolabelled tyramine and these containing fibers were found only in the fibers were labelled throughout their length. rhabdomeral lobe of the ventral photoBecause of the specific uptake of radio- receptor celP and frequently directly aplabelled tyramine, it was possible to use posed to microvilli of internal rhabdom ~'6 light microscope autoradiography to re- (Fig, 3) Areas of contact between efferent construct projections of efferent fibers in fibers and external rhabdom have not yet the ventral eye. Tracings were made of been observed. These morphological studthe locations of silver grains in autoradio- ies suggest that efferent fibers selectively graphs of 1/zm longitudinal serial sections innervate the internal rhabdom of ventral through a ventral optic nerve that had photoreceptor cells. This leads to the been incubated with radiolabelled tyra- speculation that input from efferent fibers mine. Using these tracings, a computer may modulate functions of the ventral reconstructed the efferent fiber projections. photoreceptor cell that are specific to this Fig, 2 shows a stereograph of three- light-sensitive membrane. dimensional reconstructions of efferent fibers projecting to two neighboring ven- L a t e r a l eye The lateral eye of L i m u l u s is a comtral photoreceptor cells, like those within pound eye. The carapace overlying the the boxed area of Fig, 1. Three features of efferent projections eye is transparent and modified into many become apparent in this and other recon- conical lens facets. Below each lens facet is the organizational unit of the eye called structions. (1) Each photoreceptor cell or small the ommatidium. Fig, 4 is a schematic of cluster of cells is innervated by a separate a longitudinal section through one such efferent axon. The total number of efferent ommatidium. An ommatidium contains axons in ventral optic nerves, between 70 8-12 photoreceptor or retinular cells arranged like sections of an orange around a and 200+, is consistent with this idea. (2) Efferent a.xons ramify extensively in central core. The central core area is the area of the photoreceptor cell occupied occupied by the dendrite of a second order neuron called the eccentric cell. by rhabdom. (3) Regions of the photoreceptor cell Pigment cells surround each ommatidium devoid of rhabdom do not receive efferent and form an aperture at the base of the innervation. The last two observations lens. The only neuronal cell types in the
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eye are photoreceptor and eccentric cells, both of which send axons to the brain. The eccentric cell dendrite is electrically coupled to the photoreceptor cells and when photoreceptors depolarize in response to light, action potentials can be recorded in the eccentric cell axon. It is not yet clear what kind of potential, if any, is conducted to the brain by axons of the retin,ular cells. A complex neuroplle formed by axon collaterals of eccentric cells lies between ommatidia. Interactions among ommatidia, such as lateral inhibition, occur in this neuropile and appear to be mediated via collaterals of eccentric cell axons2.7-9. A small number of efferent axons project from the brain to the lateral eye through the lateral optic nerve. As in the ventral eye, efferent fibers projecting to the lateral eye can be identified with the electron microscope by the pleomorphic dense granules they contain. Only 10-20 of these axons have been counted in lateral optic nerves ~.~ but these ramify extensively in the lateral eye to innervate all ommatidia" between 800 and 900 in the eye of an adult L i m u l u s 2. Details of the anatomical circuitry of efferents within the ommatidia and neuropile are being analysed TM. Synapses of efferent fibers have been observed on cell bodies of photoreceptor, eccentric and pigment cells and on collaterals of eccentric cell axons (Fig 4). Reciprocal synapses between eccentric cell collaterals and efferent fibers also have been observedTM. Based on this anatomy, one might expect efferent fiber activity to influence the function of all cell types in the ommatidia and to modulate interactions among ommatidia. As will be described below, studies of responses of cells in the lateral eye to efferent innervation bear out these exceptions. Function of retinal efferent fibers
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A striking feature of the activity of efferent fibers innervating L i m u l u s eyes is its cyclic nature l°. In an animal well ~\ \3 Cent,.. ', ~]l entrained to the natural light cycle, effer~Y ",,IJlJ ent fibers in the lateral optic nerve begin firing near dusk, they remain active throughout the night and stop firing near dawn. This cyclic change in activity has characteristics of a circadian rhythm. It has been Limulus Proto~:etebrum Ventralphotoreoeptots maintained in animals held in constant darkness for several days and the phase of Fig. 1. Left: outline of a dorsal view of Limulus. The compound lateral eyes and the median eyes are the cycle can be shifted l°.~t. located on the dorsal carapace The protocerebru~ circumesophageal ring, and ventral optic nerves are The efferent fibers in lateral optic near the ventral surface Center. enlarged view of the dorsal side of the protocerebrum showing the optic nerves fire synchronously, creating bursts ganglia (lamina, medulla and ocellar ganglion), lateral and median optic nerve stumps and ventral optic nerves` Ventral photoreceptor cell bodies are clustered near the brain and in the end organ and are of activity that occur at a maximum rate scattered randomly along the periphery of the ventral optic nerves. Right: enlarged view of a piece of of approximately two per second ~°. Bursts ventral optic nerve showing the large photoreceptor cell bodies, The rhabdomeral lobes, containing the of efferent activity have been recorded photosensitive membrane and arhabdomeral lobes are illustrated also in ventral and median optic nerves
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eye increases 2 0 - 1 0 0 - f o l d when efferent fibers are stimulated ~°.:1. During the natural circadian cycle of efferent activity, this increase in sensitivity occurs at night ~°.". Much of the increased sensitivity, measured as an increase in the amplitude of the electroretinogram (ERG), is the result of changes in the structure of ommatidia. These structural changes are illustrated in Fig, 4 (night-time state) ~. When efferent fibers become active, processes of distal pigment cells move outward from the base of the lens, widening and shortening the aperture. The photoreceptor cells move up toward the base of the lens and the rhabdom reorients to accommodate the enlarged aperture. In this night-time configuration, each ommatidium can collect light from a wider area and the photosensitive membrane is maximally exposed to the incident light. In addition to changes ~g. 2. Stereograph of three-dimensional reconstructions of efferent fibers projecting to two neighboring in structure, efferent activity induces ventral photoreceptor cell bodies like those enclosed within the box in the right-hand dra wing of Fig. L The changes in electrophysiological properties efferent fibers are drawn as solid lines. The dashed lines show the outlines of the cells as they appeared in of cells in the lateral eye that also conthe eighth of the sixteen serial I ttm sections used in the reconstructiotL Shaded regions show the area of tribute to increased sensitivity. The sponeach cell that was occupied by rhabdont Reconstructions were prepared from light microscope autoradiographs of ventral optic nerves that had been incubated in PHI tyramine to label spec~qcally the taneous activity of photoreceptor cells efferent fiber~ The reconstruction shows efferent fibers projecting to the internal rhabdom of ventral (noise) is reduced 14, the activity of eccenphotoreceptor cellz (Modifiedfrom Refs 1 and 6.) tric cells recorded in response to a single photon of light (gain) is increased 14 and and the bursts recorded from all optic activity exerts multiple effects on structure the strength of inhibitory interactions nerves are synchronous with one an- aiad function that combine to increase among ommatidia (lateral inhibition) is other j°.~2. One interpretation of these sensitivity 'D. The sensitivity of the lateral reduced '~. findings is that the same or tightly coupled circadian pacemaker's are driving the cell bodies of all of the efferent fibers projecting along the optic nerves ~2. Interestingly, the experiments demonstrafing this synchronous bursting activity were done using an excised brain preparation maintained in vitro 12. The preparation consists of the protocerebrum and circumesophogeal ring of the animal with proximal segments of all of the optic nerves attached (Fig, 1, left and center). Circadian efferent activity has been recorded for several days from the optic nerves of an excised brain; the preparation thus opens new possibilities for investigation of questions related to the neuronal circuitry that drives the efferent fibers. The cell bodies that give rise to retinal efferent fibers have not yet been located anatomically. But initial studies with the excised brain preparation have provided functional evidence that these cells and the circadian pacemaker(s) driving the cells are located in the protoeerebrum or circumesophageal ring and that axons of retinal efferents enter the optic nerves at the optic ganglia. Effects o f efferent f i b e r activity on the f u n c t i o n o f lateral a n d ventral eyes
Effects of efferent fiber activity on vision have been studied most extensively in the lateral eye. In this eye efferent fiber
Fig. 3. Electron microscope autoradiograph of an efferent fiber directly apposing (double arrow) microvilli of the internal rhabdom (R) of a ventral photoreceptor celL The efferent fiber is partially surrounded by glia (G) and contains large dense granules (arrow). The preparation was incubated with r l-l] tyramin~ The silver grains are concentrated only over the efferent fibers and represent newly-synthesized octopamine The bar equals l~tnt (From Ref. 1.)
T I N S - August 1984
280 Widening and shortening the aperture of the lens contributes to an increase in the quanta of light received but also increases the overlap between the receptive fields of neighboring ommatidia ~. Reducing the strength of lateral inhibition increases the overall ability of the eye to detect light but at the same time decreases its ability to detect edges tS. The increase in sensitivity is therefore accompanied by a decrease in the eye's ability to discriminate pattem. The effects described above are temporally linked to efferent activity and appear to occur in direct response to efferent input. Other processes have been identified in the lateral eye for which efferent input at night seems to be a necessary priming step. Such input is necessary in order for the eye to adapt its structure to changes in .ambient light levels the next day. If the circadian efferent input to the lateral eye is prevented by cutting the lateral optic nerve, the photomechanical movements of photoreceptor and pigment cells that normally accompany light adaptation are severely restricted ~6. Efferent input is also a prerequisite for the shedding that is part of the process o f daily renewal of photosensitive membrane t7.'8. Shedding in retinular cells of Limulus is analogous to the daily shedding of discs from vertebrate rods and cones. In Limulus, lateral eye shedding is initiated by the onset of light and normally occurs every day at dawn. If efferent input to the lateral eye is prevented during the night, shedding does not occurJT.~8.It is not clear how the activity of efferent fibers primes these processes. Efferent input may initiate metabolic changes in target cells that are required in order for the structural changes and the shedding to occur. In summary, efferent fiber input to Limulus lateral eye affects a number of functions that are basic to all primary visual organs. These include the ability to detect light, respond to light, discriminate patterns, adapt to ambient light levels and renew photosensitive membrane. Efferent input exerts its influence by modulating the structure, physiology and probably metabolism of cells in the eye. Further investigations with the lateral eye should clarify molecular mechanisms underlying the diverse effects of efferent input and yield new information on how basic aspects of vision are controlled and modulated. No information is available as yet on the functional consequences of natural efferent innervation to photoreceptors in the ventral eye. Investigations of this question have been hampered by the difficulty of recording from ventral photo-
receptors with their efferent innervation invertebrates, is a neurotransmitter of the intact. Unlike cells of the lateral eye, dense granule-containing efferent fibers ventral photoreceptors are relatively in- in Limulus ventral and lateral eyes. accessible in sit~ In-vitro experiments Octopamine synthetic activity has been traditionally have been done with ventral measured in ventral and lateral eyes and optic nerves cut from the brain and thus significant amounts of endogenous octosevered from the circadian pacemaker pamine are present in these eyes ng. Dense that drives the effefents. The difficulty of granule-containing fibers were identified studying efferent modulation of ventral as the sites o f octopamine synthesis and photoreceptor cells may be resolved by storage in analyses of electron microthe excised brain preparation mentioned scopic autoradiographs of ventral and above in which efferent activity in ventral lateral eyes incubated with radioactive optic nerves can be maintained for several tyramine, the immediate precursor of A Daytime state
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~g. 4. Schematic of a longitudinal section through an ommatidium in the lateral eye showing the major cell O'pes within the eye and the efferent projections to these cells. (A) Structure of the ommatidium during the day. (B) Structure of the ommatidium during the night when the efferent fibers are active. (Modified from Refs. 17 and 18.)
days in vitro L2. This preparation seems ideally suited for studies of direct effects of efferent innervation on photoreceptor cells without the complicating factors of efferent mediated changes in the structure and function of other surrounding cell
octopamine t,6. These experiments were done using conditions in which as much as 80% of the radioactive substances in the eyes at the end of the incubation was newly synthesized octopamine. The only labelled structures found in electron types. microscopic autoradiographs of the eye were the dense granule-containing efferent Neurotransmitter chemistry of fibers (Fig. 3). Ca2+-dependent release of Limulu.~" retinal efferent fibers newly synthesized octopamine from venTo begin to learn about mechanisms tral and lateral eyes also has been demonunderlying the diverse effects of efferent strated in vitro in response to K+-induced innervation on the function of the eye it is depolarization6.20. necessary to identify the neuroactive The effects of octopamine have been molecules released from retinal efferents. studied most extensively in the lateral There is now substantial evidence that eye. Injecting octopamine and known octopamine, a major biogenic amine of octopamine receptor agonists into the
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lateral eye during the day increases the sensitivity of the eye. Clozapine, an antagonist of octopamine receptors, blocks the increase in sensitivity induced by octopamine as well as natural efferent innervation z'. Octopamine very effectively decreases photoreceptor cell' noise '26 and increases the 'gain' of the lateral eye zt. The amine also changes the structure of ommatidia toward the night-time statea3. These and other pharmacological studies z~ indicate that functional receptors for octopamine are present in the lateral eye and that octopamine can stimulate many of the structural and functional changes that are observed with natural circadian efferent input to the lateral eye. Forskolin, a drug known to increase adenylate cyclase activity, and analogues of c A M P also modify the structure and drive up the sensitivity of the lateral eye23. Since octopamine increases levels ofintracellular c A M P in lateral eyes (Battelle, B-A., unpublished observations), a working hypothesis is that some of the effects of efferent innervation on lateral eyes may be mediated via octopamine-stimulated increases in the second messenger cAMP. It must be pointed out, however, that the increase in sensitivity observed after injecting even a high concentration of octopamine into the lateral eye never equals that achieved by stimulating the efferent fibers 2~. Structural changes observed after octopamine injections were also consistently less dramatic (Kass, L , unpublished observations). This may be due to a rapid metabolism of the injected amine ~9, but another possibility is that other neuroactive molecules released from efferent fibers may act together with octopamine to produce the total response. Serotonin ~7 and substance p34 also induce structural changes in the lateral eye and increase sensitivity. The physiological relevance of the effects of serotonin is unclear. No serotonin has been detected in lateral optic nerves or lateral eyes either biochemically (Battelle, B-A., unpublished observations) or immunocytochemically25 and the concentration of serotonin required to drive up lateral eye sensitivity is 100-fold higher than the concentration of octopamine used to initiate similar responsesaL On the other hand, a substance P-like molecule has been isolated from lateral-eyes z4 and fibers containing substance P-like immunoreactivity are present in the neuropile and between ommatidia ~4-27. The origin of fibers containing substance Plike immunoreactivity appears to be different from that of the octopamine-contalning fibers. Fibers capable of synthesizing and storing octopamine are present in the proximal ends of lateral and ventral
optic nerves at the level of the optic ganglia ~. Fibers containing substance Plike immunoreactivity are found only at the distal end of the lateral optic nervea4. No substance P-like immunoreactivity has been detected in ventral optic nerves 24.z7. Fibers containing substance P-like immunoreactivity may represent a second class of efferent fibers projecting to the lateral eye. Clearly more experiments are required to sort out the relationships between octopamine and the substance Plike molecule in producing the multiple effects associated with circadian efferent fiber activity. Although functional consequences of natural efferent innervation to ventral photoreceptors have been difficult to study, responses of these cells in vitro to octopamine are being examined. This work is just beginning, Studies already have shown that octopamine increases adenylate cyclase activity and elevates intracellular c A M P in ventral photoreceptor cells 2s. -Results of electrophysiological studies have shown that octopamine, possibly acting via cAMP, increases the rate at which ventral photoreceptor ceils recover sensitivity after a bright flash ofligh t3z. These results suggest that efferent input to ventral photoreceptor cells may modulate part of the photoresponse. Conclusion The information already gathered on the efferent innervation o f L i m u l u s eyes and the experimental preparations developed will facilitate detailed studies of several different aspects of central control of visual function. The Lirnulus excised brain preparation should prove very useful in analyses of the central circuitry that generates the circadian rhythm of efferent activity. Lateral eye preparations offer rich possibilities for examining the modulation of a variety of retinal functions. The ventral eye may be the preparation of choice for studies of biochemical and electr0physiological mechanisms underlying direct modulation of photoreceptor cells by the CNS. Identification of likely efferent fiber neurotransmitters provides new possibilities for in-vitro experiments with lateral and ventral eyes and makes available a battery of pharmacological tools that should be helpful in sorting out mechanisms underlying the complex effects of efferent input. Retinal functions similar to those modulated by efferent fiber activity in Liraulus may be influenced by efferent innervation in other animals as well. For example, efferent innervation is involved in controlling circadian pigment migration and sensitivity changes in eyes of scorpion 3°,
spiders ~ and crayfish zz. The circadian oscillator in the retinas of rats which controls the shedding of rod outer-segrnent discs may ultimately depend on efferent input from the brain to set the phase of its cycle 33. Efferent input to the retina of birds influences both the responsiveness and receptive field properties of ganglion cells. As in Liraulus lateral eye, the overall effect is to make the bird retina more responsive to a broader range of visual inputs zg. Information generated from studies of the efferent system innervating the relatively simple eyes of L i m u l u s should, therefore, result in a better understanding of the modulation of fundamental visual processes.
Acknowledgements Work done in the author's laboratory was supported in part by an NEI Postdoctoral Fellowship award to J. A. Evans. I would like to thank R. B. Barlow, Jr, P. A. Brewer, S. C. Chamberlain, J. A. Evans and D. G. Puro for their helpful comments on this manuscript.
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Springer, Berlin 10 Barlow, R. B. Jr, (1983) J. NeuroscL 3. 456870 11 Barlow, 1L B. Jr, Bolanowski, S.J. Jr. and Brachman, M. L (1977) Science 197, 86-88 12 Kass, L., Eisele, I_. E. and Barlow, R. B. Jr, (1983) lnvest OphthalmoL Visual ScL (Suppl) 24, 218 13 Barlow, R. B. Jr, Chamberlain, S. C. and Levinson, J. Z. (1980) Science 210, 1037-1039 14 Kaplan, E. and Barlow,R. B. Jr, (1980) Nature (London) 286, 393 15 Batra, IL and Barlow, R.B. Jr, (1982) Soe NeuroscL Abs~ 8, 49 16 Chamberlain, S. C. and BarlowR. B. Jr, ( 1981) Invest OphthalmoL Visual Sci (suppl) 20, 75 17 Barlow, R. B. Jr, and Chamberlain,S. C. (1980) in The Effects of Constant Light on the Visual Process (William, T. P. and Baker, B. N., eds), pp. 247-269, Plenum Press, New York 18 Chamberlain, S. C. and Barlow, R. B. Jr, (1979) Science 206, 361-363 19 Battelle, B-A. (1980) Vision Re~ 20, 911-922 20 Battelle, B-A_ and Evans, J.A. (1984) J.
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Barlow, 1L B. Jr, (1984) lnvesL OphthalmoL N e u r o c h e m . 42, 71-79 Visual Set (Suppl) 25, 288. 21 Kass, L and Badow, IL B. Jr, (1984) J. Neuro27 Chamberlain, S. C. and Engbretson, G.A. scL 4, 908-917 (1982) J. Comtx Neurol 208, 304--315 22 Page, T. L and Latimer,J. L (1975)J. Comtx 28 Kaupp,U. B., Malbon, C. C., Battelle,B-A. and Physiol 97, 81-96 Brown,J. E.(1982) VisionRex 22, 1503-1506 23 Kass, L.and Barlow, IL B. Jr, (1981) Biot 29 Miles, F. A. (1970) Science 170, 992-995 • B u l l 161,348 24 Mantillas, J. 1L and Brown, M. R. (1984) J. 30 Fteissner, (3. and Fleissner, G. (1978) Comp. Biochent 61 A, 69-71 Neuroscl 4, 832--846 25 Chamberlain, S. C., Battelle, B-A. and Wyse, 31 Yamashita,S. and Tateda, H. (1981)./. Comp. Physiol (A) 143,477--483 G. A. (1983)So," NeuroscL Abstr. 9, 76 26 Pelletier, J. L, Kass, L, Renninger, G. H. and 32 O'Day, P. M. and Lisman, J. E. (1984)Invest
OphthalmoL Visual ScL (8uppl) 25, 253 33 Tierstein, P. S., Goldman,A. J. and O'Brien,P. £(1980) Invest Ophthalraol Visual Sc[ 19, 1268-1273 34 Mantillas, J. R. and Selverston, A. L (1984)J. Neurosc[ 4, 847-859 35 Fahrenbach, W. H. (1981) Cell Tissue Res. 216, 655-659
The neuronal activity in the supplementary motor area
and hand in the absence of proximal muscle activity, some S M A neurons were found to be active TM. An example of an SMA neuron exhibiting a marked increase in activity prior to a key press movement is shown in Fig, I. These observations indicate participation of some S M A neurons in the execution of even the simplest limb movements.
of primates Jun Tanji
B-A- Battelie is at the Laboratory of Vision Research at the National Eye lnstitut¢ National Institutes of Health, Bethesda, MD 20205, USA.
Is there somatotopy in S M A ? Attempts to study somatotopic organizThe functional role o f the supplementary motor area ( S M A ) has been largely ation of SM A by electrical stimulation of unknown f o r the 30 years since its definitiort However, recent work employing the technique o f single cell recording f r o m behaving primates offers important clues as to the cortical surface have given rise to conflicting results. In Woolsey's m a p " , how activity in this area takes part in controlling motorperformanc~ Whereas S M A activity is not as related to execution o f simple movements as that in the precentral for instance, the hindlimb area of S M A is motor cortex, S M A neurons respond strongly to motor instructions that determine the located mainly beneath the tail area of precentraI motor cortex and these two way a.nimals have to respond to forthcoming sensory signalz areas extend similarly along the anteroThe term supplementary motor area active in relation to execution of simple posterior axis. This does not agree with (SMA) was first introduced by Penfleld movements and might be primarily in- observations by Penfleld and Welch 1, and Welch ~ to designate a secondary volved in control of posture or in organiz- who described the S M A hindlimb area as motor area situated rostromedially to the ation of eomplex movements. extending 10mm anteriorly from the preprimary, precentral motor area of the Results of recent single cell recording central tail area. The technique of eleccerebral cortex of primates, including experiments, however, do not support this trical surface stimulation seems inademan. The SMA is located in the superior view. Brinkman and Porter 8 found a num- quate for studying functional organization frontal gyrus on the medial wall of the ber of neurons active prior to and during of S M A for two reasons: it is difficult to hemisphere and in the upper bank of the execution of lever pulling or food taking evoke discrete movements localized to a cingulate sulcus, corresponding to the movements. Smith 9 also found SMA portion of limbs or body, and it is necesmedial part of cytoarchitechtonic areas neurons to be active during a precisiofi sary to use much higher current than in 6 ac~ and 6 aft of Vogt and Vogt2. Results of grip. Furthermore, even if the movement the precentral motor cortex and therefore ablation and electrical stimulation studies was performed exclusively by the fingers the stimulus current inevitably spreads of the SMA, as well as clinical observations on human patients have been well documented in recent reviews3-s. This article will focus on recent studies of the SMA that apply the technique of recording from single cells in animals performhag movements. This technique made it possible to ask several hitherto una answered questions about the functional organization and role of the SMA.
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Is the S M A active prior to execution of simple limb movements? Electrical stimulation of the SMA gives rise to complex, synergistic limb and body movements in man and often in monkeys ~.s. Ablation of the SMA does not affect f'me movements involving distal parts of the extremities* as does a lesion of the primary motor cortex. These facts led to an assumption that S M A might not be
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eds), pp. 311-322, New York Academy of Sciences, New York Fourcin, A. J., Douek, E. E., Moore, B. C.J., Rosen, S. M., Walliker, J. IL, Howard, D. M., Abberton, E. and Frampton, S. (1983) in Cochlear Prostheses" An International Symposium (Parkins, C. W. and Anderson, S. W., eds), pp. 280-294, New York Academy of Sciences, New York Moore, B. C. J., Fourcin, A. J., Rosen, S. M., Walliker, J. R., Howard, D. M., Abberton, E., Douek, E. E. and Frampton, S. in lOth Anniversary Conference on Cochlear lmplants: An International Symposium (Schindler, IL A., ed.), Raven Press (in press) Tong, Y. C., Blarney, P. J., Dowell, R. C. and Clark, G. M. (1983) J. Acoust Soc. Am. 74, 73-80 Rosen, S. M., Fourcin, A. J. and Moore, B. C. J.
( 1981 ) Nature (London) 291, 150-I 52 14 Douek, E. El, Fourcin, A. J., Moore, B. C. J., Rosen, S. M., Walliker, J. R., Frampton, S. and Howard, D. M. (1983) in Cochlear Prostheses: An International Symposium (Parkins, C. W. and Anderson, S. W., eds), pp. 332-336, New York Academy of Sciences, New York 15 Walliker, J. R., Douek, E. E., Frampton, S., Abberton, E., Fourcin, A. J., Howard, D. M., Nevard, S., Rosen, S. M. and Moore, B. C. J. in lOth Anniversary Conference on Cochlear Implants: An International Symposium (Schindler, R.A., ed.), Raven Press (in press) 16 Howard, D.M. and Fourcin, A.J. (1983) ElectrorL LetL 19, 776-777 17 Abberton, E., Fourcin, A.J., Rosen, S.M., Walliker. J. 1L, Howard, D. M., Moore, B. C. J., Douek~ E. E. and Frampton, S. in lOth Anniversar3, Conference on Cochlear Implants: An
Efferent innervation to Limulus eyes B-A. Battelle The visual system of the horseshoe crab Limulus polyphemus has been an important model for studies of basic aspects of visiorL Experiments investigating phototransduction and contrast enhancement by lateral inhibition using Limulus eyes are famous. Recent studies with this animal are providing new information on still another feature of the visual system common to many species: the modulation of retinal function by efferent innervation from the brain. Retinal efferent systems have been identified in a variety of animals from invertebrates to mammals (see Ref 1 for selected references) and of these the efferent system projecting to Limulus eyes is now the most thoroughly understood Studies with Limulus have revealed that efferent innervation controls circadian changes in the sensitivity of the eye and influences the eye's integration of visual informatior~ adaptation to ambient light levels, and" renewal of photosensitive membrane Postsynaptic targets of efferent projections have been identified as has an efferent fiber neurotransmitter. The retinal efferent system in Limulus has opened the wayfor detailed studies of mechanisms underlying the central control of visual function. Organization of efferent projections to Lintuhts ventral and lateral eyes Limulus has three structurally very different types of eyes that have been important experimentally, lateral, median and ventral The locations of these eyes are illustrated in the left-hand drawing of Fig, 1. The lateral and median eyes are found on the dorsal carapace; the ventral eyes lie below the cuticle on the ventral side of the animal. Efferent fibers project to all three types of eyes but as most of the work on retinal efferents thus far has been done with ventral and lateral eyes, the present discussion will focus only on these two types. The organization of efferent projections in ventral and lateral eyes reflects the relative structural and functional complexity of the eyes.
Ventral eye The ventral eye is a very simple eye. It consists of an optic nerve that extends
anteriorly from the brain and ends immediately under the animal's ventral cuticle in a structure called the end organ. Each ventral optic nerve contains a.xons of approximately 300 photoreceptor cellsa. Generally, large clusters ofphotoreceptor cell bodies are found in the end organ and near the brain. Other photoreceptor cell bodies are scattered, either singly or in small clusters, apparently randomly along the periphery of the nerve trunk3 (Fig, 1, center and right). Ventral photoreceptor cell bodies can be very large, measuring up to 200 by 60 /.tm, and they have two lobes: a rhabdomeral lobe and an arhabdomeral lobe 4.5 (Fig, 1, fight). The rhabdomeral lobe contains all of the photosensitive membrane or rhabdorn. External rhabdom, on the surface of the rhabdomeral lobe, is formed by an elaborate folding of the plasma membrane into microvillar arrays. The internal rhabdom is formed from
International Symposium (Schindler, R.A., ed.), Raven Press (in press) 18 Simmons, F. B. and Smith, L. (1983) in Cochlear Prosthesesc An International Symposium (Parkinson, C. W. and Anderson, S. W., eds), pp. 422-423, New York Academy of Sciences, New York 19 Rosen, S.M., Fourcin, A.J., Abberton, E., Walliker, J. R., Howard, D. M., Moore, B. C. J., Douek, E. E. and Frampton, S. in lOth Anniversary Conference on Cochlear Implants: An International Symposium (Schindler, R.A., ed.), Raven Press (in press)
BHan C J. Moore is a Fellow of Wolfson College Cambridge He is also a Lecturer in Experimental Psychology at the University of Cambridg~ Downing Stree~ Cambridge CB2 3EB, UK.
invaginations of the extemal rhabdom 4. The arhabdomeral lobe, located nearest the axort, contains the nucleus of the cell and other cellular organelles but no photosensitive membrane. The bilobed organization of the ventral photoreceptor cell of Limulus can be likened to that of vertebrate cones with the rhabdomeral lobe analogous to the outer segment and the arhabdomeral lobe to the inner segment. The only neuronal elements in ventral optic nerves other than the photoreceptor cell bodies and their axons are the axons of retinal efferent fibers3. These efferent fibers are very small, measuring between 0.1 and 1.2 p.m in diameter. They have the appearance of the neurosecretory fibers found in many invertebrate preparations in that large pleomorphic dense granules accumulate in their terminal regions2.3(Fig, 3 ). These efferent fibers invaginate deeply into the photoreceptor cell bodies. The fibers are normally separated from the photoreceptor cell membrane by glia, but there are also areas of direct apposition between efferents and photoreceptors L3.4(Fig, 3), suggesting that the efferent fibers may innervate the ventral photoreceptor cells directly. In recent light and electron microscopic autoradiographic studies ~, the distribution of efferent fibers within the ventral eye and the relationship between efferent fibers and ventral photoreceptor cells were examined in detail These studies, described below, revealed that efferents project specifically to the internal rhabdom of ventral photoreceptor cells ~.~. A series of experiments investigating the neurotransmitter chemistry of cells in the Lira ulus visual system (some of which will be described in a later section) revealed that radiolabelled tyramine, a precursor for the biogenic amine octopamine, was taken up very specifically into efferent fibers 1.~. Efferent fibers were the only structures labelled in light and electron
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278 microscope autoradiographs of ventral have been corff'trmed by electron microand lateral eyes that had been incubated scopic studies in which dense granulewith radiolabelled tyramine and these containing fibers were found only in the fibers were labelled throughout their length. rhabdomeral lobe of the ventral photoBecause of the specific uptake of radio- receptor celP and frequently directly aplabelled tyramine, it was possible to use posed to microvilli of internal rhabdom ~'6 light microscope autoradiography to re- (Fig, 3) Areas of contact between efferent construct projections of efferent fibers in fibers and external rhabdom have not yet the ventral eye. Tracings were made of been observed. These morphological studthe locations of silver grains in autoradio- ies suggest that efferent fibers selectively graphs of 1/zm longitudinal serial sections innervate the internal rhabdom of ventral through a ventral optic nerve that had photoreceptor cells. This leads to the been incubated with radiolabelled tyra- speculation that input from efferent fibers mine. Using these tracings, a computer may modulate functions of the ventral reconstructed the efferent fiber projections. photoreceptor cell that are specific to this Fig, 2 shows a stereograph of three- light-sensitive membrane. dimensional reconstructions of efferent fibers projecting to two neighboring ven- L a t e r a l eye The lateral eye of L i m u l u s is a comtral photoreceptor cells, like those within pound eye. The carapace overlying the the boxed area of Fig, 1. Three features of efferent projections eye is transparent and modified into many become apparent in this and other recon- conical lens facets. Below each lens facet is the organizational unit of the eye called structions. (1) Each photoreceptor cell or small the ommatidium. Fig, 4 is a schematic of cluster of cells is innervated by a separate a longitudinal section through one such efferent axon. The total number of efferent ommatidium. An ommatidium contains axons in ventral optic nerves, between 70 8-12 photoreceptor or retinular cells arranged like sections of an orange around a and 200+, is consistent with this idea. (2) Efferent a.xons ramify extensively in central core. The central core area is the area of the photoreceptor cell occupied occupied by the dendrite of a second order neuron called the eccentric cell. by rhabdom. (3) Regions of the photoreceptor cell Pigment cells surround each ommatidium devoid of rhabdom do not receive efferent and form an aperture at the base of the innervation. The last two observations lens. The only neuronal cell types in the
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A striking feature of the activity of efferent fibers innervating L i m u l u s eyes is its cyclic nature l°. In an animal well ~\ \3 Cent,.. ', ~]l entrained to the natural light cycle, effer~Y ",,IJlJ ent fibers in the lateral optic nerve begin firing near dusk, they remain active throughout the night and stop firing near dawn. This cyclic change in activity has characteristics of a circadian rhythm. It has been Limulus Proto~:etebrum Ventralphotoreoeptots maintained in animals held in constant darkness for several days and the phase of Fig. 1. Left: outline of a dorsal view of Limulus. The compound lateral eyes and the median eyes are the cycle can be shifted l°.~t. located on the dorsal carapace The protocerebru~ circumesophageal ring, and ventral optic nerves are The efferent fibers in lateral optic near the ventral surface Center. enlarged view of the dorsal side of the protocerebrum showing the optic nerves fire synchronously, creating bursts ganglia (lamina, medulla and ocellar ganglion), lateral and median optic nerve stumps and ventral optic nerves` Ventral photoreceptor cell bodies are clustered near the brain and in the end organ and are of activity that occur at a maximum rate scattered randomly along the periphery of the ventral optic nerves. Right: enlarged view of a piece of of approximately two per second ~°. Bursts ventral optic nerve showing the large photoreceptor cell bodies, The rhabdomeral lobes, containing the of efferent activity have been recorded photosensitive membrane and arhabdomeral lobes are illustrated also in ventral and median optic nerves
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eye increases 2 0 - 1 0 0 - f o l d when efferent fibers are stimulated ~°.:1. During the natural circadian cycle of efferent activity, this increase in sensitivity occurs at night ~°.". Much of the increased sensitivity, measured as an increase in the amplitude of the electroretinogram (ERG), is the result of changes in the structure of ommatidia. These structural changes are illustrated in Fig, 4 (night-time state) ~. When efferent fibers become active, processes of distal pigment cells move outward from the base of the lens, widening and shortening the aperture. The photoreceptor cells move up toward the base of the lens and the rhabdom reorients to accommodate the enlarged aperture. In this night-time configuration, each ommatidium can collect light from a wider area and the photosensitive membrane is maximally exposed to the incident light. In addition to changes ~g. 2. Stereograph of three-dimensional reconstructions of efferent fibers projecting to two neighboring in structure, efferent activity induces ventral photoreceptor cell bodies like those enclosed within the box in the right-hand dra wing of Fig. L The changes in electrophysiological properties efferent fibers are drawn as solid lines. The dashed lines show the outlines of the cells as they appeared in of cells in the lateral eye that also conthe eighth of the sixteen serial I ttm sections used in the reconstructiotL Shaded regions show the area of tribute to increased sensitivity. The sponeach cell that was occupied by rhabdont Reconstructions were prepared from light microscope autoradiographs of ventral optic nerves that had been incubated in PHI tyramine to label spec~qcally the taneous activity of photoreceptor cells efferent fiber~ The reconstruction shows efferent fibers projecting to the internal rhabdom of ventral (noise) is reduced 14, the activity of eccenphotoreceptor cellz (Modifiedfrom Refs 1 and 6.) tric cells recorded in response to a single photon of light (gain) is increased 14 and and the bursts recorded from all optic activity exerts multiple effects on structure the strength of inhibitory interactions nerves are synchronous with one an- aiad function that combine to increase among ommatidia (lateral inhibition) is other j°.~2. One interpretation of these sensitivity 'D. The sensitivity of the lateral reduced '~. findings is that the same or tightly coupled circadian pacemaker's are driving the cell bodies of all of the efferent fibers projecting along the optic nerves ~2. Interestingly, the experiments demonstrafing this synchronous bursting activity were done using an excised brain preparation maintained in vitro 12. The preparation consists of the protocerebrum and circumesophogeal ring of the animal with proximal segments of all of the optic nerves attached (Fig, 1, left and center). Circadian efferent activity has been recorded for several days from the optic nerves of an excised brain; the preparation thus opens new possibilities for investigation of questions related to the neuronal circuitry that drives the efferent fibers. The cell bodies that give rise to retinal efferent fibers have not yet been located anatomically. But initial studies with the excised brain preparation have provided functional evidence that these cells and the circadian pacemaker(s) driving the cells are located in the protoeerebrum or circumesophageal ring and that axons of retinal efferents enter the optic nerves at the optic ganglia. Effects o f efferent f i b e r activity on the f u n c t i o n o f lateral a n d ventral eyes
Effects of efferent fiber activity on vision have been studied most extensively in the lateral eye. In this eye efferent fiber
Fig. 3. Electron microscope autoradiograph of an efferent fiber directly apposing (double arrow) microvilli of the internal rhabdom (R) of a ventral photoreceptor celL The efferent fiber is partially surrounded by glia (G) and contains large dense granules (arrow). The preparation was incubated with r l-l] tyramin~ The silver grains are concentrated only over the efferent fibers and represent newly-synthesized octopamine The bar equals l~tnt (From Ref. 1.)
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280 Widening and shortening the aperture of the lens contributes to an increase in the quanta of light received but also increases the overlap between the receptive fields of neighboring ommatidia ~. Reducing the strength of lateral inhibition increases the overall ability of the eye to detect light but at the same time decreases its ability to detect edges tS. The increase in sensitivity is therefore accompanied by a decrease in the eye's ability to discriminate pattem. The effects described above are temporally linked to efferent activity and appear to occur in direct response to efferent input. Other processes have been identified in the lateral eye for which efferent input at night seems to be a necessary priming step. Such input is necessary in order for the eye to adapt its structure to changes in .ambient light levels the next day. If the circadian efferent input to the lateral eye is prevented by cutting the lateral optic nerve, the photomechanical movements of photoreceptor and pigment cells that normally accompany light adaptation are severely restricted ~6. Efferent input is also a prerequisite for the shedding that is part of the process o f daily renewal of photosensitive membrane t7.'8. Shedding in retinular cells of Limulus is analogous to the daily shedding of discs from vertebrate rods and cones. In Limulus, lateral eye shedding is initiated by the onset of light and normally occurs every day at dawn. If efferent input to the lateral eye is prevented during the night, shedding does not occurJT.~8.It is not clear how the activity of efferent fibers primes these processes. Efferent input may initiate metabolic changes in target cells that are required in order for the structural changes and the shedding to occur. In summary, efferent fiber input to Limulus lateral eye affects a number of functions that are basic to all primary visual organs. These include the ability to detect light, respond to light, discriminate patterns, adapt to ambient light levels and renew photosensitive membrane. Efferent input exerts its influence by modulating the structure, physiology and probably metabolism of cells in the eye. Further investigations with the lateral eye should clarify molecular mechanisms underlying the diverse effects of efferent input and yield new information on how basic aspects of vision are controlled and modulated. No information is available as yet on the functional consequences of natural efferent innervation to photoreceptors in the ventral eye. Investigations of this question have been hampered by the difficulty of recording from ventral photo-
receptors with their efferent innervation invertebrates, is a neurotransmitter of the intact. Unlike cells of the lateral eye, dense granule-containing efferent fibers ventral photoreceptors are relatively in- in Limulus ventral and lateral eyes. accessible in sit~ In-vitro experiments Octopamine synthetic activity has been traditionally have been done with ventral measured in ventral and lateral eyes and optic nerves cut from the brain and thus significant amounts of endogenous octosevered from the circadian pacemaker pamine are present in these eyes ng. Dense that drives the effefents. The difficulty of granule-containing fibers were identified studying efferent modulation of ventral as the sites o f octopamine synthesis and photoreceptor cells may be resolved by storage in analyses of electron microthe excised brain preparation mentioned scopic autoradiographs of ventral and above in which efferent activity in ventral lateral eyes incubated with radioactive optic nerves can be maintained for several tyramine, the immediate precursor of A Daytime state
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days in vitro L2. This preparation seems ideally suited for studies of direct effects of efferent innervation on photoreceptor cells without the complicating factors of efferent mediated changes in the structure and function of other surrounding cell
octopamine t,6. These experiments were done using conditions in which as much as 80% of the radioactive substances in the eyes at the end of the incubation was newly synthesized octopamine. The only labelled structures found in electron types. microscopic autoradiographs of the eye were the dense granule-containing efferent Neurotransmitter chemistry of fibers (Fig. 3). Ca2+-dependent release of Limulu.~" retinal efferent fibers newly synthesized octopamine from venTo begin to learn about mechanisms tral and lateral eyes also has been demonunderlying the diverse effects of efferent strated in vitro in response to K+-induced innervation on the function of the eye it is depolarization6.20. necessary to identify the neuroactive The effects of octopamine have been molecules released from retinal efferents. studied most extensively in the lateral There is now substantial evidence that eye. Injecting octopamine and known octopamine, a major biogenic amine of octopamine receptor agonists into the
281
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lateral eye during the day increases the sensitivity of the eye. Clozapine, an antagonist of octopamine receptors, blocks the increase in sensitivity induced by octopamine as well as natural efferent innervation z'. Octopamine very effectively decreases photoreceptor cell' noise '26 and increases the 'gain' of the lateral eye zt. The amine also changes the structure of ommatidia toward the night-time statea3. These and other pharmacological studies z~ indicate that functional receptors for octopamine are present in the lateral eye and that octopamine can stimulate many of the structural and functional changes that are observed with natural circadian efferent input to the lateral eye. Forskolin, a drug known to increase adenylate cyclase activity, and analogues of c A M P also modify the structure and drive up the sensitivity of the lateral eye23. Since octopamine increases levels ofintracellular c A M P in lateral eyes (Battelle, B-A., unpublished observations), a working hypothesis is that some of the effects of efferent innervation on lateral eyes may be mediated via octopamine-stimulated increases in the second messenger cAMP. It must be pointed out, however, that the increase in sensitivity observed after injecting even a high concentration of octopamine into the lateral eye never equals that achieved by stimulating the efferent fibers 2~. Structural changes observed after octopamine injections were also consistently less dramatic (Kass, L , unpublished observations). This may be due to a rapid metabolism of the injected amine ~9, but another possibility is that other neuroactive molecules released from efferent fibers may act together with octopamine to produce the total response. Serotonin ~7 and substance p34 also induce structural changes in the lateral eye and increase sensitivity. The physiological relevance of the effects of serotonin is unclear. No serotonin has been detected in lateral optic nerves or lateral eyes either biochemically (Battelle, B-A., unpublished observations) or immunocytochemically25 and the concentration of serotonin required to drive up lateral eye sensitivity is 100-fold higher than the concentration of octopamine used to initiate similar responsesaL On the other hand, a substance P-like molecule has been isolated from lateral-eyes z4 and fibers containing substance P-like immunoreactivity are present in the neuropile and between ommatidia ~4-27. The origin of fibers containing substance Plike immunoreactivity appears to be different from that of the octopamine-contalning fibers. Fibers capable of synthesizing and storing octopamine are present in the proximal ends of lateral and ventral
optic nerves at the level of the optic ganglia ~. Fibers containing substance Plike immunoreactivity are found only at the distal end of the lateral optic nervea4. No substance P-like immunoreactivity has been detected in ventral optic nerves 24.z7. Fibers containing substance P-like immunoreactivity may represent a second class of efferent fibers projecting to the lateral eye. Clearly more experiments are required to sort out the relationships between octopamine and the substance Plike molecule in producing the multiple effects associated with circadian efferent fiber activity. Although functional consequences of natural efferent innervation to ventral photoreceptors have been difficult to study, responses of these cells in vitro to octopamine are being examined. This work is just beginning, Studies already have shown that octopamine increases adenylate cyclase activity and elevates intracellular c A M P in ventral photoreceptor cells 2s. -Results of electrophysiological studies have shown that octopamine, possibly acting via cAMP, increases the rate at which ventral photoreceptor ceils recover sensitivity after a bright flash ofligh t3z. These results suggest that efferent input to ventral photoreceptor cells may modulate part of the photoresponse. Conclusion The information already gathered on the efferent innervation o f L i m u l u s eyes and the experimental preparations developed will facilitate detailed studies of several different aspects of central control of visual function. The Lirnulus excised brain preparation should prove very useful in analyses of the central circuitry that generates the circadian rhythm of efferent activity. Lateral eye preparations offer rich possibilities for examining the modulation of a variety of retinal functions. The ventral eye may be the preparation of choice for studies of biochemical and electr0physiological mechanisms underlying direct modulation of photoreceptor cells by the CNS. Identification of likely efferent fiber neurotransmitters provides new possibilities for in-vitro experiments with lateral and ventral eyes and makes available a battery of pharmacological tools that should be helpful in sorting out mechanisms underlying the complex effects of efferent input. Retinal functions similar to those modulated by efferent fiber activity in Liraulus may be influenced by efferent innervation in other animals as well. For example, efferent innervation is involved in controlling circadian pigment migration and sensitivity changes in eyes of scorpion 3°,
spiders ~ and crayfish zz. The circadian oscillator in the retinas of rats which controls the shedding of rod outer-segrnent discs may ultimately depend on efferent input from the brain to set the phase of its cycle 33. Efferent input to the retina of birds influences both the responsiveness and receptive field properties of ganglion cells. As in Liraulus lateral eye, the overall effect is to make the bird retina more responsive to a broader range of visual inputs zg. Information generated from studies of the efferent system innervating the relatively simple eyes of L i m u l u s should, therefore, result in a better understanding of the modulation of fundamental visual processes.
Acknowledgements Work done in the author's laboratory was supported in part by an NEI Postdoctoral Fellowship award to J. A. Evans. I would like to thank R. B. Barlow, Jr, P. A. Brewer, S. C. Chamberlain, J. A. Evans and D. G. Puro for their helpful comments on this manuscript.
Reading list 1 Evans, J. A., Chamberlain, S. C. and Battelle, B-A. (1983),/. Comp. NeuroL 219, 369-383 2 Fahrenbach, W. H. (1975)InrRev. CytoL 41, 385-349 3 Clark, A. W., Millecchia, R. and Mauro, A. (1969) J. GerL PhysioL 54, 289-309 4 Calmart,B. G. and Chamberlain, S. C. (1982)J. Gert PhysioL 80, 839-862 5 Stem. J., Chinn, K., Bacigalupo,J. and Lisman, J. (1982) J. Geg PhysioL 80, 825-837 6 Battelle, B-A., Evans, J. A., and Chamberlain, S. C. (1982) Science 216, 1250-1252 7 Fahrenbach, W. H. (1983) So~ NeuroscL Abstr. 9, 215 8 Fain, G. I_. and Lisman, J. E. (1981) Prog. Biophys. Mot BioL 37, 91-147 9 Hartline, K. H. and Ratliff, F. (1972) in Handbook of Sensory Physiology: Physiology of PhotoreceptorOrgans, Vol.VII/2, pp. 381--447
Springer, Berlin 10 Barlow, R. B. Jr, (1983) J. NeuroscL 3. 456870 11 Barlow, 1L B. Jr, Bolanowski, S.J. Jr. and Brachman, M. L (1977) Science 197, 86-88 12 Kass, L., Eisele, I_. E. and Barlow, R. B. Jr, (1983) lnvest OphthalmoL Visual ScL (Suppl) 24, 218 13 Barlow, R. B. Jr, Chamberlain, S. C. and Levinson, J. Z. (1980) Science 210, 1037-1039 14 Kaplan, E. and Barlow,R. B. Jr, (1980) Nature (London) 286, 393 15 Batra, IL and Barlow, R.B. Jr, (1982) Soe NeuroscL Abs~ 8, 49 16 Chamberlain, S. C. and BarlowR. B. Jr, ( 1981) Invest OphthalmoL Visual Sci (suppl) 20, 75 17 Barlow, R. B. Jr, and Chamberlain,S. C. (1980) in The Effects of Constant Light on the Visual Process (William, T. P. and Baker, B. N., eds), pp. 247-269, Plenum Press, New York 18 Chamberlain, S. C. and Barlow, R. B. Jr, (1979) Science 206, 361-363 19 Battelle, B-A. (1980) Vision Re~ 20, 911-922 20 Battelle, B-A_ and Evans, J.A. (1984) J.
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Barlow, 1L B. Jr, (1984) lnvesL OphthalmoL N e u r o c h e m . 42, 71-79 Visual Set (Suppl) 25, 288. 21 Kass, L and Badow, IL B. Jr, (1984) J. Neuro27 Chamberlain, S. C. and Engbretson, G.A. scL 4, 908-917 (1982) J. Comtx Neurol 208, 304--315 22 Page, T. L and Latimer,J. L (1975)J. Comtx 28 Kaupp,U. B., Malbon, C. C., Battelle,B-A. and Physiol 97, 81-96 Brown,J. E.(1982) VisionRex 22, 1503-1506 23 Kass, L.and Barlow, IL B. Jr, (1981) Biot 29 Miles, F. A. (1970) Science 170, 992-995 • B u l l 161,348 24 Mantillas, J. 1L and Brown, M. R. (1984) J. 30 Fteissner, (3. and Fleissner, G. (1978) Comp. Biochent 61 A, 69-71 Neuroscl 4, 832--846 25 Chamberlain, S. C., Battelle, B-A. and Wyse, 31 Yamashita,S. and Tateda, H. (1981)./. Comp. Physiol (A) 143,477--483 G. A. (1983)So," NeuroscL Abstr. 9, 76 26 Pelletier, J. L, Kass, L, Renninger, G. H. and 32 O'Day, P. M. and Lisman, J. E. (1984)Invest
OphthalmoL Visual ScL (8uppl) 25, 253 33 Tierstein, P. S., Goldman,A. J. and O'Brien,P. £(1980) Invest Ophthalraol Visual Sc[ 19, 1268-1273 34 Mantillas, J. R. and Selverston, A. L (1984)J. Neurosc[ 4, 847-859 35 Fahrenbach, W. H. (1981) Cell Tissue Res. 216, 655-659
The neuronal activity in the supplementary motor area
and hand in the absence of proximal muscle activity, some S M A neurons were found to be active TM. An example of an SMA neuron exhibiting a marked increase in activity prior to a key press movement is shown in Fig, I. These observations indicate participation of some S M A neurons in the execution of even the simplest limb movements.
of primates Jun Tanji
B-A- Battelie is at the Laboratory of Vision Research at the National Eye lnstitut¢ National Institutes of Health, Bethesda, MD 20205, USA.
Is there somatotopy in S M A ? Attempts to study somatotopic organizThe functional role o f the supplementary motor area ( S M A ) has been largely ation of SM A by electrical stimulation of unknown f o r the 30 years since its definitiort However, recent work employing the technique o f single cell recording f r o m behaving primates offers important clues as to the cortical surface have given rise to conflicting results. In Woolsey's m a p " , how activity in this area takes part in controlling motorperformanc~ Whereas S M A activity is not as related to execution o f simple movements as that in the precentral for instance, the hindlimb area of S M A is motor cortex, S M A neurons respond strongly to motor instructions that determine the located mainly beneath the tail area of precentraI motor cortex and these two way a.nimals have to respond to forthcoming sensory signalz areas extend similarly along the anteroThe term supplementary motor area active in relation to execution of simple posterior axis. This does not agree with (SMA) was first introduced by Penfleld movements and might be primarily in- observations by Penfleld and Welch 1, and Welch ~ to designate a secondary volved in control of posture or in organiz- who described the S M A hindlimb area as motor area situated rostromedially to the ation of eomplex movements. extending 10mm anteriorly from the preprimary, precentral motor area of the Results of recent single cell recording central tail area. The technique of eleccerebral cortex of primates, including experiments, however, do not support this trical surface stimulation seems inademan. The SMA is located in the superior view. Brinkman and Porter 8 found a num- quate for studying functional organization frontal gyrus on the medial wall of the ber of neurons active prior to and during of S M A for two reasons: it is difficult to hemisphere and in the upper bank of the execution of lever pulling or food taking evoke discrete movements localized to a cingulate sulcus, corresponding to the movements. Smith 9 also found SMA portion of limbs or body, and it is necesmedial part of cytoarchitechtonic areas neurons to be active during a precisiofi sary to use much higher current than in 6 ac~ and 6 aft of Vogt and Vogt2. Results of grip. Furthermore, even if the movement the precentral motor cortex and therefore ablation and electrical stimulation studies was performed exclusively by the fingers the stimulus current inevitably spreads of the SMA, as well as clinical observations on human patients have been well documented in recent reviews3-s. This article will focus on recent studies of the SMA that apply the technique of recording from single cells in animals performhag movements. This technique made it possible to ask several hitherto una answered questions about the functional organization and role of the SMA.
Light
Is the S M A active prior to execution of simple limb movements? Electrical stimulation of the SMA gives rise to complex, synergistic limb and body movements in man and often in monkeys ~.s. Ablation of the SMA does not affect f'me movements involving distal parts of the extremities* as does a lesion of the primary motor cortex. These facts led to an assumption that S M A might not be
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Key.press..... ~ ...............
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L
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500 ms Fi& 1. Discharges of an SMA neuron whoseaclivit2 increasedprior to a key press movementpeorormed in responseto a visual signal In rasterdisplays; dischargesare displayed as dots and each horizontal r o w corresponds to individual performance of the motor task. In each peri-event time histogrorg discharges are summated in successive16 ms periodx Discharges are aligned with the onset of the light signal(left) and with the movement onset (right).
1984, Elsevier SclencePublishers ~ V,, Anmterdam 0Y18-$9121841502.00