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ARTICLES
Year : 1981  |  Volume : 29  |  Issue : 3  |  Page : 263-267

Effect of optic nerve sectioning on E.R.M.


Dr. Rajendra Prasad Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi, India

Correspondence Address:
P K Khosla
Dr. Rajendra Prasad Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi
India
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Source of Support: None, Conflict of Interest: None


PMID: 6809614

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How to cite this article:
Khosla P K, Saini J S, Gahlot D K, Ratnakar K S. Effect of optic nerve sectioning on E.R.M. Indian J Ophthalmol 1981;29:263-7

How to cite this URL:
Khosla P K, Saini J S, Gahlot D K, Ratnakar K S. Effect of optic nerve sectioning on E.R.M. Indian J Ophthalmol [serial online] 1981 [cited 2020 Nov 24];29:263-7. Available from: https://www.ijo.in/text.asp?1981/29/3/263/30899

Regulation of retinal activity by extra-reti­nal centrifugal fibres has been a matter of interest and discussion for many years. The subject has been explored from various aspects and in a large number of animals but the results in most cases have remained inconclu­sive and at time contradictory.

The present experimental study in monkeys was designed to explore electrophysiologically and histologically the existence of extrareti­nal centrifugal innervation of the retina.


  Materials and methods Top


The study was undertaken in six healthy monkeys. Electrophysiological and histolo­gical methods were used to investigate the possible existence of optic nerve mediated neural centrifugal influence on retina.

Electroretinograms were recorded from both eyes on Mingograf-800 under standardized conditions before undertaking surgery.

Optic nerve on the right side was sectioned transcrainally just in front of the chiasma under direct vision. ERG were again recor­ded in both eyes soon after surgery and then on each of the postoperative days.

At the conclusion of electrophysiological study the eye balls of both sides were enuclea­ted and processed. Flat whole, retinal pre­ parations were stained with the silver impre­gnation techhique (Gallego's modification of Gros-Bielchowsky's method) with following modifications:-1. Hyalase was used to help easy removal of vitreous from the retinal surface. 2. Fixation of retinae in 20% for malin was limited to only 15 minutes which helped reduce friability of retina.


  Observations Top


The observations were recorded as follows:­

(a) Preoperative:- ERG amplitude varied from 0.18 my to 0.25 my with a mean of 0.2041+0.017 my for the right eye and 0.18mv to 0.25 my with a mean of 0.207 020 my for

the left eye.

(b) Post operative:­

  1. Immediately after sectioning the right optic nerve: ERG amplitude for the right eye varied from 0.09 my to 0.20 my (mean 0.122±0.051 mv). It differed from the preoperative E.R.G. by 0.0882 my which is statistically significant (p<0.001). E.R.G. ampli­tude for the left eye averaged 0.09 my ±0.017 my with difference of 0.117 my from preoperative amplitude and was also significant (p<0.001).
  2. First postoperative day - the mean E.R.G. amplitude for the right eye in six animals was 0.140±0.044 my and this differed from preoperative E.R.G. significantly (p<0,00I). Mean E.R.G. amplitude for the left eye was 0.114,mv and the difference from preoperative value was also significant (p<0.001).
  3. Second postoperative day - the mean amplitude in right eye was 0.15±0.057 my and differed by 0.54 my from the mean preoperative amplitude which is significant (p<0.001). Mean E.R.G. in left eye was 0.108 my and it differed from preoperative E.R.G. Significantly (p<0.001).

    The mean E.R.G. amplitude in right eye does not differ significantly from that in the left eye during the first two postoperative days.
  4. Third postoperative day - the ampli­tude in right eye averaged 0.17±0.08 mv which is only 0.034 my lower than the preoperative E.R.G. and the difference was poorly significant (p <0.02). E.R.G. amplitude for the left eye averaged 0.08 my which was significantly different from preopera­tive E.R.G. amplitude (p<0.00I).
  5. Fourth postoperative day - the mean E.R.G. amplitude for the right eye was 0.36±0.056 mv. which was 0.156 my higher than the mean preoperative E.R.G. and the difference was significant (p<0.001). The mean E.R.G. amplitude for the left eye was 0 204 my which was, however, not significantly different from the preoperative E.R.G. (p=N.S.).
  6. Fifth postoperative day - Mean E.R.G for the right eye was 0.379±0.065 mv, which was 0.175 my higher than the mean preoperative E.R.G. and this difference was significant (p<0.001). The E.R.G. amplitude for the left eye averaged 0.220 my and this did not differ significantly from the mean preoperative E.R.G. (p= N.S.).
  7. Sixth postoperative day - Mean ampli­tude for the right eye was 0.390±0.076 my which was higher and differed significantly from the mean preopera­tive E.R.G. (p<0.001). Mean E.R.G. amplitude for the left eye was 0.130+ 0.069 my which was not significantly different from the preoperative E.R.G. amplitude (p= N.S.).
  8. Seventh postoperative day - only one animal (No.2) survived upto this day and the E.R.G. amplitude in right eye was 0.310 my i.e. significantly higher from the preoperative amplitude while it was I my in the left eye i.e. not significantly different from preoperative reading.


II. Histological observation

The morphological findings in all the retinae studied were similar and can be summa­rised as follow:

Stained flat retinal preoperations show some peculiar thick intensely argyrophilic fibre like structures in addition to the regularly arranged nerve fibres converging towards papilla and blood vessels with a central lumen. Vertical sections through these areas of thicker fibres were made and studied.

In some vertical sections, fibres which were initially within the nerve fibre layer were seen to cross the ganglion cell layer and inner plexiform layer to end in the inner nuclear layer [Figure - 2]. These fibres were intensely argyraphilic and had a sinuous course. Similar deeply argyrophilic fibres were also seen coursing horizontally external to the ganglion cell layer [Figure - 3]. The sinuously coursing vertical fibres had no nuclet at their origin in the nerve fibre layer and passed clear of any nuclei present in the ganglion cell layer. No intraretinal neuronal origin could be identified and for these fibres. Therefore, the fibres described are probably centrifugal in nature and originated from extraretinal neurons.


  Discussion Top


Contradictions continue to prevail regard­ing the possible existence of centrifugal con­trol of retinal activity. None-the-less a con­siderable body of anatomical and physiologi­cal evidence has been collected which suggests that optic nerve mediated centrifugal impulse may be involved in regulating retinal acti­vity[2],[3],[4],[5].

In our study there occurred bilateral decrease in `b' wave amplitude immediately after and for two days following unilateral intracranial optic nerve sectioning. Such an observation has not been reported before. Mascetti et als described in their experiments in cats that stimulation of sympathetic fibres could inhibit E.R.G. amplitude. It is possi­ble that in our experiments surgical trauma may have caused excitation of sympathetic centres (hypothalmus) which is anatomically close to the chiasma. Therefore, direct or indirect activation of the centres for sympa­thetic nerves and the resultant increase in their known inhibitory influences on the retina might have caused bilateral decrease in the b­wave.

On the 3rd postoperative day the E.R.G. amplitude recovered to its preoperative value in the right eye (operated eye), but in the left eye (unoperated eye) it continued to be low. Thereafter from the 4th day onwards the b-wave amplitude become supernormal in the right eye (operated eye) while it recovered to its preoperative vulun in the left eye. There seem to be two sources of inhibition on the basis of available evidence (i) mediated through sympathetic fibres and (ii) mediated through centrifugal fibres. Right sided optic nerve transection removed additional source of inhibition for the right eye (centrifugal fibres) while the sympathetic system mediated inhibition remained operative for both eyes [Figure - 4]. The dual inhibition in the left eye could be responsible for differential manifest b-wave inhibition. As the b-wave is seen to become supernormal on the operated side, this necessarily implied that the act of optic nerve transection has removed some inhibitory influence on the right side. Thus the electro­physiological observations strongly suggest that optic nerve carries inhibitory impulses to the retina.

In our study histologically in transverse sections of the retina intensely argyrophilic stained fibres coursing vertically from the nerve fibre layer across the ganglion cell layer end­ing in the inner nuclear layer were seen. These fibres were probably of extraretinal origin.

The demonstration of certain extra retinal fibres in the retina could be significant in that these may be the anatominal pathway of the centrifugal inhibitory inpulses.


  Summary Top


This study employed electrophysiological and histological methods for investigating the existence of centrifugal fibres to retina in monkeys. The study used E.R.G. as the electrophysiological prarmeter before and after intracranial unilateral optic nerve transection. The 'b' was amplitude was seen to decrease significantly in both eyes immediately after the surgery and for the next two days. From the 4th day onwards `b' wave in the operated eye became supernormal but recovered to normal in the unoperated eye. The observa­tions go to show definite existence of inhibi­tory centrifugal fibres influencing the retinal activity. It appears that both optic nerve and possibly sympathetic fibres carry these centrifugal fibres. Silver impregnation stain­ing of retina showed vertically descending fibres from the nerve fibre layer to terminate in the inner nuclear layer. These are extra­ retinal centrifugal fibres. The histological evidence thus corroborates the electrophysiolo­gical results[6].

 
  References Top

1.
Brindley, G.S., 1970, Physiology of Retina and Visual pathway; At nold, London.  Back to cited text no. 1
    
2.
Lowan, W.M. and Powell, T.P.S., 1963, Proc. Roy. Soc. B. 18:232.  Back to cited text no. 2
    
3.
Sacks, J.G. and Lindenberg, R.: 1969. Amer. J. Ophthalmol. 68:691.  Back to cited text no. 3
    
4.
Jacobson, J.H. and Suzuki, T.A. Arch. Oph­thalmol. 67:791.  Back to cited text no. 4
    
5.
Hasselt, P. Van, 1969, Ophthalmologicl, 159: 65.  Back to cited text no. 5
    
6.
Mascetti, G.C.; iMarzi G.A., and Barlucchi, G. 1969, Arch. Ital. Bio. G , 107:158.  Back to cited text no. 6
    


    Figures

  [Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4]



 

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