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Neural Mechanism behind Ocular Accommodation – An Abridgment

by | Nov 1, 2020 | Manuscripts | 0 comments

Nandini Ravi, B.S Optometry

Fellowship in the Department of Binocular Vision and Vision Therapy, The Sankara Nethralaya Academy, Chennai, India

 

Preface

Accommodation is a process by which the eye changes focus from distance to near objects. This process is done with the changes in lens shape by the action of the ciliary muscles on the zonular fibres (1).

Figure 1: showing the change in lens shape during the process of ocular accommodation Picture courtesy – https://images.app.goo.gl/NMkeXAhmCtSWTBbv9

 

Evidences show that, Edinger Westphal nucleus (EWN) which is located dorsal to the oculomotor nerve (OMN) nucleus innervates the ciliary muscle (2-3), thus controlling ocular accommodation (4-6). This report gives a brief idea on the neural process controlling ocular accommodation and the clinical implications associated with it.

The Near triad

The near triad is elicited due to changes in viewing distance for maintaining clear binocular single vision (7). For a smooth near response to take place, we need the afferent and efferent neurons to be functioning effectively (8). This three component reflex initiates with the afferent inputs going to the visual cortex via the optic tract. Then the efferent pathway consists of innervating the iris sphincter muscle for pupil constriction, ciliary muscle for accommodation of the lens and the bilateral medial rectus
for convergence (9).

Figure 2: showing the near triad with the respective innervations

 

Neural pathway for ocular accommodation

The trail initiates with a stimulus to ocular accommodation, which is the blurred visual signal (10). The signal reaches the visual cortex and then to the frontal eye fields (FEF) which are in the middle frontal and pre-central gyrus (11). This pre-central gyrus is responsible for merging the oculomotor and Somato-motor space coding in the human brain (12). Anatomically, the fibres from both the OMN and EWN run parallel. The parasympathetic signals originating in the EWN are carried by the oculomotor nerve fibres and pass the lateral wall of the cavernous sinus and the superior orbital fissure reaching the ciliary ganglion. The short ciliary nerves from the ciliary ganglion supply the iris and ciliary body. The contraction of the ciliary muscle puts the zonular fibres into relaxation and making the lens get its spherical contour during accommodation (13, 14).

 Clinical relevance

Clinically, the measurement of accommodative parameters requires few factors to be considered for accuracy. Studies describe that, there is a shared neural mechanism between the process of attention and accommodation (15-17). If there is a lack of attention during accommodative dynamics measurement, there could be an inaccurate value which in a flow impacts management(18). Beatriz et al reported that, incorporating auditory feedback improves attention and aids in precise measurements(18). Further, it is crucial to look for accommodative issues in patients who have lesions pertaining to the cerebellar regions(19). In general, it is important to look for accommodative dysfunctions in subjects who had any kind of neurological insult.

 

Figure 3: Flow chart showing the neural pathway which causes the accommodative reflex

In conclusion, with the growing digital world and with the spike in gadget use, children as well as adults tend to have asthenopia related to the eye’s accommodative system (20). Thus, understanding the neural foundation behind accommodation will assist us in clear-cut diagnosis.

 References:

  1. https://www.aao.org/bcscsnippetdetail.aspx?id=0554ca9e-b088-4bfe-91ab-2fc77bb0ea67
  2. Burde, R. M., Parelman, J. J., & Luskin, M. (1982). Lack of unity of Edinger-Westpal nucleus projections to the ciliary ganglion and spinal cord: a double-labeling approach. Brain Research, 249(2), 379-382.
  3. Loewy, A. D., &Saper, C. B. (1978). Edinger-Westphal nucleus: projections to the brain stem and spinal cord in the cat. Brain research, 150(1), 1-27.
  4. Gamlin, P. D., Zhang, Y. I. H. O. N. G., Clendaniel, R. A., et al. (1994). Behavior of identified Edinger-Westphal neurons during ocular accommodation. Journal of Neurophysiology, 72(5), 2368-2382.
  5. Jampel, R. S., &Mindel, J. (1967). The nucleus for accommodation in the midbrain of the macaque. Investigative Ophthalmology & Visual Science, 6(1), 40-50.
  6. Clarke, R. J., Coimbra, C. J., &Alessio, M. L. (1985). Oculomotor areas involved in the parasympathetic control of accommodation and pupil size in the marmoset (Callithrixjacchus). Brazilian Journal of Medical and Biological Research= RevistaBrasileira de PesquisasMedicas e Biologicas, 18(3), 373-379.
  7. Schor CM, Ciuffreda KJ. Vergence Eye Movements: Basic and Clinical Aspects. Boston: Butterworth Publishers; 1983.
  8. Hunyor, AP. Reflexes and the Eye. Aust N Z J Ophthalmol.1994;22(3):155-159.
  9. Dragoi, Valentin. “Chapter 7: Ocular Motor System”. Neuroscience Online: An Electronic Textbook for the Neurosciences.Department of Neurobiology and Anatomy, The University of Texas Medical School at Houston.
  10. Agarwal, M., Ulmer, J. L., Chandra, T., et al. (2016). Imaging correlates of neural control of ocular movements. European radiology, 26(7), 2193-2205.
  11. Sato, K., &Takamori, M. (1998). Left lateral gaze paresis due to subcortical hematoma in the right precentral gyrus. RinshoShinkeigaku= Clinical Neurology, 38(3), 266-269
  12. Iacoboni, M., Woods, R. P., Lenzi, G. L., et al. (1997). Merging of oculomotor and somatomotor space coding in the human right precentral gyrus. Brain: a journal of neurology, 120(9), 1635-1645
  13. Lv, X., Chen, Y., Tan, W., Yu, Y., Zou, H., Shao, Y et al. (2020). Functional Neuroanatomy of the Human Accommodation Response to an “E” Target Varying from-3 to-6 Diopters. Frontiers in Integrative Neuroscience, 14, 29.
  14. Chaurasia, B. D. (2004). Human anatomy(p. 53). CBS Publisher.
  15. Kellermann, T., Regenbogen, C., De Vos, M., et al. (2012). Effective connectivity of the human cerebellum during visual attention. Journal of Neuroscience, 32(33), 11453-11460.
  16. Knudsen, E. I. (2011). Control from below: the role of a midbrain network in spatial attention. European Journal of Neuroscience, 33(11), 1961-1972.

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