Pathway of near reflex of eye
The pathway of near reflex is a complex neural circuit that involves the coordination of three responses: accommodation, convergence, and pupillary constriction. These responses are triggered by the visual stimulus of a near object, such as a book or a computer screen. The purpose of these responses is to bring the near object into clear focus on the retina and to maintain binocular vision.
Accommodation is the change in the shape of the lens of the eye to increase its refractive power. This allows the eye to focus on near objects that are closer than the far point of the eye. The far point is the farthest distance at which an object can be seen clearly without accommodation. The near point is the closest distance at which an object can be seen clearly with maximum accommodation. The range between the near point and the far point is called the amplitude of accommodation.
Convergence is the inward rotation of both eyes toward each other to align them with the near object. This ensures that the image of the near object falls on the corresponding points of both retinas. The corresponding points are the points on each retina that have the same visual direction. When the image falls on the corresponding points, binocular vision and stereopsis (depth perception) are achieved.
Pupillary constriction is the narrowing of the pupil size to reduce the amount of light entering the eye and to increase the depth of focus. The depth of focus is the range of distances within which an object can be seen clearly without changing accommodation. By constricting the pupil, the eye blocks out the peripheral light rays that would otherwise create a blurred image on the retina.
NEAR REFLEX at near object
Near reflex components:
It has two components
• Convergence Reflex : Contraction of pupil on convergence
• Accommodation Reflex : Contraction of pupil associated with accommodation
The accommodation reflex (or accommodation-convergence reflex) is a reflex action of the eye, in response to focusing on a near object, then looking at a distant object (and vice versa)
The change in the shape of the lens is controlled by ciliary muscles inside the eye. Changes in contraction of the ciliary muscles alters the focal distance of the eye, causing nearer or farther images to come into focus on the retina; this process is known as accommodation(1)
Convergence reflex is the ability of the eye to simultaneously demonstrate inward rotation of both eyes toward each other. This is helpful in effort to make focus on near objects clearer. Three reactions occur simultaneously; the eyes adduct, the ciliary muscles contract, and the pupils become smaller. This action involves the contraction of the medial rectus muscles of the two eyes and relaxation of the lateral rectus muscles. The medial rectus attaches to the medial aspect of the eye and its contraction adducts the eye. The medial rectus is innervated by motor neurons in the oculomotor nucleus and nerve(2)
The pathway of near reflex involves both afferent (sensory) and efferent (motor) pathways. The afferent pathway carries information from the retina to the brain, while the efferent pathway carries commands from the brain to the eye muscles.
The afferent pathway starts with photoreceptors in the retina that detect light and convert it into electrical signals. These signals are transmitted by bipolar cells and ganglion cells in the retina to form the optic nerve. The optic nerve carries these signals to the optic chiasm, where some fibers cross over to the opposite side. The fibers then continue as the optic tract to reach the lateral geniculate nucleus (LGN) in the thalamus. The LGN is a relay station that processes visual information and sends it to the primary visual cortex in the occipital lobe via the optic radiations.
However, some fibers from the optic tract branch off before the LGN and go to the pretectal area in the midbrain. The pretectal area is responsible for initiating the pupillary light reflex, which is the constriction of both pupils in response to light stimulation in either eye. The pretectal area also sends signals to the Edinger-Westphal nucleus (EWN) in the midbrain, which is responsible for initiating the accommodation-convergence reflex, which is the coordinated change in lens shape and eye position in response to viewing a near object.
The efferent pathway starts with the EWN, which contains parasympathetic neurons that control the ciliary muscle and the pupillary sphincter muscle in the eye. The ciliary muscle is responsible for changing the shape of the lens during accommodation. The pupillary sphincter muscle is responsible for constricting the pupil during pupillary constriction. The EWN sends its fibers along the oculomotor nerve (cranial nerve III) to reach the ciliary ganglion in the orbit. The ciliary ganglion is a collection of nerve cells that relay parasympathetic signals to the eye muscles via short ciliary nerves.
The efferent pathway also involves motor neurons in the oculomotor nucleus (OMN) in the midbrain, which control the medial rectus muscle in the eye. The medial rectus muscle is responsible for rotating the eye inward during convergence. The OMN sends its fibers along the oculomotor nerve to reach the medial rectus muscle directly.
Accomodation: Accomodated and unaccomodated eye diagram
How to determine the near reflex
To determine the near reflex, patient is asked to focus on a far object and then instructed suddenly to focus at an object (pencil or tip of index finger) held about 15 cm from patient’s eye. While the patient’s eye converges and focuses the near object, observe the constriction of pupil.
Pathway of near reflex
The refractive index of the eye’s cornea-lens system allows the eye to produce sharply focused images on the retina. The refractive power resides mainly in the cornea, but the finer changes in refractive power of the eye are achieved by the lens changing its shape.
As a distant object is brought closer to the eye, the image moves behind the retina, producing blurring at the retina. This blurring is minimised by squeezing the lens to a more spherical shape, which again moves the image back to the plane of the retina.
In order to fixate on a near object, the ciliary muscle contracts around the lens to decrease its diameter and increase its thickness. The suspensory zonules of Zinn relax and the radial tension around the lens is released. This causes the lens to form a more spherical shape achieving greater refractive power (3)
The pathway of near reflex is affected by various factors, such as age, fatigue, stress, and medication. As people age, their lens becomes less elastic and their ciliary muscle becomes weaker. This reduces their amplitude of accommodation and increases their near point. This condition is called presbyopia, which means “old sight”. Presbyopia can be corrected by wearing reading glasses or bifocals.
Fatigue and stress can also impair the near reflex by reducing the ability of the brain and the eye muscles to sustain the accommodation, convergence, and pupillary constriction. This can cause eye strain, blurred vision, headache, and difficulty in reading or working on a computer. To prevent this, it is recommended to take frequent breaks, blink often, adjust the lighting and the distance of the screen, and use artificial tears if needed.
Some medications can also affect the near reflex by interfering with the neurotransmitters involved in the pathway. For example, anticholinergic drugs (such as atropine) can block the action of acetylcholine, which is the neurotransmitter released by the parasympathetic neurons in the EWN and the ciliary ganglion. This can cause paralysis of accommodation (cycloplegia) and dilation of pupil (mydriasis). On the other hand, cholinergic drugs (such as pilocarpine) can mimic the action of acetylcholine and cause excessive accommodation (spasm of accommodation) and constriction of pupil (miosis).
Some eye problems related to the near reflex are convergence insufficiency and spasm of near reflex. Convergence insufficiency is a condition where the eyes have difficulty in converging when looking at near objects. This can cause double vision (diplopia), eye strain, headache, and difficulty in reading or working on a computer. Convergence insufficiency can be treated by eye exercises (orthoptics) or prism glasses.
Spasm of near reflex is a condition where the eyes have excessive or involuntary accommodation, convergence, and pupillary constriction when looking at near or distant objects. This can cause blurred vision, eye pain, headache, and photophobia (sensitivity to light). Spasm of near reflex can be caused by psychological stress, trauma, infection, inflammation, or medication. Spasm of near reflex can be treated by relaxing the eyes, using cycloplegic drops or glasses, or addressing the underlying cause.
See : Struggles of being an optometrist
Reference
1. Watson, Neil V.; Breedlove, S. Marc (2012). Mind’s Machine: Foundations of Brain and Behavior. Sunderland, MA: Sinauer Associates. p. 171. ISBN 978-0-87893-933-6. OCLC 843073456.
2. 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. Archived from the original on 2 November 2012. Retrieved 24 October 2012.
3. Khurana, AK (September 2008). “Asthenopia, anomalies of accommodation and convergence”. Theory and practice of optics and refraction (2nd ed.). Elsevier. pp. 98–99.
