A major function of the optical components of the eye is to achieve a focused image on the retina through careful refraction of incoming light.
This article will explore the specific changes that occur to facilitate this, through a process known as the accommodation reflex.
We will look at the physiology of the accommodation reflex, explore its anatomical basis and discuss its relevance to clinical practice.
Refractive Power of the Eye
Refraction describes the deflection of light rays to change their angle as they pass onto the retina, which is necessary for the formation of a focused image. The cornea contributes the majority of the refractive power of the eye, the remaining percentage being attributed to the lens.
While the refractive power of the cornea is fixed, the lens can increase its refractive power to focus on nearby objects and decrease its refractive power for focussing on more distant objects.
Accommodation refers to dynamic changes in the refractive power of the lens, which is achieved by modifying the shape of the lens. The shape of the lens is controlled by two opposing forces:
- The internal elasticity of the lens, which tends to keep the lens rounded up. A more curved lens refracts light to a greater degree.
- The force exerted by radially arranged zonule fibres, which flatten the lens when stretched. The zonule fibres are attached to the ciliary muscle.
When focusing on a distant object, the lens is flat and thin and therefore has less refractive power. To achieve this, the force exerted by the radially arranged zonule fibres exceeds the intrinsic elasticity of the lens. The tension in the zonule fibres is caused by the relaxation of the ciliary muscle.
For nearby objects, the lens becomes thicker and rounder, increasing its refractive power. Accordingly, the intrinsic elasticity of the lens prevails, while the zonule fibres relax as a consequence of the ciliary muscle contraction.
An accommodation reflex is a coordinated change which occurs when you switch focus from an object that’s far away to one that’s closer. It comprises of three separate processes: accommodation reaction, convergence and miosis, collectively known as the near triad.
Let’s discuss each component of the near triad:
- Accommodation reaction – as the object moves closer to the eye, the refractive power of the eye needs to be increased for a clear image to be formed on the retina. This process involves a contraction of the ciliary muscle, which lessens the tension of the zonular fibres. Consequently, the intrinsic elasticity prevails and the curvature of the lens increases. Hence, the lens becomes more round.
- Convergence – a simultaneous inward movement of both eyes towards each other. This movement is caused by the bilateral contraction of the medial recti. Convergence helps to maintain the focused image on each fovea and facilitates binocular vision.
- Miosis – a decrease in the diameter of the pupil. This is achieved by the contraction of the iris sphincter muscles to increase the depth of focus.
It is worth noting that each of the near triad actions can occur without the others. For instance, when plus lenses are placed in front of both eyes, miosis and convergence would occur without accommodation.
Anatomical Basis of Accommodation Reflex
The input of the accommodation reflex is the same as for the visual pathway to the occipital cortex. From the occipital cortex, the information is sent to the frontal eye fields (FEF); a region in the frontal lobe. The FEF then communicates with the oculomotor nucleus (III) and parasympathetic Edinger-Westphal (EDW) nucleus in the midbrain via the internal capsule.
The oculomotor nerve (III) innervates the medial rectus muscles causing a bilateral contraction and hence a simultaneous inward movement of both eyeballs – convergence. On the other hand, the parasympathetic fibres from the EDW nucleus travel with the oculomotor nerve (III) and follow the parasympathetic pathway, via the ciliary ganglion and short ciliary nerve, to innervate the ciliary and iris sphincter muscle, which execute accommodation reaction and miosis, respectively.
Clinical Relevance – Presbyopia
Presbyopia is an irreversible loss of ability to accommodate. Most patients with presbyopia would experience trouble reading the fine print, blurring of near vision and asthenopia (eye strain). Whilst presbyopia is a very common condition, the main cause of presbyopia is still not well understood.
One of the suggested mechanisms is the age-related change of the lens, which reduces its intrinsic elasticity. Ciliary muscle dysfunction and loss of elasticity in the posterior zonules may also play a role in the development of presbyopia. This loss in accommodation ability typically occurs in patients in their fifth decade of life.
It is important to remember these patients have reduced accommodation but the rest of the near triad remains intact.