The Normal Eye - Iris and Ciliary Body
The iris forms from the ciliary body in front of the lens. The anterior chamber lies between the anterior iris surface and the inner surface of the cornea, and the shallow posterior chamber between its inner surface and the lens. The pupil averages about 4 mm in diameter and is regulated, by opposing influences ofparasympathetic and sympathetic impulses on the sphincter and dilator muscles, according to the degree of illumination. The pupils are normally equal in size; indeed a difference in size (anisocoria) is usually of pathological significance.
Autonomic nervous control of pupil diameter starts at the retina where the photoreceptors initiate a response. Parasympathetic neurones derived from the third cranial nerve control smooth muscle contraction of the iris. Conversely dilatation of the pupil is a function of radially orientated iris dilator muscles under sympathetic nerve control. As in other parts of the body, sympathetic neurones use noradrenaline as a neurotransmitter stimulating alpha adrenoreceptors, which can be effectively blocked by alpha-adrenoreceptor antagonists (pupil constriction) or stimulated by agonists (pupil dilation). Interestingly, light-stimulated contraction of one pupil can cause a contraction in the other eye. This is known as the consensual pupil response. This reflex provides a useful diagnostic tool for assessing brain damage in comatose or unconscious patients.
The iris is composed of three layers, an inner pigment epithelium/a stromal layer and an outer endothelial layer, which is continuous with the corneal endothelium. The stromal layer is continuous with the choroid and contains a blood vessel network. The iris merges with the ciliary body although both structures have distinct and important roles. The key roles of the ciliary body in the eye are:
• the source of aqueous fluid
• correct suspension of the lens
• contraction of the ciliary muscle which allows changes in accommodation due to alteration in curvature of the lens.
Aqueous humour (also referred to as aqueous) is actively secreted from the non-pigmented epithelium via several enzymatic systems (e.g. NaVK'-ATPase pump) and accounts for about 80% of the aqueous fluid. The remainder is produced by ultrafiltration from the ciliary capillaries.
Aqueous humour is essential to feed the transparent structures of the anterior chamber but the balance between inflow and outflow needs to be very finely integrated.
The regulation of aqueous production remains ill defined and may be under control of neuronal and humoral pathways. However, it is known that increased secretion of aqueous results from the beta adrenoreceptor stimulation while alpha adrenoreceptor stimulation may actively reduce outflow by constricting blood flow through the ciliary body.