Where is Schlemm's canal located and how is it structured?
Schlemm's canal is part of the eye's drainage system. It is located in the iridocorneal angle (the angle of the anterior chamber), which lies between the iris and the cornea.
The canal consists of a chain of interconnected circular cavities located within the limbus. All these cavities are lined with simple squamous epithelium (endothelium) that is continuous with the endothelium of veins.
How does the outflow of intraocular fluid occur?
Intraocular pressure (IOP) depends on the balance between the production of aqueous humor and its outflow. Aqueous humor is produced by the ciliary body and passes through the drainage system, in which Schlemm's canal plays a key role in fluid outflow.
Outflow pathway and sequence:
- Zonular spaces and chambers of the eye (spatia zonularia, camerae anterior et posterior bulbi) — pathways for aqueous humor outflow.
- Spaces of Fontana — slit-like spaces between the trabecular meshwork (pectinate ligament). Fluid passes through them before entering the sinus.
- Schlemm's canal (sinus venosus sclerae) — receives aqueous humor from the spaces of Fontana.
- Collector channels — drain fluid from Schlemm's canal.
- Scleral venous plexus (superficial scleral veins) — the final destination of the drainage system.
Clinical significance and drug effects
The mechanism of intraocular pressure changes is related to fluid access to the eye's drainage system: the iridocorneal angle, the spaces of Fontana, and Schlemm's canal.
- M-cholinomimetics: cause contraction of the circular muscle of the iris, leading to pupil constriction (miosis). The iris is pulled away from the anterior chamber angle, opening the spaces of Fontana and improving fluid access to Schlemm's canal. Effect: increased aqueous humor outflow and decreased intraocular pressure.
- M-cholinolytics: cause pupil dilation (mydriasis). The root of the iris thickens and can block the anterior chamber angle. Effect: decreased outflow of intraocular fluid and a risk of elevated intraocular pressure.
Class and exam questions
What type of epithelium lines Schlemm's canal?
The cavities of Schlemm's canal and the spaces of Fontana are lined with simple squamous epithelium (endothelium), which is continuous with the endothelium of veins.
Where does fluid enter the scleral venous sinus from?
Aqueous humor enters the canal from the anterior chamber of the eye by passing through the slit-like spaces of Fontana within the trabecular meshwork (pectinate ligament).
Where does fluid drain from Schlemm's canal?
From Schlemm's canal, fluid flows into the collector channels and then into the scleral venous plexus (superficial scleral veins).
How do M-cholinomimetics lower intraocular pressure?
They cause miosis (pupil constriction). As a result, the iris is pulled away from the anterior chamber angle, which opens the spaces of Fontana and facilitates fluid outflow into Schlemm's canal.
What else to study on this topic
- Iridocorneal angle and spaces of Fontana
- Ciliary body and aqueous humor production
- Anterior and posterior chambers of the eyeball
- Scleral venous plexus
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