The awesomeness of ocular anatomy and physiology: the scleral membrane and the oculocephalic reflex as defenses against insult

The vestibulo-ocular reflex. A rotation of the...
The vestibulo-ocular reflex. A rotation of the head is detected, which triggers an inhibitory signal to the extraocular muscles on one side and an excitatory signal to the muscles on the other side. The result is a compensatory movement of the eyes. (Photo credit: Wikipedia)

Today brings one of those dismal events that afflict us all from time-to-time: a stupid accident that makes you deeply thankful for that combined achievement of evolution – a tough scleral membrane and the oculocephalic reflex. Moving furniture outside, I manage to stab myself in the dorsolateral aspect of my right eye while bending down to pick up something. In humans and other non-human primate species, the eye can move independently of the head. It lacks a nictitating membrane (NM: the hidden third eyelid that moves lateromedially (i.e. outside to the inside) and defensively across the eyeball. The movement of the NM can be classically conditioned – that is, events signalling (such as sound or a flash of light) that something noxious is about to occur (such as an air puff to the eyeball) can control the movement of the NM, as opposed to discrete reflexive responses post-insult. In other words, the NM reflex is plastic and can enter into the control of behaviour. The circuit is principally controlled by discrete nuclei in the cerebellum; this experimental model has taught us an astonishing amount about the neural bases of learning and memory.

In my case, no such defensive luck. Instead, I get a very fast and blurry view of the sharp and elongate thorn and reflexively my head and (more importantly) my eye move sharply to the left. The eyeballs move under the control of cranial nerves III and IV and can accelerate faster than almost any other body part . The property of being able to move the eyes independently of the head allows us to pay attention to parts of visual space other than where the eyes are foveating at a particular moment – an especially useful capacity in a social and hierarchical species consisting of small groups requiring transient and intermittent attention to other group members. Independently-moving allows you to furtively track the movements of (possibly very aggressive) conspecifics without attracting unwanted attention from them.

Anyway, the pain and nausea have subsided, and I have an unblurred view of tehe Mayo-Dublin Final; the visuomotor integration required to type appears fine too.

Update: Poor Mayo.

Author: Shane O'Mara

Neuroscientist

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