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Portrait of Pupil

Anatomy · Eye, ear & senses

Pupil

The aperture at the centre of the iris

Eye · the opening of the iris

Not a structure but a hole: the round opening in the centre of the iris through which light enters the eye. It looks black because the light passing in is absorbed inside. Its diameter is adjusted continuously by the iris muscles under the control of the autonomic nervous system, and the pupillary light reflex is one of the standard bedside tests of the brainstem.

How to spot them

  • ▸The round black opening in the middle of the coloured iris
  • ▸Narrows in bright light (miosis) and widens in dim light or with excitement (mydriasis)
  • ▸The light reflex: shine a torch in one eye and both pupils constrict
  • ▸Appears black because the light entering is absorbed by the pigmented tissues at the back of the eye

What to know

Shine a torch in one eye and both constrict; the afferent limb is the optic nerve, the efferent limb runs with the oculomotor nerve

The pupillary light reflex and its consensual response

Appears black although it is only a hole, because light entering is absorbed by the pigmented tissues at the back of the eye

The pupil

Constricted to the size of a pinhead after opioids, and dilated by atropine from the deadly nightshade, whose species name means 'beautiful lady'

Pupil size: miosis and mydriasis; Atropa belladonna

Constricts to accommodation but not to light, described in Edinburgh in 1869 and classically linked to neurosyphilis (1869)

The Argyll Robertson pupil — 'accommodates but does not react'

A constricted opening on one side, a drooping eyelid and loss of sweating on the same side of the face

Horner's syndrome, from interruption of the sympathetic supply

The red glow in flash photographs, from light bouncing back off the blood-rich layer behind the retina

Red-eye effect: light reflected through the pupil from the choroid

Round in humans, a vertical slit in domestic cats and a horizontal rectangle in goats

Pupil shape varies with the animal's lifestyle

Key ideas

  • The reflex arc runs from the retina along the optic nerve to the pretectal nuclei, then to both Edinger-Westphal nuclei, and out along the oculomotor nerves; this crossed wiring explains the consensual response.
  • A small pupil deepens the field of focus and reduces optical aberration; a large one lets in far more light, up to about sixteen times the area.

Quick-fire facts

  • •Typically 2–4 mm in bright light and 4–8 mm in the dark; the range is roughly 1.5 to 8 mm
  • •The word comes from the Latin pupilla, 'little doll', because you see a tiny reflection of yourself in another's eye
  • •Unequal pupils (anisocoria) of under 1 mm are found in roughly one in five healthy people
  • •Constriction by the sphincter pupillae is parasympathetic via the oculomotor nerve; dilation by the dilator pupillae is sympathetic
  • •A fixed, dilated pupil on one side in a patient with head injury is a warning sign of raised intracranial pressure and pressure on the oculomotor nerve
  • •Pilocarpine constricts the pupil and is used in glaucoma; tropicamide and atropine dilate it for eye examination
  • •Douglas Argyll Robertson of Edinburgh described the pupil that bears his name in 1869
  • •Pupil size also changes with emotion, interest and mental effort, and is used as a measure in psychology experiments

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Sources