Ophthalmology · MU Sofia

48. Normal and pathological pupillary responses

Covers the anatomy of the pupillary pathways, starting with the functions of the pupil and how the dilator and sphincter muscles of the iris control mydriasis and miosis. It then traces the three-neuron sympathical chain from the hypothalamus through the spinal cord and superior cervical ganglion to the dilator muscle, followed by the parasympathetic route via the Edinger-Westphal nucleus and ciliary ganglion. A closing section explains why both pupils constrict when light is shone on one eye, using the pretectal nuclei.

Inside: 1 section · 4 labelled pictures · 15 flashcards · 3 min

Topic 48 of 91 in the MU Sofia syllabus for Ophthalmology (Medicine, Year 4). The free AceMed note covers it in 1 section, about 453 words (3 min of reading), with 15 flashcards.

In this note

  1. Anatomy of the pupillary pathways and reactions.

Anatomy of the pupillary pathways and reactions.

Functions of pupil:

  • increases the depth of focus
  • it allows flow of aqueous humour from the posterior to the anterior chamber
  • the pupil determines the amount of light that enters into the eye
  • the size of the pupil is determined by:
  • dilator muscle of iris (has sympathetic innervation) -> Mydriasis
  • sphincter muscle of iris (has parasympathetic innervation) -> Miosis

There are 2 reflexes in which the pupil is involved:

  1. 1) When light is pointed towards the eye -> much light enters the eye -> this triggers a reflex -> constriction of the pupil -> less light can enter into eye
  2. 2) When it is very dark, only a little bit of light enters into eye -> this also triggers a reflex -> dilation of pupil so that as much light as possible can enter into eye.

We can very well investgate the first reflex, simply by pointing a little lamp towards the patient’s eye.

But, now we are not going to talk about the reflexes. We are going to talk what happens when the sympathetic NS becomes very active and what happens when the parasympathetic NS becomes very active.

1) Sympathetic NS activity:

  • Sympathetic NS is located between T₁ and L₂/L₃
  • results in dilation of pupil (mydriasis)
  • here we have 3 neurons:
  1. 1) First order neuron:
  • starts from the posterolateral hypothalamus and their axons run to the intermediolateral cell column of C₈ to Th₂ (ciliospinal centre of budge)
  • Here they synapse with the 2ⁿᵈ order neuron
  1. 2) Second order preganglionic neuron
  • their axons travel along the ventral roots of C8 to Th2 and leave the spinal cord
  • they run towards the superior cervical ganglion where they synapse with the 3ʳᵈ order neuron
  1. 3) Third order postganglionic neuron
  • their axons join the ophthalmic division of the trigeminal nerve, they enter into ciliary ganglion without synapsing there and finally reach the dilator muscle of iris as ‘long ciliary nerves’

2) Parasympathetic NS:

  • Parasympathetic NS is located in brain stem and spinal cord S2—S4
  • From Edinger Westphal nucleus, the preganglionic fibres run within the oculomotor nerve -> synapse at the ciliary ganglion
  • From ciliary ganglion, about 6-10 postganglionic short ciliary nerves originate, which supply the ciliary muscle and the pupillary sphincter

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