Cranial nerve reflexes
Outline the neural pathways for the pupillary light, corneal, oculomotor and gag reflexes
Pupillary light reflex
Pupillary light reflex | |
|---|---|
Sensor | Retinal photoreceptors |
Afferent limb | CN II (optic nerve) |
Integrator | Pretectal nucleus and Edinger–Westphal nucleus in the midbrain |
Efferent limb | Parasympathetic fibres in CN III → ciliary ganglion → short ciliary nerves |
Effector | Iris sphincter muscle |
Effect | Pupillary constriction (direct and consensual miosis) |
Key points
- Light entering one eye produces both a direct response in the stimulated eye and a consensual response in the opposite eye
- This occurs because the pretectal nucleus projects bilaterally to the Edinger–Westphal nuclei
- Efferent parasympathetic fibres travel with CN III to the ciliary ganglion, then via the short ciliary nerves to the sphincter pupillae
The pupillary light reflex tests integrity of the retina, optic nerve, midbrain, and parasympathetic component of CN III.
A lesion in the afferent limb abolishes both direct and consensual responses when light is shone in the affected eye, whereas an efferent lesion prevents constriction only in the affected pupil.
This is an important bedside test of midbrain function in coma and brain injury.
Corneal reflex
Corneal reflex | |
|---|---|
Sensor | Free nerve endings and mechanoreceptors in the cornea |
Afferent limb | CN V1 (ophthalmic division of trigeminal nerve) |
Integrator | Spinal trigeminal nucleus with interneuronal connections to the facial motor nucleus in the pons |
Efferent limb | CN VII (facial nerve) |
Effector | Orbicularis oculi muscle |
Effect | Bilateral eyelid closure |
Key points
- Touching either cornea normally produces bilateral blinking
- The afferent limb is CN V1 and the efferent limb is CN VII
- The reflex depends on intact pathways through the pons, so it is useful in localisation of pontine dysfunction.
The corneal reflex should be tested with a fine wisp of cotton touching the cornea, not the conjunctiva, because conjunctival stimulation may be less specific.
Loss of the reflex may reflect pathology in the ophthalmic branch of trigeminal nerve, facial nerve, or pontine brainstem connections.
In the ventilated or sedated patient, an absent corneal reflex may have implications for eye care, because poor lid closure predisposes to corneal injury.
Oculomotor / Vestibulo-ocular reflex
Vestibulo-ocular reflex (VOR) | |
|---|---|
Definition | Physiological reflex that stabilises gaze during head movement by generating equal and opposite eye movements |
Stimulus | Head movement (rotational and linear) Clinically tested by:
|
Sensor | Semicircular canals (angular acceleration) |
Afferent limb | CN VIII (vestibulocochlear nerve) |
Integrator | Vestibular nuclei → medial longitudinal fasciculus → CN III, IV, VI nuclei Coordinates conjugate eye movements via crossed brainstem pathways |
Efferent limb | CN III, CN IV, CN VI |
Effector | Extraocular muscles |
Effect | Conjugate eye movement equal and opposite to head movement, maintaining fixation |
Key points
The VOR stabilises gaze by producing equal and opposite eye movement to head movement, maintaining fixation on a target.
It is driven by a push–pull system:
- Head rotation → excitation of the ipsilateral semicircular canal and inhibition of the contralateral canal
- This asymmetry in firing drives directional eye movement.
The reflex is mediated via:
- CN VIII → vestibular nuclei → medial longitudinal fasciculus (MLF) → CN III, IV, VI
- The MLF is critical for coordinated (conjugate) eye movements.
The VOR is a pure brainstem reflex and does not require cortical input.
In clinical practice, the vestibulo-ocular pathway may be tested via the oculocephalic (doll's eyes) reflex and the oculovestibular reflex (caloric testing).
Oculocephalic reflex (doll’s eye)
- Passive head rotation → eyes move opposite to head if the brainstem is intact
- Absent response (eyes move with head) → suggests brainstem dysfunction
- Testing this reflex is contraindicated if cervical spine injury is suspected
Oculovestibular reflex (caloric testing)
- Cold/warm water flushes in the ear induce endolymph flow, stimulating the vestibulo-ocular pathway
- Normal response
- a cold water flush induces slow eye movement towards the stimulus
- a warm water flush induces slow eye movement away the stimulus
Testing this reflex is contraindicated if the tympanic membrane is not intact.
Gag reflex
Gag reflex | |
|---|---|
Sensor | Mechanoreceptors in the posterior pharynx and oropharynx |
Afferent limb | CN IX (glossopharyngeal nerve) |
Integrator | Nucleus ambiguus in the medulla |
Efferent limb | CN X (vagus nerve) |
Effector | Pharyngeal constrictor muscles and palatal musculature |
Effect | Elevation and contraction of the pharynx producing gagging |
Key points
- The gag reflex is triggered by stimulation of the posterior pharyngeal wall
- The afferent limb is CN IX and the efferent limb is CN X
- It is integrated in the medulla, principally through the nucleus ambiguus
The gag reflex assesses integrity of the bulbar cranial nerves and medullary brainstem function.
An absent gag reflex may occur with lesions affecting CN IX, CN X, nucleus ambiguus, or the neuromuscular apparatus of the pharynx.
However, the gag reflex is variable in healthy people, so its absence alone does not reliably predict aspiration risk or severe neurological injury.