Cranial Nerve Reflexes Flashcards
What are the protective reflexes in the nervous system?
Corneal Blink
Pupillary Light reflex
What reflexes help to coordinate eye movements?
Gaze
Vestibulo-ocular reflex
Accommodation
What is the function of the corneal blink reflex?
Protective reflex to ensure cornea is lubricated to remove foreign particles from the eye
Outline the corneal blink reflex
- Ophthalmic Division of Trigeminal Nerve (CN V1) provides sensory innervation to the cornea
- Synapse in spinal trigeminal and chief sensory nuclei
- Interneurons synapse bilaterally with facial motor nucleus
- Axons from facial motor nucleus innervate Orbicularis Oculi for blinking
What is the function of the Pupillary Light Reflex?
Limits the amount of light that falls onto the retina.
Pupils constrict in bright light conditions through contraction of constrictor pupillae muscle
Outline the pupillary light reflex
- Afferent limb begins with photoreceptors of the retina
- Sends via optic nerve (CN II) and optic tract through superior colliculus to pretectal area
- Projects ipsilaterally and contra laterally via Posterior commissure to Edinger-Westphal Nucleus
- Preganglionic parasympathetic fibers travel with E-W nucleus together with oculomotor nerve to orbits and synapse in ciliary ganglia
- Innervate constrictor pupillae muscles
What are the cranial nerves that control eye movement?
Oculomotor Nerve (CN III), Trochlear Nerve (CN IV), Abducens Nerve (CN VI)
What somatic muscles does the oculomotor nerve (CN III) innervate?
Superior, Medial, Inferior Rectus muscles
Inferior oblique
Levator Palpebrae superioris
What visceral muscles does CN III innervate?
Intraocular muscles like constrictor pupillae and ciliary muscles
What muscle does the trochlear nerve (CN IV) innervate?
Superior oblique muscle
What muscle does the Abducens nerve (CN VI) innervate?
Lateral rectus muscle
What is gaze?
Coordinated, Synergistic movement of both eyes to focus the image onto the fovea
What structures does gaze involve?
Coordinated movement of 2 eyes
6 extrinsic muscles of each eye
3 pairs of cranial nuclei
Medial Longitudinal Fasciculus
What is the function of medial longitudinal fasciculus?
Allows coordinated movement of both eyes and adjusts eye position in response to head movement
What is the descending MLF also known as?
Medial vestibulospinal tract
What are the 2 general categories of eye movement?
Conjugate and Vergence
What is the function of saccades?
Redirects gaze to an object of interest so that image falls on fovea
What cortical and subcortical structures initiate saccades?
Frontal eye field
Supplementary eye field
Parietal eye field
Where do fibers that initiate saccades project to for horizontal saccades?
Contralateral Paramedian Pontine Reticular formation +/- ipsilateral superior colliculus
Where do fibers that initiate saccades project to for vertical saccades?
Rostral Midbrain reticular formation (RMRF)
Outline the coordination of horizontal conjugate eye movement (saccades)
- Abducens and Oculomotor nerve receives input from PPRF to direct gaze IPSILATERALLY
- Cell bodies of somatic motor neurons send projections to ipsilateral lateral rectus
- Axons from interneurons cross midline and ascend in MLF to contralateral Oculomotor nerve
- Oculomotor neuron innervates ipsilateral medial rectus so eye moves to contralateral side
Results in abduction of left eye by the left lateral rectus and adduction of right eye by right medial rectus
What happens to horizontal gaze when there is a lesion in the tegmentum of caudal pons?
Loss of conjugate gaze on side of lesion
Eyes deviated away from side of lesion
What happens to horizontal gaze when there is cortical lesion?
Loss of conjugate gaze away from side of lesion
Eyes deviate to side of lesion
What structure provides main input for reflexive saccades?
Superior colliculus
