Ch 13 Pt 2 Flashcards
PS pathway : ciliary muscles of lens
Adjusts thickness of the lens in response to changing viewing distances
• The lens focuses the image on
the retina
Visual cortex signals pretectal nuclei that image is out of focus Parasympathetics to the ciliary muscle are activated to adjust the shape of the lens (to keep the image in focus)
sympathetic pathway: pupillary dilation
Visual cortex sends signal to autonomic nuclei in the hypothalamus —>
Descending neurons travel in lateral brainstem and cervical cord to the T1-T2 level —>
Synapse occurs on preganglionic sympathetic neurons in the intermediolateral cell column (T1-T2 level) —>
Preganglionic sympathetic neurons enter the sympathetic trunk (chain) —>
Ascend to synapse on superior cervical ganglion —>
Postganglionic sympathetic neurons ascend beside the ICA to the orbit to then synapse on pupillary dilator muscle
Other sympathetic fx
Superior tarsal muscle
• Elevates upper eyelid during increased sympathetic outflow
Orbitalis muscle
• Prevents eye from sinking back in
the orbit
Cutaneous arteries and sweat glands of face and neck
Pupillary abnormality causes
Peripheral or central lesions, sympathetic or
parasympathetic lesions, or disorders of the iris muscle or visual pathways
Anisocoria
Pupillary asymmetry
CN III lesion : pupillary
Complete lesion, pupil is very large
Ptosis (drooping of upper eyelid) and eyemovement abnormalities associated w/ CN III Fox
Why is aniscoria more obvious in a room w/ light than a dark room?
In a room w/ light, it should be constricting, but will stay open due to loss of PS for pupillary constriction
Horner’s syndrome
Lesion affecting sympathetic pathway to face and eye
Ipsilateral:
Ptosis (mild)
• Miosis -Decreased pupillary size
Anhidrosis - Decreased
sweating
of face and neck
Honors syndrome can be caused by
Lesion anywhere that affects the sympathetic pathway to face and eye
Common causes of ptosis
Horner’s syndrome (mild)
CN III palsy (weakness of levator palpebrae superior)
Also could be due to myasthenia
gravis (MG), an orbital mass, or
excess skin folds
widened palpebral fissure
Can be mistaken for ptosis
Sagging of lower face due to UMN lesion can slightly widen the palpebral fissure and pull the eyelid down slightly as well • Examine entire eyelid using iris as a reference point • Examine the facial muscles below the eye (UMN lesion)
Supranuclear pathways
pathways in the CNS for eye movement control via activation of CN III, IV, and VI nuclei
Three known circuits in supranuclear -
Control CN III, IV, and VI to produce
- Horizontal eye movements
- Vertical eye movements
- Vergence eye movements
Supranuclear pathways also generate eye movements for other purposes
• Saccades • Smooth Pursuit • Vergence • Reflex Eye Movements - (Optokinetic) Nystagmus -Vestibulo-Ocular Reflex (VOR)
Actions of CN III, IV, and VI nuclei are coordinationed through
Medial longitudinal fasciculus
CN VI nucleus acts as
Horizontal gaze center
Controls horizontal movement of both eyes via the MLF • To move eyes to the Left: • Neurons from L CN VI nucleus project to L lateral rectus • Other neurons from L CN VI nucleus project to R CN III nucleus (via MLF) to R medial rectus
Paramedics pontine reticular formation
Also serves as a horizontal gaze center via input to CN VI nuclei
Vertical eye movements
- Superior and inferior recti
* Superior and inferior obliques
Brainstem centers for controlling vertical eye movements
upper midbrain reticular formation and pretectal area
• Anterior portion - Mediates downgaze
• Posterior portion -Mediates upgaze
Convergence eye movements
Medial recti
Divergence eye movements
Lateral recti
Vergence movements
- exact control centers in brainstem have not been defined
- likely controlled by neurons in midbrain reticular formation
Cortical control of eye movements influence by
Visual inputs arriving at the PVC and visual association cortex
Multiple pathways descend form cortex to control eye movement circuits
These travel directly to the brainstem centers for horizontal, vertical, or
vergence eye movements, or through the the midbrain superior collicul
Frontal eye fields
Best-known cortical area that controls eye movements (for contralateral eye movements)
Saccades
- rapid eye movements to switch gaze from one object to another
- reach velocities of 700 deg/sec
Smooth pursuit
Allows for stable viewing of moving objects
-reaches velocities of 100 deg/sec
Vergence
Movement of eyes toward or away from midline as targets move toward and away from the viewer
-reach velocities of 20 deg/sec
Nystagmus
Slow eye movement in one direction interrupted repeatedly by fast movements in opposite direction
Normal occurs during attempts to view a visual scene moving in front of the eyes
Nystagmus is abnormal when
It occurs at rest w/out changing visual or vestibular inputs
Vestibulospinal-ocular reflex
Stabilizes the eyes on a visual image during head movements
- inputs from vestibular nuclei travel in the MLF to control the extraocular eye muscle nuclei
- cerebellum and proprioception of cervical SC also influence VOR
