Lecture 19: Extrageniculate Visual Pathway

Extrageniculate Visual Pathway Overview

• Visual pathways transmit signals from the retina to various brain areas, but extrageniculate pathways bypass the LGN, serving distinct functions.
• Critical roles include pupillary response control, certain eye movements, and contributions to phenomena like 'blindsight'.

Pupillary Light Reflex Pathways

• Constriction of pupils occurs in response to light through parasympathetic activation, while dilation occurs due to sympathetic activity in darkness.
• Bilateral projections from pretectal nuclei to Edinger-Westphal nuclei are essential for coordinated pupillary response, allowing both pupils to constrict when light is shone in one eye.
• The near triad involves three functions: pupillary constriction, lens accommodation, and convergence of both eyes to focus on nearby objects.

Dilation Mechanism

• Pupil dilation results from the sympathetic nervous system during low light conditions, where sphincter muscle inhibition allows for dilator muscle activation.
• Three-neuron pathway includes:
  • Neuron 1: Long projection from brain to spinal cord, disruptions can lead to Horner’s Syndrome.
  • Neuron 2: Located in thoracic sympathetic trunk.
  • Neuron 3: Transmits noradrenaline to activate dilator muscles.

Horner's Syndrome

• Characterized by pupil constriction due to suppressed sympathetic signals, leading to drooping eyelids and disrupted sweating.
• Results from disrupted sympathetic innervation affecting eyelids, sweat glands, and local blood vessel dilation.

Accessory Optic System (AOS)

• Comprises nuclei responsible for processing larger, slow-moving visual stimuli.
• Plays a role in optokinetic nystagmus (OKN) and vestibulo-ocular reflex (VOR), crucial for motion perception.
• Optokinetic system generates sustained responses to low-frequency motion stimuli, while the vestibular system responds transiently to high-frequency stimuli.

Superior Colliculus

• Located in the midbrain, it is essential for eye movements and integrates various sensory inputs including visual, auditory, and tactile signals.
• Key in directing gaze toward novel stimuli and receives direct retinal inputs alongside connections from the visual and parietal cortices.
• Serves dual pathways: dorsal for spatial awareness and ventral for processing visual intent.

Pulvinar Nucleus

• Functions as a hub for integrating information from different visual pathways, projecting to visual cortical and parietal areas.
• Modulates information transmission based on attentive demands; lesions may impair emotion recognition through visual stimuli.
• Contains units sensitive to emotional expressions and visual stimuli, indicating its role in processing significant social cues.

Circadian Rhythm and ipRGCs

• Inputs from intrinsically photosensitive retinal ganglion cells (ipRGCs) are vital for regulating circadian rhythms, influencing behavioral and physiological cycles.

Conclusion

• Multiple pathways exist beyond the classic LGN → V1 route, facilitating diverse visual functions:
  • Pretectal nucleus input directs pupillary reflex.
  • Accessory optic system is involved in large-field motion detection.
  • Superior colliculus plays a minimal role in conscious vision but is crucial for eye movement execution.
  • Pulvinar integrates visual processing and attention mapping.
  • ipRGCs underpin circadian regulation.