Retinofugal Projections and Visual Pathways

Retinofugal Projections

  • Axons of ganglion cells in the retina form the optic nerve.
  • These axons are called retinofugal projections (retino- = from the retina, -fugal = fleeing the retina).
  • Targets: Thalamus (lateral geniculate nucleus - LGN) and non-thalamic regions.
  • Approximately 90% of retinofugal projections go to the thalamus (LGN).
  • Approximately 10% go to non-thalamic targets.

Optic Nerve and Optic Chiasm

  • The optic nerve is formed at the optic disc, a region at the back of the eyeball (blind spot).
  • The optic nerve travels posteriorly to the optic chiasm, located in front of the pituitary gland.
  • Partial decussation (crossing of axons) occurs at the optic chiasm.
  • Axons from the nasal retina cross at the optic chiasm.
  • Axons from the temporal retina do not cross.

Optic Tract and Hemifields

  • The optic tract starts at the optic chiasm and contains both crossed and uncrossed axons.
  • Hemifield: Half of the visual field.
  • Left Hemifield:
    • Image projects onto the nasal retina of the left eye and the temporal retina of the right eye.
    • Axons from the left nasal retina cross, while axons from the right temporal retina do not.
    • The right optic tract contains information from the left hemifield.
    • Information from the left visual field is processed by the right side of the brain.
  • Right Hemifield:
    • Image projects onto the nasal retina of the right eye and the temporal retina of the left eye.
    • Axons from the right nasal retina cross, while axons from the left temporal retina do not.
    • The left optic tract contains information from the right hemifield.
    • Information from the right visual field is processed by the left side of the brain.

Binocular Visual Field

  • The left and right hemifields overlap in the center, forming the binocular visual field.
  • Each hemifield consists of a monocular portion on its respective side and a binocular portion in the center.

Lesions in the Optic Pathway

  • The location of a lesion affects the specific visual deficits.
  • Example: Cutting the left optic tract after the optic chiasm results in loss of information from the right visual field, while the left visual field remains intact.

Thalamic Target: Lateral Geniculate Nucleus (LGN)

  • Axons of ganglion cells synapse on neurons in the LGN.
  • Axons of LGN neurons synapse in the visual cortex.
  • Axons from the LGN to the visual cortex form the optic radiations.
  • This pathway mediates conscious visual perception.

Structure of the LGN

  • The LGN consists of six layers, numbered 1 to 6.
  • Retinofugal projections terminate in different layers based on the type of ganglion cells.
    • P-type ganglion cells project to layers 3, 4, 5, and 6, synapsing on parvocellular LGN cells.
    • M-type ganglion cells project to layers 1 and 2, synapsing on magnocellular LGN cells.
    • Non-M, non-P ganglion cells project to layers K-1 to K-6, synapsing on koniocellular LGN cells.

LGN Receptive Fields

  • LGN receptive fields are similar to those of the ganglion cells that feed them.
  • Parvocellular LGN cells have small center-surround receptive fields.
  • Magnocellular LGN neurons have large center-surround receptive fields.

Non-Thalamic Targets

  • Hypothalamus: Role in biological rhythms (sleep and wakefulness).
  • Pretectum: Control of pupil size and certain eye movements.
  • Superior Colliculus: Control of eye orientation in response to new stimuli.

Striate Cortex (V1, Area 17)

  • First cortical target of visual information.
  • Located in the occipital lobe, specifically on the medial aspect of the brain, in the upper and lower banks of the calcarine fissure.
  • Rich in myelinated fibers, giving it a striate appearance.
  • Also referred to as the primary visual cortex.

Retinotopic Organization

  • Visual information follows a retinotopic organization from the retina to the LGN and then to V1.
  • Adjacent cells in the retina project to adjacent cells in the LGN, which project to adjacent cells in V1.
  • Essential for the formation of images, such that the image formed in the visual cortex is similar to the one detected by the retina.

Cytoarchitecture of the Striate Cortex

  • Similar to other regions of the neocortex, with six layers labeled from 1 to 6.
  • Layer 4 receives the bulk of information from the LGN.
  • Parvocellular, magnocellular, and koniocellular LGN cells synapse on neurons in layer 4.

Ocular Dominance Columns

  • Stripes of neurons in the striate cortex that respond preferentially to input from one eye or the other.
  • Demonstrated via trans-neuronal autoradiography:
    1. Inject a radioactive tracer into one eye.
    2. Axons of ganglion cells become radioactive.
    3. Radioactive LGN axons synapse in layer 4 of the striate cortex.
    4. Some layer 4 neurons become radioactive, while others do not, forming the ocular dominance columns.

Cortical Outputs: Layer 3 and Blobs

  • The striate cortex sends information to other regions of the brain.
  • Neurons in layer 3 project to other cortical areas and play a key role in visual processing.
  • Neurons in layer 3 are organized in patches called blobs.
  • Blobs are also present in layer 2.
  • Each blob is centered on an ocular dominance column in layer 4.
  • Blobs are made of neurons sensitive to color.

Extra-Striate Cortical Areas and Visual Streams

  • Beyond V1, there are a dozen extra-striate cortical areas.
  • Two main paths for visual information:
    • Dorsal Stream: Analysis of visual motion and visual control of action.
    • Ventral Stream: Perception of the visual world and recognition of objects.

Visual Areas

  • Most visual areas are found in the medial aspect of the occipital lobe.
  • V1 receives visual information from the LGN.
  • Extra-striate areas: V2, V3, V4, V5, etc., each with specific functions (e.g., motion, shapes, color).

Dorsal Stream

  • Starts in V1 and projects to V2 and V3 in the occipital lobe.
  • Information from V3 projects to area MT (V5) in the temporal lobe, which is specialized in detecting motion of objects.
  • Area MT projects to area MST in the parietal lobe, involved in motion perception.
  • Damage to area MST can impair motion perception (e.g., difficulty knowing when to stop pouring coffee).

Ventral Stream

  • Starts in V1 and projects to V2, V3, and V4 in the occipital lobe.
  • V4 is involved in color perception; damage leads to achromatopsia (inability to perceive color).
  • Information from V4 projects to area IT in the temporal lobe.
  • Area IT is specialized in color perception and face recognition; damage can lead to prosopagnosia (face blindness).

Central Auditory Processes

  • Inner ear: Inner hair cells detect the signal, outer hair cells amplify it.
  • Signal is sent to the brain via the auditory nerve (cranial nerve VIII), made of axons of spiral ganglion cells.

Auditory Pathway

  • Afferents from spiral ganglion cells synapse ipsilaterally on dorsal and ventral cochlear nucleus neurons in the brainstem.
  • Cochlear nuclei neurons synapse on neurons of the superior olive in the brainstem, bilaterally.
  • Superior olive neurons synapse on inferior colliculi neurons bilaterally.
  • Axons of superior olive neurons travel in the lateral lemniscus to reach the thalamus.
  • Inferior colliculi neurons synapse bilaterally on a thalamic nucleus called the Medial Geniculate Nucleus (MGM).
  • MGM neurons synapse on auditory cortex neurons bilaterally.
  • Axons of MGM neurons use the internal capsule (acoustic radiation).
  • Bilateral Innervation: Beyond the superior olive, the left auditory pathway carries information from the right ear and vice versa.
  • Damage along the auditory pathway after the superior olive does not cause hearing loss on one side.

Response Properties of Neurons

  • Spiral ganglion cells in the inner ear respond differently to different frequencies (characteristic frequency).
  • Some neurons fire action potentials in response to high-intensity sounds, others to low-intensity sounds.

Auditory Cortex

  • Located in the temporal lobe and made of different regions.
  • Primary auditory cortex (A1) has a similar structure to the striate cortex.
  • Secondary auditory areas have similar structure to extra-striate areas.
  • Tonotopic organization: Regions in the basilar membrane and auditory cortex respond to different frequencies.

Vestibular System

  • Enables awareness of body position and movement without conscious effort.
  • Maintains posture and coordinates eye movements with head movements.
  • Located in the vestibular labyrinth on each side of the head.

Structures of the Vestibular Labyrinth

  • Otolith Organs: Detect forces of gravity and tilt of the head.
  • Semicircular Canals: Detect rotation of the head.
  • Both systems use hair cells to detect changes.

Otolith Organs

  • Detect head angle changes and linear acceleration (vertical and horizontal).
  • Two otolithic organs on each side: saccule and utricle.
  • Macula: Specialized epithelium containing hair cells embedded in a gelatinous cap and otoconia (little rocks).
  • Tilting the head displaces the otoconia, which moves the gelatinous cap and deflects the hair cells.
  • Saccule detects vertical linear acceleration.
  • Utricle detects horizontal linear acceleration.

Semicircular Canals

  • Detect turning movements and angular acceleration of the head.
  • Crista: Specialized epithelium containing hair cells embedded in a gelatinous structure called cupula, surrounded by endolymph.
  • Spinning the head causes the endolymph to move, displacing the hair cells and affecting their membrane potential.

Vestibular Nerve and Pathways

  • Signals detected by hair cells are transmitted to the brain via the vestibular nerve.
  • Axons from the vestibular nerve synapse ipsilaterally on vestibular nuclei neurons and cerebellar neurons.
  • Vestibular nuclei receive inputs from the cerebellum and the visual system.

Vestibulo-Ocular Reflex (VOR)

  • Allows fixation on an object even when the head is moving.
  • The vestibular system senses rotations of the head and commands compensatory movements of the eyes in the opposite direction.
  • Rotation of the head detected by semicircular canals sends a message to the vestibular nuclei and then to cranial nerve nuclei.
  • Extraocular muscles contract and adjust the gaze.