Visual system | Anatomy2Medicine
visual system

Visual system

The Visual System

    • optic nerve, CN II is a special somatic afferent (SSA) nerve.
    • The Retina

 

  • is derived from the optic vesicle of the diencephalon.

 

      • contains efferent fibers that give rise to the optic nerve
      • optic nerve is actually a fiber tract of the diencephalon.

 

  • is sensitive to wavelengths from 400 nm to 700 nm.

 

    • Structures of the ocular fundus
      • Optic disk (optic papilla)

 

  • is located 3.5 mm nasal to the fovea centralis.

 

        • contains unmyelinated axons from the ganglion cell layer of the retina
        • is the blind spot (contains no rods or cones).

 

  • contains a central cup, a peripheral disk margin, and retinal vessels.
  • Macula lutea

 

        • is a yellow-pigmented area that surrounds the fovea centralis.

 

  • Fovea centralis

 

        • is located within the macula lutea, 2.5 disk diameters temporal to the optic disk
        • contains only cones
        • site of highest visual acuity.
        • is avascular
        • receives nutrients by diffusion via the choriocapillaris.
        • subserves color or day (photopic) vision.
      • Retinal blood supply
        • choriocapillaris of the choroid layer
        • the central retinal artery, a branch of the ophthalmic artery.
    • Cells of the retina
      • constitute a chain of three neurons that project visual impulses via the optic nerve and the lateral geniculate body (LGB) to the visual cortex.

 

  • Rods and cones

 

        • are first-order receptor cells that respond directly to light stimulation.
        • generate only graded potentials.
        • utilize glutamate as a neurotransmitter.

 

  • Rods (100 million)

 

          • contain rhodopsin (visual purple)
          • are sensitive to low-intensity light.
          • subserve night (scotopic) vision.

 

  • Cones (7 million)

 

          • contain the photopigment iodopsin.
          • operate only at high illumination levels.
          • are concentrated in the fovea centralis.

 

  • are responsible for day (photopic) vision, color vision, and high visual acuity.
  • Bipolar neurons
  • are second-order neurons that relay stimuli from the rods and cones to the ganglion cells
  • generate only graded potentials.

 

        • utilize glutamate as a neurotransmitter.

 

  • Ganglion cells
  • are third-order neurons that form the optic nerve (CN II).

 

        • are retinal cells with voltage-gated sodium channels that generate action potentials.

 

  • project directly to the hypothalamus, superior colliculus, pretectal nucleus, and LGB

 

        • utilize glutamate as a neurotransmitter.

 

  • Interneurons
  • Horizontal cells

 

          • interconnect photoreceptors and bipolar cells.

 

  • inhibit neighboring photoreceptors (lateral inhibition).

 

          • generate only graded potentials.
          • utilize gamma-aminobutyric acid (GABA) as a neurotransmitter.
          • play a role in the differentiation of colors.

 

  • Amacrine cells
  • are small cells that have no axons and few dendrites.

 

          • receive input from bipolar cells
          • project inhibitory signals to ganglion cells

 

  • mediate lateral interactions at the bipolar–ganglion cell synapse.

 

          • utilize GABA, glycine, dopamine, and acetylcholine (ACh) as neurotransmitters.

 

  • Müller cells

 

        • are radial glial cells

 

  • have a support function similar to that of astrocytes

 

        • extend from the inner limiting layer to the outer limiting layer.
    • Meridional divisions of the retina

 

  • Temporal hemiretina
  • receives image input from the nasal visual field.
  • has ganglion cells that project to the ipsilateral LGB, layers 2, 3, and 5.
  • Nasal hemiretina
  • receives image input from the temporal visual field.
  • has ganglion cells that project to the contralateral LGB, layers 1, 4, and 6.
  • Upper retinal quadrants

 

        • receive image input from the lower visual fields.

 

  • have ganglion cells that project via the LGB to the upper banks of the calcarine fissure.
  • Lower retinal quadrants

 

        • receive image input from the upper visual fields.

 

  • have ganglion cells that project via the LGB to the lower banks of the calcarine fissure

 

    • Concentric divisions of the retina and retinotopy

 

  • Macular area

 

        • is a small area surrounding the fovea centralis
        • serves central vision (high visual acuity).
        • contains cones.

 

  • predominantly projects to the posterior part of the visual cortex.
  • Paramacular area

 

        • is a large area surrounding the macular area
        • contains predominantly rods.

 

  • projects to the visual cortex anterior to the macular representation.
  • Monocular area

 

        • represents the peripheral monocular field.
        • projects to the visual cortex anterior to the paramacular representation
        • Lesions result in a contralateral crescentic defect.

 

  • Visual Pathway
  • transmits visual impulses from the retina to the LGB and from the LGB to the primary visual cortex (area 17) of the occipital lobe.

 

      • Ganglion cells
        • constitute the ganglion cell layer of the retina
        • contain axons that form the optic nerve, CN II
        • project from the nasal hemiretina to the contralateral LGB.
        • project from the temporal hemiretina to the ipsilateral LGB.
      • Optic nerve (CN II)
        • is a myelinated tract of CNS( diencephalon)
        • it is not a true nerve.
        • is invested by the pia–arachnoid and dura mater.
        • receives its blood supply from the central retinal artery, pial arteries, posterior ciliary arteries, and the arterial circle of Willis.
        • is surrounded by the subarachnoid space.
        • is incapable of regeneration.
        • Compression results in optic atrophy.
        • Transection at the optic chiasma results in
          • ipsilateral blindness
          • contralateral upper temporal scotoma (junction scotoma); inferior nasal fibers loop into the contralateral optic nerve
      • Optic chiasm
        • is part of the diencephalon.
        • lies dorsal to the hypophysis and diaphragma sellae.
        • contains decussating fibers from the two nasal hemiretinae.
        • contains noncrossing fibers from the two temporal hemiretinae.
        • receives its blood supply from the anterior cerebral and internal carotid arteries.
        • Midsagittal transection or pressure results in bitemporal hemianopia (pituitary tumor).
        • Bilateral lateral compression results in binasal hemianopia (calcified internal carotid arteries).
      • Optic tract

 

  • contains fibers from the ipsilateral temporal hemiretina and the contralateral nasal hemiretina.

 

        • contains pupillary reflex fibers.

 

  • projects to the LGB and via the brachium of the superior colliculus to the pretectal nuclei and superior colliculus.
  • receives its blood supply from the posterior communicating artery and the anterior choroidal artery.

 

        • Transection results in
          • contralateral homonymous hemianopia

 

  • transsynaptic degeneration of the ipsilateral LGB.

 

      • Lateral geniculate body (LGB)
        • is a thalamic relay nucleus subserving vision.

 

  • receives fibers from the ipsilateral temporal hemiretina, which terminate in layers 2, 3, and 5.
  • receives fibers from the contralateral nasal hemiretina, which terminate in layers 1, 4, and 6.

 

        • projects, via the geniculocalcarine tract, the visual radiation to the primary visual cortex (area 17).

 

  • The stripe of Gennari marks the calcarine cortex.
  • is irrigated by branches of the posterior cerebral artery and the anterior choroidal artery.

 

        • Destruction results in a contralateral homonymous hemianopia.
      • Geniculocalcarine tract (visual radiation)
        • extends from the LGB to the banks of the calcarine sulcus, the visual cortex (area 17).

 

  • is irrigated by branches of the middle cerebral artery, anterior choroidal artery, and calcarine artery (a branch of the posterior cerebral artery).

 

        • Transection results in contralateral homonymous hemianopia.
        • Upper division

 

  • projects to the upper bank of the calcarine sulcus, the cuneus.
  • contains input from the superior retinal quadrants, representing inferior visual field quadrants.

 

          • Transection results in contralateral lower homonymous quadrantanopia.
        • Lower division
          • loops from the LGB anteriorly (Meyer loop), then posteriorly
          • terminate in the lower bank of the calcarine sulcus, the lingual gyrus.

 

  • contains input from the inferior retinal quadrants, representing superior visual field quadrants.

 

          • Transection of Meyer loop results in a contralateral upper homonymous quadrantanopia.
      • Visual (striate) cortex (area 17)
        • is located on the banks of the calcarine sulcus.
        • receives retinal input via the ipsilateral LGB.

 

  • receives its blood supply from the calcarine artery, a branch of the posterior cerebral artery

 

        • anastomosis with the middle cerebral artery may be substantial (macular sparing).
        • Lesions result in a contralateral homonymous hemianopia with macular sparing.
        • Bilateral destruction of both cunei results in a lower altitudinal hemianopia

 

  • Bilater- al destruction of the lingual gyri results in an upper altitudinal hemianopia.

 

        • Retinotopic organization of the visual cortex includes:

 

  • Posterior third of the visual cortex

 

            • receives macular input (central vision).

 

  • Intermediate area of the visual cortex
  • receives paramacular input (peripheral input).
  • Anterior area of the visual cortex

 

            • receives monocular input.

 

  • Pupillary Light Reflexes and Pathway

 

      • Pupillary light reflexes
        • result when light shined into one eye causes both pupils to constrict.
        • Direct pupillary light reflex – response in the stimulated eye.
        • Consensual pupillary light reflex –  response in the unstimulated eye.
      • Pupillary light reflex pathway
        • comprises an afferent limb, CN II, and an efferent limb, CN III.
        • Ganglion cells of the retina
          • project bilaterally to the pretectal nuclei.
        • Pretectal nucleus of the midbrain

 

  • projects crossed (in the posterior commissure) and uncrossed fibers to the rostral Edinger-Westphal nucleus.

 

        • Edinger-Westphal nucleus of the midbrain
          • gives rise to preganglionic parasympathetic fibers, which exit the midbrain with the oculomotor nerve and synapse with postganglionic parasympathetic neurons of the ciliary ganglion.

 

  • Ciliary ganglion of the orbit
  • gives rise to postganglionic parasympathetic fibers, which innervate the sphincter muscle of the iris.
  • Pupillary Dilation Pathway

 

        • is mediated by the sympathetic division of the autonomic nervous system (ANS).
        • Interruption at any level results in Horner syndrome.

 

  • has neurons that project directly to the ciliospinal center (T1–T2) of the intermediolateral cell column
  • Ciliospinal center of the spinal cord
  • projects preganglionic sympathetic fibers via the sympathetic trunk to the superior cervical ganglion.
  • Superior cervical ganglion

 

          • projects postganglionic sympathetic fibers via the perivascular plexus of the carotid system to the dilator muscle of the iris and to the palpebral muscles of Müller.
          • Postganglionic sympathetic fibers pass through the cavernous sinus and enter the orbit via the superior orbital fissure.
    • The Convergence-Accommodation Reaction
      • is essential for visual fixation and acuity at close range.
      • is initiated by conscious visual fixation on a near object or by a blurred retinal image.
      • with accommodative effort, three reflex changes are evoked

 

  • Convergence

 

          • occurs as the eyes focus on a near point.
          • is mediated by medial recti innervation via the oculomotor nerve (CN III).

 

  • Accommodation

 

          • is adjustment of the eyes for various distances.
          • occurs as contraction of the ciliary muscle

 

  • results in a thickening of the lens and an increase in refractive power.

 

          • is mediated by the caudal Edinger-Westphal nucleus via CN III.

 

  • Pupillary constriction

 

          • results in an increase in depth of field and depth of focus.
          • is mediated by the rostral Edinger-Westphal nucleus via CN III.
      • The convergence-accommodation pathway

 

  • Visual cortex (area 17)

 

          • projects to the visual association cortex (area 19).

 

  • Visual association cortex (area 19)
  • projects via the corticotectal tract to the pretectal area of the midbrain.
  • Pretectal area

 

          • projects to Perlia nucleus.

 

  • Perlia nucleus of the oculomotor complex, CN III

 

          • projects to the rostral and caudal Edinger-Westphal nuclei and the medial rectus subnuclei of CN III.

 

  • Centers for Ocular Motility

 

      • Frontal eye field
        • is located in the caudal part of the middle frontal gyrus (area 8).
        • Is a cortical center for voluntary eye movements
        • Mediates fast,saccadic,searching movements.
        • Stimulation (irritative lesion) results in contralateral conjugate deviation of the eyes.
        • Destruction (destructive lesion) results in transient ipsilateral conjugate deviation of the eyes.
      • Occipital eye fields (area 18 and 19)
        • are the cortical centers for involuntary pursuit or tracing movements
        • Stimulation results in contralateral conjugate deviation of the eyes.
        • Lesions result in difficulty following a slow-moving object.
      • Subcortical center for vertical conjugate gaze
        • is located at the level of the posterior commissure.
        • includes the rostral interstitial nucleus of the medial longitudinal fasciculus (MLF),which projects to the oculomotor and trochlear nuclei.
        • is involved in Parinaud syndrome
      • Subcortical center for lateral conjugate gaze

 

  • is located in the abducent nucleus of CN VI and parapontine reticular formation (PPRF)

 

        • receives input from the contralateral frontal eye field.

 

  • projects via the contralateral MLF to the medial rectus subnucleus of the oculomotor complex.

 

        • projects via abducent fibers to the ipsilateral lateral rectus muscle.
        • Damage to the MLF between the abducent and oculomotor nuclei results in medial rectus palsy
    • MLF syndrome (internuclear ophthalmoplegia [INO])
      • damage (demyelination) to the MLF between the abducent and oculomotor nuclei.
      • results in
      • medial rectus palsy on attempted lateral conjugate gaze
      • monocular horizontal nystagmus in the abducting eye (convergence is normal).
    • One-and-a-half syndrome
      • consists of
        • bilateral MLF lesion

 

  • unilateral lesion of the abducent nucleus.

 

On attempted lateral conjugate gaze, the only muscle that functions is the intact lateral rectus.