VISUAL SYSTEM
 Perception of
 shape
 motion
 color
 Two pathways
 retina – cortex
• visual perception
 retina – brainstem, diencephalon
• eye movements
• circadian photoentrainment
• accommodation
• pupillary reflexes
Light passes through the cornea,
aqueous humor, lens, and vitreous body
to form an image on the retina.
Macula lutea + fovea centralis
= areas of the highest visual acuity
Fundus oculi
RETINA
10 layers: mainly separated by cell bodies (nuclear layers)
and axons (plexiform layers)
 5 main cell types:
 photoreceptors
 bipolar cells
 horizontal cells
 amacrine cells
 ganglion cells
 Photoreceptors:
 rods and cones
 involved in
transduction
converting the
light signal into a
nerve impulse
 neurons with serial (vertical)
connection
 the main visual pathway
 photoreceptors → bipolar cells
→ ganglion cells
 neurons with parallel (horizontal)
connection
 modulation of the visual information
by retina
 horizontal cells
 amacrine cells
 Cones (7 million)
 cluster at fovea
(macula lutea)
 detect color in bright light
= photopic vision
 Rods (100 million)
 outside the fovea
 sensitive to shape and
movement
= scotopic vision
CONES
 3 different types with
three different
photopigments:
blue, green and red
 Each type is maximally
sensitive to the wavelength
that corresponds to
the specific color range
(spectral sensitivity)
GANGLION CELLS
 P cells (80%)
 ganglion cells that monitor cones
 smaller, more numerous
 axons end on parvocellular laminae of LGN
 provide information about fine detail and color
 M cells (10%)
 ganglion cells that monitor rods
 relatively large
 axons end on magnocellular laminae of LGN
 provide information about a general form of an object,
motion, and shadows in dim light
 non-P non-M cells (10%)
 projection to subcortical nuclei, koniocellular cells of LGN
PRIMARY VISUAL PATHWAY
 The primary visual pathway connects the retina with
lateral geniculate nucleus and primary visual cortex
(retinogeniculostriate pathway)
 It is responsible for detection of shape, movement
and color
1st neuron (photoreceptors)
2nd neuron (bipolar cells)
3rd neuron (ganglion cells) LGNOptic chiasm
Primary
visual
cortex
CN II Optic tract
Optic
radiation
 LGN is composed of 6 layers
 Layers 1 and 2 contain
larger neurons
 Layers 3 - 6 contain
smaller neurons
LATERAL GENICULATE NUCLEUS (LGN)
 Ipsilateral input enters
layers 2,3 and 5
 Contralateral input enters
layers 1, 4 and 6
 LGN contains the topographic
representation of what the retina
“ sees”. This retinotopic map
is sent to the cortex.
 LGN modulates and regulates
the flow of visual information
to the primary visual cortex
 cortex can control efficiency of
thalamic input
optic radiation (geniculocalcarine fibres) runs
under the temporal lobe to the occipital lobe
GENICULOSTRIATE PATHWAY
RETINOTOPIC REPRESENTATION
 Nasal and temporal visual fields
 Reversed to opposite halves of retinal
representative fields (hemiretinas)
 Inverted and reversed
 Nasal visual fields project to
temporal hemiretinas and their axons
do not cross at the optic chiasm
 Temporal visual fields project to
nasal hemiretinas and their axons
cross at the optic chiasm
RETINOTOPY
 Most of the visual field is shared
by the two eyes (binocular field)
 Representation of different parts
of the visual field is disproportionate
in size
VISUAL CORTEX
PRIMARY VISUAL CORTEX (V1)
 Most LGN axons terminate in V1
 All V1 neurons respond to visual
stimuli exclusively
 Ablating V1 results in blindness
in the contralesional hemifield
(homonymous hemianopsia)
 Electrical stimulation of V1 elicits
visual sensations
VISUAL ASSOCIATION CORTEX
Dorsal Stream
 spatial orientation
 binocular fusion/depth perception
 the location, the movement
and the movement direction
and velocity of objects in space
Ventral Stream
 recognize objects and colors
 read text
 learn and remember visual objects
(e.g., words and their meanings)
VISUAL PATHWAYS TO SUBCORTICAL
STRUCTURES
 to the suprachiasmatic nucleus of hypothalamus
 to the pretectum of the midbrain
 to the superior colliculus
1st order neuron
 bipolar neuron of the spiral ganglion
 dendrites make synapses with hair cells
 axons form the cochlear part of CN VIII
2nd order neuron
 ventral cochlear nucleus → trapezoid body → lateral lemniscus
 dorsal cochlear nucleus → lateral lemniscus
3rd order neuron
 nucleus of inferior colliculus
→ brachium c.i.
4th order neuron
 medial geniculate nucleus
→ radiatio acustica
(internal capsule)
PRIMARY AUDITORY CORTEX
gyrus temporalis superior (gyri temporales transversi of Heschl)
- area 41 + 42
Two functionally significant features:
 tonotopical organization
 bilateral projection
DESCENDING PATHWAYS
 feedback system processing ascending information
 enhance signals
 supress noise
 mainly functions of the
superior olivary complex
 focus on a particular speaker and
inhibit other voices
 changes in the motion of the head (kinetic) and
in the position of the head with respect to gravity (static)
 3 afferent sources: the eyes, general proprioceptive
receptors throughout the body, and the vestibular
receptors in the inner ear
 to maintain equilibrium, to direct the gaze of the eyes,
and to preserve a constant plane of vision
VESTIBULAR APPARATUS
 Labyrinth of static apparatus
 macula utriculi – orientation in horizontal position
 macula sacculi – orientation in vertical position
 Labyrinth of kinetic apparatus
 cristae ampullares of semicircular ducts
 Hair cells in the maculae of the saccule and the utricle
respond to linear acceleration (gravity).
 Hair cells in the cristae ampullares in the semicircular
ducts respond to angular acceleration (rotation of the head).
VESTIBULAR PATHWAY
 1st order neuron –
vestibular ganglion
(utriculoampullar nerve,
saccular nerve,
posterior ampullar nerve)
 2nd order neuron –
vestibular nuclei
(superior, inferior,
medial, lateral)
Connections with the cerebellum
 vestibular portion of the CN VIII – inferior cerebellar
peduncles – ipsilateral vestibulocerebellum
 vestibular nuclei – inferior cerebellar peduncles –
vestibulocerebellum
maintenance of balance
Connections with the spinal cord
to motoneurons that innervate axial and proximal limb muscles
 lateral vestibulospinal tract
 from lateral vestibular nucleus
 uncrossed
 terminating at all levels of the spinal cord
 excitatory influences for extensors
 medial vestibulospinal tract
 from medial vestibular nucleus
 uncrossed
 descends in the MLF
 terminates mainly at cervical levels
 coordination of head position and eye movements
Connections with the brain stem
 ascending portion of MLF
 CN III, IV, VI
 Darkschewitsch and Cajal nuclei
 coordination of eye movements in response
to head movements
Connection with the thalamus (cortex)
conscious perception of movement and gravity
Olfactory region
1st order neuron – bipolar olfactory neurons
2nd order neuron – mitral cells –
olfactory tract
3rd order neuron – olfactory tubercle
4th order neuron – dorsomedial nucleus of thalamus
Orbitofrontal cortex (perception of olfactory information)
Taste buds
 receptor cells
(replaced about every 9-10 days
by differentiating basal cells)
 supportive columnar cells
 basal cells
1st order neuron –
 CN VII –geniculate ganglion
 via lingual nerve and chorda tympani
 via greater petrosal nerve
 CN IX – inferior ganglion of CN IX
 CN X – inferior ganglion of CN X
2nd order neuron - rostral part of
the solitary nucleus
3rd order neuron – ventral
posteromedial nucleus of thalamus
Primary gustatory cortex
 a. 43 in the postcentral gyrus
 insula
Illustrations were copied and lecture was prepared from:
Noback CH.R. et al: The Human Nervous System:
Structure and Function. Humana Press, Totowa, New
Jersey, Sixth ed.
Neuroscience Online, the Open-Access Neuroscience
Electronic Textbook
Department of Neurobiology and Anatomy
University of Texas Medical School at Houston