THE VISUAL SYSTEM

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THE VISUAL SYSTEM

• D. C. MIKULECKY
• PROFESSOR OF PHYSIOLOGY
• AND
• FACULTY MENTORING PROGRAM

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THE VISUAL SYSTEM SENSES ELEECTROMAGNETIC
RADIATION

• ELECTROMAGNETIC RADIATION (EMR) SPANS THE
  ELECTROMAGNETIC SPECTRUM (EMS)
• FROM RADIO WAVES (VERY LONG) TO ?-RADIATION
  (VERY SHORT)
• VISIBLE LIGHT IS A SMALL PORTION OF THE SPECTRUM
• PHOTONS OF LIGHT INTERACT WITH MATTER

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ANATOMICAL ORGANIZATION

• THE EYE
• CORNEA AND LENS BEND LIGHT RAYS AND FOCUS THEM
  ON THE RETINA
• CILLIARY MUSCLES LOSSEN OR TIGHTEN TO ADJUST LENS
  THICKNESS
• RETINA SITE OF PHOTORECEPTORS
• FOVEA MOST SENSITVE PART OF RETINA

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SCLERA
TEAR DUCT AND DRAINAGE CANAL
PUPIL
IRIS
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STRUCTURE OF THE EYE
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THE PHOTORECEPTORS

• RODS CYLLINDRICALLY SHAPED- BROAD RANGE OF
  WAVELENGTHS, NIGHT
• CONES CONICALLY SHAPED-NARROW WAVELENGTH RANGE,
  COLOR

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BEFORE A PHOTON ARRIVES

• SODIUM IS PUMPED AND LEAKS BACK THROUGH OPEN
  CHANNEL
• CHANNELS OPENNESS DEPENDENT ON cGMP
• MEMBRANE POLARIZED NORMALLY

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AFTER A PHOTON ARRIVES

• RHODOPSIN ABSORBS PHOTON AND CHANGES SHAPE
• TRANSDUCIN ACTIVATED BY RHODOPSIN
• PHOSPHODIESTERASE ACTIVATED BY TRANSDUCIN
• cGMP HYDROLIZED
• SODIUM CHANNEL CLOSES
• MEMBRANE HYPERPOLARIZED

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RETINAS NEURAL NETWORK

• GANGLION CELLS DEPOLARIZED BY BIPOLAR CELLS,
  FORM OPTIC NERVE
• BIPOLAR CELLS DEPOLARIZED BY PHOTORECEPTOR CELLS
• HORIZONTAL CELLS INHIBITED BY PHOTORECEPTORS -
  DEPOLARIZE ADJACENT PHOTORECEPTORS(INHIBITING
  THEM)

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HORIZONTAL CELLS INHIBIT ADJACENT PHOTORECEPTORS
PHOTORECEPTORS
PHOTORECEPTORS
A
B
A
B
GANGLION CELL
LOW RATE OF FIRING
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RECEPTIVE FIELDS

• THE AREAS OF THE RETINA THAT EXCITE OR INHIBIT A
  GANGLION CELL MAKE UP THE RECEPTIVE FIELD FOR
  THAT CELL
• RETINAL GANGLION CELLS HAVE ANNULAR RECEPTIVE
  FIELDS WITH DIFFERENT PATTERNS

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PATERNS OF EXCITATION/INHIBITION

PHOTORECEPTOR HORIZONTAL CELLS BIPOLAR
CELLS
- - --
-

-

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PATHWAYS FOR VISUAL INFORMATION

• OPTIC NERVE (GANGLION CELLS FROM RETINA)
• LEAVES THROUGH BLIND SPOT
• LATERAL GENICULATE NUCLEUS PROJECTS TO CORTEX
• PRIMARY VISUAL CORTEX

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ORGANIZATION OF PHOTORECEPTORS
ROD
TO OPTIC NERVE
HORIZONTAL CELL
AMICRINE CELL
CONE
BIPOLAR CELLS
GANGLION CELLS
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RECEPIVE FIELDS IN THE LATERAL GENICULATE AND
SENSORY CORTEX

• LATERAL GENICULATELIKE GANGLION CELLS
  ANNULAR(EXCITATORY RINGS OR INHIBITORY RINGS)
• OVERLAP OF ANNULAR FIELDS FROM LG RESULT IN
  RECTANGULAR FIELDS ON THE CORTEX

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VISUAL ORIENTATION COLUMNS

• CELLS IN VARIOUS COLUMNS OF CORTEX RESPOND TO
  DIFFERENT ORIENTATIONS
• THESE DEVELOP DURING THE EARLY VISUAL EXPERIENCES
  OF YOUNG ANIMALS

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COLOR VISION

• TRICHROMATIC RED, BLUE, GREEN
• PARVOCELLULAR NEURONS CARRY INFORMATION
• DIFFERENT CELL TYPES
• COLOR CONTRAST

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THREE KINDS OF CONES

• RED, BLUE, AND GREEN
• CONNECT TO SMALL GANGLION CELLS
• TRANSMIT COLOR INFORMATION TO PARVOCELLULAR
  NEURONS IN LGN

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COLOR NEURONS

• BROAD BAND SINGLE COLOR, INSIDE, - OUT
• SINGLE - OPPONENT EXITED BY ONE COLOR IN CENTER,
  INHIBITED WHEN ANOTHER COLOR IN PERIPHERY
• DOUBLE OPPONENT OPPOSING COLORS IN BOTH CENTER
  AND PERIPHERY
• ANNULAR FIELDS OVERLAP TO RECTANGULAR IN CORTEX

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LENS DEFECTS

• FOCUSING IN FRONT OF RETINA
• NEARSIGHTEDNESS (MYOPIA)
• USUALLY DUE TO WEAK CILIARY MUSCLES
• FOCUSING BEHIND THE RETINA
• FARSIGHTEDNESS(HYPEROPIA)
• LENS TOO STIFF (AGING)

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NEARSIGHTEDNESS (MYOPIA)
UNCORRECTED
CORRECTED
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FARSIGHTEDNESS (HYPEROPIA)
UNCORRECTED
CORRECTED
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VISUAL FIELD DEFECTS

• OPTIC NERVEVISUAL FIELD ON SAME SIDE
• OPTIC CHIASMOUTER HALF OF BOTH VISUAL FIELDS
• OPTIC TRACT OPPOSITE HALF IN BOTH VISUAL FIELDS
• OPTIC RADIATIONSLOWER OR UPPER QUADRANT ON
  OPPOSITE SIDE