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Special Senses

  • Special sensory receptors
    • Distinct, localized receptor cells in head
  • Vision
  • Taste
  • Smell
  • Hearing
  • Equilibrium

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The Eye and Vision

  • 70% of body's sensory receptors in eye
  • Most of eye protected by cushion of fat and bony orbit

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Figure 15.1a The eye and accessory structures.

© 2013 Pearson Education, Inc.

Eyebrow

Eyelid

Eyelashes

Site where

conjunctiva

merges with

cornea

Palpebral

fissure

Lateral

commissure

Iris

Eyelid

Surface anatomy of the right eye

Pupil

Sclera

(covered by

conjunctiva)

Lacrimal

caruncle

Medial

commissure

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Structure of the Eyeball

  • Wall of eyeball contains three layers
    • Fibrous
    • Vascular
    • Inner
  • Internal cavity filled with fluids called humors
  • Lens separates internal cavity into anterior and posterior segments (cavities)

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Fibrous Layer

  • Outermost layer; dense avascular connective tissue
  • Two regions: sclera and cornea

1. Sclera

      • Opaque posterior region
      • Protects, shapes eyeball; anchors extrinsic eye muscles
      • Continuous with dura mater of brain posteriorly

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Fibrous Layer

2. Cornea

      • Transparent anterior 1/6 of fibrous layer
      • Bends light as it enters eye
      • Sodium pumps of corneal endothelium on inner face help maintain clarity of cornea
      • Numerous pain receptors contribute to blinking and tearing reflexes

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Vascular Layer (Uvea)

  • Middle pigmented layer
  • Three regions: choroid, ciliary body, and iris

1. Choroid region

      • Posterior portion of uvea
      • Supplies blood to all layers of eyeball
      • Brown pigment absorbs light to prevent light scattering and visual confusion

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Vascular Layer

2. Ciliary body

      • Ring of tissue surrounding lens
      • Smooth muscle bundles (ciliary muscles) control lens shape
      • Capillaries of ciliary processes secrete fluid
      • Ciliary zonule (suspensory ligament) holds lens in position

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Vascular Layer

3. Iris

      • Colored part of eye
      • Pupil—central opening that regulates amount of light entering eye
        • Close vision and bright light—sphincter pupillae (circular muscles) contract; pupils constrict
        • Distant vision and dim light—dilator pupillae (radial muscles) contract; pupils dilate – sympathetic fibers
        • Changes in emotional state—pupils dilate when subject matter is appealing or requires problem-solving skills

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Inner Layer: Retina

  • Originates as outpocketing of brain
  • Delicate two-layered membrane
    • Outer Pigmented layer
      • Single-cell-thick lining
      • Absorbs light and prevents its scattering
      • Phagocytize photoreceptor cell fragments
      • Stores vitamin A

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Inner Layer: Retina

    • Inner Neural layer
      • Transparent
      • Composed of three main types of neurons
        • Photoreceptors, bipolar cells, ganglion cells
      • Signals spread from photoreceptors to bipolar cells to ganglion cells
      • Ganglion cell axons exit eye as optic nerve

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The Retina

  • Optic disc (blind spot)
    • Site where optic nerve leaves eye
    • Lacks photoreceptors
  • Quarter-billion photoreceptors of two types
    • Rods
    • Cones

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Photoreceptors

  • Rods
    • Dim light, peripheral vision receptors
    • More numerous, more sensitive to light than cones
    • No color vision or sharp images
    • Numbers greatest at periphery

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Photoreceptors

  • Cones
    • Vision receptors for bright light
    • High-resolution color vision
    • Macula lutea exactly at posterior pole
      • Mostly cones
      • Fovea centralis
        • Tiny pit in center of macula with all cones; best vision

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Figure 15.7 Part of the posterior wall (fundus) of the right eye as seen with an ophthalmoscope.

© 2013 Pearson Education, Inc.

Central

artery

and vein

emerging

from the

optic disc

Optic disc

Macula

lutea

Retina

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Lens

  • Biconvex, transparent, flexible, and avascular
  • Changes shape to precisely focus light on retina
  • Two regions
    • Lens epithelium anteriorly; Lens fibers form bulk of lens
    • Lens fibers filled with transparent protein crystallin
    • Lens becomes more dense, convex, less elastic with age
      • cataracts (clouding of lens) consequence of aging, diabetes mellitus, heavy smoking, frequent exposure to intense sunlight

​

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Cataracts

  • Clouding of lens
    • Consequence of aging, diabetes mellitus, heavy smoking, frequent exposure to intense sunlight
    • Some congenital
    • Crystallin proteins clump
    • Vitamin C increases cataract formation
    • Lens can be replaced surgically with artificial lens

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Figure 15.9 Photograph of a cataract.

© 2013 Pearson Education, Inc.

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Light And Optics: Wavelength And Color

  • Eyes respond to visible light
    • Small portion of electromagnetic spectrum
    • Wavelengths of 400-700 nm
  • Light
    • Packets of energy (photons or quanta) that travel in wavelike fashion at high speeds
    • Color of light objects reflect determines color eye perceives

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Light And Optics: Refraction And Lenses

  • Refraction
    • Bending of light rays
      • Due to change in speed when light passes from one transparent medium to another
      • Occurs when light meets surface of different medium at an oblique angle
    • Curved lens can refract light

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Refraction and Lenses

  • Light passing through convex lens (as in eye) is bent so that rays converge at focal point
    • Image formed at focal point is upside-down and reversed right to left
  • Concave lenses diverge light
    • Prevent light from focusing

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Focusing Light on The Retina

  • Pathway of light entering eye: cornea, aqueous humor, lens, vitreous humor, entire neural layer of retina, photoreceptors
  • Light refracted three times along pathway
    • Entering cornea
    • Entering lens
    • Leaving lens
  • Majority of refractory power in cornea
  • Change in lens curvature allows for fine focusing

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Focusing For Distant Vision

  • Eyes best adapted for distant vision
  • Far point of vision
    • Distance beyond which no change in lens shape needed for focusing
      • 20 feet for emmetropic (normal) eye
      • Cornea and lens focus light precisely on retina
  • Ciliary muscles relaxed
  • Lens stretched flat by tension in ciliary zonule

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Focusing For Close Vision

  • Light from close objects (<6 m) diverges as approaches eye
    • Requires eye to make active adjustments using three simultaneous processes
      • Accommodation of lenses
      • Constriction of pupils
      • Convergence of eyeballs

​

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Focusing For Close Vision

  • Accommodation
    • Changing lens shape to increase refraction
    • Near point of vision
      • Closest point on which the eye can focus
    • Presbyopia—loss of accommodation over age 50
  • Constriction
    • Accommodation pupillary reflex constricts pupils to prevent most divergent light rays from entering eye
  • Convergence
    • Medial rotation of eyeballs toward object being viewed

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Functional Anatomy Of Photoreceptors

  • Rods and cones
    • Modified neurons
    • Receptive regions called outer segments
      • Contain visual pigments (photopigments)
        • Molecules change shape as absorb light
    • Inner segment of each joins cell body

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Photoreceptor Cells

  • Vulnerable to damage
  • Degenerate if retina detached
  • Destroyed by intense light
  • Outer segment renewed every 24 hours
    • Tips fragment off and are phagocytized

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Rods

  • Functional characteristics
    • Very sensitive to light
    • Best suited for night vision and peripheral vision
    • Contain single pigment
      • Perceived input in gray tones only
    • Pathways converge, causing fuzzy, indistinct images

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Cones

  • Functional characteristics
    • Need bright light for activation (have low sensitivity)
    • React more quickly
    • Have one of three pigments for colored view
    • Nonconverging pathways result in detailed, high-resolution vision
    • Color blindness–lack of one or more cone pigments

© 2013 Pearson Education, Inc.

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© 2013 Pearson Education, Inc.

Table 15.1 Comparison of Rods and Cones

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Chemistry Of Visual Pigments

  • Retinal
    • Light-absorbing molecule that combines with one of four proteins (opsins) to form visual pigments
    • Synthesized from vitamin A
    • Retinal isomers: 11-cis-retinal (bent form) and all-trans-retinal (straight form)
      • Bent form 🡪 straight form when pigment absorbs light
      • Conversion of bent to straight initiates reactions 🡪 electrical impulses along optic nerve

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Phototransduction: Capturing Light

  • Deep purple pigment of rods–rhodopsin
    • 11-cis-retinal + opsin 🡪 rhodopsin
    • Three steps of rhodopsin formation and breakdown
      • Pigment synthesis
      • Pigment bleaching
      • Pigment regeneration

​

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Phototransduction: Capturing Light

  • Pigment synthesis
    • Rhodopsin forms and accumulates in dark
  • Pigment bleaching
    • When rhodopsin absorbs light, retinal changes to all-trans isomer
    • Retinal and opsin separate (rhodopsin breakdown)
  • Pigment regeneration
    • All-trans retinal converted to 11-cis isomer
    • Rhodopsin regenerated in outer segments

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Phototransduction In Cones

  • Similar as process in rods
  • Cones far less sensitive to light
    • Takes higher-intensity light to activate cones

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Light Transduction Reactions

  • Light-activated rhodopsin activates G protein transducin
  • Transducin activates PDE, which breaks down cyclic GMP (cGMP)
  • In dark, cGMP holds channels of outer segment open 🡪 Na+ and Ca2+ depolarize cell
  • In light cGMP breaks down, channels close, cell hyperpolarizes
    • Hyperpolarization is signal!

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Information Processing In The Retina

  • Photoreceptors and bipolar cells only generate graded potentials (EPSPs and IPSPs)
  • When light hyperpolarizes photoreceptor cells
    • Stop releasing inhibitory neurotransmitter glutamate
    • Bipolar cells (no longer inhibited) depolarize, release neurotransmitter onto ganglion cells
    • Ganglion cells generate APs transmitted in optic nerve to brain

© 2013 Pearson Education, Inc.

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© 2013 Pearson Education, Inc.

Figure 15.18 Signal transmission in the retina (1 of 2).

Slide 2

In the dark

cGMP-gated channels

open, allowing cation influx.

Photoreceptor depolarizes.

1

Photoreceptor

cell (rod)

Bipolar

Cell

Ganglion

cell

−40 mV

−40 mV

Ca2+

Na+

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© 2013 Pearson Education, Inc.

Figure 15.18 Signal transmission in the retina (1 of 2).

Slide 3

In the dark

cGMP-gated channels

open, allowing cation influx.

Photoreceptor depolarizes.

1

Voltage-gated Ca2+

channels open in synaptic

terminals.

Photoreceptor

cell (rod)

Bipolar

Cell

Ganglion

cell

Ca2+

−40 mV

−40 mV

2

Ca2+

Na+

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© 2013 Pearson Education, Inc.

Figure 15.18 Signal transmission in the retina (1 of 2).

Slide 4

In the dark

cGMP-gated channels

open, allowing cation influx.

Photoreceptor depolarizes.

1

Voltage-gated Ca2+

channels open in synaptic

terminals.

Neurotransmitter is

released continuously.

Photoreceptor

cell (rod)

Bipolar

Cell

Ganglion

cell

Ca2+

−40 mV

−40 mV

2

3

Ca2+

Na+

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© 2013 Pearson Education, Inc.

Figure 15.18 Signal transmission in the retina (1 of 2).

Slide 5

In the dark

cGMP-gated channels

open, allowing cation influx.

Photoreceptor depolarizes.

1

Voltage-gated Ca2+

channels open in synaptic

terminals.

Neurotransmitter is

released continuously.

Neurotransmitter causes

IPSPs in bipolar cell.

Hyperpolarization results.

Photoreceptor

cell (rod)

Bipolar

Cell

Ganglion

cell

Ca2+

−40 mV

−40 mV

2

3

4

Ca2+

Na+

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© 2013 Pearson Education, Inc.

Figure 15.18 Signal transmission in the retina (1 of 2).

Slide 6

In the dark

cGMP-gated channels

open, allowing cation influx.

Photoreceptor depolarizes.

1

Voltage-gated Ca2+

channels open in synaptic

terminals.

Neurotransmitter is

released continuously.

Neurotransmitter causes

IPSPs in bipolar cell.

Hyperpolarization results.

Hyperpolarization closes

voltage-gated Ca2+ channels,

inhibiting neurotransmitter

release.

Photoreceptor

cell (rod)

Bipolar

Cell

Ganglion

cell

Ca2+

−40 mV

−40 mV

2

3

4

5

Ca2+

Na+

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© 2013 Pearson Education, Inc.

Figure 15.18 Signal transmission in the retina (1 of 2).

Slide 7

In the dark

cGMP-gated channels

open, allowing cation influx.

Photoreceptor depolarizes.

1

Voltage-gated Ca2+

channels open in synaptic

terminals.

Neurotransmitter is

released continuously.

Neurotransmitter causes

IPSPs in bipolar cell.

Hyperpolarization results.

Hyperpolarization closes

voltage-gated Ca2+ channels,

inhibiting neurotransmitter

release.

No EPSPs occur in

ganglion cell.

Photoreceptor

cell (rod)

Bipolar

Cell

Ganglion

cell

Ca2+

−40 mV

−40 mV

2

3

4

5

6

Ca2+

Na+

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© 2013 Pearson Education, Inc.

Figure 15.18 Signal transmission in the retina (1 of 2).

Slide 8

In the dark

cGMP-gated channels

open, allowing cation influx.

Photoreceptor depolarizes.

1

Voltage-gated Ca2+

channels open in synaptic

terminals.

Neurotransmitter is

released continuously.

Neurotransmitter causes

IPSPs in bipolar cell.

Hyperpolarization results.

Hyperpolarization closes

voltage-gated Ca2+ channels,

inhibiting neurotransmitter

release.

No EPSPs occur in

ganglion cell.

No action potentials occur

along the optic nerve.

Photoreceptor

cell (rod)

Bipolar

Cell

Ganglion

cell

Ca2+

−40 mV

−40 mV

2

3

4

5

6

7

Ca2+

Na+

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© 2013 Pearson Education, Inc.

Figure 15.18 Signal transmission in the retina. (2 of 2).

Slide 2

−70 mV

cGMP-gated channels

close, so cation influx

stops. Photoreceptor

hyperpolarizes.

1

Photoreceptor

cell (rod)

Bipolar

Cell

Ganglion

cell

In the light

Light

−70 mV

Below, we look at a tiny column of retina.

The outer segment of the rod, closest to the

back of the eye and farthest from the

incoming light, is at the top.

Light

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© 2013 Pearson Education, Inc.

Figure 15.18 Signal transmission in the retina. (2 of 2).

Slide 3

−70 mV

cGMP-gated channels

close, so cation influx

stops. Photoreceptor

hyperpolarizes.

Voltage-gated Ca2+

channels close in synaptic

terminals.

1

Photoreceptor

cell (rod)

Bipolar

Cell

Ganglion

cell

In the light

Light

−70 mV

2

Below, we look at a tiny column of retina.

The outer segment of the rod, closest to the

back of the eye and farthest from the

incoming light, is at the top.

Light

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© 2013 Pearson Education, Inc.

Figure 15.18 Signal transmission in the retina. (2 of 2).

Slide 4

−70 mV

No neurotransmitter

is released.

cGMP-gated channels

close, so cation influx

stops. Photoreceptor

hyperpolarizes.

Voltage-gated Ca2+

channels close in synaptic

terminals.

1

Photoreceptor

cell (rod)

Bipolar

Cell

Ganglion

cell

In the light

Light

−70 mV

2

3

Below, we look at a tiny column of retina.

The outer segment of the rod, closest to the

back of the eye and farthest from the

incoming light, is at the top.

Light

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© 2013 Pearson Education, Inc.

Figure 15.18 Signal transmission in the retina. (2 of 2).

Slide 5

−70 mV

No neurotransmitter

is released.

cGMP-gated channels

close, so cation influx

stops. Photoreceptor

hyperpolarizes.

Lack of IPSPs in bipolar

cell results in depolarization.

Voltage-gated Ca2+

channels close in synaptic

terminals.

1

Photoreceptor

cell (rod)

Bipolar

Cell

Ganglion

cell

In the light

Light

−70 mV

2

3

4

Below, we look at a tiny column of retina.

The outer segment of the rod, closest to the

back of the eye and farthest from the

incoming light, is at the top.

Light

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© 2013 Pearson Education, Inc.

Figure 15.18 Signal transmission in the retina. (2 of 2).

Slide 6

−70 mV

No neurotransmitter

is released.

Depolarization opens

voltage-gated Ca2+ channels;

neurotransmitter is released.

cGMP-gated channels

close, so cation influx

stops. Photoreceptor

hyperpolarizes.

Lack of IPSPs in bipolar

cell results in depolarization.

Voltage-gated Ca2+

channels close in synaptic

terminals.

1

Photoreceptor

cell (rod)

Bipolar

Cell

Ganglion

cell

In the light

Light

Ca2+

−70 mV

2

3

4

5

Below, we look at a tiny column of retina.

The outer segment of the rod, closest to the

back of the eye and farthest from the

incoming light, is at the top.

Light

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© 2013 Pearson Education, Inc.

Figure 15.18 Signal transmission in the retina. (2 of 2).

Slide 7

−70 mV

No neurotransmitter

is released.

Depolarization opens

voltage-gated Ca2+ channels;

neurotransmitter is released.

EPSPs occur in ganglion

cell.

cGMP-gated channels

close, so cation influx

stops. Photoreceptor

hyperpolarizes.

Lack of IPSPs in bipolar

cell results in depolarization.

Voltage-gated Ca2+

channels close in synaptic

terminals.

1

Photoreceptor

cell (rod)

Bipolar

Cell

Ganglion

cell

In the light

Light

Ca2+

−70 mV

2

3

4

5

6

Below, we look at a tiny column of retina.

The outer segment of the rod, closest to the

back of the eye and farthest from the

incoming light, is at the top.

Light

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© 2013 Pearson Education, Inc.

Figure 15.18 Signal transmission in the retina. (2 of 2).

Slide 8

−70 mV

No neurotransmitter

is released.

Depolarization opens

voltage-gated Ca2+ channels;

neurotransmitter is released.

EPSPs occur in ganglion

cell.

Action potentials

propagate along the

optic nerve.

cGMP-gated channels

close, so cation influx

stops. Photoreceptor

hyperpolarizes.

Lack of IPSPs in bipolar

cell results in depolarization.

Voltage-gated Ca2+

channels close in synaptic

terminals.

1

Photoreceptor

cell (rod)

Bipolar

Cell

Ganglion

cell

In the light

Light

Ca2+

−70 mV

2

3

4

5

6

7

Below, we look at a tiny column of retina.

The outer segment of the rod, closest to the

back of the eye and farthest from the

incoming light, is at the top.

Light

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© 2013 Pearson Education, Inc.

Figure 15.18 Signal transmission in the retina. (2 of 2).

Slide 1

−70 mV

No neurotransmitter

is released.

Depolarization opens

voltage-gated Ca2+ channels;

neurotransmitter is released.

EPSPs occur in ganglion

cell.

Action potentials

propagate along the

optic nerve.

cGMP-gated channels

close, so cation influx

stops. Photoreceptor

hyperpolarizes.

Lack of IPSPs in bipolar

cell results in depolarization.

Voltage-gated Ca2+

channels close in synaptic

terminals.

1

Photoreceptor

cell (rod)

Bipolar

Cell

Ganglion

cell

In the light

Light

Ca2+

−70 mV

2

3

4

5

6

7

Below, we look at a tiny column of retina.

The outer segment of the rod, closest to the

back of the eye and farthest from the

incoming light, is at the top.

Light

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Light Adaptation

  • Move from darkness into bright light
    • Both rods and cones strongly stimulated
      • Pupils constrict
    • Large amounts of pigments broken down instantaneously, producing glare
    • Visual acuity improves over 5–10 minutes as:
      • Rod system turns off
      • Retinal sensitivity decreases
      • Cones and neurons rapidly adapt

© 2013 Pearson Education, Inc.

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Dark Adaptation

  • Move from bright light into darkness
    • Cones stop functioning in low-intensity light
    • Rod pigments bleached; system turned off
    • Rhodopsin accumulates in dark
    • Transducin returns to outer segments
    • Retinal sensitivity increases within 20–30 minutes
    • Pupils dilate

© 2013 Pearson Education, Inc.

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Night Blindness

  • Nyctalopia
  • Rod degeneration
    • Commonly caused by vitamin A deficiency
    • If administered early vitamin A supplements restore function
    • Also caused by retinitis pigmentosa
      • Degenerative retinal diseases that destroy rods

© 2013 Pearson Education, Inc.

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Visual Pathway To The Brain

  • Axons of retinal ganglion cells form optic nerve
  • Medial fibers of optic nerve decussate at optic chiasma
  • Most fibers of optic tracts continue to lateral geniculate body of thalamus
  • Fibers from thalamic neurons form optic radiation and project to primary visual cortex in occipital lobes

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Visual Pathway

  • Fibers from thalamic neurons form optic radiation
  • Optic radiation fibers connect to primary visual cortex in occipital lobes
  • Other optic tract fibers send branches to midbrain, ending in superior colliculi (initiating visual reflexes)

© 2013 Pearson Education, Inc.

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Visual Pathway

  • A small subset of ganglion cells in retina contain melanopsin (circadian pigment), which projects to:
    • Pretectal nuclei (involved with pupillary reflexes)
    • Suprachiasmatic nucleus of hypothalamus, timer for daily biorhythms

© 2013 Pearson Education, Inc.

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Depth Perception

  • Both eyes view same image from slightly different angles
  • Depth perception (three-dimensional vision) results from cortical fusion of slightly different images
  • Requires input from both eyes

© 2013 Pearson Education, Inc.

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Problems Of Refraction

  • Myopia (nearsightedness)
    • Focal point in front of retina, e.g., eyeball too long
    • Corrected with a concave lens
  • Hyperopia (farsightedness)
    • Focal point behind retina, e.g., eyeball too short
    • Corrected with a convex lens
  • Astigmatism
    • Unequal curvatures in different parts of cornea or lens
    • Corrected with cylindrically ground lenses or laser procedures

© 2013 Pearson Education, Inc.

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