Sensation Properties and Types of Sensory Receptors General Senses Chemical Senses Hearing and Equilibrium Vision Definitions



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Ch 14 and 16 Sensation


Sensation

  • Properties and Types of Sensory Receptors

  • General Senses

  • Chemical Senses

  • Hearing and Equilibrium

  • Vision

Definitions

  • sensory input is vital to the integrity of personality and intellectual function

  • sensory deprivation – withholding sensory stimulation

  • sensory receptor - a structure specialized to detect a stimulus

    • bare nerve ending

    • sense organs - nerve tissue surrounded by other tissues that enhance response to certain type of stimulus

      • added epithelium, muscle or connective tissue

Motor Divisions of PNS

  • motor (efferent) division – carries signals from the CNS to gland and muscle cells that carry out the body’s response

      • effectors – cells and organs that respond to commands from the CNS

    • somatic motor division – carries signals to skeletal muscles

      • output produces muscular contraction as well as somatic reflexes – involuntary muscle contractions

    • visceral motor division (autonomic nervous system) - carries signals to glands, cardiac muscle, and smooth muscle

      • involuntary, and responses of this system and its receptors are visceral reflexes

      • sympathetic division

        • tends to arouse body for action

        • accelerating heart beat and respiration, while inhibiting digestive and urinary systems

      • parasympathetic division

        • tends to have calming effect

        • slows heart rate and breathing

        • stimulates digestive and urinary systems

Sensory Divisions of PNS

  • sensory (afferent) division – carries sensory signals from various receptors to the CNS

    • informs the CNS of stimuli within or around the body

    • somatic sensory division – carries signals from receptors in the skin, muscles, bones, and joints

    • visceral sensory division – carries signals from the viscera of the thoracic and abdominal cavities

      • heart, lungs, stomach, and urinary bladder

General Properties of Receptors

  • transduction – the conversion of one form of energy to another

    • fundamental purpose of any sensory receptor

    • conversion of stimulus energy (light, heat, touch, sound, etc.) into nerve signals

    • sense organ, gasoline engine, light bulb are all transducers

  • receptor potential – small, local electrical change on a receptor cell brought about by an initial stimulus

      • results in release of neurotransmitter or a volley of action potentials that generates nerve signals to the CNS

  • sensation – a subjective awareness of the stimulus

    • most sensory signals delivered to the CNS produce no conscious sensation

      • filtered out in the brainstem

      • some do not require conscious awareness like pH and body temperature

Unencapsulated Nerve Endings

  • dendrites not wrapped in connective tissue

  • free nerve endings

    • for pain and temperature

    • skin and mucous membrane

  • tactile discs

    • for light touch and texture

    • associated with Merkel cells at base of epidermis

  • hair receptors

    • wrap around base hair follicle

    • monitor movement of hair

Encapsulated Nerve Endings

  • dendrites wrapped by glial cells or connective tissue

  • connective tissue enhances sensitivity or selectivity of response

  • tactile (Meissner) corpuscles

  • Krause end bulb

    • tactile; in mucous membranes

  • lamellated (pacinian) corpuscles - phasic

    • deep pressure, stretch, tickle and vibration

    • periosteum of bone, and deep dermis of skin

  • bulbous (Ruffini) corpuscles - tonic

    • heavy touch, pressure, joint movements and skin stretching

Nature of Reflexes

  • reflexes - quick, involuntary, stereotyped reactions of glands or muscle to stimulation

    • automatic responses to sensory input that occur without our intent or often even our awareness

  • four important properties of a reflex

    • reflexes require stimulation

      • not spontaneous actions, but responses to sensory input

    • reflexes are quick

      • involve few if any interneurons and minimum synaptic delay

    • reflexes are involuntary

      • occur without intent and difficult to suppress

      • automatic response

    • reflexes are stereotyped

      • occur essentially the same way every time

Pain

  • pain – discomfort caused by tissue injury or noxious stimulation, and typically leading to evasive action

    • important since helps protect us

    • lost in diabetes mellitus – diabetic neuropathy

  • somatic pain - from skin, muscles and joints

  • visceral pain - from the viscera

    • stretch, chemical irritants or ischemia of viscera (poorly localized)

Chemical Sense – Taste

  • gustation (taste) – sensation that results from action of chemicals on taste buds

    • 4000 - taste buds mainly on tongue

    • inside cheeks, and on soft palate, pharynx, and epiglottis

  • lingual papillae

    • filiform - no taste buds

      • important for food texture

    • foliate - no taste buds

      • weakly developed in humans

    • fungiform

      • at tips and sides of tongue

    • vallate (circumvallate)

      • at rear of tongue

      • contains 1/2 of all taste buds

Taste Bud Structure

  • all taste buds look alike

  • lemon-shaped groups of 40 – 60 taste cells, supporting cells, and basal cells

  • taste cells

    • have tuft of apical microvilli (taste hairs) that serve as receptor surface for taste molecules

    • taste pores – pit in which the taste hairs project

    • taste hairs are epithelial cells not neurons

    • synapse with and release neurotransmitters onto sensory neurons at their base

  • basal cells

    • stem cells that replace taste cells every 7 to 10 days

  • supporting cells

    • resemble taste cells without taste hairs, synaptic vesicles, or sensory role

Physiology of Taste

  • to be tasted, molecules must dissolve in saliva and flood the taste pore

  • five primary sensations

    • salty – produced by metal ions (sodium and potassium)

    • sweet – associated with carbohydrates and other foods of high caloric value

    • sour – acids such as in citrus fruits

    • bitter – associated with spoiled foods and alkaloids such as nicotine, caffeine, quinine, and morphine

    • umami – ‘meaty’ taste of amino acids in chicken or beef broth

  • taste is influenced by food texture, aroma, temperature, and appearance

    • mouthfeel - detected by branches of lingual nerve in papillae

  • hot pepper stimulates free nerve endings (pain), not taste buds

  • regional differences in taste sensations on tongue

    • tip is most sensitive to sweet, edges to salt and sour, and rear to bitter

  • two mechanisms of action

    • activate 2nd messenger systems

      • sugars, alkaloids, and glutamate bind to receptors which activates G proteins and second-messenger systems within the cell

    • depolarize cells directly

      • sodium and acids penetrate cells and depolarize it directly

      • either mechanism results in release of neurotransmitters that stimulate dendrites at base of taste cells

Projection Pathways for Taste

  • facial nerve collects sensory information from taste buds over anterior two-thirds of tongue

  • glossopharyngeal nerve from posterior one-third of tongue

  • vagus nerve from taste buds of palate, pharynx and epiglottis

  • all fibers reach solitary nucleus in medulla oblongata

  • from there, signals sent to two destinations

    • hypothalamus and amygdala control autonomic reflexes – salivation, gagging and vomiting

    • thalamus relays signals to postcentral gyrus of cerebrum for conscious sense of taste

      • sent on to orbitofrontal cortex to be integrated with signals from nose and eyes - form impression of flavor and palatability of food

Smell – Anatomy

  • olfactory cells

    • are neurons

    • shaped like little bowling pins

    • head bears 10 – 20 cilia called olfactory hairs

    • have binding sites for odorant molecules and are nonmotile

    • lie in a tangled mass in a thin layer of mucus

    • basal end of each cell becomes the axon

    • axons collect into small fascicles and leave cranial cavity through the cribriform foramina in the ethmoid bone

    • fascicles are collectively regarded as Cranial Nerve

Smell – Physiology

  • humans have a poorer sense of smell than most other mammals

    • women more sensitive to odors than men

    • highly important to social interaction

    • odorant molecules bind to membrane receptor on olfactory hair

    • hydrophilic - diffuse through mucus

    • hydrophobic - transported by odorant-binding protein in mucus

    • activate G protein and cAMP system

  • opens ion channels for Na+ or Ca2+

    • depolarizes membrane and creates receptor potential

  • Human Pheromones

    • human body odors may affect sexual behavior

    • a person’s sweat and vaginal secretions affect other people’s sexual physiology

      • dormitory effect

    • presence of men seems to influence female ovulation

    • ovulating women’s vaginal secretions contain pheromones called copulines, that have been shown to raise men’s testosterone level

Olfactory Projection Pathways

  • olfactory cells synapse in olfactory bulb

    • on dendrites of mitral and tufted cells

    • dendrites meet in spherical clusters called glomeruli

      • each glomeruli dedicated to single odor because all fibers leading to one glomerulus come from cells with same receptor type

  • tufted and mitral cell axons form olfactory tracts

    • reach primary olfactory cortex in the inferior surface of the temporal lobe

    • secondary destinations –hippocampus, amygdala, hypothalamus, insula, and orbitofrontal cortex

      • identify odors, integrate smell with taste, perceive flavor, evoke memories and emotional responses, and visceral reactions

    • fibers reach back to olfactory bulbs where granule cells inhibit the mitral and tufted cells

      • reason why odors change under different conditions

      • food smells more appetizing when you are hungry

Hearing and Equilibrium

  • hearing – a response to vibrating air molecules

  • equilibrium – the sense of motion, body orientation, and balance

  • both senses reside in the inner ear, a maze of fluid-filled passages and sensory cells

  • fluid is set in motion and how the sensory cells convert this motion into an informative pattern of action potentials

The Nature of Sound

  • sound – any audible vibration of molecules

    • a vibrating object pushes on air molecules

    • in turn push on other air molecules

    • air molecules hitting eardrum cause it to vibration

Anatomy of Ear

  • ear has three sections outer, middle, and inner ear

    • first two are concerned only with the transmission of sound to the inner ear

    • inner ear – vibrations converted to nerve signals

Outer (External) Ear

  • outer ear – a funnel for conducting vibrations to the tympanic membrane (eardrum)

    • auricle (pinna) directs sound down the auditory canal

      • shaped and supported by elastic cartilage

    • auditory canal – passage leading through the temporal bone to the tympanic membrane

    • external acoustic meatus – slightly s-shaped tube that begins at the external opening and courses for about 3 cm

      • guard hairs protect outer end of canal

      • cerumen (earwax) – mixture of secretions of ceruminous and sebaceous glands and dead skin cells

        • sticky and coats guard hairs

        • contains lysozyme with low pH that inhibits bacterial growth

        • water-proofs canal and protects skin

        • keeps tympanic membrane pliable

Middle Ear

  • middle ear - located in the air-filled tympanic cavity in temporal bone

    • tympanic membrane (eardrum) – closes the inner end of the auditory canal

      • separates it from the middle ear

      • about 1 cm in diameter

      • suspended in a ring-shaped groove in the temporal bone

      • vibrates freely in response to sound

      • innervated by sensory branches of the vagus and trigeminal nerves

        • highly sensitive to pain

    • tympanic cavity is continuous with mastoid air cells

      • space only 2 to 3 mm wide between outer and inner ears

      • contains auditory ossicles

    • auditory (eustachian) tube connects middle ear cavity to nasopharynx

      • equalizes air pressure on both sides of tympanic membrane

      • normally flattened and closed and swallowing and yawning opens it

      • allows throat infections to spread to the middle ear

    • auditory ossicles

      • malleus - attached to inner surface of tympanic membrane

      • incus - articulates in between malleus and stapes

      • stapes - footplate rests on oval window – inner ear begins

    • stapedius and tensor tympani muscles attach to stapes and malleus

Inner (Internal) Ear

  • bony labyrinth - passageways in temporal bone

  • membranous labyrinth - fleshy tubes lining the bony labyrinth

    • filled with endolymph - similar to intracellular fluid

    • floating in perilymph - similar to cerebrospinal fluid

Details of Inner Ear

  • labyrinth - vestibule and three semicircular ducts

  • cochlea - organ of hearing

    • 2.5 coils around an screwlike axis of spongy bone, the modiolus

    • threads of the screw form a spiral platform that supports the fleshy tube of the cochlea

Anatomy of Cochlea

  • cochlea has three fluid-filled chambers separated by membranes:

    • scala vestibuli – superior chamber

      • filled with perilymph

      • begins at oval window and spirals to apex

    • scala tympani – inferior chamber

      • filled with perilymph

      • begins at apex and ends at round window

        • secondary tympanic membrane – membrane covering round window

    • scala media (cochlear duct) – triangular middle chamber

      • filled with endolymph

      • separated from:

        • scala vestibuli by vestibular membrane

        • scala tympani by thicker basilar membrane

      • contains spiral organ - organ of Corti - acoustic organ – converts vibrations into nerve impulses

Spiral Organ (Organ of Corti)

  • spiral organ has epithelium composed of hair cells and supporting cells

  • hair cells have long, stiff microvilli called stereocilia on apical surface

      • gelatinous tectorial membrane rests on top of stereocilia

  • spiral organ has four rows of hair cells spiraling along its length

    • inner hair cells – single row of about 3500 cells

      • provides for hearing

    • outer hair cells – three rows of about 20,000 cells

      • adjusts response of cochlea to different frequencies

      • increases precision

Physiology of Hearing - Middle Ear

  • tympanic membrane

    • has 18 times area of oval window

    • ossicles concentrate the energy of the vibrating tympanic membrane on an area 1/18 the size

    • ossicles create a greater force per unit area at the oval window and overcomes the inertia of the perilymph

    • ossicles and their muscles have a protective function

      • lessen the transfer of energy to the inner ear

  • tympanic reflex

    • during loud noise, the tensor tympani pulls the tympanic membrane inward and tenses it

    • stapedius muscle reduces the motion of the stapes

    • muffles the transfer of vibration from the tympanic membrane to the oval window

    • middle ear muscles also help to coordinate speech with hearing

      • dampens the sound of your own speech

Excitation of Cochlear Hair Cells

  • stereocilia of outer hair cells

    • bathed in high K+ fluid, the endolymph

      • creating electrochemical gradient

      • outside of cell is +80 mV and inside about – 40 mV

    • tip embedded in tectorial membrane

    • stereocilium on inner hair cells

    • single transmembrane protein at tip that functions as a mechanically gated ion channel

      • stretchy protein filament (tip link) connects ion channel of one stereocilium to the sidewall of the next taller stereocilium

      • tallest one is bent when basilar membrane rises up towards tectorial membrane

      • pulls on tip links and opens ion channels

      • K+ flows in – depolarization causes release of neurotransmitter

      • stimulates sensory dendrites and generates action potential in the cochlear nerve

Sensory Coding

  • for sounds to carry meaning, we must distinguish between loudness and pitch

  • variations in loudness (amplitude) cause variations in the intensity of cochlear vibrations

    • soft sound produces relatively slight up-and-down motion of the basilar membrane

    • louder sounds make the basilar membrane vibrate more vigorously

      • triggers higher frequency of action potentials

      • brain interprets this as louder sound

  • pitch depends on which part of basilar membrane vibrates

    • at basal end, membrane attached, narrow and stiff

      • brain interprets signals as high-pitched

    • at distal end, 5 times wider and more flexible

    • brain interprets signals as low-pitched

Cochlear Tuning

  • increases ability of cochlea to receive some sound frequencies

  • outer hair cells shorten (10 to 15%) reducing basilar membrane’s mobility

    • fewer signals from that area allows brain to distinguish between more and less active areas of cochlea

  • pons has inhibitory fibers that synapse near the base of inner hair cells

    • inhibiting some areas and increases contrast between regions of cochlea

Auditory Projection Pathway

  • sensory fibers begin at the bases of the hair cells

    • somas form the spiral ganglion around the modiolus

    • axons lead away from the cochlea as the cochlear nerve

    • joins with the vestibular nerve to form the vestibulocochlear nerve, Cranial Nerve VIII

  • each ear sends nerve fibers to both sides of the pons

    • end in cochlear nuclei

    • synapse with second-order neurons that ascend to the nearby superior olivary nucleus

    • superior olivary nucleus issues efferent fibers back to the cochlea

      • involved with cochlear tuning

  • binaural hearing – comparing signals from the right and left ears to identify the direction from which a sound is coming

    • function of the superior olivary nucleus

  • fibers ascend to the inferior colliculi of the midbrain

    • helps to locate the origin of the sound, processes fluctuation in pitch, and mediate the startle response and rapid head turning in response to loud noise

    • third-order neurons begin in the inferior colliculi and lead to the thalamus

  • fourth-order neurons complete the pathway from thalamus to primary auditory complex

    • involves four neurons instead of three unlike most sensory pathways

  • primary auditory cortex lies in the superior margin of the temporal lobe

    • site of conscious perception of sound

  • because of extensive decussation of the auditory pathway, damage to right or left auditory cortex does not cause unilateral loss of hearing

Equilibrium

  • equilibrium – coordination, balance, and orientation in three-dimensional space

  • vestibular apparatus – constitutes receptors for equilibrium

    • three semicircular ducts

      • detect only angular acceleration

    • two chambers

      • anterior saccule and posterior utricle

      • responsible for static equilibrium and linear acceleration

  • static equilibrium – the perception of the orientation of the head when the body is stationary

  • dynamic equilibrium - perception of motion or acceleration

      • linear acceleration - change in velocity in a straight line (elevator)

      • angular acceleration - change in rate of rotation (car turns a corner

Macula Utriculi and Macula Sacculi

  • static equilibrium - when head is tilted, heavy otolithic membrane sags, bending the stereocilia, and stimulating the hair cells

  • dynamic equilibrium – in car, linear acceleration detected as otoliths lag behind, bending the stereocilia, and stimulating the hair cells

  • because the macula sacculi is nearly vertical, it responds to vertical acceleration and deceleration

Semicircular Ducts

  • rotary movements detected by the three semicircular ducts

  • bony semicircular canals of temporal bone hold membranous semicircular ducts

  • each duct filled with endolymph and opens up as a dilated sac (ampulla) next to the utricle

  • each ampulla contains crista ampullaris, mound of hair cells and supporting cells

Vision and Light

  • vision (sight) – perception of objects in the environment by means of the light that they emit or reflect

  • light – visible electromagnetic radiation

    • human vision - limited to wavelengths of light from 400 -750 nm

    • ultraviolet radiation - < 400 nm; has too much energy and destroys macromolecules

    • infrared radiation - > 750 nm; too little energy to cause photochemical reaction, but does warm the tissues

    • light must cause a photochemical reaction to produce a nerve signal

Conjunctiva

  • conjunctiva – a transparent mucous membrane that lines eyelids and covers anterior surface of eyeball, except cornea

  • richly innervated and vascular (heals quickly)

    • secretes a thin mucous film that prevents the eyeball from drying

Anatomy of the Eyeball

  • three principal components of the eyeball

    • three layers (tunics) that form the wall of the eyeball

    • optical component – admits and focuses light

    • neural component – the retina and optic nerve

Tunics of the Eyeball

  • tunica fibrosa – outer fibrous layer

    • sclera – dense, collagenous white of the eye

    • cornea - transparent area of sclera that admits light into eye

  • tunica vasculosa (uvea) – middle vascular layer

    • choroid – highly vascular, deeply pigmented layer behind retina

    • ciliary body – extension of choroid that forms a muscular ring around lens

      • supports lens and iris

      • secretes aqueous humor

    • iris - colored diaphragm controlling size of pupil, its central opening

      • melanin in chromatophores of iris - brown or black eye color

      • reduced melanin – blue, green, or gray color

  • tunica interna - retina and beginning of optic nerve

Optical Components

  • transparent elements that admit light rays, refract (bend) them, and focus images on the retina

    • cornea

      • transparent cover on anterior surface of eyeball

    • aqueous humor

      • serous fluid posterior to cornea, anterior to lens

      • reabsorbed by scleral venous sinus (canal of Schlemm)

      • produced and reabsorbed at same rate

    • lens

      • lens fibers – flattened, tightly compressed, transparent cells that form lens

      • suspended by suspensory ligaments from ciliary body

      • changes shape to help focus light

        • rounded with no tension or flattened with pull of suspensory ligaments

    • vitreous body (humor) fills vitreous chamber

      • jelly fills space between lens and retina

Neural Components

  • includes retina and optic nerve

  • retina

    • forms as an outgrowth of the diencephalon

    • attached to the rest of the eye only at optic disc and at ora serrata

    • pressed against rear of eyeball by vitreous humor

    • detached retina causes blurry areas in field of vision and leads to blindness

  • examine retina with opthalmoscope

    • macula lutea – patch of cells on visual axis of eye

    • fovea centralis – pit in center of macula lutea

    • blood vessels of the retina Test for Blind Spot

Test for Blind Spot

optic disk - blind spot

    • optic nerve exits posterior surface of eyeball

    • no receptor cells at that location

  • blind spot - use test illustration above

    • close eye, stare at X and red dot disappears

  • visual filling - brain fills in green bar across blind spot area

Formation of an Image

  • light passes through lens to form tiny inverted image on retina

  • iris diameter controlled by two sets of contractile elements

    • pupillary constrictor - smooth muscle encircling the pupil

      • parasympathetic stimulation narrows pupil

    • pupillary dilator - spokelike myoepithelial cells

      • sympathetic stimulation widens pupil

  • pupillary constriction and dilation occurs in two situations

    • when light intensity changes

    • when our gaze shifts between distant and nearby objects

  • photopupillary reflex – pupillary constriction in response to light

    • consensual light reflex because both pupils constrict even if only one eye is illuminated

Principle of Refraction

  • refraction – the bending of light rays

  • light slows down from 300,000 km/sec in air, water, glass or other media

  • refractive index of a medium is a measure of how much it retards light rays relative to air

  • angle of incidence at 90° light slows but does not change course

  • any other angle, light rays change direction (it is refracted)

  • greater the refractive index and greater the angle of incidence, the more refraction

Refraction in the Eye

  • light passing through the center of the cornea is not bent

  • light striking off-center is bent towards the center

  • aqueous humor and lens do not greatly alter the path of light

  • cornea refracts light more than lens does

    • lens merely fine-tunes the image

    • lens becomes rounder to increase refraction for near vision

The Near Response

  • emmetropia – state in which the eye is relaxed and focused on an object more than 6 m (20 ft) away

    • light rays coming from that object are essentially parallel

    • rays focused on retina without effort

    • light rays coming from a closer object are too divergent to be focused without effort

  • near response – adjustments to close range vision requires three processes

    • convergence of eyes

      • eyes orient their visual axis towards object

    • constriction of pupil

      • blocks peripheral light rays and reduces spherical aberration (blurry edges)

    • accommodation of lens – change in the curvature of the lens that enables you to focus on nearby objects

      • ciliary muscle contracts, lens takes convex shape

      • light refracted more strongly and focused onto retina

      • near point of vision – closest an object can be and still come into focus

Sensory Transduction in the Retina

  • conversion of light energy into action potentials occurs in the retina

  • structure of retina

    • pigment epithelium – most posterior part of retina

      • absorbs stray light so visual image is not degraded

    • neural components of the retina from the rear of the eye forward

      • photoreceptor cells – absorb light and generate a chemical or electrical signal

        • rods, cones, and certain ganglion cells

        • only rods and cones produce visual images

      • bipolar cells – synapse with rods and cones and are first-order neurons of the visual pathway

      • ganglion cells – largest neurons in the retina and are the second-order neurons of the visual pathway

Photoreceptor Cells

  • light absorbing cells

    • derived from same stem cells as ependymal cells of the brain

    • rod cells (night - scotopic vision or monochromatic vision)

      • outer segment – modified cilium specialized to absorb light

        • stack of 1,000 membranous discs studded with globular proteins, the visual pigment, rhodopsin

      • inner segment – contains organelles sitting atop cell body with nucleus

    • cone cells (color, photopic, or day vision)

Histology - Layers of Retina

  • pigment epithelium

  • rod and cone cells

  • bipolar cells

    • rods & cones synapse on bipolar cells

    • bipolar cells synapse on ganglion cells

  • ganglion cells contain sensory pigment - melanopsin

    • single layer of large neurons near vitreous

    • axons form optic nerve

    • absorb light and transmit signals to brainstem

      • detect light intensity only

Schematic Layers of the Retina

  • 130 million rods and 6.5 million cones in retina

  • only 1.2 million nerve fibers in optic nerve

  • neuronal convergence and information processing in retina before signals reach brain

    • multiple rod or cone cells synapse on one bipolar cell

    • multiple bipolar cells synapse on one ganglion cell

Nonreceptor Retinal Cells

  • horizontal cells and amacrine cells

  • do not form layers within retina

  • horizontal and amacrine cells form horizontal
    connections between cone, rod and bipolar cells

    • enhance perception of contrast, the edges of
      objects, moving objects, and changes in light intensity

  • much of the mass of the retina is astrocytes and other glial cells

Visual Pigments

  • rods contain visual pigment - rhodopsin (visual purple)

    • two major parts of molecule

      • opsin - protein portion embedded in disc membrane of rod’s outer segment

      • retinal (retinene) - a vitamin A derivative

    • has absorption peak at wavelength of 500 nm

      • can not distinguish one color from another

  • cones contain photopsin (iodopsin)

    • retinal moiety same as in rods

    • opsin moiety contain different amino acid sequences that determine wavelengths of light absorbed

    • 3 kinds of cones, identical in appearance, but absorb different wavelengths of light to produce color vision

Generating the Optic Nerve Signal Rhodopsin Bleaching/Regeneration

  • rhodopsin absorbs light, converted from bent shape in dark (cis-retinal) to straight (trans-retinal)

    • retinal dissociates from opsin (bleaching)

    • 5 minutes to regenerate 50% of bleached rhodopsin

  • cones are faster to regenerate their photopsin – 90
    seconds for 50%

Generating Optic Nerve Signals

  • in dark, rods steadily release the neurotransmitter, glutamate from basal end of cell

  • when rods absorb light, glutamate secretion ceases

  • bipolar cells sensitive to these on and off pulses of glutamate secretion

    • some bipolar cells inhibited by glutamate and excited when secretion stops

      • these cells excited by rising light intensities

    • other bipolar cells are excited by glutamate and respond when light intensity drops

    • when bipolar cells detect fluctuations in light intensity, they stimulate ganglion cells directly or indirectly

  • ganglion cells are the only retinal cells that produce action potentials

  • ganglion cells respond to the bipolar cells with rising and falling firing frequencies

  • via optic nerve, these changes provide visual signals to the brain

Light and Dark Adaptation

  • light adaptation (walk out into sunlight)

    • pupil constriction and pain from over stimulated retinas

    • pupils constrict to reduce pain & intensity

    • color vision and acuity below normal for 5 to 10 minutes

    • time needed for pigment bleaching to adjust retinal sensitivity to high light intensity

    • rod vision nonfunctional

  • dark adaptation (turn lights off)

    • dilation of pupils occurs

    • rod pigment was bleached by lights

    • in dark, rhodopsin regenerates faster than it bleaches

    • in a minute or two night (scotopic) vision begins to function

    • after 20 to 30 minutes the amount of regenerated rhodopsin is sufficient for your eyes to reach maximum sensitivity

Dual Visual System

  • duplicity theory of vision explains why we have both rods and cones

    • a single type of receptor can not produce both high sensitivity and high resolution

  • it takes one type of cell and neural circuit for sensitive night vision

  • it takes a different cell type and neuronal circuit for high resolution daytime vision

Scotopic System (Night Vision)

  • rods sensitive – react even in dim light

    • extensive neuronal convergence

    • 600 rods converge on 1 bipolar cell

    • many bipolar converge on each ganglion cell

    • results in high degree of spatial summation

      • one ganglion cells receives information from 1 mm2 of retina producing only a coarse image

  • edges of retina have widely-spaced rod cells, act as motion detectors

    • low resolution system only

    • cannot resolve finely detailed images

Color Vision Photopic System (Day Vision)

  • fovea contains only 4000 tiny cone cells (no rods)

    • no neuronal convergence

    • each foveal cone cell has “private line to brain”

  • high-resolution color vision

    • little spatial summation so less sensitivity to dim light

Color Vision

  • primates have well developed color vision

    • nocturnal vertebrates have only rods

  • three types of cones are named for absorption peaks of their photopsins

    • short-wavelength (S) cones peak sensitivity at 420 nm

    • medium-wavelength (M) cones peak at 531 nm

    • long-wavelength (L) cones peak at 558 nm

  • color perception based on mixture of nerve signals representing cones of different absorption peaks

Color Blindness

  • color blindness – have a hereditary atteration or lack of one photopsin or another

  • most common is red-green color blindness

    • results from lack of either L or M cones

    • causes difficulty distinguishing these related shades from each other

    • occurs in 8% of males, and 0.5% in females (sex-linkage)

Stereoscopic Vision (Stereopsis)

  • stereoscopic vision is depth perception - ability to judge distance to objects

    • requires two eyes with overlapping visual fields which allows each eye to look at the same object from different angles

    • panoramic vision has eyes on sides of head (horse or rodents – alert to predators but no depth perception)

  • fixation point - point in space in which the eyes are focused

    • looking at object within 100 feet, each eye views from slightly different angle

    • provides brain with information used to judge position of objects relative to fixation point

Visual Projection Pathway

  • bipolar cells of retina are first-order neurons

  • retinal ganglion cells are second-order neurons whose axons form optic nerve

    • two optic nerves combine to form optic chiasm

    • half the fibers cross over to the opposite side of the brain (hemidecussation) and chiasm splits to form optic tracts

      • right cerebral hemisphere sees objects in the left visual field because their images fall on the right half of each retina

      • each side of brain sees what is on side where it has motor control over limbs

  • optic tracts pass laterally around the hypothalamus with most of their axons ending in the lateral geniculate nucleus of the thalamus

  • third-order neurons arise here and form the optic radiation of fibers in the white matter of the cerebrum

    • project to primary visual cortex of occipital lobe where conscious visual sensation occurs

    • a few optic nerve fibers project to midbrain and terminate in the superior colliculi and pretectal nuclei

      • superior colliculi controls visual reflexes of extrinsic eye muscles

      • pretectal nuclei are involved in photopupillary and accommodation reflexes

Visual Information Processing

  • some processing begins in retina

    • adjustments for contrast, brightness, motion and stereopsis

  • primary visual cortex is connected by association tracts to visual association areas in parietal and temporal lobes which process retinal data from occipital lobes

    • object location, motion, color, shape, boundaries

    • store visual memories (recognize printed words)



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