Chapter 8Chapter 8Special SensesSpecial Senses

Sensory systemSensory system• Sensory system serves as a protective Sensory system serves as a protective

mechanism for an organism by mechanism for an organism by

• detecting changes in the environmentdetecting changes in the environment• An environmental change becomes a stimulus and An environmental change becomes a stimulus and • initiates nerve impulse to CNS by sensory neurons.initiates nerve impulse to CNS by sensory neurons.• The stimulus is then interpreted by cerebral cortexThe stimulus is then interpreted by cerebral cortex

• Senses are classified according to the Senses are classified according to the distribution of the receptors as distribution of the receptors as

• somatic (general) senses:somatic (general) senses: touch, pain, pressure, touch, pain, pressure, temperaturetemperature

• special senses :special senses :vision, taste, smell, hearing and vision, taste, smell, hearing and balance balance 

  

            A special senseA special sense is located in a special sense organ is located in a special sense organ

e.g.e.g.

• Chemoreceptors: two typesChemoreceptors: two types• Taste :Taste : receptors on tongue receptors on tongue• Smell :Smell : receptors in the upper nasal cavity receptors in the upper nasal cavity

• Photoreceptors : Photoreceptors : • Vision :Vision : receptors of eye receptors of eye

• Mechanoreceptors: Mechanoreceptors: • Hearing and Equilibrium:Hearing and Equilibrium: receptors of internal ear receptors of internal ear

Chemical Senses – Taste and Chemical Senses – Taste and SmellSmell

Both senses use Both senses use chemoreceptorschemoreceptors

Stimulated by chemicals in solutionStimulated by chemicals in solutionTaste has four types of receptorsTaste has four types of receptorsSmell can differentiate a large range of Smell can differentiate a large range of

chemicalschemicalsBoth senses complement each other and Both senses complement each other and

respond to many of the same stimulirespond to many of the same stimuli

Sense of Smell: or olfactionSense of Smell: or olfaction     occurs in response to occurs in response to

odorsodors

The special The special nasal nasal epitheliumepithelium of nasal of nasal cavity is called cavity is called olfactory epithelium olfactory epithelium

Airborne molecules Airborne molecules enter nasal cavity and enter nasal cavity and

stimulate chemo stimulate chemo receptors present in receptors present in olfactory epitheliumolfactory epithelium

   Stimulation creates action potential sending impulses to Stimulation creates action potential sending impulses to brain brain

through through olfactory nerve (I)olfactory nerve (I) to cerebral cortex to cerebral cortex Between Between frontal and temporal lobefrontal and temporal lobe above hypothalamus above hypothalamus These receptors degenerate with ageThese receptors degenerate with age

Sense of Smell: or olfactionSense of Smell: or olfaction

   During common cold, nasal mucous producing cells is During common cold, nasal mucous producing cells is

inflammated inflammated • preventing the odor from reaching the olfactory neurons of preventing the odor from reaching the olfactory neurons of

nose and nose and • thus preventing the sense of smellthus preventing the sense of smell

Sense of Smell: or olfactionSense of Smell: or olfaction

Sense of TasteSense of Taste The sense structures that The sense structures that

detect detect gustatory or taste gustatory or taste stimulistimuli are called taste are called taste buds buds 

Taste buds are oval Taste buds are oval structures embedded in the structures embedded in the epithelium of tongue and epithelium of tongue and mouth mouth

Known as papillae Known as papillae 

• Food particles Food particles dissolved in saliva dissolved in saliva stimulate taste buds stimulate taste buds

• And send impulse to And send impulse to

brain throughbrain through

• Medulla byMedulla by• facial nerve (VII)facial nerve (VII)• glassopharyngeal glassopharyngeal

nerve (IX) nerve (IX) • vagus (X).vagus (X).

Sense of TasteSense of Taste

  

  There are four primary taste There are four primary taste sensations detected by taste buds:sensations detected by taste buds:

a. a. Bitter: Bitter: e.g. coffee. Taste buds e.g. coffee. Taste buds are at the back of the tongueare at the back of the tongue

b. Sour: b. Sour: due to acids . Taste due to acids . Taste buds are on either side of tonguebuds are on either side of tongue

c. Salty :c. Salty : salts like NaCl . Taste salts like NaCl . Taste buds are on the sides and front of buds are on the sides and front of tonguetongue

d. Sweet :d. Sweet : Sugars give sweet Sugars give sweet taste. Taste buds are in front of taste. Taste buds are in front of the tonguethe tongue

Sense of TasteSense of Taste

70% of all sensory receptors are in the 70% of all sensory receptors are in the eyeseyes

Each eye has over a million nerve fibersEach eye has over a million nerve fibersProtection for the eyeProtection for the eye

Most of the eye is enclosed in a bony orbitMost of the eye is enclosed in a bony orbitA cushion of fat surrounds most of the eyeA cushion of fat surrounds most of the eye

The Eye and VisionThe Eye and Vision

Eyelids and eyelashesEyelids and eyelashesConjunctivaConjunctivaLacrimal apparatus Lacrimal apparatus Extrinsic eye musclesExtrinsic eye muscles

Accessory Structures of the EyeAccessory Structures of the Eye

Accessory Structures of the EyeAccessory Structures of the Eye

Eyelids: Eyelids: Eyelids protect Eyelids protect eye from foreign eye from foreign objectsobjects

Tarsal glands in Tarsal glands in eyelids lubricate the eyelids lubricate the eyeeye

Eyebrows:Eyebrows: prevent prevent sweat from entering sweat from entering eyes and help in eyes and help in shading eyesshading eyes

Accessory Structures of the EyeAccessory Structures of the Eye

ConjunctivaConjunctiva Delicate membrane Delicate membrane

that lines the eyelidsthat lines the eyelids

Connects to the Connects to the surface of the eyesurface of the eye

Secretes mucus to Secretes mucus to lubricate the eyelubricate the eye

Accessory Structures of the EyeAccessory Structures of the Eye Lacrimal apparatus:Lacrimal apparatus:

Consists of lacrimal glandConsists of lacrimal gland

Lacrimal gland—Lacrimal gland— produces produces lacrimal fluid, located above the lacrimal fluid, located above the lateral end of eyelateral end of eye

Properties of lacrimal fluidProperties of lacrimal fluid Dilute salt solution (tears)Dilute salt solution (tears) Contains antibodies and lysozymeContains antibodies and lysozyme

Protects, moistens, and lubricates Protects, moistens, and lubricates the eyethe eye

Empties into the nasal cavityEmpties into the nasal cavity

Accessory Structures of the EyeAccessory Structures of the Eye

Extrinsic eye muscles Extrinsic eye muscles Six muscles attach to Six muscles attach to

the outer surface of the outer surface of the eyethe eye

Produce eye Produce eye movementsmovements

Structure of the EyeStructure of the Eye

Eye is enclosed in three layers Eye is enclosed in three layers or tunicsor tunics

Fibrous layerFibrous layer Outside layer Outside layer

Vascular layerVascular layer Middle layer Middle layer

Sensory layerSensory layer Inside layerInside layer

Structure of the Eye: The Fibrous LayerStructure of the Eye: The Fibrous Layer

ScleraSclera White connective tissue White connective tissue

layerlayer Seen anteriorly as the Seen anteriorly as the

“white of the eye”“white of the eye” CorneaCornea

Transparent, central Transparent, central anterior portionanterior portion

Allows for light to pass Allows for light to pass throughthrough

Repairs itself easilyRepairs itself easily The only human tissue that The only human tissue that

can be transplanted can be transplanted without fear of rejectionwithout fear of rejection

Structure of the Eye: Vascular LayerStructure of the Eye: Vascular Layer ChoroidChoroid is a blood-rich nutritive layer is a blood-rich nutritive layer

in the posterior of the eye contains a in the posterior of the eye contains a dark pigmentdark pigment

Light is focused by choroid Light is focused by choroid pigment onto retina thus pigment onto retina thus preventing scattering of the lightpreventing scattering of the light

Modified anteriorly into two Modified anteriorly into two structuresstructures

Ciliary body—Ciliary body— smooth muscle smooth muscle attached to lensattached to lens

Iris—Iris— regulates amount of light regulates amount of light entering eyeentering eye

Pigmented layer that gives Pigmented layer that gives eye coloreye color

Pupil—Pupil— rounded opening in rounded opening in the iristhe iris

Iris is regulated by: Iris is regulated by:

a. sympathetic a. sympathetic nervous system by nervous system by dilating itdilating it

b. parasympathetic b. parasympathetic nervous system by nervous system by constricting itconstricting it      

Structure of the Eye: Vascular LayerStructure of the Eye: Vascular Layer

Structure of the Eye: Sensory LayerStructure of the Eye: Sensory Layer

Retina contains two Retina contains two layerslayers Outer pigmented layerOuter pigmented layer

They contain photosensitive They contain photosensitive pigmentspigments

Involved in the conversion Involved in the conversion of light into nerve impulsesof light into nerve impulses Inner neural layerInner neural layer

Contains receptor cells Contains receptor cells Biopolar cellsBiopolar cells Ganglion cellsGanglion cells

Light sensitiveLight sensitive pigments arepigments are

Rods:Rods: contain contain rhodopsin pigment rhodopsin pigment differentiates differentiates between light and between light and darknessdarkness

Cones:Cones: detect colors detect colors (red, blue and green) (red, blue and green) of the visual light.of the visual light.

Structure of the Eye: Sensory LayerStructure of the Eye: Sensory Layer

Cone SensitivityCone Sensitivity

There are three There are three types of conestypes of cones

Different cones are Different cones are sensitive to sensitive to different different wavelengthswavelengths

Color blindness is Color blindness is the result of lack of the result of lack of one cone typeone cone type

Fovea centralis:Fovea centralis: region region where there are more where there are more conescones

Blind spot:Blind spot: region where region where the optic nerve exits the the optic nerve exits the eyeeye

Structure of the Eye: Sensory LayerStructure of the Eye: Sensory Layer

o Lens Lens is present behind is present behind the pupilthe pupil

o Lens focuses the light Lens focuses the light on the retinaon the retina

o Lens is made of Lens is made of transparent intercellular transparent intercellular materialmaterial

o It is suspended by It is suspended by ligamentsligaments

o Lens regulates the Lens regulates the focusing with the help focusing with the help of suspensory ligamentsof suspensory ligaments

LensLens

o Lens divides the eye Lens divides the eye cavity into cavity into

o Anterior cavity or Anterior cavity or aqueous humoraqueous humor

o Posterior cavity or Posterior cavity or vitreous humorvitreous humor

o It is made of water It is made of water and mucoprotein.and mucoprotein.

o It maintains the It maintains the pressure to support pressure to support retinaretina

LensLens

Physiology of visionPhysiology of vision·  ·  Light reflected from the object enters cornea towards lens to retina.Light reflected from the object enters cornea towards lens to retina.  ·   Retina has cones, which detect colors (red, blue and green), and rods ·   Retina has cones, which detect colors (red, blue and green), and rods detect tone of object and night vision.detect tone of object and night vision.  ·   Photoreceptors of rods and cones send impulse to brain through ·   Photoreceptors of rods and cones send impulse to brain through bipolar cells to optic nervesbipolar cells to optic nerves

optic nerve pass through thalamus optic nerve pass through thalamus cross path at ( optic chiasma), cross path at ( optic chiasma),

connecting to connecting to occipital lobe of the cerebral cortexoccipital lobe of the cerebral cortex Thus responding to light Thus responding to light

stimulationstimulation And visual interpretation, or seeing And visual interpretation, or seeing

occursoccurs Human vision is binocular

Provides depth perception

Lens AccommodationLens Accommodation

Light must be focused to a Light must be focused to a point on the retina for point on the retina for optimal visionoptimal vision

The eye is set for distance The eye is set for distance vision vision (over 20 ft away)(over 20 ft away)

The lens must change The lens must change shape to focus for closer shape to focus for closer objectsobjects

•Short sight or near sightedness:Short sight or near sightedness: Eye ball is Eye ball is stretched in length thus focusing the object in stretched in length thus focusing the object in front of retina rather than on retina.front of retina rather than on retina.

concave lens correct the mistake concave lens correct the mistake

•Long Sight or far sightedness:Long Sight or far sightedness: Eye ball is compressed such that Eye ball is compressed such that the light is focused behind the retina instead of on the retinathe light is focused behind the retina instead of on the retina

Convex lens fixes the mistakeConvex lens fixes the mistake

EarEar

Is the sense Is the sense organ important organ important for hearing and for hearing and balancebalance The ear is The ear is divided into divided into three parts:three parts:

Outer earOuter ear Middle earMiddle ear Inner earInner ear

a. a. Outer ear :Outer ear : Is composed of Is composed of

Auricle (pinna)Auricle (pinna) which which directs sound waves into directs sound waves into earear

And And External auditory External auditory maetus (canal)maetus (canal) lined with lined with epithelial cells of skin epithelial cells of skin called called ceruminous glands,ceruminous glands,

They produce wax They produce wax

Wax collects the dust Wax collects the dust particles particles 

a. a. Outer ear :Outer ear : Tympanic membrane Tympanic membrane

(or eardrum) connects (or eardrum) connects outer ear to middle ear outer ear to middle ear

Sound waves reach Sound waves reach tympanic membrane tympanic membrane through pinna andthrough pinna and

cause it to vibratecause it to vibrate

  b. Middle ear:b. Middle ear: Connects the outer ear to inner earConnects the outer ear to inner ear Filled with air cavity within the temporal bone Filled with air cavity within the temporal bone Consists of two passages:Consists of two passages: One passage opens into One passage opens into Mastoid air cells of temporal boneMastoid air cells of temporal bone and and Second passage opens into pharynx through Second passage opens into pharynx through auditoryauditory or or EustachianEustachian tubes tubes Auditory tube equalizes air pressure between outside air and middle air cavity Auditory tube equalizes air pressure between outside air and middle air cavity

    b. Middle ear: b. Middle ear: Is made ofIs made of• three small bones three small bones

called auditory ossicles called auditory ossicles • Malleus Malleus (hammer)(hammer)• Incus (Incus (anvil) and anvil) and • StapesStapes (stirrup)  (stirrup) • These bones are These bones are

connected to the inner connected to the inner ear ear

• They amplify the They amplify the vibrations from vibrations from tympanic membrane tympanic membrane and transfer the and transfer the vibration to malleus vibration to malleus anvil anvil stirrup stirrup inner earinner ear

  

Inner earInner ear Includes sense organs for hearing and balanceIncludes sense organs for hearing and balance consists of bony labyrinth:consists of bony labyrinth: vestibulevestibule cochlea (snail like structure) and cochlea (snail like structure) and 3 semicircular canals3 semicircular canals And filled with perilymph (plasma like fluid)And filled with perilymph (plasma like fluid)

  

Organ of CortiOrgan of Corti Located within the cochleaLocated within the cochlea

Cochlea is a bony structure Cochlea is a bony structure

Has modified cells forming a Has modified cells forming a structure called spiral organs or structure called spiral organs or organ of corti. organ of corti.

Organ of corti is specialized Organ of corti is specialized sensory haircells and filled with sensory haircells and filled with endolymphendolymph..

Organs of HearingOrgans of HearingOrgans of HearingOrgans of Hearing

Physiology of hearingPhysiology of hearing Vibrations of air medium generate sound waves. Vibrations of air medium generate sound waves. The sound waves are collected by pinna and The sound waves are collected by pinna and

directed to external auditory maetus. directed to external auditory maetus.

The sound waves cause vibrations The sound waves cause vibrations of tympanic membrane,of tympanic membrane,

which moves ossicles which moves ossicles

resulting sound waves being resulting sound waves being amplified. amplified.

Amplified sound waves create Amplified sound waves create waves in endolymph of the waves in endolymph of the cochlea cochlea

and activate haircells of spiral and activate haircells of spiral organorgan

Physiology of hearingPhysiology of hearing

This creates action This creates action potential, which is potential, which is transmitted through transmitted through vestibulocochlear nerve tovestibulocochlear nerve to

pons( of brain stem) pons( of brain stem)

to thalamus.to thalamus.

From thalamus , impulse From thalamus , impulse is directed to auditory is directed to auditory cortex of temporal lobe cortex of temporal lobe

and sound is perceivedand sound is perceived

Sound is measured in decibelsSound is measured in decibels

Diseases related to sensory systemDiseases related to sensory system

Sensory Sensory functionfunction

diseasedisease Specific Specific structurestructure

Function Function affectedaffected

Sense of smellSense of smell Common coldCommon cold Nasal epitheliumNasal epithelium Cannot smellCannot smell

visualvisual glaucomaglaucoma Build up of aqueous Build up of aqueous fluid due to blockage fluid due to blockage of ducts in the of ducts in the choroidchoroid

Blindness caused Blindness caused due to damage to due to damage to nervesnerves

visionvision Macular degenerationMacular degeneration Degeneration of fova Degeneration of fova centralis( cones) due centralis( cones) due to age advancementto age advancement

Blind spot, faded Blind spot, faded colors colors

visionvision Color blindnessColor blindness Cone proteins not Cone proteins not mademade

Cannot distinguish Cannot distinguish red, blue and greenred, blue and green

auditoryauditory Conduction deafnessConduction deafness Fused middle ear Fused middle ear bonesbones

Cannot hear from Cannot hear from birthbirth

auditoryauditory Nerve deafnessNerve deafness Cochlear cilia of Cochlear cilia of nerve cell receptors nerve cell receptors degenerate due to degenerate due to age advancementage advancement

Cannot hear Cannot hear