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Special Senses. Dr. Michael P. Gillespie. Special Senses. Receptors for the special senses – smell, taste, vision, hearing, and equilibrium – are anatomically distinct from one another. These receptors are concentrated in very specific locations in the head.

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Special senses

Special Senses

Dr. Michael P. Gillespie

Special senses1
Special Senses

  • Receptors for the special senses – smell, taste, vision, hearing, and equilibrium – are anatomically distinct from one another.

  • These receptors are concentrated in very specific locations in the head.

  • They are usually embedded in the epithelial tissue within complex sensory organs such as the eyes and ears.

  • The neural pathways for the special senses are more complex than those of the general senses.

Smell taste
Smell & Taste

  • Smell and taste are chemical senses. They involve the interaction of molecules with the receptors.

  • Impulses from these sense propagate to the limbic system and higher cortical areas. Consequently, they evoke emotional responses and memories.


  • A chemosensor, also known as a chemoreceptor, is a sensory receptor that transduces a chemical signal into an action potential.

  • A chemosensor detects certain chemicals in the environment.

Olfactory receptors
Olfactory Receptors

  • There are between 10 – 100 million receptors for olfaction (sense of smell).

    • They are contained in the olfactory epithelium (5cm2)

Olfactory receptors1
Olfactory Receptors

  • 3 Kinds of cells

    • Olfactory receptors

      • 1st order neurons – bipolar

      • Olfactory hairs – cilia that project from the dendrite – respond to chemicals called odarants

    • Supporting cells

      • Columnar epithelium of mucous membrane

      • Support, nourish, detoxify chemicals

    • Basal cells

      • Stem cells – produce new olfactory cells (live approx. 1 month)

Olfactory glands bowman s
Olfactory Glands (Bowman’s)

  • Bowman’s glands are in the connective tissue that supports the epithelium.

  • They produce mucous which moistens the epithelial surface and dissolves the odorants.


  • Branches of the facial nerve (CN VII) innervate the supporting cells and olfactory glands.

  • They stimulate the olfactory glands and the lacrimal glands.

  • The lacrimal glands produce tears from pepper and ammonia.

Physiology of olfaction
Physiology of Olfaction

  • There are hundreds of “primary” odors.

  • Humans can recognize about 10,000 different odors.

  • Different combinations of olfactory receptors stimulate different patterns of activity in the brain.

Odor thresholds and adaptation
Odor Thresholds and Adaptation

  • All special senses have a low threshold including olfaction.

    • Methyl mercaptan can be detected with as little as 1/25 billionth of a milligram/ mL air.

  • Adaptation is a decrease in sensitivity. It occurs rapidly. 50% of the decrease occurs within the first second or so and then very slowly after that.

Olfactory pathway
Olfactory Pathway

  • There are approximately 20 olfactory foramina on either side of the nose in the cribiform plate of the ethmoid bone.

  • 40 or so bundles of axons form right and left olfactory nerves (CN I).

    • They terminate in the olfactory bulbs – below the frontal lobes of the cerebrum.

Olfactory pathway1
Olfactory Pathway

  • Axons of the olfactory bulbs form the olfactory tract which projects to the primary olfactory area of the cerebral cortex.

    • Some project into the limbic system and hypothalamus (emotional and memory evoked responses.

  • Olfactory sensations are the only sensations that reach the cerebral cortex without first synapsing in the thalamus.

Olfactory pathway2
Olfactory Pathway

  • The primary olfactory area has axons that extend to the orbitofrontal area (frontal lobe) – region for odor identification.


  • Hyposmia is a reduced ability to smell.

  • Women have a keener sense of smell than men, especially at ovulation.

  • Smoking impairs the sense of smell.

  • Age deteriorates the olfactory receptors.

    • Affects 50% over 65 and 75% over 80 years of age.


  • Neurological changes impair the receptors.

    • Head injury, Alzheimer’s, Parkinson’s.

  • Medications impair receptors.

    • Antihistamines, analgesics, steroids.

  • Aromatherapy

    • Effects of smells on our psychology have been claimed.

    • Lavender, Orange Blossom, Rose, and Sage are said to be calming.

    • Sandlewood, Patachouli and Jasmine are said to alleviate mild depression.

    • Association areas of the brain.

    Survival function
    Survival Function

    • Our sense of smell serves a survival function to help us select non-poisonous foods.

    • There are very few naturally occurring toxic vapors that are odorless.

    • Synthetic vapors often give false impressions to our senses. Our natural preferences can no longer be relied upon.

    Gustation taste
    Gustation (Taste)

    • Chemical sense.

    • There are only 5 primary tastes that can be distinguished.

      • Sour, sweet, bitter, salty, umani (receptors stimulated by MSG)

      • All other flavors are combinations of the 5 primary tastes and smell.

    Taste receptor cells
    Taste Receptor Cells

    • Contrary to popular belief, there is no tongue ‘map’.

    • Responsiveness to the five basic modalities – bitter, sour, sweet, salty, and umami – is present in all areas of the tongue.

    • The taste receptor cells are tuned to detect each of the five basic tastes.

    • A given gustatory receptor may respond more strongly to some tastants than others.

    Taste buds and papillae
    Taste Buds and Papillae

    • There are approximately 10,000 taste buds.

      • Most are on the tongue.

      • There are some on the soft palate, pharynx, and epiglottis.

    • Each taste bud is an oval body with 3 kinds of epithelial cells.

      • Supporting cells.

      • Gustatory receptor cells (life span of approx. 10 days).

      • Basal cells.

    Epithelial cells on taste buds
    Epithelial Cells on Taste Buds

    • The Supporting Cells surround approximately 50 gustatory receptor cells in a taste bud.

      • The Gustatory Receptor Cells synapse with 1st order neurons. ! 1st order neurons contacts many gustatory receptor cells.

    • The Gustatory Hair (microvillus) projects through the taste pore.

    • The Basal Cells are stem cells at the periphery of the taste bud. They produce supporting cells which will develop into gustatory cells.

    Taste buds1
    Taste Buds

    • The taste buds are found in elevations on the tongue.

    • Vallate (circumvallate) papillae

    • Fungiform papillae

    • Foliate papillae

    • Filiform papillae

    Vallate circumvallate papillae
    Vallate (Circumvallate) Papillae

    • About 12 very large circular vallate papillae form an inverted V-shaped row at the back of the tongue.

    • Each of these papillae contains approximately 100-300 taste buds.

    Fungiform papillae
    Fungiform Papillae

    • The Fungiform (mushroom like) papillae are mushroom shaped elevations scattered over the entire surface of the tongue.

    • They contain about 5 tastebuds each.

    Foliate papillae
    Foliate Papillae

    • The foliate (leaflike) papillae are located in small trenches on the lateral margins of the tongue, but most of their taste buds degenerate in early childhood.

    Filiform papillae
    Filiform Papillae

    • Filiform papillae cover the entire surface of the tongue.

    • They are pointed, threadlike structures that contain tactile receptors but no taste buds.

    • They increase friction between the tongue and the food, making it easier for the tongue to move food into the oral cavity.


    • Tastants are chemicals that stimulate gustatory receptors.

    • Tastants dissolve in saliva. They can then make contact with the plasma membrane of the gustatory hairs, which are the sites of taste transduction.

    • This generates a receptor potential, which in turn triggers nerve impulses with first-order sensory neurons.

    Tastant stimulation of gustatory receptors
    Tastant Stimulation of Gustatory Receptors

    • Different tastants stimulate the gustatory receptors in different ways to generate the receptor potential.

    • The sodium ions in salty foods enter the gustatory receptor cells via Na+ channels in the membrane.

    • The hydrogen ions in sour tastants flow in through H+ channels.

    Tastant stimulation of gustatory receptors1
    Tastant Stimulation of Gustatory Receptors

    • Other tastants (sweet, bitter, and umami) do not enter the gustatory receptor cells. They bind to receptors on the plasma membrane. They trigger second messengers in the cell.

    • All tastants ultimately result in the release of neurotransmitters from the gustatory receptor cell.

    • Different foods taste different because of the patterns of nerve impulses in groups of first-order neurons that synapse with the receptors.

    Taste thresholds
    Taste Thresholds

    • The threshold for taste varies for each of the primary tastes.

    • The threshold for bitter substances (i.e. quinine) is the lowest.

    • Poisonous substances are often bitter.

    • The threshold for sour substances (i.e. lemon) is somewhat higher.

    • The thresholds for salty substances and sweet substances are similar and higher than the others.

    Taste adaptation
    Taste Adaptation

    • Complete adaptation of a taste can occur in 1-5 minutes if continuous stimulation.

    Gustatory pathway
    Gustatory Pathway

    • Three cranial nerves contain axons for the gustatory pathways.

      • Facial Nerve (CN VII) – serves taste buds in anterior 2/3 of the tongue.

      • Glossopharyngeal Nerve (CN IX) – serves taste buds in the posterior 1/3 of the tongue.

      • Vagus Nerve (CN X) – serves taste buds in the throat and epiglottis.

    • Impulses propagate to the gustatory nucleus in the medulla oblongata.

    Gustatory pathway1
    Gustatory Pathway

    • Some axons carrying taste signals project into the limbic system and the hypothalamus.

    • Others project to the thalamus and from there to the primary gustatory area in the parietal lobe of the cerebral cortex.

      • This allows us to perceive taste.

    Taste aversion
    Taste Aversion

    • The taste projections to the hypothalamus and limbic system account for the strong association between taste and emotions.

    • Sweet foods evoke reactions of pleasure, while bitter foods can evoke reactions of disgust. This is true even in newborn babies.

    • Animals learn to avoid foods that upset the digestive system. This is known as taste aversion.

    • Certain medications cause upset stomach and can cause taste aversion to all foods.


    • Sight is extremely important for human survival.

    • More than half of the sensory receptors in the human body are located in the eyes.

    Electromagnetic radiation
    Electromagnetic Radiation

    • Electromagnetic radiation is energy in the form of waves that radiate from the sun.

    • Many types:

      • Gamma rays, X-rays, UV rays

      • Visible light

      • Infrared radiation, Microwaves, Radio waves

    • The range of electromagnetic radiation is known as the electromagnetic spectrum.


    • The distance between two consecutive peaks of an electromagnetic wave is the wavelength.

    • Wavelengths range from short to long.

      • Gamma rays – short than a nanometer.

      • Radio waves – greater than a meter

    • The eyes are responsible for the detection of visible light.


    • The color of visible light depends upon its wavelength.

      • Wavelength of 400 nm is violet

      • Wavelength of 700 nm is red

    • An object will absorb certain wavelengths of light and reflect others. It will appear the color of the wavelengths it reflects.

    • White – reflects all wavelengths of visible light.

    • Black – absorbs all wavelengths of visible light.

    Accessory structures of the eye
    Accessory Structures of the Eye

    • Eyelids

    • Eyelashes

    • Eyebrows

    • Lacrimal apparatus

    • Extrinsic eye muscles


    • The palpebrae or eyelids (palprbra – singlular) shade the eyes during sleep, protect the eyes from excessive light and foreign objects, and spread lubricating secretions over the eyeballs.

    • The upper eyelid contains the levator palpebrae superioris muscle and is more moveable than the lower.

    • The lacrimal caruncle is a small, reddish elevation on the medial border and contains sebaceous (oil) and sudoriferous (sweat) glands.


    • The Meibomian glands are embedded in the eyelids and secrete fluid that prevents the eyelids from adhering to each other. Infection of the glands produces a cyst known as a chalazion.

    • The bulbar conjunctiva passes from the eyelids to the surface of the eyeball and covers the sclera (“white” of the eye). The conjunctiva is vascular. Irritation or infection cause bloodshot eyes.

    Eyelashes and eyebrows
    Eyelashes and Eyebrows

    • The eyelashes and eyebrows help protect the eyeballs from foreign objects, perspiration, and the direct rays of the sun.

    • Sebaceous ciliary glands are located at the base of the hair follicles of the eyelashes. The release a lubricating fluid into the follicles.

    • Infection of these glands results in a sty.

    Lacrimal apparatus
    Lacrimal Apparatus

    • The lacrimal apparatus is a group of structures that produces and drains lacrimal fluid or tears.

    • The lacrimal ducts empty tears onto the surface of the conjunctiva of the upper lid.

    Lacrimal apparatus1
    Lacrimal Apparatus

    • The fluid passes into through the lacrimal puncta, into lacrimal canals, to the lacrimal sac, and then into the nasolacrimal duct.

    • The lacrimal glands are supplied by parasympathetic fibers of the facial nerves (VII).

    • Tears are cleared away by either evaporation or by passing into the lacrimal ducts.

    Extrinsic eye muscles
    Extrinsic Eye Muscles

    • Six extrinsic eye muscles move each eye:

      • Superior rectus, inferior rectus, medial rectus, inferior oblique (CN III - Oculomotor).

      • Superior oblique (CN IV – Trochlear).

      • Lateral rectus (CN VI – Abducens).

    • They are supplied by cranial nerves III, IV, VI. SO4LR6.

    • Motor units in these muscles tend to be small with each motor neuron serving only 2 or 3 muscle fibers. This permits smooth, precise, and rapid movements.

    Anatomy of the eyeball
    Anatomy of the Eyeball

    • The adult eyeball is about 2.5 cm (1 inch) in diameter.

    • Only 1/6 of the surface area is exposed. The remainder is protected by the orbit.

    • The wall of the eyeball consists of three layers:

      • Fibrous tunic

      • Vascualr tunic

      • Retina

    Fibrous tunic
    Fibrous Tunic

    • The fibrous tunic is the superficial layer of the eyeball and consists of the anterior cornea and posterior sclera.


    • The cornea is a transparent coat that covers the colored iris.

    • It is curved and helps to focus light.

    • The central part of the cornea receives oxygen from the outside air.


    • The sclera (“white” of the eye) covers the entire eyeball except the cornea.

    • The sclera gives shape to the eyeball, makes it more rigid, protects its inner parts, and serves as a site of attachment for the extrinsic eye muscles.

    Canal of schlemm
    Canal of Schlemm

    • At the junction of the sclera and cornea is an opening known as the scleral venous sinus (canal of Schlemm).

    • The aqueous humor drains into this sinus.

    Vascular tunic
    Vascular Tunic

    • The vascular tunic or uvea is the middle layer of the eyeball.

    • It is composed of three parts:

      • Choroid

      • Ciliary body

      • Iris


    • The choroid is highly vascularized.

    • It provides nutrients to the posterior surface of the retina.

    • It contains melanocytes which produce the pigment melanin.


    • The melanin absorbs stray light rays, which prevents reflection and scattering of light within the eyeball.

    • Consequently, the image cast on the retina by the cornea and the lens remains sharp and clear.

    • Albinos lack melanin in all parts of the body, therefore bright light is perceived as a bright glare due to scattering.

    Ciliary body
    Ciliary Body

    • In the anterior portion of the vascular tunic, the choroid becomes the ciliary body.

    • It contains melanin producing melanocytes.

    • The ciliary processes produce aqueous humor.

    • Zonular fibers extend from the ciliary processes and attach to the lens.

    Ciliary body1
    Ciliary Body

    • The ciliary muscle is a circular band of smooth muscle that controls the tightness of the zonular fibers.

    • Contraction or relaxation of the ciliary muscle changes the tightness on the zonular fibers, which alters the shape of the lens, adapting it for near or far vision.


    • The iris (= rainbow) is the colored portion of the eyeball.

    • It is shaped like a flattened doughnut.

    • It is suspended between the cornea and the lens.

    • It contains melanocytes. The amount of melanin produced determines eye color.

    • It contains circular and radial smooth muscle fibers.


    • A principle function is to regulate the amount of light entering the eyeball through the pupil, the hole in the center of the iris.

    • When bright light stimulates the eye, parasympathetic fibers of the oculomotor nerve (CN III) stimulate the circular muscles (sphincter pupillae) to contract causing a decrease in pupil size (constriction).

    • In dim light, sympathetic neurons stimulate the radial muscles (dilator pupillae) to contract, causing an increase in the pupil’s size (dilation).


    • The retina is the inner layer of the eyeball.

    • It is the beginning of the visual pathway.

    • We can view the anatomy of the retina through an ophthalmoscope.

    • Landmarks visible through the ophthalmoscope:

      • Optic disc – the site where the optic nerve (CN II) exits the eyeball.

      • Central retinal artery and central retinal vein.

      • Macula lutea.

      • Fovea centralis.


    • Photoreceptors are specialized cells that begin the process by which light rays are converted to nerve impulses.

    • Two types:

      • Rods (approximately 120 million per retina)

        • Allow us to see in dim light.

        • Do not provide color vision.

      • Cones (approximately 6 million per retina)

        • Stimulated in brighter light.

        • Produce color vision.

    Three types of cones
    Three Types of Cones

    • There are three types of cones in the retina:

      • Blue cones – sensitive to blue light.

      • Green cones – sensitive to green light.

      • Red cones – sensitive to red light.

    • Color vision results from the stimulation of various combinations of these three types of cones.

    Rods and cones
    Rods and Cones

    • Most of our experiences are mediated by the cone system, the loss of which produces legal blindness.

    • A person who loses rod vision mainly has difficulty seeing in dim light.

    Blind spot
    Blind Spot

    • The optic disc is the site where the optic nerve exits the eyeball.

    • The optic disc is also called the blind spot.

    • There are no rods or cones where the blind spot is.

    • We are typically not aware of having a blind spot because the two eyes compensate for one another.

    Detached retina
    Detached Retina

    • A detached retina may occur due to trauma, such as a blow to the head, in various eye disorders, or as a result of age-related degeneration.

    • Detachment occurs between the neural portion of the retina and the pigment epithelium.

    • Fluid accumulates between these layers and forces the retina outward.

    • This results in distorted vision and blindness in the corresponding fields.

    • Laser surgery or cryosurgery can correct this.

    Age related macular degeneration amd
    Age-related Macular Degeneration (AMD)

    • Age-related macular disease (AMD), also known as macular degeneration, is a degenerative disorder of the retina in persons 50 years of age or older.

    • Abnormalities occur in the region of the macula lutea, which is ordinarily the most acute area of vision.

    • Victims of AMD retain their vision but lose the ability to look straight ahead.

    • They cannot see facial features to identify a person in front of them.

    Age related macular degeneration amd1
    Age-related Macular Degeneration (AMD)

    • AMD is the leading cause of blindness in those over age 75, afflicting 13 million Americans.

    • AMD is 2.5 times more common in 1 pack / day smokers.


    • The lens is behind the pupil and the iris, within the cavity of the eyeball.

    • Proteins called crystallins, arranged like layers of an onion, make up the refractive media of the lens.

    • The lens is normally perfectly transparent and lacks blood vessels.

    • The lens helps focus images on the retina to facilitate clear vision.

    Interior of the eyeball
    Interior of the Eyeball

    • The lens divides the interior of the eyeball into two cavities: the anterior cavity and vitreous chamber.

    • The anterior cavity – the space anterior to the lens – consists of two chambers.

      • Anterior chamber – lies between the cornea and iris.

      • Posterior chamber – lies behind the iris and in front of the lens.

    • Both chambers of the anterior cavity are filled with aqueous humor, a transparent watery fluid that nourishes the lens and the cornea.

    Interior of the eyeball1
    Interior of the Eyeball

    • The posterior cavity of the eyeball is the vitreous chamber, which lies between the lens and the retina.

    • The vitreous body lies within the vitreous chamber. The vitreous body is a transparent jellylike substance that holds the retina flush against the choroid, giving the retina an even surface for the reception of clear images.

    Interior of the eyeball2
    Interior of the Eyeball

    • Occasionally, collections of debris may cast a shadow on the retina and create the appearance of specks that dart in and out of the field of vision. These are known as vitreous floaters.

    Intraocular pressure
    Intraocular Pressure

    • The pressure of the eye is referred to as intraocular pressure.

    • It is produced mainly by the aqueous humor and partly by the vitreous body.

    • It is normally about 16 mmHg.

    • It helps to maintain the shape of the eyeball and prevent it from collapsing.

    • Punctures of the eyeball can cause a loss of aqueous humor and the vitreous body, thereby decreasing intraocular pressure, a detached retina, and sometimes blindness.

    Image formation
    Image Formation

    • In many ways the eye operates like a camera.

    • It has optical elements which focus as image on the retina.

    • Three processes help to focus the image:

      • The refraction or bending of light by the lens and cornea.

      • The change in shape of the lens (accommodation).

      • The constriction or narrowing of the pupil.

    Refraction of light rays
    Refraction of Light Rays

    • When light rays pass from one substance (air) to another substance with a different density (water), they bend at the junction between the two substances.

    • This bending is known as refraction.

    • Light is refracted at both the cornea and the lens so that it comes into exact focus on the retina.

    Refraction of light rays1
    Refraction of Light Rays

    • Images focused on the retina are inverted (upside down). They also undergo light to left reversal.

    • 75% of the refraction occurs at the cornea.

    • 25% occurs at the lens, which also changes the focus to view either distant or near objects.

    Accomodation and the near point of vision
    Accomodation and the Near Point of Vision

    • Convex – surface that curves outward.

    • When a lens is convex, it will refract incoming light rays towards one another.

    • Concave – surface that curves inward.

    • When a lens is concave, it refract incoming light rays away from each other.

    • The lens of the eye is convex on both the anterior and posterior surfaces.

    Accomodation and the near point of vision1
    Accomodation and the Near Point of Vision

    • As the curvature becomes greater, its focusing power increases.

    • When the eye is focusing on a close object, the lens becomes more curved, causing greater refraction of the light rays.

    • This increase in the curvature of the lens is called accommodation.

    • The near point of vision is the minimum distance from the eye that an object can be clearly focused with the maximum accommodation.

    Accomodation and the near point of vision2
    Accomodation and the Near Point of Vision

    • When viewing distant objects, the ciliary muscle is relaxed and the lens is flatter because the taught zonular fibers are stretching it in all directions.

    • When viewing close objects, the ciliary muscle contracts, which pulls the ciliary processes towards the lens. This releases tension on the lens and zonular fibers. The lens is elastic and then becomes more spherical.

    • Parasympathetic fibers of CN III (oculomotor) innervate the ciliary muscle.

    Refraction abnormalities
    Refraction Abnormalities

    • Emmetropic eye – normal eye – can sufficiently refract light rays from objects 6 m (20 ft) away so that a clear image is focused on the retina.

    • Myopia – nearsightedness – can see close objects clearly, but not distant objects.

    • Hyperopia (hypermetropia) – farsightedness – can see distant objects clearly, but not close ones.

    • Astigmatism – either the cornea or the lens has an irregular curvature. Parts of the image are out of focus.

    Constriction of the pupil
    Constriction of the Pupil

    • Constriction of the pupil is a narrowing of the diameter of the hole through which light enters the eye due to contraction of the circular muscles of the iris.

    • This occurs automatically during accommodation to prevent light from entering at the periphery of the lens.

    • The pupil also constricts in bright light.


    • Binocular vision is focusing on one set of objects with both eyes.

    • This allows us to perceive depth and the three dimensional nature of objects.

    • When we look ahead at an object, light is refracted to comparable spots on the retinas of both eyes.

    • As we move closer to an object, the eyes must rotate medially towards the object being viewed.

    • Convergence is the medial movement of the two eyeballs so that both are pointed towards the object.

    Photoreceptors and photopigments
    Photoreceptors and Photopigments

    • Rods and cones were named for the different appearance of the outer segment – the distal end next to the pigmented layer.

    • The outer segment of rods are cylindrical or rod-shaped; those of cones are tapered or cone-shaped.

    • Transduction of light energy into a receptor potential occurs in the outer segment.

    • Photopigments are integral proteins in the plasma membrane.

    Photoreceptors and photopigments1
    Photoreceptors and Photopigments

    • In cones, the plasma membrane is folded back and forth in a pleated fashion.

    • In rods, the outer segment contains a stack of about 1000 discs, piled up like coins in a wrapper.

    • The inner segment contains the cell nucleus, Golgi complex, and many mitochondria.

    • The proximal end expands into synaptic terminals filled with synaptic vesicles.

    Photoreceptors and photopigments2
    Photoreceptors and Photopigments

    • The photopigment undergoes a structural change when it absorbs light, which leads to a receptor potential.

    • Rods contain the pigment rhodopsin.

    • Cones contain three different photopigments, one for each of the three types of cones.

    • Different colors of light activate different cone pigments.

    • All photopigments contain the glycoprotein opsin and a derivative of vitamin A called retinal.

    Light and dark adaptation
    Light and Dark Adaptation

    • When you emerge from dark surroundings into the light, light adaptation occurs. Your visual system adjusts within seconds to the brighter surroundings.

    • When you enter a darkened room, dark adaptation occurs. Your sensitivity increases slowly over several minutes.

    Rods do not see red
    Rods Do Not See Red

    • The light response of the rods peaks sharply in blue light. They respond little to red light.

    • In bright light, the color sensitive cones predominate. At twilight, the less-sensitive cones begin to shut down and most of the vision comes from the rods.

    • The attainment of optimum night vision can take up to a half hour.

    • You can view things with red light at night without activating the cones and therefore, you will not lose your night vision.

    Release of neurotransmitter by photoreceptors
    Release of Neurotransmitter by Photoreceptors

    • A ligand known as cyclic GMP (guanosine monophosphate) or cGMP allows the inflow of Na+ ions to depolarize the photoreceptor.

    • Light causes a hyperpolarizing receptor potential in photoreceptors, which decreases release of an inhibitory neurotransmitter (glutamate).

    • The cGMP channels close.

    • The photoreceptor cells become excited and stimulate the ganglion cells to form action potentials.

    Visual pathway
    Visual Pathway

    • Visual signals from the retina exit the eyeball as the optic nerve (CN II) and proceed to the brain.

    Processing of visual input in the retina
    Processing of Visual Input in the Retina

    • Visual input is processed in the retina before proceeding to the optic nerve.

    • There are 126 million photoreceptors in the human eye, but only 1 million ganglion cells.

    • Some features of visual input are enhanced, while others are discarded.

    Processing of visual input in the retina1
    Processing of Visual Input in the Retina

    • Between 6 and 600 rods synapse with a single bipolar cell; a cone more often synapses with a single bipolar cell.

    • Convergence of many rods onto a single bipolar cell increases the light sensitivity, but may slightly blur the image.

    • Cone vision is less sensitive, but sharper due to the one to one synapse with the bipolar cell.

    Brain pathway and visual fields
    Brain Pathway and Visual Fields

    • 1. Axons of all retinal ganglion cells in one eye exit the eyeball at the optic disc and form the optic nerve on that side.

    • 2. At the optic chiasm, axons from the temporal half of each retina do not cross but continue directly to the lateral geniculate nucleus of the thalamus on the same side.

    • 3. Axons from the nasal half of each retina cross the optic chiasm and continue to the opposite hypothalamus.

    Brain pathway and visual fields1
    Brain Pathway and Visual Fields

    • 4. Each optic tract consists of crossed and uncrossed axons that project from the optic chiasm to the thalamus on one side.

    • 5. Axon collateral extend to the midbrain to govern pupil constriction and to the hypothalamus to govern patterns of sleep and other circadian rhythms relevant to light and darkness.

    • 6. Axons of thalamic neurons form the optic radiations and project to the primary visual area of the cortex on the same side.

    Hearing and equilibrium
    Hearing and Equilibrium

    • The ear can transduce sound vibrations with amplitudes as small as the diameter of an atom of gold (0.3 nm) into electrical signals 1000 times faster than photoreceptors can respond to light.

    • The ear also contains receptors for equilibrium.

    Anatomy of the ear
    Anatomy of the Ear

    • The ear is divided into three main regions:

      • External ear – collects sound waves and channels them inward.

      • Middle ear – conveys sound vibrations to the oval window.

      • Internal ear – houses the receptors for hearing and equilibrium.

    External ear
    External Ear

    • The external (outer) ear consists of the auricle, external auditory canal, and eardrum.

    • The tympanic membrane (eardrum) is a thin, semitransparent partition between the external auditory canal and the middle ear.

    • Tearing of the tympanic membrane is called a perforated eardrum.

      • Pressure from a cotton swab, trauma, or a middle ear infection can cause perforation. It usually heals within 1 month.

    External ear1
    External Ear

    • The membrane can be examined using an otoscope.

    • Ceruminous glands secrete cerumen (earwax).

    • Cerumen and hairs help prevent dust and foreign particles from collecting in the ear.

    • Impacted cerumen can impair hearing.

    Middle ear
    Middle Ear

    • The middle ear contains the auditory ossicles.

    • The malleus attaches to the internal surface of the tympanic membrane.

    • The head of the malleus articulates with the incus.

    • The incus articulates with the head of the stapes.

    • The stapes fits into the oval window which is enclosed by a secondary tympanic membrane.

    • The stapedius muscles (supplied CN VII) dampens vibrations of the stapes due to loud noises.

    Middle ear eustachean tube
    Middle Ear (Eustachean Tube)

    • The auditory (pharyngotympanic) tube, also known as the eustachian tube connects the middle ear to the nasopharynx.

    • It helps to equalize pressure in the middle ear.

    • If pressure is not equalized, intense pain, hearing impairment, ringing in the ears, and vertigo can develop.

    • It is a route for pathogens to enter and cause otitis media.

    Internal ear
    Internal Ear

    • The internal ear is also called the labyrinth because of its complicated series of canals including the semicircular canals and cochlea.

    • The spiral organ or Corti contains supporting cells including approximately 16,000 hair cells, which are the receptors for hearing.

    Nature of sound waves
    Nature of Sound Waves

    • Sound waves are alternating high and low pressure regions traveling in the same direction through some medium (such as air).

    • The frequency of a sound wave is the pitch.

    • The human ear most acutely detects sounds waves between 500 and 5000 hertz (Hz).

    Nature of sound waves1
    Nature of Sound Waves

    • The audible range extends between 20 and 20,000 Hz.

    • Sounds of speech are between 100 and 3000 Hz.

    • The larger the intensity (size or amplitude) of the vibration, the louder the sound.

    • Sound intensity is measured in units called decibels (dB).

    Physiology of hearing
    Physiology of Hearing

    • 1. The auricle directs sound waves into the external auditory canal.

    • 2. Sound waves strike the tympanic membrane causes it to vibrate back and forth. The distance it moves depends upon the intensity and frequency of the waves.

    • 3. The central eardrum connects to the malleus, which also starts to vibrate. This vibration is then transmitted to the incus and stapes.

    • . As the stapes moves back and forth, it pushes the membrane of the oval window in and out.

    Physiology of hearing1
    Physiology of Hearing

    • 5. The movement of the oval window sets up fluid pressure waves in the perilymph of the cochlea.

    • 6. Pressure waves are transmitted to the round window, causing it to bulge outward.

    • 7. These waves in turn create pressure waves in the endolymph of the cochlear duct.

    • 8. This causes the basilar membrane to vibrate, which moves the hair cells leading to receptor potentials and ultimately nerve impulses.

    Auditory pathway
    Auditory Pathway

    • Bending of the stereocilia of the hair cells of the spiral organ causes the release of a neurotransmitter, which causes nerve impulses in the sensory neurons.

    • These nerve impulses pass along the axons to form the cochlear branch of the vestibulocochlear nerve (CN VIII).

    • They synapse in the cochlear nuclei in the medulla oblongata on the same side.

    Auditory pathway1
    Auditory Pathway

    • Some axons decussate in the medulla and terminate in the midbrain on the opposite side.

    • Other axons continue to the pons on the same side.

    • Slight differences in the timing of the impulses allow us to locate the source of the sound.

    • The axons are then conveyed to the thalamus and ultimately to the primary auditory area of the cerebral cortex in the temporal lobe.