Eyebrows, which shade and protect the eye Eyelids

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Chapter 15: Special Senses

Eye and Vision

  • The eye is enclosed in the boney orbit and cushioned by fat.

  • Accessory structures include

    • Eyebrows, which shade and protect the eye

    • Eyelids, protect and lubricate the eyes by reflex blinking

      • Within the eyelids are the orbicularis oculi and levator palpebrae muscles and modified sebaceous and sweat glands

    • The conjunctiva is a mucosa that lines the eyelids and covers the anterior eyeball surface

      • Its mucus lubricates the eyeball surface

  • The lacrimal apparatus consists of the lacrimal gland (produces saline solution containing mucus, lysozyme, and antibodies) the lacrimal canaliculi, the lacrimal sac, and the nasolacrial duct.

  • The extrinsic eye muscles (superior, inferior, lateral and medial rectus and superior and inferior oblique) move the eyeballs.

Structure of the eyeball

  • The wall of the eyeball is made up of 3 layers

      • The sclera protects the eye and gives it shape

      • The cornea allows light to enter the eye

    • The middle, pigmented vascular layer (uvea) consists of the choroid, the ciliary body, and the iris

      • The choroid provides nutrients to the eye and prevents light scattering within the eye

      • The ciliary muscles of the ciliary body control lens shape

      • The iris controls the size of the pupil

    • The sensory layer, or retina, consists of an outer pigmented layer and an inner neural layer.

      • The neural layer contains the photoreceptors (rods and cones), bipolar cells, and ganglion cells.

        • Ganglion cell axons form the optic nerve which exits the eye via the optic disk (“blind spot”)

        • The outer segments of the photoreceptors contain the light-absorbing pigment membrane –bounded discs

    • The posterior segment of the eyeball, behind the lens, contains vitreous humor, which helps support the eyeball and keep the retina in place.  

    • The anterior segment, anterior to the lens, is filled with aqueous humor, formed by capillaries in the ciliary processes and drained into the scleral venous sinus.  

      • Aqueous humor is a major factor in maintaining intraocular pressure

    • The biconvex lens is suspended within the eye by the ciliary zonule attached to the ciliary body.  It is the only adjustable refractory structure of the eye.

Physiology of Vision:

  • Light is made up of those wavelengths of the electromagnetic spectrum that excite the photoreceptors - ROYGBIV. Red wavelengths are the longest and have the lowest energy and violet are the shortest and the most energetic.

  • Light is refracted (bent) when passing from one transparent medium to another of a different density.

    • Concave lens disperse the light, convex lens converge the light and bring its rays to a focal point.

      • The greater the lens curvature, the more light bends

  • As light passes through the eye, it is bent by the cornea and the lens and focused on the retina.  

  • The cornea accounts for most of the refraction.

    • The lens allows for active focusing for different distances

      • Focusing for distance vision requires no special movements of the eye structures.

      • Focusing for close-up vision requires accommodation (bulging of the lens), pupillary constriction, and convergence of the eyeballs.  

      • Accommodation is the process that increases the refractory power of the lens

      • Pupillary constriction prevents the most divergent light rays from entering the eye. These rays would pass through the extreme edge of the lens causing blurred vision

      • Convergence is the medial rotation of the eyeballs by the medial rectus muscles so that each is directed toward the object being viewed

  • Refractory problems include presbyopia, myopia, hyperopia and astigmatism

  • 99% of refractory problems are related to the eyeball shape, either too long of too short

  • Presbyopia, (old people’s vision), the eyeball is non-accommodating – the lens losing its elasticity. Having to hold a book at arm’s length

  • Myopia (short vision) occurs when distant objects are focused not on, but in front of, the retina.  Eyeball is too long

  • Myopic people can see close objects without problems but distant objects are blurred.  Nearsighted

  • Hyperiopia, (far vision) or far- sightedness. When distant objects are focused behind the retina. These people can see far objects well because the ciliary muscles contract continuously to increase the light bending power of the lens, which brings the focal point back to the retina.

        • Astigmatism, unequal curvatures of different parts of the cornea and/or the lens causes blurry images.

Functional Anatomy of Photoreceptors

  • Photoreceptors are modified neurons

  • Rods respond to low intensity light and provide night and peripheral vision

  • Cones are bright light, high discrimination receptors that provide color vision

    • Anything that must be viewed precisely is focused on the cone-rich fovea centralis

  • Rods and cones have an outer segment (receptor region) and an inner segment (connecting to the cell body)

  • These cells are highly vulnerable to damage and immediately begin to degenerate if the retina becomes detached.

  • Rods and cones contain unique visual pigments that absorb different wavelengths of light.

    • They also have different thresholds for activation

      • Rods are very sensitive and respond to very dim light ( a single photon) making them best suited for night vision and peripheral vision

      • They contain a single pigment so it is perceived as tones of gray

      • Cones need bright light, have 3 different pigments that provide a vividly colorful view

  • Light absorbing molecule is called Retinal which is chemically related to Vitamin A and is made from it.

    • Retinal combines with a proteins called Opsins to form 4 types of visual pigments.

      • Rods visual pigment is rhodopsin

      • Three types of cones all contain retinal but each has a different type of opsin. Each responding to different wavelengths of light – red, blue, green

  • Color blindness is the absence of one of more cones. Usually red or green, it is an X-linked trait and is more prevalent in males than females

  • Night blindness is a condition in which rod function is severely hampered impairing the ability to drive safely at night. Most common cause is a vitamin A deficiency which leads to rod degeneration.

Visual Pathways to Brain:

  • Begins with the optic nerve fibers (ganglion cell axons) from the retina.

  • To the optic chiasma (cross)

  • To the optic tracts to synapse with the thalamus neurons which connect to the primary visual cortex in the occipital lobes where seeing occurs.

Chemical Senses: Taste and Smell

  • Taste and smell are primitive senses that alert us to stuff to be savored or avoided

  • Taste (gustation) and Smell (olfaction: olfact = to smell) are chemoreceptors

    • Respond to chemicals in an aqueous solution

    • They complement each other and respond to different classes of chemicals

    • Taste receptors are excited by food chemicals dissolved in the saliva

    • Smell receptors, by the airborne chemicals that dissolve in the fluids coating the nasal membranes

  • Olfactory epithelium is located on the roof of the nasal cavity.

    • Receptor cells are ciliated (increasing surface area) bipolar neurons.

  • Individual neurons show a range of responsiveness to different chemicals

    • Smell is difficult to research because any given odor made up of hundreds of different chemicals.

    • Some studies have identified 1000 smell genes that are active only in the nose. Each gene encodes for a specific receptor protein

    • Olfactory neurons are exceptionally sensitive with some taking a few molecules to activate them.

  • For us to smell a particular odor, it must be volatile, that means it must be in a gaseous state as it enters the nasal cavity.

    • It must also dissolve in the fluid coating of the olfactory epithelium

  • Pathways:

    • Action potentials of olfactory nerve filaments are transmitted to the olfactory bulb where to filaments synapse with mitral cells. The mitral cells send impulses via the olfactory tract to the olfactory cortex. Fibers carrying impulses from the olfactory receptors also project to the limbic system. (our emotional, or affective (feelings) center in the brain)


  • Taste buds are scattered in the oral cavity and pharynx but are most abundant on the tongue papillae

  • Gustatory cells, the epithelial receptor cells of the taste buds, have gustatory hairs (microvilli) that serve the receptor regions.

  • We have 5 basic taste qualities: Sweet, Sour, Salty, Bitter and Umami (allows you to experience the “beef taste” of steak, the tang of aging cheese

    • Most taste buds respond to 2 or more taste qualities

    • There are only slight differences in the localization of specific taste receptors in the different regions of the tongue

  • Pathway: through the cranial nerves VII, IX, and X which send impulses to the nucleus of the medulla. From there to the thalamus and the taste cortex.

Hearing and Balance:

  • Our hearing apparatus allows us to hear an extraordinary range of sound.

  • Our equilibrium receptors keep our brain informed of head movements and position.

  • Structurally, these are interconnected but they respond to different stimuli and are activated independently of one another.

Structure of the Ear:

  • The ear is divided into 3 major areas: external ear, middle ear and internal ear.

    • The external and middle ear are involved in hearing only (relatively simple structurally)

    • The internal ear is involved with hearing and balance and is extremely complex

  • The external ear consists of the auricle and external acoustic meatus. The auricle (pinna) is what most people call the ear.

    • Composed of elastic cartilage covered with thin skin and an occasional hair

    • Function is to direct sound waves to the external auditory meatus (auditory canal)

    • Sound waves entering the auditory meatus hit the tympanic membrane (eardrum), the boundary between the external ear and the middle ear.

      • Thin translucent membrane shaped like a flattened cone. Vibrations then transmit the sound waves to the bones of the middle ear.

  • The middle ear or tympanic cavity, is a small, air filled, mucosa lined cavity in the temporal bone

    • Connected by the pharyngotympanic tube to the naso-pharynx.

    • The ossicles, which help amplify the sound, span the middle ear cavity and transmit sound vibrations from the eardrum to the oval window (vestibular window).

  • The internal ear

    • Consists of the boney labyrinth (maze), within which the membranous labyrinth (continuous series of membranous sacs and ducts) is suspended

    • Boney labyrinth chambers contain perilymph (fluid similar to cerebral spinal fluid)

    • The membranous sacs and ducts contain endolymph (K+ rich intracellular fluid)

      • These two fluids conduct the sound vibrations involved in hearing and respond to the mechanical forces occurring changes in body position and acceleration

    • The vestibule is the central egg shaped cavity of the boney labyrinth.

      • Contains the saccule and utricle which house receptor regions called maculae that respond to the pull of gravity and changes in head position.

      • The semicircular canals extend posteriorly from the vestibule in 3 planes. These contain the semicircular ducts which respond to angular rotational movements of the head.

    • The Cochlea

      • Houses the cochlea duct (scala media), containing the spiral organ (of Corti)

        • This is receptor organ for hearing

        • Within the cochlear duct are hair cells (receptors) which rest on the basilar membrane, hairs project into the gelatinous tectorial membrane

Physiology of Hearing:

  • Sound originates from a vibrating object and travels is waves consisting of alternating areas of compression and rarefaction of a medium

  • The distance from crest to crest on a sine wave is the sound’s wavelength. The shorter the wavelength, the higher the frequency (pitch) (measured in hertz)

  • The amplitude of sound is the height of the peaks of the sine wave, which reflects the sounds intensity (loudness). Sound intensity is measured in decibels.

  • Auditory processing is analytic; each tone is perceived separately.

    • Perception of pitch is related to the position of the excited hair cells along the basilar membrane.

    • Intensity perception reflects the fact that as sound intensity increases, basilar membrane mobility is increased and the frequency of impulse transmission to the cortex is enhanced.

    • Cues for sound localization include the intensity and timing of sound arriving at each ear.

Homeostatic imbalances:

  • Conduction deafness results from interference with the conduction of sound vibrations to the fluids of the internal ear

  • Sensorineural deafness reflects damage to the neural structures

  • Tinnitus is an early sign of sensorineural deafness; it may also result from use of certain drugs

Equilibrium and Orientation:

  • The equilibrium receptor regions of the internal ear are called the vestibular apparatus

  • Receptors for static equilibrium are the maculae of the saccule and utricle

  • The dynamic equilibrium receptor, the crista ampullaris within each semicircular duct, responds to angular movement in one plane.

Developmental Aspects of the Special Senses:

  • Chemical senses are sharpest at birth and gradually decline with age as replacement of the receptor cells slows

  • The eye reaches adult size by age 8-9.

  • With age, the lens loses elasticity and clarity, there is a decline in the iris ability to dilate, and visual acuity decreases

  • Response to sound in infants is reflexive. By 5 months an infant can locate sound. Critical listening develops in toddlers

  • Deterioration of the spiral Organ of Corti occurs throughout life

  • Age-related loss of hearing (presbycusis) occurs in the 60’s – 70’s

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