Special Senses

• Special senses include:

  • Smell

  • Taste

  • Sight

  • Hearing

  • Equilibrium

• Special sense receptors

  • Large, complex sensory organs

  • Localized clusters of receptors

The Eye and Vision

• 70% of all sensory receptors are in the eyes

• Each eye has over 1 million nerve fibers carrying information to the brain

Anatomy of the Eye

• Accessory structures include the:

  • Extrinsic eye muscles (operating from the outside)

  • Eyelids

  • Conjunctiva

  • Lacrimal apparatus

External and Accessory Structures

• Eyelids

  • Meet at the medial and lateral commissure (canthus)

• Eyelashes

  • Tarsal glands produce an oily secretion that lubricates the eye

  • Ciliary glands are located between the eyelashes

• Conjunctiva

  • Membranes that lines the eyelids and eyeball

  • Connects with the transparent cornea

  • Secretes to lubricate the eye and keep it moist

• Lacrimal apparatus = lacrimal gland + ducts

  • Lacrimal gland—produces lacrimal fluid (tears); situated on lateral end of each eye

  • Tears drain across the eye into the lacrimal canaliculi, then the lacrimal sac, and into the nasolacrimal duct-, which empties into the nasal cavity

• Tears contain:

  • Dilute salt solution (saline)

  • Mucus

  • Antibodies

  • Lysozyme (enzyme that destroys bacteria)

• Function of tears

  • Cleanse, protect, moisten, lubricate the eye

• Extrinsic eye muscles

  • 6 muscle attach attach to the outer surface of the eye

  • Produce gross eye movements

Internal Structures: The Eyeball

• Three layers, or tunics, form the wall of the eyeball

  • Fibrous layer: outside layer

  • Vascular layer: middle layer

  • Sensory layer: inside layer

• Humors are fluids that fill the interior of the eyeball

• Lens divides the eye into two chambers

• Fibrous layer = sclera + cornea

  • Sclera

    • White connective tissue layer ”white of the eye”

  • Cornea

    • Transparent, central anterior portion

    • Allows for light to pass through

    • Repairs itself easily

    • The only human tissue that can be transplanted without fear of rejection

• Vascular layer

  • Choroid is a blood-rich nutritive layer that contains a pigment(prevents light from scattering) & is modified anteriorly into two smooth muscle structures

    • Ciliary body

    • Iris -—regulates amount of light entering eye

      • Pigmented layer—gives eye color

      • Pupil—rounded opening in the iris

• Sensory layer

  • Retina contains two layers

    • Outer pigmented layer absorbs light and prevents it from scattering

    • Inner neural layer contains receptor cells (photoreceptors)

      • Rods

      • Cones

  • Electrical signals pass from photoreceptors via a two-neuron chain

    • Bipolar neuronsGanglion cells

  • Signals leave the retina toward the brain through the optic nerve

  • Optic disc- (blind spot) is where the optic nerve leaves the eyeball

    • Cannot see images focused on the optic disc

  • Rods

    • Most are found toward the edges of the retina

    • Allow vision in dim light and peripheral vision

    • All perception is in gray tones

  • Cones

    • Allow for detailed color vision

    • Densest in the center of the retina

    • Fovea centralis–lateral to blind spot

      • Area of the retina with only cones

      • Visual acuity(sharpest vision) is here

      • No photoreceptor cells are at the optic disc, or blind spot

  • Cone sensitivity

    • Three types of cones

    • Each cone type is sensitive to different wavelengths of visible light

• Lens

  • Flexible, biconvex (convex on both sides) crystal-like structure

  • Held in place by a suspensory ligament attached to the ciliary body

• Lens divides the eye into two chambers

  • Anterior (aqueous) segment

    • Anterior to the lens

    • Contains aqueous humor, a clear, watery fluid

  • Posterior (vitreous) segment

    • Posterior to the lens

    • Contains vitreous humor, a gel-like substance

• Aqueous humor

  • Watery fluid found between lens and cornea

  • Similar to blood plasma

  • Helps maintain intraocular pressure

  • Provides nutrients for the lens and cornea

  • Reabsorbed into venous blood through the scleral venous sinus, or canal of Schlemm

• Vitreous humor

  • Gel- like substance posterior to the lens

  • Prevents the eye from collapsing

  • Helps maintain intraocular pressure

• Ophthalmoscope

  • Instrument used to illuminate the interior of the eyeball and fundus (posterior wall)

  • Can detect diabetes, arteriosclerosis, degeneration of the optic nerve and retina

Physiology of Vision

• Path of light through eye & light refraction

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

  • Light is bent, or refracted, by the cornea, aqueous humor, lens, and vitreous humor

  • The eye is set for distant vision (over 20 feet away)

  • Accommodation—the lens must change shape to focus on closer objects (less than 20 feet away)

• Pathway of light through the eye and light refraction (continued)

  • Image formed on the retina is a real image

  • Real images are:

    • Reversed from left to right

    • Upside down

    • Smaller than the object

• Visual fields and visual pathways to brain

  • Optic nerve

    • Bundle of axons that exit the back of the eye carrying impulses from the retina

  • Optic chiasma

    • Location where the optic nerves cross

    • Fibers from the medial side of each eye cross over to the opposite side of the brain

• Visual fields & visual pathways to the brain

  • Optic tracts

    • Contain fibers from the lateral side of the eye on the same side and the medial side of the opposite eye

    • Synapse with neurons in the thalamus- (relaying of sensory signals, including motor signals, to the cerebral cortex, and the regulation of consciousness, sleep, and alertness)

  • Optic radiation

    • Axons from the thalamus run to the occipital lobe

    • Synapse with cortical cells, and vision interpretation (seeing) occurs

• Summary of the pathway of impulses from the retina to the point of visual interpretation

  • Optic nerve

  • Optic chiasma

  • Optic tract

  • Thalamus

  • Optic radiation

  • Optic cortex in occipital lobe of brain

• Visual fields

  • Each eye “sees” a slightly different view

  • Field of view overlaps for each eye

• Binocular vision results and provides:

  • Depth perception (three-dimensional vision)

A Closer Look

• Emmetropia—eye focuses images correctly on the retina

• Myopia (nearsightedness)

  • Distant objects appear blurry

  • Light from those objects fail to reach the retina and are focused in front of it

  • Results from an eyeball that is too long

• Hyperopia (farsightedness)

  • Near -objects are blurry, whereas distant objects are clear

  • Distant objects are focused behind the retina

  • Results from an eyeball that is too short or from a “lazy lens”

• Astigmatism

  • Images are blurry

  • Results from light focusing as lines, not points, on the retina because of unequal curvatures of the cornea or lens

  • Convergence: reflexive movement of the eyes medially when we focus on a close object

  • Photopupillary reflex: bright light causes pupils to constrict

  • Accommodation pupillary reflex: viewing close objects causes pupils to constrict

The Ear: Hearing and Balance

• Ear houses two senses

  1. Hearing

  2. Equilibrium (balance)

• Receptors are mechanoreceptors (respond to touch or feel)

• Different organs house receptors for each sense

Anatomy of the Ear

• The ear is divided into three areas

1. External (outer) ear

2. Middle ear

3. Internal (inner) ear

• External (outer) ear

  1. Auricle (pinna)

  2. Ext. acoustic meatus (auditory canal)

     • Narrow chamber in the temporal bone

     • Lined with skin and ceruminous (earwax) glands

     • Ends at the tympanic membrane (eardrum)

• External ear is involved only in collecting sound waves

• Middle ear cavity (tympanic cavity)

• Air filled, mucosa-lined cavity within the temporal bone

• Involved only in the sense of hearing

• Located between tympanic membrane and oval window and round window

• Pharyngotympanic tube (auditory tube)

• Links middle ear cavity with the throat

• Equalizes pressure in the middle ear cavity so the eardrum can vibrate

• Middle ear cavity (tympanic cavity)

• Three bones (ossicles) span the cavity

• Malleus(hammer), Incus(anvil), Stapes(stirrup)

• Function

• Transmit vibration from tympanic membrane to the fluids of the inner ear

• Vibrations travel: hammer -> anvil -> stirrup -> oval window of inner ear

• Internal (inner) ear

• Sense organs for hearing and balance

• Bony labyrinth (osseous labryrinth) consists of:

• Cochlea, vestibule, semicircular canals

• Bony labyrinth is filled with perilymph

• Membranous labyrinth is suspended in perilymph and contains endolymph

Equilibrium

• Equilibrium receptors of the inner ear are called the vestibular apparatus

• Vestibular apparatus has two functional parts

  1. Static equilibrium

  2. Dynamic equilibrium

Static Equilibrium

• Maculae—receptors in the vestibule

  • Report on the position of the head

  • Help us keep our head erect

  • Send information via the vestibular nerve (division of cranial nerve VIII) to the cerebellum of the brain

• Anatomy of the maculae

  • Hair cells are embedded in the otolithic membrane

  • Otoliths (tiny stones) float in a gel around hair cells

  • Movements cause otoliths to roll and bend hair cells

Dynamic Equilibrium

• Crista ampullaris

  • Responds to angular or rotational of the head

  • In ampulla of each semicircular canal

  • Tuft of hair cells covered with cupula (gelatinous cap)

  • If the head moves, the cupola drags against the endolymph

  • Hair cells are stimulated, impulse travels vestibular n. to the cerebellum

Hearing

• Spiral organ of Corti

  • Located within the cochlear duct

  • Receptors = hair cells on the basilar membrane

  • Gel-like tectorial membrane is capable of bending hair cells

  • Cochlear nerve attached to hair cells transmits nerve impulses to auditory cortex on temporal lobe

• Pathway of vibrations from sound waves

  • Ear drumossiclesoval window

  • Sound is amplified by the ossicles

  • Pressure waves cause vibrations in the basilar membrane in the organ of Corti

  • Hair cells of the tectorial membrane are bent when the basilar membrane vibrates against it

  • An action potential starts in the cochlear nerve (cranial nerve VIII), and the impulse travels to the temporal lobe

• High pitched sounds disturb the short, stiff fibers of the basilar membrane

  • Receptor cells close to the oval window are stimulated

• Low pitched sounds disturb the long, floppy fibers of the basilar membrane

  • Specific hair cells further along the cochlea are affected

Hearing and Equilibrium Deficits

• Deafness is any degree of hearing loss

  • Conduction deafness results when the transmission of sound vibrations through the external and middle ears is hindered

  • Sensorineural deafness results from damage to the nervous system structures involved in hearing

  • Meniere’s affects inner ear and causes progressive deafness and perhaps vertigo (sensation of spinning)

Chemical Senses: Smell & Taste

• Chemoreceptors

  • Stimulated by chemicals in solution

  • Taste has five types of receptors

  • Smell can differentiate a wider range of chemicals

• Both senses complement each other and respond to many of the same stimuli

Olfactory Receptors/Sense of Smell

• Olfactory receptors in roof of nasal cavity

  • Olfactory receptor cells (neurons) with long cilia (olfactory hairs) detect chemicals

  • Chemicals must be dissolved in mucus for detection by chemoreceptors called olfactory receptors

• Impulses are transmitted via the olfactory filaments to the olfactory nerve (I)

• Smells interpreted in the olfactory cortex

Taste Buds and Sense of Taste

• Taste buds house the receptor organs

• Locations of taste buds

  • Most are on the tongue

  • Soft palate

  • Superior part of the pharynx

  • Cheeks

• The tongue is covered with projections called papillae that contain taste buds

  • Vallate (circumvallate) papillae

  • Fungiform papillae

  • Filiform papillae

• Gustatory cells are the taste receptors

  • Possess gustatory hairs (long microvilli)

  • Gustatory hairs protrude through a taste pore

  • Hairs are stimulated by chemicals dissolved in saliva

• Impulses are carried to the gustatory complex by several cranial nerves because taste buds are found in different areas

  • Facial nerve (cranial nerve VII)

  • Glossopharyngeal nerve (cranial nerve IX)

  • Vagus nerve (cranial nerve X)

• Taste buds are replaced frequently by basal cells

• Five basic taste sensations

  • Sweet receptors respond to sugars, saccharine, some amino acids

  • Sour receptors respond to H+ ions or acids

  • Bitter receptors respond to alkaloids

  • Salty receptors respond to metal ions

  • Umami receptors respond to the amino acid glutamate or the beefy taste of meat

Developmental Aspects of the Special Senses

• Special sense organs are formed early in embryonic development

• Maternal infections during the first 5 or 6 weeks of pregnancy may cause visual abnormalities as well as sensorineural deafness in the developing child

• Vision requires the most learning

• The infant has poor visual acuity (is farsighted) and lacks color vision and depth perception at birth

• The eye continues to grow and mature until age 8 or 9

• Age-related eye issues

  • Presbyopia—“old vision” results from decreasing lens elasticity that accompanies aging

    • Difficulty to focus for close vision

  • Lacrimal glands become less active

  • Lens becomes discolored

  • Dilator muscles of iris become less efficient, pupils remain constricted

• The newborn infant can hear sounds, but initial responses are reflexive

• By the toddler stage, the child is listening critically and beginning to imitate sounds as language development begins

• Age-related ear problems

  • Presbycusis—type of sensorineural deafness that may result from otosclerosis (ear ossicles fuse)

  • Congenital ear problems usually result from missing pinnas and closed or missing external acoustic meatuses

• Taste and smell are most acute at birth and decrease in sensitivity after age 40 as the number of olfactory and gustatory receptors decreases