Chapter 8- Special Senses

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

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