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