Receptors: chemoreceptors, thermoreceptors, nociceptors, mechanoreceptors, and proprioceptors
Mechanoreceptors that react to touch, pressure, and vibration stimuli.
Located in the dermis and the subcutaneous tissue.
Most numerous type of receptor.
free nerve endings : pain and temperature mainly.
root hair plexuses : hair movement
tactile (Merkel) discs : light touch
Encapsulated : covered by either connective tissue or glial cells
Krause bulbs : light pressure and vibration
located primarily in the mucous membranes
Lamellated (Pacinian) corpuscles : deep pressure and vibration
Ruffini corpuscles : continuous pressure and distortion of the skin and in some joint capsules (tonic)
Tactile (Meissner’s) corpuscles : light touch in dermal papillae of hairless skin
Smell is the least complex
Vision is the most complex
Gustation – Sense of Taste
Gustatory receptors are housed in specialized taste buds on the dorsal surface of the tongue.
4 Types of Papillae
Vallate (circumvallate) papillae are the
least numerous yet the largest
arranged in an inverted V shape on the posterior dorsal surface of the tongue
most of our taste buds are housed within the walls of these
Filiform papillae have no taste buds
on the anterior two-thirds of the tongue surface
Fungiform papillae have only a few taste buds
primarily located on the tip and sides of the tongue
Foliate papillae are not well developed in humans
Five basic taste sensations:
umami - deliciousness - amino acids such as glutamate or aspartate
Taste maps have a broader region than originally believed
Not everyone tastes equally
Gustatory cells are taste receptors
Live 7-10 days
Chemoreceptors - food must be dissolved in saliva to be tasted
Facial nerve for anterior 2/3 of tongue
Glossopharyngeal for posterior 1/3 of tongue
Gustatory cortex is in the insula
The conscious perception of taste integrates taste info with those of temperature, texture, and smell
Sense of smell or remote chemoreception
Not highly developed in humans
Olfactory neurons (or receptor cells)
in 5 cm square area of nasal epithelium
sandwich the olfactory neurons and sustain and maintain the receptors
function as stem cells to replace olfactory epithelium components
Lamina propria, internal to the epithelium, has olfactory glands secreting mucin and blood vessels and nerves
Olfactory Receptor Cells
Bipolar cells that are highly differentiated and modified
Have olfactory hairs in mucus: receptor proteins there detect molecules
Adaptation occurs rapidly
Eight different primary odors that all smell as well as thousands of other chemical stimuli that not everyone smells
Undergo mitosis to replace aged cells!!!
As with gustatory receptors, the number decreases with age
The olfactory nerves are the olfactory neuron axons.
Project through the olfactory foramina into olfactory bulbs and synapse.
Olfactory tracts go to the olfactory cortex in the temporal lobe and not to thalamus initially !
Later, they can project to the thalamus and then to the frontal lobes for more specific discrimination.
Widespread olfactory associations with the hypothalamus and limbic system. Smells often initiate behavioral and emotional reactions.
The Sense of Vision
Photoreceptors in the eyes to detect light, color, and movement.
About 70% of all the receptors are in the eye
Binocular vision gives depth perception
1 inch in diameter, only 1/6 visible
Conjunctiva - a superficial covering over part of its anterior exposed surface and blood vessels to nourish eye
Eyebrows, eyelashes, and eyelids protect against foreign objects
Lacrimal glands keep the exposed surface clean, lubricated, and helps prevent bacterial infection because of antibiotic like enzyme, lysozyme
Tarsal (Meibomian) glands are sebaceous glands that produce a secretion to prevent tear overflow from the open eye and keep the eyelids from sticking together
Ciliary glands are modified sweat glands that form the thick secretory products that contribute to “sleep”
Six extrinsic muscles move eye
Accessory Structures 19-
May be viral, bacterial, allergenic, etc.
Causes inflammation and redness, called “pink eye”
Trachoma is a chronic, contagious form caused by Chlamydia trachomatis .
A common cause of neonatal blindness in developing countries.
Most common transplant. Also plastic artificial ones can be used.
Rejection rare because there are no blood vessels in cornea
Rods for vision in dim light (120 million)
Found more towards periphery
Cones for color vision and visual acuity (6 million)
Only work in bright light
Three types: red, green, and blue
Overlap of three ranges gives us all the colors
Color blindness is sex-linked (males)
Separation of outer pigmented and inner neural layers of retina
Cause: trauma or may be no overt cause
Higher risk: nearsighted, diabetics, and older people
Retina gets starved of nutrients from choroid layer
The optic disc lacks photoreceptors, therefore, it is the blind spot.
The macula lutea , lateral to the optic disc, is a rounded, yellowish region of the retina containing a pit called the fovea centralis (the area of sharpest vision with the highest proportion of cones and almost no rods.
A leading cause of blindness in developed countries
Increased risk with smoking and hypertension
Amsler grid: check in lab may detect loss of visual acuity, etc.
Laser photocoagulation may destroy abnormally proliferating blood vessels and help slow progression
Macular Degeneration 19-
Cavities and Chambers of the Eye
The eye is subdivided by the lens into two separate cavities.
anterior cavity -between the lens and the cornea
anterior chamber between the iris and cornea
posterior chamber between the lens and the iris
Lens becomes cloudy
A major cause of blindness worldwide
Causes: aging mainly, but other factors are diabetes, infections, UV, and glaucoma
Ultrasound fragments and then pieces removed and replaced with an artificial lens usually
The anterior cavity contains aqueous humor.
removes waste products and helps maintain the chemical environment within the anterior and posterior chambers of the eye
secreted into the posterior chamber
then it flows through the posterior chamber
down through the pupil
into the anterior chamber
Increased intraocular pressure from aqueous humor
May dislocate lens, compress choroid layer and thereby constrict blood vessels nourishing retina and cause blindness from retinal degeneration
May not be noticed early
Early detection is important
Posterior cavity is posterior to the lens and anterior to the retina.
Transparent, gelatinous vitreous body which completely fills the space between the lens and the retina.
Each optic nerve conducts visual stimulus information.
At the optic chiasm , some axons from the optic nerve decussate.
The optic tract on each side then contains axons from both eyes.
Visual stimulus information is processed by the thalamus and then interpreted by visual association areas in the cerebrum .
Note: the eye labels have been reversed!
(page 582 and figure on next page)
Emmetropia = normal vision (20/20)
Hyperopia = far-sightedness (short eyeball)
Myopia = near-sightedness (long eyeball)
Astigmatism = unequal curvature of the cornea or lens
Presbyopia (“old eyes”) = loss of the ability to accommodate for near vision
How Vision Can Be Functionally Impaired 19-
Glasses and contacts
Corneal incision - radial keratotomy flattens the cornea for myopia
Laser vision correction changes the shape of the cornea for either myopia or hyperopia
Hearing and Equilibrium 19-
Hearing and Equilibrium
The external ear is located mostly on the outside of the body
The middle and inner areas are housed within the petrous portion of the temporal bone.
The external and middle ear are for hearing only
Movements of the inner ear fluid result in the sensations of hearing and equilibrium, or balance.
The Middle Ear
Contains an air-filled tympanic cavity.
Medially, a bony wall that houses the oval window and round window separates the middle ear from the inner ear.
The Middle Ear
Tympanic cavity maintains an open connection with the atmosphere through the auditory tube ( pharyngotympanic tube or Eustachian tube ).
opens into the nasopharynx (upper throat) from the middle ear - normally closed
air movement allows the pressure to equalize on both sides of the tympanic membrane
Tympanic cavity of the middle ear houses the auditory ossicles.
malleus (hammer), the incus (anvil), and the stapes (stirrup)
Middle ear infection
Common esp. in young children: horizontal, short, and undeveloped auditory tubes
Fluid accumulates causing pressure, pain, and maybe impaired hearing
Possible complications: meningitis, fusion of the ossicles leading to impaired hearing
Antibiotics (not used much currently to avoid antibiotic resistance) or, if necessary, myringotomy, a small drainage tube in eardrum
The Inner Ear
In the petrous portion of the temporal bone, within a bony labyrinth, are membrane-lined, fluid-filled tubes and spaces, called the membranous labyrinth.
The Inner Ear
Contains two saclike, membranous labyrinth parts—the utricle and the saccule - interconnected through a narrow passageway
Used for static equilibrium and linear acceleration
The membranous labyrinth is called the semicircular ducts
Used for dynamic equilibrium
The membranous labyrinth is called the cochlear duct
Used for hearing
Rotation of the head causes endolymph within the semicircular canal to push against the cupula covering the hair cells, resulting in bending of their stereocilia and the initiation of a nerve impulse.
The vestibular complex of the inner ear is sending impulses to the brain that conflict with the visual reference
Structures for Hearing
Housed within the cochlea in both inner ears.
snail-shaped spiral chambers in the bone of the inner ear
Membranous labyrinth houses the organ of Corti for hearing .
Sound Wave Pathways
Sound waves enter the external auditory canal and make the tympanic membrane vibrate
Tympanic membrane vibration causes movement by the auditory ossicles; sound waves are amplified. The stapes moves within the oval window; pressure waves are generated.
Pressure waves begin at the oval window and travel through the scala vestibuli.
Sound Wave Pathways
The sound waves displace one region of the basilar membrane causing hair cells in the spiral organ to distort when they push against the tectorial membrane. This stimulus is converted to a nerve impulse, which travels through the cochlear nerve.
Remaining pressure wave vibrations are transferred to the scala tympani and exit the inner ear via the round window.
Conduction deafness is due to middle ear conditions
Sensorineural deafness is due to problems within the cochlea or along the auditory nerve pathway. Hard to treat with hearing aides.
Electronic device that compensates for damaged or nonfunctioning parts of the inner ear. May help person learn to speak.