Ch. 16 - Special Senses

Hearing

a response to vibrating air molecules

• sense resides in inner ear

Equilibrium

the sense of motion, balance, and body orientation in 3D space

• sense resides in inner ear

Sound

any audible vibration of molecules

• A vibrating object pushes on air molecules

• These air molecules push on other air molecules

• the air molecules hitting the eardrum cause it to vibrate

Pitch

our sense of whether a sound is “high” or “low”

• Determined by vibration frequency: hertz or cycles/second

• in hearing loss, we can hear the outer ranges of sound, but struggle with the middle range

  • Most hearing loss with age is in the range of 250 to 2,050 Hz

Loudness

the perception of sound energy, intensity, or amplitude of the vibration

• Expressed in decibels (dB)

• Prolonged exposure to sounds > 90 dB can cause damage

Outer, Middle, and Inner Ear

Outer - what we can see

Middle - starts at ear drum and goes to auditory tube

Inner - cochlea and vestibule

What happens in the outer, middle, and inner ear?

• the outer and middle ear only deal with the transmission of sound to the inner ear

• The inner ear deals with vibrations and converts them to nerve signals

Otitis Media

Middle-ear infection

• is common in children

• Auditory tube is short and horizontal

  • infections easily spread from throat to tympanic cavity and mastoid air cells

• Symptoms

  • Fluid accumulates in tympanic cavity producing pressure, pain, and impaired hearing

  • Can spread, leading to meningitis

  • Can cause fusion of ear ossicles and hearing loss

  • makes patient have a hard time hearing bc fluid build up doesn't allow eardrum to work properly

Components of Inner Ear

• Bony Labyrinth

• Membranous Labyrinth

• Labyrinth

• Cochlea

Bony Labyrinth

passageways in temporal bone

Membranous Labyrinth

fleshy tubes lining bony labyrinth

• Filled with endolymph: similar to intracellular fluid

• Floating in perilymph: similar to cerebrospinal fluid

Labyrinth

Vestibule and three semicircular ducts

Cochlea

organ of hearing

• Winds coils around a screw-like axis of spongy bone called the modiolus

• Threads of the screw form a spiral platform that supports the tube of the cochlea

• Cochlea has three fluid-filled chambers separated by membranes

  • Scala vestibuli

  • Scala tympani

  • Scala media

Scala vestibuli

superior/top chamber

• Filled with perilymph

• Begins at oval window and spirals to apex

Scala tympani

inferior/bottom chamber

• Filled with perilymph

• Begins at apex and ends at round window

  • Secondary tympanic membrane: covers round window

Scala media (cochlear duct)

middle chamber

• Filled with endolymph

• is lined with spiral organs

Spiral Organ

acoustic organ that converts vibrations into nerve impulses

• has epithelium composed of 4 rows of hair cells and supporting cells

  • Hair cells have stereocilia

Stereocilia

long, stiff microvilli on apical surface

• has a tectorial membrane

Tectorial Membrane

Gelatinous membrane that rests on top of stereocilia

Inner Hair Cells

One row of about 3,500 cells

• Provides for hearing

• cannot tell difference between sounds like outer hair cells

Outer Hair Cells

three rows of about 20,000 cells

• Adjusts response of cochlea to different frequencies

• Increases precision

• help us tell where sounds are coming from

• tips of stereocilia of outer hair cells are imbedded in the tectorial membrane

Tympanic Membrane

eardrum

• Ossicles (bones of the middle ear) concentrate the energy of the vibrating tympanic membrane to a smaller area

• Ossicles create a greater force per unit area at the oval window and overcome the inertia of the perilymph

• Ossicles and their muscles have a protective function

  • Lessen the transfer of energy to the inner ear

Vibration of ossicles causes…

vibration of the basilar membrane under hair cells

• hair cells move with the basilar membrane

Endolymph

a high K+ fluid that bathes the stereocilia of outer hair cells

• the high concentration of K+ creates a gradient and makes the outside of the cell positive and the inside negative

Stimulation of Stereociliia of Inner Hair Cells:

1. Stretchy protein filament (called a tip link) connects the ion channel of one stereocilium to the sidewall of the next

2. Tallest stereocilium is bent when the basilar membrane rises up toward the tectorial membrane

3. the bending pulls on tip links and opens ion channels

4. K+ flows in —depolarization causes release of a neurotransmitter

5. Stimulates sensory dendrites and generates action potential in the cochlear nerve

Variations in loudness (amplitude) cause…

variations in the intensity of cochlear vibrations

• Soft sound produces a slight up-and-down motion of the basilar membrane

• Louder sounds make the basilar membrane vibrate more aggressively

  • Triggers higher frequency of action potentials

  • Brain interprets this as louder sound

Pitch depends on …

which part of the basilar membrane is vibrating

• low pitch noises go more towards the distal end, farther down the cochlea

• high pitch noises go towards the proximal end, the beginning area of the cochlea

Conductive Deafness

conditions interfere with transmission of vibrations to inner ear

• caused by damaged tympanic membrane, otitis media, blockage of auditory canal, and otosclerosis

Otosclerosis

fusion of auditory ossicles that prevents their free vibration

Sensorineural (nerve) Deafness

death of hair cells or any nervous system elements concerned with hearing

• Common in factory workers, musicians, construction workers

Vestibular Apparatus

makes up the receptors for equilibrium

• Three semicircular ducts

  • Detect only angular acceleration (dynamic equilibrium)

• Two chambers

  • Anterior saccule and posterior utricle

  • Responsible for static equilibrium and linear acceleration

Static Equilibrium

the perception of the orientation of the head when the body is stationary

• our idea of where we are in space (upright, where is up and down, etc)

• when head is tilted, the heavy otolithic membrane sags, bending the stereocilia and stimulating the hair cells

Dynamic Equilibrium

the perception of motion or acceleration

• the equilibrium of movement

• in car, linear acceleration detected as otoliths lag behind, bending the stereocilia and stimulating the hair cells

• Linear acceleration—change in velocity in a straight line

• Angular acceleration—change in rate of rotation

Macula

a 2 x 3 mm patch of hair cells and supporting cells in the saccule and utricle

• Macula sacculi: lies vertically on wall of saccule

• Macula utriculi: lies horizontally on floor of utricle

• used for static equilibrium

• Each hair cell has 40 to 70 stereocilia and one true cilium called a kinocilium

  • embedded in a gelatinous otolithic membrane

Are the macula sacculi and macula utriculi parallel or perpendicular?

they are perpendicular to each other so they can bend in their directions

• Because macula sacculi is vertical, it responds to vertical acceleration and deceleration

Otoliths

calcium carbonate–protein granules (little tiny bits of bones) imbedded in a gel like substance, so when they bend, they pull of the gel like substance, thereby pulling the hairs and opening mechanically gated channels

• add to the weight and inertia and enhance the sense of gravity and motion

Semicircular Ducts

• there are 3 of them

• detect rotary movements

• are held by the bony semicircular canals of the temporal bone

• Each duct is filled with endolymph and opens up as a dilated sac (ampulla) next to the utricle

• the 3 ducts are perpendicular to each other

  • allows them to detect movement in 3d space

• Each ampulla contains crista ampullaris

Crista Ampullaris

mound of hair cells and supporting cells

• Consists of hair cells with stereocilia and a kinocilium buried in a mound of a gelatinous membrane called the cupula

  • one in each duct

Vision

perception of objects in the environment using the light they emit or reflect

• components include the retina and the optic nerve

Light

visible electromagnetic radiation

• wavelengths of light range from 400 to 700 nm in humans

• Light must cause a photochemical reaction to produce a nerve signal

Ultraviolet Radiation

< 400 nm

• has too much energy and destroys macromolecules

Infrared Radiation

> 700 nm

• too little energy to cause photochemical reaction, but does warm the tissues

Retina

attached to eye only at the optic disc (back exit of the optic nerve) and ora serrata (front edge of retina)

• is pressed against the rear of the eyeball by the vitreous humor

• Detached retina causes blurry areas of vision and can lead to blindness

• includes the macula lutea and fovea centralis

• retina has blood vessels

Vitreous humor

a jelly-like substance that keeps retina in place

Macula lutea

patch of cells on the visual axis of the eye

Fovea centralis

pit in center of macula lutea

Optic Disc

blind spot

• where the optic nerve exits retina

• there are no receptors there

Visual Filling

brain fills in the “picture” across the blind spot area

• Brain ignores unavailable information until fast eye movements redirect gaze

Formation of an Image

• Light passes through the lens to form tiny inverted image on retina

• the iris diameter is controlled by two sets of contractile elements

  • pupillary constrictor

  • pupillary dilator

• when we see something, it is first upside down, but our brain turns it back to right-side-up

Pupillary Constrictor

smooth muscle encircling the pupil

• Parasympathetic stimulation narrows the pupil

Pupillary Dilator

spoke-like myoepithelial cells

• Sympathetic stimulation widens pupil

Pupillary constriction and dilation occurs when…

• light intensity changes

• when gaze shifts between distant and nearby objects

Photopupillary Reflex

the constriction of the pupil in response to light

• Mediated by autonomic reflex arc

  • Brighter light is signaled to the pretectal region of the midbrain

  • Excites parasympathetic fibers in oculomotor nerve that travels to ciliary ganglion in orbit

  • Postganglionic parasympathetic fibers stimulate pupillary constrictor

Refraction

the bending of light rays

• change in speed of light causes change in direction of light

Refractive Index

A measure of how much it reduces light rays relative to air

• Angle of incidence at 90° light slows but does not change course

• Any other angle, light rays change direction (are refracted)

• The greater the refractive index and the greater the angle of incidence, the more refraction

Refraction in the Eye

• the light passing through the center of the cornea is not bent

• any light striking off-center is bent toward the center

• the aqueous humor and the lens do not greatly alter the path of light

• Cornea refracts light more than lens does

  • Lens merely fine-tunes image

  • Lens becomes rounder to increase refraction for near vision

Emmetropia

state in which the eye is relaxed and focused on an object more than 6 m (20 ft) away

• Light rays coming from that object are parallel, so we can dilate without damage

• Rays focused on retina without effort

  • Light rays coming from a closer object are too divergent to be focused without effort

Adjustment to close-range vision requires 3 things:

1. Convergence of eyes

• we have to angle our eyes to the thing were looking at

2. Constriction of pupil

   • Blocks peripheral light rays and reduces spherical aberration (blurry edges)

3. Accommodation of lens

   • change in the curvature of the lens lets you focus on nearby objects

Accommodation of Lens:

Ciliary muscle contracts, suspensory ligaments slacken, and lens takes a thicker shape

• causes light to be refracted more strongly and focused onto the retina

Near Point of Vision

closest an object can be and still come into focus

• lengthens with age because the lens of our eyes become stiffer as we age

Retina converts light energy into…

action potentials

Structure of the Retina

• Pigment epithelium

  • Most posterior part of retina

  • Absorbs stray light so visual image is not degraded

• Rod Cells

• Cone Cells

Rod Cells

light-absorbing cell best for night vision or monochromatic vision

• Uses visual pigment rhodopsin

• a photoreceptor

• anterior segments are what pick up light

• more sensitive to light

Cone Cells

light-absorbing cells best for color, photopic, or day vision

• good for higher resolution vision

• not as sensitive to light as rods are

• contain photopsin (iodopsin)

• a photoreceptor

Histology of the Retina

• Pigment epithelium

• Rod and cone cells

• Bipolar cells

  • Rods and cones synapse on bipolar cells

  • Bipolar cells synapse on ganglion cells

• Ganglion cells

Ganglion Cells in the Retina

Single layer of large neurons near vitreous (front of the eye)

• Axons form the optic nerve

• Some absorb light with the pigment melanopsin and transmit signals to the brainstem

• Detect light intensity for pupil control and circadian rhythms

• do not contribute to visual image

• Ganglion cells are the only retinal cells that produce action potentials

How light changes rhodopsin:

pigment receives light, high energy electrons of the photon cause pigment to change shape

• In the dark, retinal is bent (cis-retinal) and retinal and opsin are together

• In the light, the retinal molecule straightens (trans-retinal), and retinal separates from opsin

  • called bleaching of rhodopsin

  • To reset, it takes five minutes to regenerate 50% of bleached rhodopsin

How light changes photopsin

function similarly to rhodopsin

• But are faster in regenerating their photopsin

  • 90 seconds for 50% of bleached photopsin

What neurotransmitter do rods release when it is dark and how does that change when it is light?

• rods constantly release glutamate

• glutamate goes from axons of rods to the bipolar cells

  • acts as an inhibitor to the bipolar cells

  • bipolar cells will be hyperpolarized

  • makes it so that bipolar cells do not release neurotransmitters to our ganglion cells

• When light hits, rods stop releasing glutamate, which allows neurotransmitters to be sent to ganglion cells

  • bipolar cells are excited by increasing light intensity

• when rods are not activated they are releasing neurotransmitters

• when rods are activated, they stop releasing neurotransmitters

Generating the Optic Nerve Signal

1. When bipolar cells detect fluctuations in light intensity, they stimulate ganglion cells directly or indirectly

2. Ganglion cells respond to the bipolar cells with rising and falling firing frequencies

   • ganglion cells are only retinal cells that produce action potentials

3. Using the optic nerve, these changes provide visual signals to the brain

Duplicity Theory of Vision

explains why we have both rods and cones

• A single type of receptor cannot produce both high sensitivity and high resolution

• It takes one type of cell and neural circuit for sensitive night vision

• It takes a different cell type and neuronal circuit for high-resolution daytime vision

Characteristics of Rods

• are very sensitive and react even in dim light

• have extensive neural convergence

  • lots of rod cells converge onto one bipolar cell

  • many bipolar cells converge onto a single ganglion cell

• results in a high degree of spatial summation

  • multiple signals from different locations are being combined to produce a stronger overall effect

• rods are on the outer sides of the retina

  • made for low resolution

  • cannot focus finely detailed images

Fovea Centralis

• has only cones, no rods

• No neuronal convergence

• Each foveal cone cell has a “private line to the brain”

• concentration of cones gives us high-resolution color vision

  • little spatial summation so less sensitivity to dim light

3 Types of Cones

named for absorption peaks of their photopsins

• Short-wavelength (S) cones

  • peak sensitivity at 420 nm

• Medium-wavelength (M) cones

  • peak at 531 nm

• Long-wavelength (L)cones

  • peak at 558 nm

Color perception is based on..

the mixture of nerve signals representing cones of different absorption peaks

Stereoscopic Vision

depth perception

• ability to judge distance to objects

• Requires 2 eyes with overlapping visual fields which allows each eye to look at the same object from different angles

Fixation Point

the point in space that the eyes are focused on

• Looking at an object within 100 feet, each eye views it from a slightly different angle

• Provides brain with information used to judge the position of objects relative to the fixation point

Senescence of Vision

• Loss of flexibility of lenses (presbyopia)

• Cataracts (cloudiness of lenses) becomes common

• Night vision is impaired due to fewer receptors, vitreous body less transparent, pupil dilators atrophy, and enzymatic reactions become slower

  • half-lives increase as we age, making the enzymatic reactions slower

• Glaucoma risks increase

Senescence of Hearing

• Tympanic membrane and ossicle joints stiffen

• Hair cells and auditory nerve fibers die

• Death of vestibular neurons results in dizziness

• Taste and smell are blunted as receptors decline

• we can regenerate our taste and smell receptors