Chapter 10: Sensory Physiology

10.1 Taste and Smell

Introduction: Tase and Smell

• Chemoreceptors

  • Taste responds to chemicals disolved in food + drink

  • Smell - chemical molec from air

  • Olfaction greatly influences gustation

Taste

• Aka gustation

• Receptors - taste buds

  • Consist of 50-100 specialized epithelial cells w long microvilli that extend out throug the pore in the taste bud to the environment of the mouth, wehere they are bathed in saliva

    • Ep cells arent neurons, but become depolarized whern stimulated, produce AP and release NT to stimluate sensory neurons - called neuroepithelial cells

  • Located in bumps on  the tongue called papillae

• Five categories

  • Salty

  • Sweet

  • Sour

  • Bitter

  • Umami

• Each tase bud has tase cells for the 5 categorties

• Influenced by:

  • Temp and texture of substance

• [chemical]

• Stimulation of olfactory receptors

Smell (Olfaction)

• Olfactory apparatus

  • Olfactory recptors located in olfactory epithelium of the nasal cavity

  • Consists of bipolar olfactor sensory neurons, supporting (sustentacular) cells and basal stem cells

    • Sustentacular cells oxiidze hydrophobic volatile odors

    • Basal stem cells replace receptors damaged by environ

• Olfactory receptors

  • Bipolar neurons w one dendrite projecting into the nasal cacvity that ends in a ciliated knob

  • Proteins in cilia bind to odorant molec

  • Abt 380 genes code for abt 380 diff olfactory receptors

  • 1 odorant molecule stimulates 1 protein in 1 sensory neuron

• How smell works

  • G protein coupled

  • Odor binding acitviates adenylate cyclase --> cAMP and Ppi (pyrophosphate)

  • cAMP opens Na2+ and Ca2+ channels ---> graded depolarization

    • Graded depolarization stimulates action potential

  • Up to 50 G-proteins may be associated w 1 receptor proteins

    • Gives great sesnitivty though amplificatoin

10.2 Vestibular Apparatus and Equilibrium

Vestibular Apparatus

• Provides sense of Eq

• Located in inner ear

• Consists of

  • Otolith organs

    • Utricle and saccule: linear A

  • Semicircular canals: rotational A

• Inner Ear

  • Consists of:

    • Bony labyrinth surrounding membranous labyrinth

    • In between them is perilymph fluid

    • In membranous labyrinth is endolymph fluid

Sensory Hair Cells

• Modified epithelial cells (vestibular hair cells) with 20-50 hairlike extensions (stereocilia… fake cilia) and one kinocilium (true cilium)

  • Stereocilia - modified microvilli arranged in rows of increasing height

  • Kinocilium- taller cilium touching the stereocilia of the highest row

• Stereocilia bend towards kinocilium = depolarized hair cell

• Hair cells release NT that depolarizes sensory dendrites in vestibulocochlear nerve

• Bending away from kinocilium = hyperpolarization = less NT released

• Frequency of AP in sensory neurons that innervate the hair cells carries info about the direction of movements

Utricle and Saccule

• Provide info about linear A

  • Utricle: horizontal

  • Saccule: vertical

• Specialized epithelium (macula) houses hair cells

  • Stereocilia embedded in gelatinous otolithic membrane

  • Gel also contains crystals of calcium carbonate (otoliths aka ear stones)

Semicircular Canals

• Project along 3 planes to detect rotation

  • Each  canal contains a semicircular duct filled w endolymph

  • At base of each duct is enlarged area (ampulla)

  • Hair cells embedded in crista ampullaris, with stereocilia stuck into a gelatinous cupula

• Rotation makes endolymph circulate, pushing cupula and bending hair cells

10.3 Ears and Hearing

Sound Waves

• Characterized by

  • Frequency

    • Hertz (Hz) cycles/sec

    • Higher frequency = higher pitch

    • Human range: 20-20kHz

  • Intensity/Loudness

    • Decibels (dB)

    • Related to amplitude of the wave

    • Human range: 0-80dB

Outer Ear

• Sound waves funneled by pinna (auricle) into external auditory meatus, which channels them into the tympanic membrane (eardrum)

Middle Ear

• Air filled cavity between tympanic membrane and cochlea

• Contains 3 bodies (auditory ossicles)

  • Malleus (connected to eardrum) --> incus --> stapes

  • Vibrations transmitted + amplified along the bones

  • Stapes attached to oval window which transfers vibrations into the cochlea

  • Stapedius muscle dampens the stapes if sound is too intense

The Cochlea

• Hearing part of the inner ear

• 3 Chambers:

  1. Upper chamber - portion of bony labyrinth (scala vestibuli)

  2. Lower bony chamber - scala tympani

  3. Both filled with perilymph

  4. Scala media (cochlear duct)  - portion of membranous labyrinth 

     • Filled with endolymph

     • Middle chamber that coils in 3 turns

• Helicotrema - small canal that connects scala vestibuli to scala tympani

• Sound Transmission

  1. Vibrations from oval window of mid ear displace perilymph in scala vestibuli

  2. Vibrations pass through vestibular membrane into cochlear duct through the endolymph

     • They then pass through the basilar membrane into the perilymph of the scala tympani

  3. They leave the inner ear via the round window

  4. Sound waves transmitted through cochlear ducts at locations that depend on frequency of sound

     • Low frequency travels further down the spiral of the cochlea to the apex

     • High frequency sounds are closer to the base

Spiral Organs (Organ of Corti)

• Mechanosensory hair cells (stereocilia) located on basilar membrane, projecting into endolymph of cochlear duct

  • Inner hair cells

    • 3500 form one row that runs length of basilar membrane

    • Each innervated by 6-20 sensory neurons of cranial nerve VIII and relay sound

  • Outer hair cells

    • 11,500 arranged in rows with 3 rows/turn

    • Innervated by motor neurons

      • Shorten when depolarized

      • Elongate when hyperpolarized

  • Hairs are stereocilia that are large microvilli arranged in bundles

    • Stereocilia in each bundle increase in size stepwise + are interconnected

      • Embedded in gelatinous tectorial membrane

• Made up of:

  • Basilar membrane

  • Inner hair cells with sensory fibers

  • Tectorial membrane

• How hearing works

  • Sound waves enter scale media --> tectorial membrane vibrates --> bends stereocilia

    • K+ channels facing endolymph open

    • K+ rushes in --> depolarization

      • Releases glutamate onto sensory neurons

    • K+ returns passively to perilymph at base of stereocilia

10.4 The Eyes and Vision

Structures of the Eye

• General pathway of light through the eye

  • Light --> cornea --> anterior chamber of eye ---> pupil --> lens -->posterior chamber and vitreous body --> retina  --> absorbed by pigmented choroid layer

    • Pupil can change shape (due to pigmented iris muscle) to allow more/less light in

    • Lens - changes shape to focus image

    • Retina - location of photoreceptors

• Pupil and Iris

  • Iris can increase/decrease diameter of pupil

    • Constriction: contraction of circular muscles via parasympathetic stimulation (oculomotor nerve)

    • Dilation: contraction of radial muscles (dilator papillae muscle) via sympathetic stimulation

  • Iris also has pigmented epithelium for eye color

• Lens

  • Composed of layers of living cells that are normally completely clear

  • Avascular

  • Cell metabolism is very low and anaerobic

  • Attached to muscles (ciliary bodies)

  • Suspensory ligaments support lens by way of zonular fibers

• Aqueous Humor

  • Fills anterior + posterior chambers between cornea + lens

  • Clear, watery liquid secreted by ciliary bodies to provide nourishment to lens + cornea

  • Drains into scleral venous sinus (canal of schlemm) back into the venous blood

  • Inadequate drainage = glaucoma

Lens Accommodation

• Accommodation- the ability of a lens to keep an object focused on the retina as the distance between the eye and the object moves

  • Contraction of ciliary muscles - suspensory ligaments relax, lens thickens and roundup

    • Good for close vision

  • Relaxation of ciliary muscles - suspensory ligaments pulled, lens thin and flatten

    • Distant vision

Visual Acuity

• Sharpness of vision that depends upon resolving power - ability to distinguish between two closely spaced objects

• Measured at 20ft with Snellen Eye Chart

• Myopia - nearsightedness

  • Distant images brought to point of focus in front of retina

  • Often due to elongated eyeball

  • Corrected by concave lenses

• Hyperopia - farsightedness

  • Distant images brought to point of focus behind retina

  • Often due to short eyeball

  • Corrected by convex lenses

• Astigmatism

  • Asymmetry of cornea and/or lens curvature

  • Several points of focus on retina

  • Corrected by cylindrical lenses

10.5 The Retina

Introduction to Retina

• Forward extension of the brain so neural layers face outward toward incoming light

• Neuron axons in retina gathered at a point (optic disc aka blind spot) and exit as the optic nerve

• Blood vessels also enter + leave here

Layers of the Retina

• Photoreceptors (rods and cones) in inner layer (towards vitreous body)

• Synapse on a middle layer of bipolar cells, which synapse on the outer layer of retinal ganglion cells

• Horizontal cells and amacrine cells within layers

Rods and Cones

• Consist of:

  • Outer segment - full of flattened discs w photopigment molecules

  • Inner segment contains cell organelles

Effects of Light on the Rods

• Allow black and white vision in low light

• Contain purple pigment (rhodopsin) - absorbs green light best (abt 500nm)

  • Absorption = rhodopsin dissociation --> retinaldehyde + opsin

  • Retinaldehyde (aka retinal/retinene) derived from Vitamin A

  • Called bleaching reaction

• Visual Cycle of Retinal

  • In rhodopsin, retinal exists in an 11-cis form

  • After bleaching

    • Retinal is in all-trans form

    • Separates from opsin

      • Changes ionic permeability of rod = production of nerve impulse

      • To be reincorporated into retinal, must be converted back to 11-cis

  • Occurs in pigment epithelial cells

Electrical Activity of Retinal Cells

• Dark current

  • In dark, photoreceptors inhibit/hyperpolarize bipolar cells

  • Na+ channels in rods and cones are always open, depolarizing photoreceptors; called dark current

  • Allows photoreceptor to release inhibiting NT in the dark

  • Light inhibits photoreceptors from releasing inhibitory NT = stimulates bipolar cells

• When light hits photoreceptors

  • Dissociation of rhodopsin = activation of G-protein/2nd messenger system --> Na+ channels close

    • G-proteins: transducers

    • Alpha transducin activates enzyme phosphodiesterase = converts cGMP --> GMP

      • Closes cGMP -gated Na+ channels = inhibits dark current

  • Photoreceptors hyperpolarized and inhibition on bipolar cells lifted

  • Bipolar cells activate ganglion cells that transmit AP to the brain

Cones + Color Vision

• Less sensitive to light, but allow color vision + greater visual acuity

• Trichromatic vision involves 3 types of cones

  • S: short wavelengths, blue

  • M: medium wavelength, green

  • L: long wavelength, red

• Instead of opsin, photopigments have photopsins with retinene

  • They vary in each type of cone

• Cone response depends on:

  • Wavelength

  • Light intensity

Visual Acuity and Sensitivity

• Vision best at one point in retina - fovea centralis - within macula lutea

  • Here, other layers of retina are pushed aside = light falls directly on group of cones

  • Each cone has 1:1 relationship with ganglion cell (usually its 105:1) = great visual acuity

  • Only works in good light

• Convergence of lots of rods onto single ganglion = increased light sensitivity

• Saccadic eye movements continually shift parts of visual field onto the fovea