The PNS: Afferent Nervous System

• two kinds of pathways– 1. Somatic: sensory/afferent information from skeletal

muscle• receptors are scattered at the body surface• can become specialized = Special senses

– 2. Visceral: sensory information from the internal viscera• receptors are scattered throughout the viscera (organs located in a

cavity)• e.g. blood pressure, body fluid concentration, respiratory gas

concentration• never reaches a conscious level

Perception & Sensation

• Sensation: response to environment via generation of nerve impulse• sensation occurs upon arrival of nerve impulse at cerebral cortex• before nerve impulse is generated - sensory receptors integrate

or sum up the incoming signals• several types of integration: one type is adaptation - decrease in

response to a stimulus– role of the thalamus?? (gatekeeper??)

• sensory nerve impulses are sent via ascending tracts in spinal cord to specific sensory areas in the cerebral cortex

• Perception: our conscious interpretation of the external world– created by the brain based on information it receives from sensory

receptors– interpretation of sensation

Sensation

• each type of sensation = sensory modality• one type of neuron carries only one type of

modality• modalities can be grouped into two classes

– 1. general senses – includes both the somatic and visceral senses

• tactile (touch, pressure), thermal, pain and proprioception

– 2. special senses: sight, sound, hearing, taste

Sensation• 1. stimulation of the sensory receptor

– alters the permeability of the neuron’s PM– usually does this through non-specific opening of small ion

channels

• 2. transduction of the stimulus– increased influx of positive ions (usually Na+) depolarization

of the sensory receptor– known as a graded receptor potential

• 3. generation of the nerve impulse– increase in graded receptor potential past threshold -> Action

Potential– AP propagates toward the CNS using ascending sensory tracts

• 4. integration of the sensory input– receipt of sensory information by a particular region in the

cerebral cortex– integration of sensation and perception

Sensory Pathways

• sensory pathways enter in the posterior side of the spinal cord

• enter into the spinal nerve and continue along the dorsal root into the posterior gray horn

• where they go from there depends on the pathway (i.e. the kind of modality)

Sensory Pathways• these pathways consist of three neurons

– 1. first order neurons – conduct sensory information from the receptor into the CNS

• cranial nerves conduct information from the face, mouth, eyes, ears and teeth

• spinal nerves conduct information from the neck, trunk and limbs

– 2. second order neurons – conduct information from the brain and SC into the thalamus

• many of these neurons decussate (cross over) within the thalamus

– 3. third order neurons – conduct information from the thalamus to the primary areas within the cerebral cortex

• for integration

Sensory Pathways• sensory pathways enter the SC and ascend to the cerebral

cortex via:– 1. the posterior column-medial lemniscus path

• for conscious proprioception and most tactile/touch sensations

• two tracts of white matter: posterior column and the medial lemniscus

• first order neurons = sensory receptors in the trunk and limbs - form the posterior columns in the spinal cord

• second order neurons = start in the medulla oblongata and run to the thalamus

– 2nd order neurons cross to the opposite side of the medulla and enter the medial lemniscus in the thalamus

• third order neurons = run from thalamus to the cortex (primary somatosensory area)

– fine touch– stereostegnosis – ability to recognize shapes,

sizes and textures by feeling– proprioception– vibratory sensations

– 2. the anterolateral/spinothalmic path• first order neurons = synapse with

sensory receptors in the neck, trunk or limbs

– cell bodies of these neurons are located in the dorsal root ganglion

– axons run through the dorsal root and into the PGH

• second order neurons = originate in the posterior gray horn

• second order neurons than cross to the opposite side of the SC and pass upward to the brain stem in either the:

– lateral spinothalmic tract: pain and temperature

– anterior spinothalmic tract: information for tickle, itch, crude touch and pressure

• third order neurons = go from thalamus to primary somatosensory area

Sensory Pathways• two main tracts: posterior spinocerebellar and anterior spinocerebellar• major routes for proprioceptive impulses from lower limbs that reach the

cerebellum• not consciously perceived• critical for posture, balance and coordination• first order neurons: known as muscle spindles and tendon organs• second order neurons: synapse with the 1st order neurons in the muscles and

tendons– cell bodies are found in dorsal gray horn, axons enter the dorsal root and travel up to end in the

thalamus• third order neurons: thalamus to cerebellum via cerebellar peduncles (no

decussation)• Posterior tract = info from muscle spindles and tendon organs• Anterior tract = info from tendon organs

Primary Somatosensory area

• specific areas of the cerebral cortex receive somatic sensory input from various parts of the body

• precise localization of these somatic sensations occurs when they arrive at the primary somatosensory area

• some regions provide input to large regions of this area (e.g. cheeks, lips, face and tongue) while others only provide input to smaller areas (trunk and lower limbs)

-sensory receptors: can either be a1) specialized ending of an afferent neuron2) a separate cells closely associated with an

afferent neurons

-can classify a sensory receptor based on:1. its microscopic features2. its location3. the type of stimulus that activates it

1. microscopic features:a. free nerve endings: bare dendrites associated with pain, heat, tickle, itch and some touchb. encapsulated nerve endings: dendrites enclosed in a connective tissue capsule - touch

e.g. Pacinian corpusclec. separate cells: individual receptors that synapse with first-order afferent neurons

e.g. gustatory cells (taste)2. receptor location:

a. exteroceptors: located at or near the body surface, responds to information coming in from the environment (taste, touch, smell, vision, pressure, heat and pain)

b. interoceptors: located in blood vessels, visceral organs and the nervous system; provide information about internal environment

c. proprioceptors: located in inner ear, skeletal muscle and joints; provides information about position of limbs and head

3. type of stimulus:1. Chemoreceptors2. Mechanoreceptors3. Nociceptors/pain receptors4. Thermoreceptors5. Photoreceptors6. Osmoreceptors

Proprioceptive Sensation

Proprioceptors

-located in muscles, joints and tendons -position of limbs and degree of muscle relaxation needed for contraction/moving the load-high concentration in postural muscles (body position), tendons (muscle contraction)-allow us to estimate weight of a load and to determine how much muscular effort is required

-also located in the inner ear – position of head- “hair cells” – position relative to the ground and movement

Proprioceptive Sensation

• three types of proprioceptors– 1. muscle spindles

• monitor changes in muscle length• used by the brain to set an overall level

of involuntary muscle contraction = motor tone

• consists of several sensory nerve endings that wrap around specialized muscle fibers = intrafusal muscle fibers

– sensory nerve endings = dendrites of 1st order neurons

– stretching of the muscle stretches the intrafusal fibers, stimulating the 1st order neuron

– info sent to the CNS via the posterior spinocerebellar tract and/or posterior column tract

– in response the gamma motor neurons adjusts the tension in a muscle spindle

• also have extrafusal muscle fibers which are innervated by alpha motor neurons (LMNs)

– response to a stretch reflex– produce normal contraction

Sensory neuron

(1st order neuron)

Sensory neuron (1st order neuron)

Sensory dendrites

(1st order neuron)

Sensory neuron

(1st order neuron)

– 2. tendon organs• located at the junction of a tendon and a

muscle• protect the tendon and muscles from damage

due to excessive tension• consists of a thin capsule of connective tissue

enclosing a few bundles of collagen– penetrated by sensory nerve endings that

intertwine among the collagen fibers

• information carried via anterior spinocerebellar tract and posterior column tract

Proprioceptive Sensation

– 3. joint receptors (joint kinesthetic receptors)• several types• located in and around the articular capsules of synovial

joints• free nerve endings and mechanoreceptors found –

detect pressure within the joint• also can find Pacinian corpuscles which detect the

speed of joint movement

Proprioceptive Sensation

Tactile SensationsCutaneous receptors -located in skin-dermis: pressure, temperature, touch (fine and crude) and pain-impulse sent to somatosensory areas of brain-touch receptors: Meissner’s (fingertips, lips, tongue, nipples, penis/clitoris) – for fine touch (1 st order neuron) - Merkel disks (epidermis/dermis) – fine touch, slowly adapting

-Root hair plexus (root of hair) - crude touch receptors-pressure receptors: Pacinian corpuscles – connective tissue capsule over the dendrites-temp receptors: free nerve endings that respond to cold OR warmth - pain

-also: Krause end bulbs, Ruffini endings (also for stretching, slowly adapting)

Pain• injured cells produce kinins – pain chemicals• white blood cells at site of injury produce prostaglandins

– prostaglandins produced upon injury can sensitize neurons to the kinins

• analgesia: relief from pain• drugs: aspirin, ibuprofen – block formation of prostaglandins

that stimulate the nociceptors • novocaine – block nerve impulses along pain nerves

– blocks opening of voltage-gated sodium channels

• morphine, opium & derivatives (codeine) – pain is felt but not perceived in brain (blocks morphine and opiate receptors in pain centers)

Pain pathways• First order neuron: nociceptor in

skin – travels via dorsal root into the posterior gray horn

• Second order neuron: starts in the in PGH– second order neuron crosses over

(decussates) and travels up lateral white column (spinothalmic tract) to thalamus

• Third order neuron: run from thalamus to primary somatosensory area for initial processing

• Nociceptors in gums and teeth travel via a different pathway– First order: nociceptors in gingiva and

tooth – synapses with 2nd order in the brain stem

– Second order: trigeminal nucleus within the pons – travels to thalamus via spinothalmic tract

– Third order: thalamus to primary somatosensory area

Taste-taste buds: salty, sweet, bitter and sour

-10,000 taste buds found on tongue, soft palate & larynx

-buds found associated with projections called papillae-taste bud opens at a taste pore bitter

salty

sour

taste papillae:1. foliate2. fungiform3. circumvallate4. filliform (texture)

Anatomy of Taste Buds

• taste bud = an oval body consisting of 50 receptor cells (taste cells) surrounded by supporting cells

• a single gustatory hair projects upward through the taste pore

• the gustatory hair bear receptors proteins for specific chemicals

• basal cells develop into new receptor cells every 10 days.

• taste cell synapse with the 1st order neurons that form the cranial nerves of taste

Physiology of Taste• receptor-ligand interaction – ligand is the chemical from the food and the

receptor is on the taste cell

• binding leads to a change in the graded receptor potential of the taste cell action potential if threshold is reached

• stimulates exocytosis of NTs from the taste cell

• NT binds to a first order neuron (axons make up cranial nerves VII, IX and X)

• pathway is distinct for different chemicals– e.g. salty foods – Na enters the gustatory cell via ligand-gated channels –

depolarization Action Potential• similar mechanism for sour foods – entrance of H+ ions which opens Na channels

– other tastants do NOT enter the cell but bind to the PM – bind to G protein coupled receptors and trigger the production of a second messenger which than causes a depolarization and action potential

• Complete adaptation in 1 to 5 minutes

• Thresholds for tastes vary among the 4 primary tastes– most sensitive to bitter (poisons)

– least sensitive to salty and sweet

Gustatory Pathway• gustatory fibers: axons of the 1st order neurons

• found in cranial nerves VII, IX and X– VII (facial) serves anterior 2/3 of tongue

– IX (glossopharyngeal) serves posterior 1/3 of tongue

– X (vagus) serves palate & epiglottis

• 1st order neurons run from taste bud to brain stem – in a specific nucleus called the nucleus of solitary tract

• 2nd order neurons terminate in thalamus

• 3rd order neurons extend from the thalamus to the insula (limbic system) and the primary gustatory area on parietal lobe of the cerebral cortex

– provides conscious perception of taste

• there are specific tracts that will carry specific tastes– e.g. salty tract, sweet tract

• taste aversion – because of the link between the hypothalamus and the limbic system – conscious and strong connection between taste and emotion

• taste can be affected by several factors– always taste better when you are hungry

– genetics

– previous tastes – orange juice after brushing your teeth

Medical application

• alopecia areata– auto-immune disease of the hair– initially appearing as a rounded bare patch about an

inch across– affects both men and women equally – 1/100

Americans– often experienced first in childhood– associated with several other conditions

• e.g. vitiligo• e.g. loss of taste

-olfactory cells - located within olfactory epithelium in the nasal cavity

-covers superior nasal cavity (superior nasal conchae) and cribriform plate-are modified neurons-neurons bear microvilli with receptor proteins for odor molecules

Olfaction

• Olfactory receptors– bipolar neurons with cilia or

olfactory hairs for chemical binding

• Supporting cells – columnar epithelium

• Basal cells = stem cells– replace receptors monthly

• Olfactory glands– produce mucus to dissolve

odorant chemicals

Olfactory Epithelium

Olfaction: Sense of Smell• Odorants bind to receptors located on the

receptor’s cilia– 1000 different types of olfactory receptor

neurons

– each receptor neuron can have 1000 different types of receptor proteins (responding to 1000 different chemicals)

– total of 10 million olfactory receptors

• Na+ channels open & depolarization occurs

• Action potential is triggered

• NTs released to bind onto 1st order neurons

• some odorants bind the olfactory receptor and trigger the activation of a G protein – second messenger production, opening of Na channels and depolarization

Olfactory Pathway

• has a very low threshold to trigger perception• axons from olfactory receptors form olfactory nerves that synapse with the 1st

order neurons in the olfactory bulb– inputs from similar olfactory neurons will travel to the same cells within the bulb

• axons of the 1st order neurons within the olfactory bulb form the olfactory tract of Cranial Nerve I

• 1st order axons eventually synapse on the primary olfactory area of temporal lobe – complicated pathway

– conscious awareness of smell begins– doesn’t pass through thalamus until AFTER it reaches primary

olfactory area• after processing by the primary olfactory area - other pathways lead to the

frontal lobe (Brodmann area 11) where identification of the odor occurs

Vision

Eye: tough outer covering - sclera (white, cornea) -middle choroid layer - vessels, melanin pigment (light absorption)

-front of eye it becomes the iris (aperture), -inner nerve layer – retina -sight is generated by the bending and focusing of light onto the retina - done by the lens (shape changes controlled by tiny ciliary muscles)

Major Processes of Image Formation

• Refraction of light– by cornea & lens – light rays must fall upon

the retina

• Accommodation of the lens– changing shape of lens so

that light is focused

• Constriction of the pupil– less light enters the eye

• Refraction = bending of light as it passes from one substance (air) into a 2nd substance with a different density(cornea)

Vision

• Anterior cavity (anterior to lens)– filled with aqueous humor

• produced by ciliary body• continually drained• replaced every 90 minutes

– 2 chambers• anterior chamber between cornea and iris• posterior chamber between iris and lens

• Posterior cavity (posterior to lens)– filled with vitreous body (jellylike)– formed once during embryonic life– floaters are debris in vitreous of older individuals

Accessory Structures of Eye• Eyelids or palpebrae

– protect & lubricate

– epidermis, dermis, CT, orbicularis oculi m., tarsal plate, tarsal glands & conjunctiva

• Tarsal glands– oily secretions keep lids

from sticking together

• Conjunctiva– palpebral & bulbar

– stops at corneal edge

– dilated BV--bloodshot

Lacrimal Apparatus

• About 1 ml of tears produced per day. Spread over eye by blinking. Contains bactericidal enzyme called lysozyme.

Tunics (Layers) of Eyeball

• Fibrous Tunic(outer layer)

• Vascular Tunic (middle layer)

• Nervous Tunic(inner layer)

CORNEA

• Transparent

• Helps focus light (refraction)– astigmatism

• 3 layers– nonkeratinized stratified squamous (outer)

– collagen fibers & fibroblasts

– simple squamous epithelium

• Nourished by tears & aqueous humor

Fibrous Tunic

SCLERA

• “White” of the eye

• Dense irregular connective tissue layer -- collagen & fibroblasts

• Provides shape & support

• Posteriorly pierced by Optic Nerve (CNII)

Vascular Tunic

• Choroid– pigmented epithelial cells (melanocytes) & blood vessels

– provides nutrients to retina via blood vessels

– black pigment in melanocytes absorb scattered light

•Ciliary body

–choroid extends to the front of the eye as ciliary muscles and processes – for controlling the shape of the lens

–ciliary processes

•folds on ciliary body

•secrete aqueous humor

–ciliary muscle

•smooth muscle that alters shape of lens

•attach to the ciliary processes

Aqueous Humor

• Continuously produced by ciliary body

• Flows from posterior chamberinto anterior through the pupil

• Scleral venous sinus

– canal of Schlemm

– opening in white of eyeat junction of cornea & sclera

– drainage of aqueous humor from eye to bloodstream

• Glaucoma– increased intraocular pressure that could produce blindness

– problem with drainage of aqueous humor

Vascular Tunic – The Lens• Focusing is done through

changing the shape of the lens

• Lens:

• Focuses light on fovea (center of the retina)

• Avascular

• Crystallin proteins arranged like layers in onion

• Clear capsule & perfectly transparent

• Lens held in place by suspensory ligaments which attach to the ciliary processes/muscles

• View a distant object – lens needs to be flattened-this is done by increasing the tension of the suspensory ligaments & by relaxing the ciliary muscles

• View a close object – lens needs to be round-ciliary muscle is contracted & decreases the tension on the suspensory ligaments on the lens - elastic lens thickens as the tension is removed from it

• Emmetropic eye (normal)

– can refract light from 20 ft away

• Myopia (nearsighted)– eyeball is too long

from front to back

– glasses concave

• Hypermetropic (farsighted)

– eyeball is too short

– glasses convex (coke-bottle)

• Astigmatism– corneal surface wavy

– parts of image out of focus

Near Point of Vision and Presbyopia

• Near point is the closest distance from the eye an object can be & still be in clear focus– 4 inches in a young adult– 8 inches in a 40 year old

• lens has become less elastic

– 31 inches in a 60 to 80 year old

• Reading glasses may be needed by age 40– presbyopia– glasses replace refraction previously provided by

increased curvature of the relaxed, youthful lens

Nervous Tunic Retina

• Posterior 3/4 of eyeball• Optic disc

– optic nerve exiting back of eyeball

– attachment of retina to optic nerve - optic disc (blind spot)

• central depression in retina - fovea centralis

• Detached retina– trauma (boxing)

• fluid between layers

• distortion or blindness

View with Ophthalmoscope

Photoreceptors

-rod and cone cells

-visual pigment: rhodopsin-rhodopsin = opsin and retinal-visual pigment is folded into “discs” found in the outer segment of the photoreceptor-shape of the outer segment resulted in their name – rod & cone-inner segment - cell body-synaptic endings for the release of neurotransmitter

Rods and Cones• Rods----rod shaped

– shades of gray in dim light, peripheral vision – 120 million rod cells– discriminates shapes & movements– distributed along periphery of the retina

• Cones---cone shaped– sharp, color vision– 6 million– 3 types: blue, red and yellow/green colour (differences in

opsin structure)– found in the fovea of macula lutea (fovea centralis)

• densely packed region of cones• at exact visual axis of eye• sharpest resolution or acuity• sharpest colour vision

Retinal cells• Pigmented epithelium

– non-visual portion– absorbs stray light & helps

keep image clear• 3 layers of neurons

(outgrowth of brain)– photoreceptor layer– bipolar neuron layer– ganglion neuron layer –

axons form cranial nerve II• 2 other cell types modify the

signal– horizontal cells – inhibits

transmission to other bipolars– amacrine cells – change in

illumination-reflective coating in retina of nocturnal animals = tapetum lucidum

-reflects light back through the retina – increases vision at night-contributes to “red eye” effect in humans

•photopigment – rhodopsin

–undergoes structural changes when it absorbs light

–made of opsin and retinal

–retinal – vitamin A derivative with two forms: cis-retinal and trans-retinal

–opsin – glycoprotein responsible for the absorption of light wavelengths

•e.g. red cones – opsin for the absorption of red wavelengths

–in dark –cis-retinal fits snugly with opsin

–upon light – the cis-retinal conformation straightens out into trans-retinal = isomerization

–results in the separation of trans-retinal from opsin – the opsin is said to be bleached

–opsin now acts as an enzyme which acts to inhibit the molecular machinery underlying vision

–the trans retinal eventually gets converted back into cis-retinal by retinal isomerase

–cis-retinal is free to rebind with opsin

• vitamin A deficiency results in lower formation of rhodopsin = night blindness

• loss of one cone type with one opsin type = color blindness

Formation of Receptor Potentials• In darkness

– Na channels open – Na ions flow through ligand-gated Na channels

– the photoreceptor becomes depolarized to threshold release of NT glutamate

– glutamate binds its target – bipolar neuron

• IPSP results at the bipolar cell

• prevents transmission of signal through the retina to the optic nerve

– receptors are always partially depolarized in the dark leading to a continuous release of inhibitory neurotransmitter onto bipolar cells

Formation of Receptor Potentials• In light

– isomerization of retinal from cis to trans

– opsin becomes an enzyme that activates a membrane protein called transducin

– transducin activates an enzyme called cGMP Phosphodiesterase

– phosphodiesterase breaks down of a compound called cGMP

– decrease of cGMP closes the Na+ channels in the outer segment

– results in a hyperpolarized receptor potential (-70mV)

– release of neurotransmitters is stopped

– bipolar cells become excited and a nerve impulse will travel towards the brain = image

Dark vs. Light

• No activated rhodopsin• No activation of transducin• No activation of cGMP

phosphodiesterase• Increased levels of cGMP within

the photoreceptor• Opening of cGMP-gated ion

channels (sodium)• Action potential and glutamate

release• Inhibition of bipolar cell AP and

ganglion cell AP• PC “ON”, 1st, 2nd, 3rd order

neurons “OFF”• NO IMAGE FORMATION

• Activated rhodopsin – bleached opsin and trans-retinal

• Activation of transducin• Activation of cGMP

phosphodiesterase• Decreased levels of cGMP

within the photoreceptor• Closing of cGMP-gated ion

channels (sodium)• NO Action potential and

glutamate release• Action potentials by bipolar

cell AP and ganglion cell• PC “OFF”, 1st 2nd, 3rd order

neurons “ON”• IMAGE FORMATION

Light and Dark Adaptation

• Light adaptation– adjustments when emerge from the dark into the light

– decreases its sensitivity

– increases the bleaching of rhodopsin

– decreases light sensitivity

• Dark adaptation– adjustments when enter the dark from a bright situation

– light sensitivity increases as photopigments regenerate• during first 8 minutes of dark adaptation, only cone pigments are

regenerated, so threshold burst of light is seen as color

• after sufficient time, sensitivity will increase so that a flash of a single photon of light will be seen as gray-white

Visual Pathways

• visual field of each eye is divided into two halves: nasal half (central half) and a temporal half (peripheral half)

• bipolar cells are the first order neurons• ganglion cells are the second order neurons – axons form the optic nerve and end in

the thalamus• the axons of the optic nerve enter the optic chiasma

– most signals cross over at this structure– signals from the temporal half of the retina do not cross over

• after passing the chiasma- the axons are now part of the optic tract which enters the brain and ends at the lateral geniculate nucleus of the thalamus

• the axons coming from the temporal half of the retina do NOT cross over in the chiasma – continue to the thalamus portion on the same side of the eye receiving the info– BUT the nasal axons cross and continue to the opposite thalamus

• third order neurons – thalamus to primary visual cortex in occipital lobe• information is processed by three areas of the cerebral cortex

– one for color discrimination– one for object shape– one for movement, location and orientation

Visual Pathways

-PCs “temporal” retina-first order - bipolar cells-second order – ganglion cells, endin thalamus NO CROSSING OVER-third order – thalamus to occipital lobe(right)

-PCs “nasal” retina-first order - bipolar cells-second order – ganglion cells, endin thalamus CROSSING OVER-third order – thalamus to occipital lobe(left)

Hearing & Equilibrium

-outer ear: pinna - cartilage and skin -for collection of sound waves-middle ear: tympanic membrane and 3 ossicles (malleus, incus, stapes)

-transmission of sound waves to inner ear-inner ear: cochlea (hearing), saccule, utricle & three semicircular canals (balance)

External Ear• Function = collect sounds• Structures

– auricle or pinna• elastic cartilage covered with

skin

– external auditory canal • curved 1” tube of cartilage & bone

leading into temporal bone• ceruminous glands produce cerumen

= ear wax

– tympanic membrane or eardrum• epidermis, collagen & elastic fibers, simple cuboidal epith.

• Perforated eardrum (hole is present) – at time of injury (pain, ringing, hearing loss, dizziness)– caused by explosion, scuba diving, or ear infection

Middle Ear Cavity• Air filled cavity in the temporal bone

• Separated from external ear by eardrum and from internal ear by oval & round window

• 3 ear ossicles connected by synovial joints– malleus attached to eardrum, incus &

stapes attached by foot plate to membrane of oval window

– stapedius and tensor tympani muscles attach to ossicles

• Auditory tube leads to nasopharynx– helps to equalize pressure on both sides of

eardrum

Inner Ea

• Bony labyrinth = set of tubelike cavities in temporal bone semicircular canals, vestibule & cochlea lined with periosteum & filled with perilymph– surrounds & protects Membranous Labyrinth

• Membranous labyrinth = set of membranous tubes containing sensory receptors for hearing & balance and filled with endolymph– utricle, saccule, ampulla, 3 semicircular ducts & cochlea

Cochlear Anatomy

• 3 fluid filled channels found within the cochlea– scala vestibuli, scala tympani and cochlear duct

• Vibration of the stapes upon the oval window sends vibrations into the fluid of the scala vestibuli

• Fluid vibration dissipated at round window which bulges

• Partitions that separate the channels are Y shaped– vestibular membrane above &

basilar membrane below form the central fluid filled chamber (cochlear duct)

• within the cochlear duct – organ of hearing = Organ of Corti

• hair cells with stereocilia (microvilli ) project from the basilar membrane and are covered

with a tectorial membrane

• endolymph flowing through the cochlear duct bends the hair cells, results in a receptor

potentials – inner hair cells transmit these potentials to 1st order sensory neurons whose cell body is in spiral ganglion

Physiology of Hearing• sound waves are alternating high and low pressure regions that travel through air or through another medium like a

fluid• the frequency of sound = number of waves that pass a point per time period

– higher the frequency – the higher the pitch of the sound• 1) Auricle collects sound waves• 2) Sound waves hit the tympanic membrane = vibration

– slow vibration in response to low-pitched sounds– rapid vibration in response to high-pitched sounds

• 3) Ossicles vibrate since malleus attached to eardrum• 4) Attachment of the stapes to the oval window within the cochlea transfers these vibrations into the fluid of the inner ear• 5) Movement of the oval window leads to fluctuations in fluid pressure

6) Pressure changes in the scala vestibuli and tympani7) The pressure changes in these scala push against the cochlear duct 8) Causes the basilar membrane to vibrate back and forth which bends

the hair cells against the tectorial membrane• Microvilli of the hair cells are bent producing

receptor potentials-bending opens mechanically-gated Na channels

• Cochlear branch of CN VIII sends signals to cochlear and superior olivary nuclei within medulla oblongata

• Fibers ascend to the– thalamus– primary auditory cortex in the

temporal lobe (areas 41 & 42)

Static equilibrium: Saccule & Utricle

• Thickened regions called macula within the saccule & utricle • two macula per inner ear – perpendicular to one another• Cell types in the macula region

– hair cells with microvilli called stereocilia– supporting cells that secrete gelatinous layer

• Gelatinous otolithic membrane contains calcium carbonate crystals called otoliths that move when you tip your head

• head movement and otolith movement bends the hair cells and results in receptor potentials via mechanically-gated Na channels

• bending of stereocilia in one direction generates an AP, bending in the opposite results in repolarization and loss of AP

• depolarization -> faster NT release and faster nerve impulses through VIII• repolarization -> slower NT release and slower nerve impulse through VIII• hair cells synapse with first order neurons in the vestibular branch of cranial nerve VIII – end in

medulla (4 vestibular nuclei within the MO)• second order = MO to thalamus

• pathway is part of the vestibulospinal tract• third order = thalamus to temporal lobe

(first order neurons)

Dynamic equilibrium: Semicircular Ducts

• role is to keep eyes still while moving, sense direction of movement

• Small elevation within the ampulla of each of three semicircular ducts– anterior, posterior & horizontal ducts detect different movements

• Hair cells covered with cupula of gelatinous material

• When you move, fluid in canal bends cupula stimulating hair cells that release NTs

• First order neurons synapse with hair cells – make up part of CNVIII (vestibular branch)

• first order neurons terminate in vestibular nuclei of MO

• these nuclei also receive inputs from cerebellum

• rest of the pathway can be quite complex

• semicircular canals also connect to:– cranial nerves that control eye and head and neck movements (III,IV,VI & XI)– vestibulospinal tract from the saccule and utricle - adjusts postural skeletal muscle contractions in response to

head movements

– motor cortex can adjust its signals to maintain balance