Transcript No Slide Title

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 a
1) specialized ending of an afferent neuron
2) a separate cells closely associated with an afferent
neurons
-can classify a sensory receptor based on:
1. its microscopic features
2. its location
3. the type of stimulus that activates it
1.
2.
3.
microscopic features:
a.
free nerve endings: bare dendrites associated with pain, heat, tickle, itch and some touch
b. encapsulated nerve endings: dendrites enclosed in a connective tissue capsule - touch
e.g. Pacinian corpuscle
c.
separate cells: individual receptors that synapse with first-order afferent neurons
e.g. gustatory cells (taste)
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
type of stimulus:
1. Chemoreceptors
2. Mechanoreceptors
3. Nociceptors/pain receptors
4. Thermoreceptors
5. Photoreceptors
6. 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
Sensory
neuron
(1st order
neuron)
Sensory
neuron
(1st order
neuron)
• also have extrafusal muscle fibers which
are innervated by alpha motor neurons
(LMNs)
– response to a stretch reflex
– produce normal contraction
Proprioceptive Sensation
– 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
Sensory
dendrites
(1st order
neuron)
Sensory
neuron
(1st order
neuron)
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
Tactile Sensations
Cutaneous 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 (1st 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
salty
-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
sour
taste papillae:
1. foliate
2. fungiform
3. circumvallate
4. 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
Olfaction
-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
Olfactory Epithelium
• 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
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)
Fibrous Tunic
CORNEA
• Transparent
• Helps focus light (refraction)
– astigmatism
• 3 layers
– nonkeratinized stratified squamous (outer)
– collagen fibers & fibroblasts
– simple squamous epithelium
• Nourished by tears & aqueous humor
SCLERA
• “White” of the eye
• Dense irregular connective tissue layer
-- collagen & fibroblasts
• Provides shape & support
• Posteriorly pierced by Optic Nerve
(CNII)
Vascular Tunic
•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
• Choroid
– pigmented epithelial cells (melanocytes) & blood vessels
– provides nutrients to retina via blood vessels
– black pigment in melanocytes absorb scattered light
Aqueous Humor
• Continuously produced
by ciliary body
• Flows from posterior chamber
into anterior through the pupil
• Scleral venous sinus
– canal of Schlemm
– opening in white of eye
at 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 (cokebottle)
•
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
View with Ophthalmoscope
– trauma (boxing)
• fluid between layers
• distortion or blindness
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: cisretinal 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 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
Visual Pathways
-PCs “temporal” retina
-first order - bipolar cells
-second order – ganglion cells, end
in thalamus NO CROSSING OVER
-third order – thalamus to occipital lobe
(right)
-PCs “nasal” retina
-first order - bipolar cells
-second order – ganglion cells, end
in 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
–
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1) Auricle collects sound waves
2) Sound waves hit the tympanic membrane = vibration
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higher the frequency – the higher the pitch of the sound
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 tympani
7) 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
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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
(first order neurons)
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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
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
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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:
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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