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
• although you can become aware of pain
• this information is critical form determining the appropriate efferent
output to maintain homeostasis
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??)
•
-nerve impulses sent via ascending tracts in spinal cord to the
brain
• 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 Na ions – depolarization – called a graded
receptor potential
– therefore the sensory receptor converts (transduces) the energy of
the stimulus into a graded potential
• 3. generation of the nerve impulse
– increase in graded receptor potential past threshold -> Action
Potential
– AP propagates toward the CNS
• 4. integration of the sensory input
– receipt of sensory information by a particular region in the CNS
– integration of sensation and perception
Sensory Pathways
• these pathways consist of thousands of sets of
neurons – grouped into threes
– 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
• these neurons decussate (cross over) within the
thalamus
– 3. third order neurons – conduct information from
the thalamus to the primary somatosensory areas
within the cerebral cortex
• for integration
Sensory Pathways
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 sensations
• two tracts of white matter: posterior
column and the medial lemniscus
• first order neurons from sensory
receptors in the trunk and limbs form
the posterior columns in the spinal
cord
• synapse with second order neurons in
the medulla oblongata
• these then cross to the opposite side of
the medulla and enter the medial
lemniscus in the thalamus – synapse
with the third order neurons that travel
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 receive
impulses from receptors in the
neck, trunk or limbs
• receptors end in dorsal root
ganglion
• synapse with the 1st order neurons
are located in the dorsal root
ganglion
• synapse with second order neurons
in the posterior gray horn
• second order neurons than cross to
the opposite side of the SC and
pass to the primary somatosensory
area in either the:
• second order neurons pass through
the brain stem as two possible
tracts:
– lateral spinothalmic tract:
pain and temperature
– anterior spinothalmic tract:
information for tickle, itch,
crude touch and pressure
Sensory
Pathways
Sensory Pathways
• two 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
• posterior spinocerebellar routes are degraded upon advanced syphillis
– severe uncoordination
• first order neurons: muscle spindles and tendon organs
• second order neurons: cell bodies in dorsal gray horn via thalamus to
the cuneate nucleus of basal ganglia
• third order neurons: thalamus to cerebellum (no decussation)
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.
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)
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. 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
-located in the inner ear – position of head
-”hair cells” – position relative to the ground and movement
-allow us to estimate weight and to determine how much muscular effort is needed for
a task
-high concentration in postural muscles (body position), tendons (muscle contraction)
-Patellar reflex:
muscle stretch, proprioceptor
fires impulse to spinal cord,
reflex arc results, muscle fiber
response
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
– very plentiful in muscles that produce
very fine movements – fingers, eyes
– stretching of the muscle stretches the
intrafusal fibers, stimulating the
sensory neurons – info to the CNS
– IFMs also receive incoming
information from gamma motor
neurons – end near the IFMs and
adjust the tension in a muscle spindle
according to the CNS
• also have extrafusal muscle fibers
which are innervated by alpha motor
neurons
– response to a stretch reflex
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
– 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
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
• analgesia: relief from pain
• drugs: aspirin, ibuprofen – block formation of
prostaglandins that stimulate the nociceptors
• novocaine – block nerve impulses along pain
nerves
• morphine, opium & derivatives (codeine) – pain is
felt but not perceived in brain (blocks morphine
and opiate receptors in pain centers)
Taste
-Taste requires dissolving of substances
salty
-taste buds: salty, sweet, bitter and sour
-10,000 taste buds found on tongue, soft palate & larynx
-found associated with projections called papillae
bitter
sour
-open at a taste pore
-taste cells are associated with support cells and connect with sensory
nerve fibers
-tips of taste cells are microvilli - receptors proteins for specific chemicals
Anatomy of Taste
Buds
• An oval body consisting
of 50 receptor cells
surrounded by
supporting cells
• A single gustatory hair
projects upward through
the taste pore
• Basal cells develop into
new receptor cells every
10 days.
taste buds:
1. foliate
2. fungiform
3. circumvallate
4. filliform (texture)
The Tongue & Papillae
foliate
fungiform
filiform
fungiform
filiform
circumvallate
Physiology of Taste
• receptor-ligand interaction – ligand is the chemical from the food and the
receptor is the taste cell
• binding leads to a change in the receptor potential – action potential
• stimulates exocytosis from the taste cell – binds to a first order neuron
• pathway is distinct for different chemicals
– e.g. salty foods – Na enters the gustatory cell via ligand-gated channels –
depolarization – direct method
• depolarization opens calcium channels – exocytosis
• 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 messanger which than
causes a depolarization and action potential – indirect methods
• 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 found in cranial nerves
– VII (facial) serves anterior 2/3 of tongue
– IX (glossopharyngeal) serves posterior 1/3 of tongue
– X (vagus) serves palate & epiglottis
• Signals travel to thalamus or limbic system &
hypothalamus
• Taste fibers extend from the thalamus to the primary
gustatory area on parietal lobe of the cerebral cortex
– providing conscious perception of taste
• taste aversion – because of the link between the
hypothalmus and the limbic system – conscious and strong
connection between taste and emotion
Smell
-olfactory cells - located within olfactory epithelium in the nasal cavity
-Covers superior nasal cavity (superior nasal conchae) and cribriform plate
-are modified neurons
-end in microvilli with receptor proteins for odor molecules
-each olfactory cell is specific for one odor molecule - specific neuron types
-olfactory nerves make connections with the limbic system (emotions and memory)
• Olfactory receptors
– bipolar neurons with cilia or
olfactory hairs
• Supporting cells
– columnar epithelium
• Basal cells = stem cells
– replace receptors monthly
• Olfactory glands
– produce mucus
• Both epithelium & glands
innervated by cranial nerve
VII.
Olfaction: Sense of Smell
•
•
•
•
•
Odorants bind to receptors
Na+ channels open
Depolarization occurs
Nerve impulse is triggered
some odors bind the 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 the olfactory nerves
(Cranial nerve I) that synapse in the olfactory bulb
– pass through 40 foramina in cribriform plate
• neurons within the olfactory bulb form the olfactory tract
that synapses on the primary olfactory area of temporal
lobe
– conscious awareness of smell begins
• Other pathways lead to the frontal lobe (Brodmann area
11) where identification of the odor occurs
• hyperosmia – keener sense of smell then others
– seen in women (time of ovulation)
– opposite is hyposmia –reduction in the sense of smell
Adaptation & Odor Thresholds
• Adaptation = decreasing sensitivity
• Olfactory adaptation is rapid
– 50% in 1 second
– complete in 1 minute
• Low threshold
– only a few molecules need to be present
e.g. methyl mercaptan added to natural gas as warning
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)
• 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
•
Choroid
– pigmented epithelial cells
(melanocytes) & blood vessels
– provides nutrients to retina
– 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
Lens:
–ciliary muscle
•
Avascular
•
Crystallin proteins arranged like layers in onion
•smooth muscle that alters shape of
•
Clear capsule & perfectly transparent
lens
•
Lens held in place by suspensory ligaments which attach to the
•attach to the ciliary processes
ciliary processes
-Suspensory ligaments attach lens to ciliary process
-Ciliary muscle controls tension on ligaments & lens
•
Focuses light on fovea (center of the retina)
Vascular Tunic
Aqueous Humor
•
•
•
•
•
–
Iris
–
–
–
–
–
–
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
is a coloured extension off the ciliary processes
Constrictor pupillae muscles (circular muscles)
are innervated by parasympathetic fibers while
Dilator pupillae muscles (radial muscles)
are innervated by sympathetic fibers.
Response varies with different levels of light
–
increased intraocular pressure
that could produce blindness
problem with drainage of
aqueous humor
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
Definition of Refraction
• Bending of light as it passes from one substance (air)
into a 2nd substance with a different density(cornea)
• In the eye, light is refracted by the anterior & posterior
surfaces of the cornea and the lens
Refraction by the Cornea & Lens
• Image focused on retina is inverted &
reversed from left to right
• Brain learns to work with that
information
• 75% of Refraction is done by
cornea -- rest is done by the lens
• Light rays from > 20’ are nearly
parallel and only need to be bent
enough to focus on retina
• Light rays from < 6’ are more
divergent & need more refraction
– extra process needed to get additional
bending of light is called accommodation
•
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
Accommodation & the Lens
• Convex lens refracts light rays towards each
other
• Lens of eye is convex on both surfaces
• View a distant object
– lens is nearly flat by pulling of suspensory ligaments
• View a close object
– ciliary muscle is contracted & decreases the pull of
the suspensory ligaments on the lens
– elastic lens thickens as the tension is removed from it
– increase in curvature of lens is called accommodation
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
-rod cells: black and white, bright and dark
-cone cells: color vision
-visual pigment: opsin and retinal
-visual pigment is folded into “discs” = outer
segment of the photoreceptor
-shape of the outer segment resulted in their name
-inner segment - cell body
-synaptic endings
Rods and Cones
• Rods----rod shaped
–
–
–
–
shades of gray in dim light
120 million rod cells
discriminates shapes & movements
distributed along periphery
• Cones---cone shaped
–
–
–
–
sharp, color vision
6 million
3 types: blue, red and yellow/green colour (differences in opsin structure)
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
• 2 other cell types (modify
the signal)
– horizontal cells – inhibits
transmission to other bipolars
– amacrine cells
•photopigment – rhodopsin in rods, photopsin in
cones
–undergoes structural changes when it absorbs light
–opsin – glycoprotein
•responsible for the absorption of light wavelengths
•e.g. red cones – opsin for the absorption of red
wavelengths
•loss of one cone type with one opsin type =
color blindness
•retinal – vitamin A derivative
–in darkness –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 colourless = bleaching
–opsin now acts as an enzyme which acts on
the molecular machinery underlying vision –
inhibits this machine
–the trans retinal gets converted back into cisretinal by retinal isomerase
–cis-retinal is free to rebind with opsin
–vitamin A deficiency results in lower formation of
rhodopsin = night blindness
Formation of Receptor Potentials
•
In darkness
– Na channels open – Na ions flow through Na
ligand-gated channels that bind cGMP
– the photoreceptor becomes depolarized –
release of NT which then binds its target –
bipolar neurons
• glutamate??
• IPSP results at the post-synaptic neuron (bipolar
cell)
• prevents transmission of signal from 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
•
In light
– isomerization of retinal from cis to trans
– this activates enzymes that breakdown cGMP
– closing of Na+ channels producing a
hyperpolarized receptor potential (-70mV)
– release of inhibitory neurotransmitter is stopped
– bipolar cells become excited and a nerve
impulse will travel towards the brain = image
Photochemistry mechanism
1.In the dark - Na channels in the outer segment are held open by cGMP
2.Na influx causes depolarization that triggers continual release of glutamate
neurotransmitter in rods
3.Glutamate hyperpolarizes (inhibits) bipolar cells.
4.Inner segment has pumps that continuously pump Na out and K in, K diffuses out
5.In the light – photons pass through retinal layers and reaches rods
6.Cis-retinal is tightly attached to opsin
7.Cis-retinal absorbs light and shifts to trans-retinal form (isomerization)
8.Trans-retinal separates from opsin becoming colorless (bleaching)
9.Opsin activates transducin (a G protein) in the cell membrane
10.Transducin activates cGMP Phosphodiesterase
11.This enzyme breaks down cGMP – decrease in cGMP levels closes gated Na
channels
12.This decreases Na influx into the rod while pump continues – more Na+ out than
Na+ flowing in
13.Rod becomes hyperpolarized and ceases glutamate release
14.Bipolar cells are not inhibited and release neurotransmitter at synapse with ganglion
cells resulting in action potential being sent along optic nerve
15.Retinal isomerase shifts trans-retinal back to cis-retinal form
16.Cis-retinal rebinds with opsin (regeneration)
17.Transducin is deactivated and Na channels are reopened
18.Rods regenerate at about same rate as bleaching occurs in daylight. Cones
regenerate very fast.
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
• NO IMAGE FORMATION
• PC ON – 1st, 2nd, 3rd order
neurons OFF
• 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 ON-Center
bipolar cells and ganglion cells
• IMAGE FORMATION
• PC OFF – 1st, 2nd, 3rd order
neurons ON
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
Retinal Processes of Image Formation
• bipolar cells: provide 30% of input to
ganglion cells
– rod bipolar cells – only one type
– cone bipolar cells – ten forms classified as
ON-center and OFF-center
– ON responds to decreased glutamate upon
light by depolarizing (action potential =
eventual image)
• responds in the dark to increased glutamate by
hyperpolarizing (no action potential = no
image)
– OFF responds to decreased glutamate upon
light by hyperpolarization (no action potential)
• responds in dark to increased glutamate by
depolarizing (action potential)
• between 6 to 600 rods synapse with a single
bipolar cells = convergence
– increases the sensitivity of rod vision – but
slightly blurs the image
• usually only one cone synapses with a single
bipolar cell – less sensitive but sharper vision
Retinal Processes of Image Formation
•
horizontal cells: inhibit the transmission of the visual
signal to bipolar cells lateral to the targeted one
– found in the outer plexiform/synaptic layer
– concentrates the stimulation to a specific area of the
retina - more contrast to the image and increases spatial
resolution
– three types – H1, H2, H3
– H2 converges rods
– cones converge on all three types – cone-specific??
– light – photoreceptor hyperpolarization – reduction in
glutamate release – hyperpolarization of bipolar cells
and horizontal cells
– inhibited horizontal cells decrease their release of
GABA – reduction in GABA allows depolarization of
photoreceptor (feedback)
•
amacrine cells: provide 70% of input to ganglion
cells
–
–
–
–
–
–
other 30% comes from bipolar-ganglion synapses
regulate bipolar to ganglion transmission
40 different types – most with no axons
laterally gather BP cell input
most are inhibitory to transmission
help supplement horizontal cell function
Visual Pathways
•
•
•
visual field of each eye is divided into two halves: nasal half (central half) and a
temporal half (peripheral half)
ganglion cells synapse with the neurons of the optic nerve
the axons of the optic nerve enter the optic chiasma
–
–
•
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
–
–
•
some axons cross over within this structure (signals from the same side of the retina)
but some axons are processed by the same side (signals from the temporal half of the retina are
processed in the same side of the brain)
the axons coming from the temporal half of the retina (i.e. nasal side of visual field) 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 (detecting temporal visual field) cross and continue to the opposite
thalamus
information is processed by three areas of the cerebral cortex
– one for color discrimination
– one for object shape
– one for movement, location and orientation
Nasal half (right eye)
Temporal half (right eye)
-PCs “temporal” retina
-PCs “nasal” retina
-first order - bipolar cells
-first order - bipolar cells
-second order – ganglion cells, end
-second order – ganglion cells, end
in thalamus NO CROSSING OVER in thalamus CROSSING OVER
-third order – thalamus to occipital lobe -third order – thalamus to occipital lobe
(right)
(leftt)
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
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)
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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
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Physiology of Hearing
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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 = first order neurons
Fibers ascend from MO to the thalamus = second order
third order neurons travel from thalamus to
primary auditory cortex in the
temporal lobe (areas 41 & 42)
Physiology of Equilibrium (Balance)
• Static equilibrium
– maintain the position of the body (head) relative to the force of gravity
– macula receptors within saccule & utricle
• Dynamic equilibrium
– maintain body position (head) during
sudden movement of any type--rotation,
deceleration or acceleration
– crista receptors within ampulla
of semicircular ducts
Static equilibrium: Saccule & Utricle
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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
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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
hair cells synapse with first order
neurons in the vestibular branch of
cranial nerve VIII – end in medulla
second order = MO to thalamus
third order
Dynamic equilibrium: Semicircular Ducts
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Small elevation within the
ampulla of each of three
semicircular ducts
– anterior, posterior & horizontal
ducts detect different
movements
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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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Fibers from vestibulocochlear nerve
(VIII) end in vestibular nuclei and the
cerebellum
Fibers from these areas connect to:
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cranial nerves that control eye and head
and neck movements (III,IV,VI & XI)
vestibulospinal tract that adjusts postural
skeletal muscle contractions in response
to head movements
motor cortex can adjust its signals to
maintain balance
Homeostatic Imbalances in Vision
and Hearing
• page 610
• http://en.wikipedia.org/wiki/Otitis_media
• http://en.wikipedia.org/wiki/Menieres_disea
se