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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

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the pns afferent nervous system
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
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
  • 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
  • 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
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 pathways7
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
sensory pathways8
Sensory Pathways
  • 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 pathways9
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
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:
  • microscopic features:
    • free nerve endings: bare dendrites associated with pain, heat, tickle, itch and some touch
    • encapsulated nerve endings: dendrites enclosed in a connective tissue capsule - touch
        • e.g. Pacinian corpuscle
    • separate cells: individual receptors that synapse with first-order afferent neurons’
    • e.g. gustatory cells (taste)
  • receptor location:
    • exteroceptors: located at or near the body surface, responds to information coming in from the environment (taste, touch, smell, vision, pressure, heat and pain)
    • interoceptors: located in blood vessels, visceral organs and the nervous system; provide information about internal environment
    • 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


-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


proprioceptive sensation
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 sensation14
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)
  • 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 requires dissolving of substances

-taste buds: salty, sweet, bitter and sour

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

-found associated with projections called papillae

-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:
  • foliate
  • fungiform
  • circumvallate
  • filliform (texture)
the tongue papillae
The Tongue & Papillae








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


  • -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


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

  • Transparent
  • Helps focus light (refraction)
    • astigmatism
  • 3 layers
    • nonkeratinized stratified squamous (outer)
    • collagen fibers & fibroblasts
    • simple squamous epithelium
  • Nourished by tears & aqueous humor
  • “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
    • ciliary muscle
      • smooth muscle that alters shape of lens
      • attach to the ciliary processes
  • -Suspensory ligaments attach lens to ciliary process
  • -Ciliary muscle controls tension on ligaments & lens
  • Lens:
  • 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
  • Focuses light on fovea (center of the retina)

Vascular Tunic

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
  • Iris
    • 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

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 (coke-bottle)
  • 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
    • trauma (boxing)
      • fluid between layers
      • distortion or blindness

View with Ophthalmoscope



-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 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
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 cis-retinal 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

      • In the dark - Na channels in the outer segment are held open by cGMP
      • Na influx causes depolarization that triggers continual release of glutamate neurotransmitter in rods
      • Glutamate hyperpolarizes (inhibits) bipolar cells.
      • Inner segment has pumps that continuously pump Na out and K in, K diffuses out
      • In the light – photons pass through retinal layers and reaches rods
      • Cis-retinal is tightly attached to opsin
      • Cis-retinal absorbs light and shifts to trans-retinal form (isomerization)
      • Trans-retinal separates from opsin becoming colorless (bleaching)
      • Opsin activates transducin (a G protein) in the cell membrane
      • Transducin activates cGMP Phosphodiesterase
      • This enzyme breaks down cGMP – decrease in cGMP levels closes gated Na channels
      • This decreases Na influx into the rod while pump continues – more Na+ out than Na+ flowing in
      • Rod becomes hyperpolarized and ceases glutamate release
      • Bipolar cells are not inhibited and release neurotransmitter at synapse with ganglion cells resulting in action potential being sent along optic nerve
      • Retinal isomerase shifts trans-retinal back to cis-retinal form
      • Cis-retinal rebinds with opsin (regeneration)
      • Transducin is deactivated and Na channels are reopened
      • Rods regenerate at about same rate as bleaching occurs in daylight.  Cones regenerate very fast.
dark vs light
Dark vs. Light
  • 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
  • PC OFF – 1st, 2nd, 3rd order neurons ON
  • 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

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 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
    • 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)
  • 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 (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)

-PCs “temporal” retina

-first order - bipolar cells

-second order – ganglion cells, end

in thalamus NO CROSSING OVER

-third order – thalamus to occipital lobe


Temporal half (right eye)

-PCs “nasal” retina

-first order - bipolar cells

-second order – ganglion cells, end

in thalamus CROSSING OVER

-third order – thalamus to occipital lobe



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
External Ear
  • Function = collect sounds
  • Structures
    • auricle or pinna
      • elastic cartilage covered with


    • 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
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
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
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 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
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
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
  • 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
Dynamic equilibrium: Semicircular Ducts
  • Fibers from vestibulocochlear nerve (VIII) end in vestibular nuclei and the cerebellum
  • Fibers from these areas connect to:
    • 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
  • 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
homeostatic imbalances in vision and hearing
Homeostatic Imbalances in Vision and Hearing
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