Sensory%20Function%20of%20the%20Nervous%20System

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Chapter 31

• Sensory Function of the Nervous System

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Contents

• Sensory receptors and sensory organs
• Pain
• The visual system
• The Auditory Systems

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Section 1Sensory Receptor and Sensory Organ
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Part 1 Sensory Receptors

• specialized nerve cells that transduce energy
  into neural signals
• mode specific
• Law of Specific Nerve Energies(????????)
  sensory messages are carried on separate channels
  to different areas of the brain
• detect a small range of energy levels
• Eye 400-700 nM
• Ear 20-20,000 Hz
• Taste buds specific chemicals

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Spectrum of the Electromagnetic Wave
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Somatic Sensory Nerve Endings
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Free Nerve Endings

• dendrites interspersed among other cells/tissues
• pain, temperature, touch

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Encapsulated Nerve Endings

• dendrites with special supporting structures
• mechanoreceptors and proprioceptors

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Classification of Receptors Location

• Exteroceptors
• Located on the body surface or specialized to
  detect external stimuli
• Pressure, pain, temp, touch, etc.
• Visceroceptor
• within internal organs, detect internal stimuli
• Blood pressure, pain, fullness.
• Proprioceptors
• Limb and body position and movement.
• in the joints and muscles
• in the vestibular structures and the semicircular
  canals of the inner ear.

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Classification of Receptors Modalities

• Mechanoceptive
• Detects stimuli which mechanically deform the
  receptor
• Pressure, vibration, touch, sound.
• Thermoceptive
• Detects changes in temperature
• hot/cold
• Nociceptive (pain)
• Detects damage to the structures
• Photoreceptors
• Detect light vision,
• retinal of the eye
• Chemoceptive
• Detect chemical stimuli
• CO2 and O2 in the blood, glucose, smell, taste

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Classification of Receptors Complexity

• Simple receptors
• Usually a single modified dendrite
• General sense
• Touch, pressure, pain, vibration, temperature
• Complexity
• High modified dendrites, organized into complex
  structures
• ear, eye.
• Special senses
• Vision, hearing, smell, taste

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Which receptor?
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Sequence of Events in a Receptor
Basic Function
Stimulus
Amplification
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Part 2 Properties of the Receptors

• Adequate Stimulus
• Transduction
• Adaptation
• Encoding

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1. Adequate Stimulus

• The type of stimulus the receptor is highly
  sensitive
• Receptor specially designed for one kind of
  stimulus
• The lowest threshold
• Insensitive to other stimulation

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2. Transduction

• A process by which an environmental stimulus
  becomes encoded as a sequence of nerve impulses
  in an afferent nerve fiber
• Transduce sensory energy into neural
  (bioelectrical) energy
• Receptor potentials Changes in the transmembrane
  potential of a receptor caused by the stimulus.
• Generator Potential A receptor potential that is
  strong enough (reaches threshold) to generate an
  action potential
• The stronger the sitmulus (above threshold) the
  more APs are fired over a given time period
• translated by the CNS as a strong sensation

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Receptor Potential and Generator Potential
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3. Adaptation
The sensory receptor adapt to any constant
stimulus after a period of time Phasic receptors
quickly adapt. Most exteroceptors Tonic
receptors adapt slowly or not at all. Most
visceroceptors
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Phasic and Tonic Receptors

• Phasic Receptor
• alert us to changes in sensory stimuli
• cease paying attention to constant stimuli
• Tonic Receptor
• useful in situations requiring maintained
  information about a stimulus.

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4. Encoding

• The quality of the stimulus is encoded in the
  frequency of the action potentials.

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Stretch Receptors Weak stretch causes low
impulse frequency on neuron leaving
receptor. Strong stretch causes high impulse
frequency on neuron leaving receptor.
Frequency Code
Membrane potential
Time
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Summary

• The external internal environments are
  monitored by sensory receptors.
• Each type of receptor is excited most effectively
  by only one modality of stimulus known as the
  adequate stimulus.
• The stimulus is converted into an electrical
  potential.
• Stimuli are detected as either static or dynamic
  events.
• The intensity duration of the stimulus is
  frequency coded as bursts of action potentials in
  the primary afferent nerve.

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Section 2. Pain
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• Pain is an unpleasant sensory and emotional
  experience associated with actual or potential
  tissue damage or described in terms of such
  damage
• International Association for the Study
  of Pain

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Why feel pain?

• Gives conscious awareness of tissue damage
• Protection
• Remove body from danger
• Promote healing by preventing further damage
• Avoid noxious stimuli
• Elicits behavioural and emotional responses

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1. Nociceptors

• special receptors that respond only to noxious
  stimuli and generate nerve impulses which the
  brain interprets as "pain".

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Nociopectors

• Adequate Stimulation
• Temperature
• Mechanical damage
• Chemicals (released from damaged tissue)
• Bradykinin, serotonin, histamine, K, acids,
  acetylcholine, proteolytic enzymes can excite the
  chemical type of pain.
• Prostaglandins and substance P enhance the
  sensitivity of pain endings but do not directly
  excite them.

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• Hyperalgesia
• ?The skin, joints, or muscles that have already
  been damaged are unusually sensitive.
• A light touch to a damaged area may elicit
  excruciating pain
• Primary hyperalgesia occurs within the area of
  damaged tissue
• Secondary hyperalgesia occurs within the tissues
  surrounding a damaged area

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2. Localization of Pain

• Superficial somatic pain arises from skin areas
• Deep somatic pain arises from muscle, joints,
  tendons (??) fascia (??)
• Visceral Pain arises from receptors in visceral
  organs

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3. Fast and Slow Pain

• Most pain sensation is a combination of the two
  types of message.
• sharp pain conducted by the A fibres
• dull pain conveyed along C fibres

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• Fast pain (acute)
• occurs rapidly after stimuli (.1 second)
• sharp pain like needle puncture or cut
• not felt in deeper tissues
• larger A nerve fibers
• Slow pain (chronic)
• begins more slowly increases in intensity
• in both superficial and deeper tissues
• smaller C nerve fibers

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• Impulses transmitted to spinal cord by
• Myelinated Ad nerves fast pain (80 m/s)
• Unmyelinated C nerves slow pain (0.4 m/s)

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• Impulses ascend to somatosensory cortex via
• Spinothalamic pathway (fast pain)
• Reticular formation (slow pain)

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4. Notable Features of Visceral pain

• Caused by
• distension of hollow organs
• ischemia
• inflammation
• localized mechanical trauma may be painless
• Poorly localized
• may be referred
• Often accompanied by strong autonomic and/or
  somatic reflexes

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Afferent innervation of the viscera.

• anatomical separation
• nociceptive innervation in sympathetic nerves
• non-nociceptive predominantly in vagus
• Many visceral afferents are specialized
  nociceptors,
• small (Ad and C) fibers involved.
• Large numbers of silent/sleeping nociceptors,
  awakened by inflammation.
• Nociceptor sensitization (hyperalgesia) well
  developed

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Referred pain

• Pain originating from organs perceived as coming
  from skin
• Site of pain may be distant from organ

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Convergence theory of Referred pain

• both visceral and somatic afferents converge on
  the same interneurons in the pain pathways.
• Excitation of the somatic afferent fibers is the
  more usual source of afferent discharge
• the location of visceral receptor activation was
  referred to the somatic source.
• The perception is incorrect.

The convergence of nociceptor input from the
viscera and the skin.
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5. Pain Gate Theory

• Melzack Wall (1965)
• A gate, where pain impulses can be gated
• The synaptic junctions between the peripheral
  nociceptor fiber and the dorsal horn cells in the
  spinal cord are the sites of considerable
  plasticity.
• A gate can stop pain signals arriving at the
  spinal cord from being passed to the brain
• Reduced pain sensation
• Natural pain relief (analgesia)

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How does pain gate work?

• The gate spinal cord interneurons that release
  opioids.
• The gate can be activated by
• Simultaneous activity in other sensory (touch)
  neurons
• Descending nerve fibers from brain

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Applications of pain gate

• Stimulation of touch fibres for pain relief
• TENS (transcutaneous electrical nerve
  stimulation)
• Acupuncture
• Massage
• Release of natural opioids
• Hypnosis
• Natural childbirth techniques

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6. Pain Relief

• Aspirin and ibuprofen (???) block formation of
  prostaglandins that stimulate nociceptors
• Novocain (????) blocks conduction of nerve
  impulses along pain fibers
• Morphine lessen the perception of pain in the
  brain.

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Section 3

• The Visual System

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Part 1. Structure of the Eyeball
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Part2 Focusing on the Retina

• The images of objects in the environment are
  focused on the retina.

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1. Principle of Optics

• Light rays are bent (refracted) when they pass
  from one medium into a medium of a different
  density.
• Parallel light rays striking a biconvex lens are
  refracted to a point (principal focus) behind the
  lens.
• The principle focus is on a line passing through
  the centers of a curvature of the lens, at the
  principal focal distance.

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Light rays from an object that strike a lens more
than 20 ft (6 m) away are considered to be
parallel. The rays from an object closer than 20
ft are diverging and are brought to a focus
farther back than the principal focus. Biconcave
lenses cause light rays to diverge.
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2. Emmertropia

• The refractive system of the human eye
• cornea, aqueous humor, crystalline lens, and
  vitreous humor.
• When light coming from an object is brought to a
  focus, an image is formed.
• Emmertropia
• parallel rays of light are focused to an image on
  the retina.
• normal human eye

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3. Accommodation

• Emmertropia ciliary muscle relax during seeing
  object farther than 6 m.
• objects closer than 6 m are brought to a focus
  behind the retina,
• the objects appear blurred.
• Problem solved by accommodation
• near objects are brought to a sharp focus on the
  retina

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(1) Accommodation of lens

• Increase bulging (refraction) of lens
• Via contraction of ciliary muscle, relaxes the
  suspensory ligaments (parasympathetic fibers)

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Focusing
Muscles relaxed Lens less spherical Focus far
Muscles working Lens more spherical Focus near
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Near Point

• The power of accommodation is limited
• The nearest distance of the eye at which an
  object can seen distinctly
• the visual accommodation is at a maximum
• Decline in the amplitude of accommodation in
  human with advancing age

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Diopter???
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(2) Pupillary reflex

• Reduces the amount of light entering the eye
• Restricts the light to the central part of the
  lens for more accurate vision
• Increase the depth of focus (??)
• Decrease the spherical aberration and chromatic
  aberration

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(3) Convergence of eyeballs

• Viewing near object causes both eyes to move
  inward
• Move the images on the corresponding position on
  the retina of the two eyes

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4. Error of Refraction

• Caused by
• shape of eye
• power of lens

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Farsightedness

• less common
• eye too short and/or lens too weak
• light focuses behind retinal
• correct with convex lens to add power

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FARSIGHTEDNESS (HYPEROPIA)
UNCORRECTED
CORRECTED
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Nearsightedness

• more common
• eye is too long and/or lens is too powerful
• light focuses in front of retina
• correct with concave lens to reduce power

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NEARSIGHTEDNESS (MYOPIA)
UNCORRECTED
CORRECTED
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• Astigmatism
• abnormal curvature of the cornea
• Light from vertical and horizontal direction do
  not focuses in the same point

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Oldsightedness (Presbyopia)

• The crystalline lens tends to harden and the
  capsule itself becomes less elastic with age
• The near point of distinct vision moves further
  and further away from the eye with age.
• The far point is normal
• May be compensated by placing a converging lens
  in front of the eye.

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The loss in power of accommodation is most
significant and dramatic between the ages of 40
and 50
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Part 3 Function of the Retina
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I. Rod system and cone system

• rod system
• rods and subsequent bipolar cells and ganglion
  cells
• cone system
• cones and subsequent bipolar cells and ganglion
  cells.

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Distribution of the cones and rods on the retina.
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Rods

• located mainly in periphery of retina
• responsible for night vision
• detail not detected
• see black, white, and gray (no color)
• several rods share 1 bipolar and 1 ganglion cell
• rod vision lacks detail, but see in low light

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Cones

• located mainly in fovea
• work best in bright light
• enable us to see fine detail
• responsible for color vision
• each cone has its own bipolar and ganglion cell
• this allows us to see detail but bright light is
  needed

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Cones see detail but require bright light Rods
see in low light but lack detail
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Evidence of Two Photoreceptor System

• the nocturnal and diurnal animals
• nocturnal animals have a preponderance of rods
• diurnal animals have a preponderance of cones
• visual pigment
• one classes (rhodopsin) in the rods
• three classes in the cones

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II Transduction of Light Energy by Rod Cell
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1. Photochemical Reaction and Metabolism of
Rhodopsin
Rhodopsin the visual pigment (light-sensitive
pigment). combination of a protein part,
scotopsin (opsin) a carotenoid pigment, 11-cis
retinal Rhodopsin cis form of the retinal bind
with scotopsin.
11-cis retinal
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Retinal Light Sensitive Pigment11-cis-Retinal
- All-trans-Retinal
Light
Dark
all-trans retinal (straight chain form)
11-cis retinal (bent shape form)
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Converting the All-trans Retinal into 11-cis
Retinal

• Occurs under the dark environment
• Requires metabolic energy
• Catalyzed by the retinal isomerase.
• Under the dim light, the 11-cis and 11-trans keep
  dynamic balance

Light
Dark
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11-cis retinal opsin
all-trans retinal opsin
rhodopsin
isomerase
All-trans retinal
11-cis retinal
opsin
opsin
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Vitamin A and Retinal (???)

• All-trans retinol is one form of vitamin A
• Vitamin A is present both
• in the cytoplasm of the rods
• in the pigment layer of the retina
• Vitamin A is always available to form new retinal
  when needed.
• Severe vitamin A deficiency - Night blindness.

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2. Receptor Potential of Rods Hyperpolarization
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III Color Vision
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Photochemistry of Color Vision by Cones

• Three kinds of proteins on the photopigments
• Sensitive to three kinds of light
• Trichromatic Theory

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Visible spectrum 380-760 nm (nm is a billionth
of a meter)
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Trichromatic Theory of Color Vision

• Occurs at the receptor level
• Each cone is coated by one of 3 photopigments
• Short-wave (blue)
• Medium-wave (green)
• Long-wave (red)
• Ratio of activated cones color differentiation

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Color Sensitivity of Different Cones
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Primary Colors

• Red, Green and Blue
• can be mixed to produce any other color

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Color Blindness

• Sex-linked conditions Genes on X chromosome, so
  more common in men.
• Protanopia, missing red photopigment
• Deuteranopia, missing green photopigment
• Non-sex-linked condition
• Tritanopia, missing blue photopigment o
• monochromats people who are totally colorblind,
  more severe

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Color Vision Systems
Tritanopia deuteranopia protanopia
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IV Dark Adaptation and Light Adaptation
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Range of luminance to which the human eye respond
Millilambert,???
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Dark Adaptation

• The decline in visual threshold in a dimly
  lighted environment
• First phase
• Neural adaptation
• Second phase
• chemical adaptation
• depleted of rhodopsin in bright light
• replenish their rhodopsin in dim light

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Light Adaptation When one passes suddenly from a
dim to a brightly lighted environment, the light
seems intensely and even uncomfortably bright
until the eyes adapt to the increased
illumination and the visual threshold rises.
Mechanism?
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Section IV The Auditory System
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Part I Structure of the Ear
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• 1. Outer Ear
• Pinna (auricle) directs sound waves into the
  auditory canal

• Auditory Canal conducts sound to the eardrum
• Tympanic membrane (Eardrum) thin membrane that
  vibrates in response to sound, transfers sound
  energy to bones of the middle ear

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• 2. Middle Ear three tiny bones amplify sound
  and transfer sound energy to the inner ear

• A Malleus
• B Incus
• C Stapes
• Ossicles are smallest bones in the body
• Act as a lever system
• Footplate of stapes enters oval window of the
  cochlea

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• 3. InnerWhere Sound Energy is Transduced
• Cochlea snail shaped fluid-filled structure
• Oval window thin membrane, transfers vibrations
  from stapes to fluid of cochlea

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• Basilar membrane runs the length of the cochlea
• Organ of Corti rests on basilar membrane,
  contains receptor cells
• Round window absorbs energy and equalizes
  pressure in the cochlea

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4. Pathway Transmitting Sound Wave from External
Environment to Inner Ear
Air Conduction
Sound wave
Sound wave
Auditory Canal
Auditory Canal
Air in tympanic cavity
Tympanic membrane
Bone Conduction
Ossicular chain
Round window
Sound wave
Oval window
Inner ear
Vibration of skull
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Part 2. Properties of Sound

• Sound travels in waves as does light
• 1. Pitch determined by frequency, the number
  of cycles per second of a sound wave, measured in
  hertz (Hz)
• 2. Loudness determined by amplitude (height)
  of the sound wave, measured in decibels (dB)
• 3. Timbre determined by complexity and shape
  of the sound wave, gives each sound its unique
  quality

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Loudness of Sound

• 0 dB hearing threshold
• 50 dB normal conversation
• 90 dB danger zone
• 120 dB Rock concert
• 130 dB Pain threshold

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Part 3 Role of Middle Ear in Sound Transmission
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Mechanisms Involved in Transformer Process

• Size difference between Tympanic Membrane and
  Stapes Footplate
• Lever action

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First Component of Middle Ear Transformer Action

• Size Difference
• Tympanic membrane
• .59 cm2
• Stapes footplate
• .032 cm2
• Pressure formula
• Pressure force/area
• Impact on sound transmission

Pressure gain 0.59/0.032 18.4 (times)
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Transformer Action of Middle EarLever Action

• Fulcrum Effect pressure gain 1.3
  times

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TRANSFORMER ACTION AMOUNT OF AMPLIFICATION
Pressure Gain Contribution from 18.4 TM
(Tympanic Membrane) to stapes footplate
1.3 Lever action 23.9 Total pressure
gain (18.6 x 1.3)
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Part 4 Function of Organ of Corti

• a structure rests atop the basilar membrane along
  its length
• contains approx. 16,000 cochlear hair cells

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1. How to discriminate the frequency of the
sound? --- Traveling Wave Theory
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Vibration of Basilar Membrane and the Traveling
Wave Theory

• Sound wave entering at the oval window cause the
  basilar membrane to vibrate
• different frequencies cause vibrations at
  different locations (places) along basilar
  membrane
• higher frequencies at base, lower frequencies at
  top

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2. Electrical Potentials

• DC vs. AC
• Direct Current (DC) stimulus doesnt change
  with time, constant i.e. battery
• Alternating Current (AC) always changing over
  time, looks like a sine wave

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Cochlea

• Perilymph-
• similar in composition to extracellular fluid.
• High in Na and low in K.
• Endolymph-
• found in the scala media.
• Similar to intracellular fluid. High in K and
  low in Na

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Two DC Potentials (EP)

• Endocochlear Potential (EP)
• 80 mV potential with respect to a neutral point
  on the body
• due to the Stria Vascularis (????????)

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80 mV
Reticular Lamina
-80 mV
Two DC Potentials (IP)

• Intracellular Potential (IP) or organ of corti
  potential (resting potential)
• Recorded -80 mV inside cells of organ of corti

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Hair Cell in the Organ of Corti
When the basilar membrane moves, a shearing
action between the tectorial membrane and the
organ of Corti causes hair cells to bend
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There are mechanical gates on each hair cell that
open when they are bent.
K comes into the hair cell and depolarizes the
hair cell.
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Two AC Potentials

• Cochlear Microphonic Potential
• Reproduces frequency and waveform of a sinusoid
  perfectly
• Generated from hair cell
• Action Potential (AP)
• Electrical activity from the VIII Nerve
• Can be measured from anywhere in the cochlea or
  in the auditory nerve

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Part 5 Deafness

• Conduction deafness -
• possible causes include perforated eardrum,
  inflammation, otosclerosis
• Sensineural deafness - nerve damage