CHAPTER%203%20SENSORY%20SYSTEMS

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CHAPTER 3SENSORY SYSTEMS
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Disorders of sensory systems

• Deficits
• Sensory organ
• Sensory nerves
• Central nervous system
• Hyperactivity
• Central neuropathic pain
• Tinnitus
• Tingling
• Normal response that is redirected
• Pain from touch
• Dizziness and vertigo from head movements

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Disorders of sensory systems

• Reduced sensitivity
• Hearing loss
• Visual impairment
• Incorrect response
• Hyperacusis
• Distorted sounds
• Allodynia
• Hyperpathia

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Disorders of sensory systems

• Impaired conduction of the physical stimulus to
  the receptors
• Impaired function of receptors
• Impaired function of sensory nerves
• Impaired or changed function of the central
  nervous system

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Reduced sensitivity

• Often caused by disorders of the sense organs
• Injury to afferent nerves

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Hyperactive sensory disorders

• Increased sensation of physical stimuli
• Altered sensation of physical stimuli
• Sensation without any physical stimulation

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General organization of sensory systems

• Conduction of the physical stimulus to the
  receptors
• Sensory receptors
• Sensory nerves
• Central nervous system

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Sensory transduction

• A physical stimulus generates a receptor
  potential
• The receptor potential is a graded potential
• The receptor potential is conducted
  electrotonically to the spike generation site

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Bipolar receptor cells (taste)
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Initiation of nerve impulses

• Occurs at the first node of Ranvier

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Two different types of receptors, with bipolar
nerve fibers
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Sensory transduction (mechanoreceptor in a muscle)
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Central nervous system
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HUMAN
Mouse
Chick
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Auditory nervous system Ascending auditory
pathways
From Møller, 2005
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Two different ascending sensory pathways have
been identified

• The classical systems
• The non-classical systems

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Classical auditory pathways
Non-classical auditory pathways
From Møller Sensory Systems, 2003
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Non-classical auditory pathways Receive input
from the somatosensory system Use the dorsal
part of the MGB
From Møller, 2005
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The classical ascending pathways

• The number of nuclei is different in different
  sensory systems
• Use ventral thalamic nuclei that project to
  primary sensory cortices
• Neurons processes only input from of one sensory
  modality

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Visual system Classical ascending pathways
Non-classical ascending pathways
From Møller, 2005
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The nonclassical pathways

• Use dorsal and medial thalamic nuclei that
  project to secondary cortices and to other parts
  of the CNS
• Receive input from more than one sense

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Somatosensory pathways Classical pathways
Non-classical pathways
From Møller, 2005
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Processing after primary sensory cortices

• Integration of input from different sensory
  systems occurs in association cortices
• Parallel processing
• Stream segregation

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The neocortex has six layers
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Simplified diagram of the connections to and from
the different layers of the cerebral cortex
From Møller Sensory Systems, 2002
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Maps

• Tonotopic
• Somatotopic

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SURFACE VIEW
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LOWER BODY IS REPRE-SENTED NEAR THE MIDLINE
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Tonotopic organization in the CN of a cat, as an
example of tonotopic organization in the auditory
system
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Tonotopic organization in the CN of a cat, as an
example of tonotopic organization in the auditory
system
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Parallel processingStream segregation
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Parallel processing Cochlear nucleus
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Function of sensory nervous systems

• Processing of sensory input at the peripheral
  level
• Convergence (spatial integration)
• Interplay between inhibition and excitation

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Spatial integration Receptive field of a
dorsal column nucleus cell
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Convergence of input to a secondary neuron
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Lateral inhibition
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Central processing of sensory information

• Each stage enhances or suppress specific
  Information

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Parallel processingThe same information is
processed in different structuresStream
segregationDifferent kinds of information is
processed in different structures(What and
Where)
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Processing after primary sensory cortices

• Integration of input from different sensory
  systems occurs in association cortices

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Stream segregation Cortical circuitry
Dorsal stream where
Ventral stream what
From Møller Sensory Systems, 2003
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Sensory information can reach other regions than
sensory regions

• Motor systems
• Memory
• Emotional brain (limbic system)

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Two different routes to the Amygdala from a
sensory system
From Møller, 2005
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Connections from a sensory system to the
amygdala
the high route
From Møller Sensory Systems, 2003
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Connections from a sensory system to the
amygdala the low
route
From Møller Sensory Systems, 2003
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Connections from the amygdala
From Møller Sensory Systems, 2003
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Hypoactive sensory disorders

• Loss of sensitivity
• Hearing loss
• Poor vision
• Numbness
• Loss of vestibular (balance) function

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HEARING LEVEL AT 4 kHz
NOISE IMMISSION LEVEL
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Genetic, epigenetic and environmental Causes (and
a stochastic component ?)
Courtesy of M. Charles Liberman
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Age-related hearing loss
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Normal variations in hearing loss of 70 year old
individuals
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Variations in speech discrimination in 70 year
old individuals
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Hearing loss in Ménière's disease
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Effect of surgical injuries to the auditory
nerve Large decrease in speech discrimination
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Hyperactive sensory disorders

• Tinnitus
• Paresthesia
• Phosphenes
• Phantom sensations
• Central neuropathic pain

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Subjective and objective tinnitus

• Different forms of tinnitus have very different
  effects on an individuals life

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Similarities between chronic pain and severe
tinnitus
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There are many forms of tinnitus

• Mild tinnitus
• Does not interfere noticeably with everyday life
• Moderate tinnitus
• May cause some annoyance and may be perceived as
  unpleasant
• Severe tinnitus
• Affects a persons entire life in major ways
• Patients own perception varies between mild,
  moderate and severe (disabling)

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Important to have words for disorders 

• We cannot think about matters that do not have
  names
• The same words is used to describe very different
  forms of tinnitus and pain
• Using the same names for fundamentally different
  disorders is a disadvantage in treating these
  disorders

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How prevalent is severe tinnitus? 

• Some statistics show 50 million people have
  tinnitus in the USA
• The prevalence of severe (bothersome) tinnitus
  is infrequent at young age it reaches 12-14 for
  people at age 65 according to one study

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How prevalent is severe pain? 

• Some pain was reported by 86 of individuals
  above the age of 65
• (Iowa study, 1994)
• The prevalence of severe pain was 33 for people
  at age 77 and above (Swedish study, 1996)

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Severe tinnitus affects a persons entire life in
major ways

•  
• Prevents or disturbs sleep
• Interferes with or prevents
• Intellectual work
• Often accompanied by altered perception of sound
•         

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Severe pain affects a persons entire life in
major ways

• Prevent or disturb sleep
• Interfere with or prevents intellectual work 
• May cause suicide
•   May involve limbic structures causing affective
  reactions
• Often accompanied by abnormal sensations from
  touch

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Severe tinnitus is often accompanied by altered
perception of sound

• Sounds are distorted
• Sounds have exaggerated loudness (hyperacusis)
• Sounds are unpleasant
• Sounds are painful and arouse fear (phonophobia)

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Little is known about the cause of subjective
tinnitus

• Noise exposure
• Ototoxic antibiotic
• Acoustic tumors

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The sympathetic nervous system is involved in
some forms of severe tinnitus

•  
• Some forms of tinnitus
• can be cured by sympathectomy

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Deprivation of sound can cause changes in neural
processing such as change in temporal integration

• Expression of neural plasticity

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The anatomical location of the abnormality that
cause chronic pain and tinnitus may be different
from that to which the pain or the tinnitus is
referred 
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The abnormal neural activity that causes symptoms
are not generated at the location where the
symptoms are felt

• Examples
• Phantom pain
• Tinnitus with severed auditory nerve

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The tinnitus in some patients can be modulated by
stimulation of the somatosensory systems (such as
by electrical stimulation of the median
nerve)cross-modal interaction
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Non-classical auditory pathways Receive input
from the somatosensory system Use the dorsal
part of the MGB
From Møller, 2005
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Other signs of involvement of the somatosensory
system

• Gaze related tinnitus
• Neck muscles and tinnitus
• TMJ and tinnitus
• Sensation of sound from touching the skin

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Connections between spinal C2 segment and the
dorsal cochlear nucleus

• Can explain why electrical stimulation of the
  skin behind the ears can modulate tinnitus

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Symptoms and signs of neuropathic pain and
severe tinnitus

• Strong emotional components
• Depression
• High risk of suicide

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Severe tinnitus is often associated with
affective (mood) disorders

• Depression
• Phonophobia

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The amygdala is involved in fear and other mood
disorders
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Connections from the auditory system to the
amygdala

• Cortical-cortical connections (the high route)
• Subcortical connections
• (the low route)

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From Møller, 2005
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CONCLUSION

• ACTIVATION OF NON-CLASSICAL ASCENDING SENSORY
  PATHWAYS CAN CAUSE SYMPTOMS AND SIGNS OF SEVERAL
  DISEASES

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Neural plasticity play greater role in generating
symptoms and signs than previously assumed

• Plastic changes are reversible
• Treatments without medicine and surgery may
  alleviate pain and tinnitus

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Therapy

• There is no treatment for tinnitus that is
  comparative to common pharmacological treatment
  of pain. Treatment of tinnitus has been mainly
  benzodiazepines (GABAA agonists)

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Reversal of neural plasticity

• TENS (transderm electric nerve stimulation) has
  been used for many years in treatment of chronic
  pain
• Recently sound stimulation in various forms have
  been introduced in treatment of severe tinnitus

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Stimulation of somatosensory system can relieve
tinnitus

• Electrical stimulation
• of the ear and
• of the skin behind the ears have been used to
  treat tinnitus
• Electrical stimulation of the auditory cortex is
  in a stage of development
• Few systematic studies of efficacy have been
  published

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