Transcript Document 7280141

CHAPTER 3
SENSORY SYSTEMS
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
Disorders of sensory systems
• Reduced sensitivity
– Hearing loss
– Visual impairment
• Incorrect response
– Hyperacusis
– Distorted sounds
– Allodynia
– Hyperpathia
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
Reduced sensitivity
• Often caused by disorders of the sense
organs
• Injury to afferent nerves
Hyperactive sensory disorders
• Increased sensation of physical stimuli
• Altered sensation of physical stimuli
• Sensation without any physical stimulation
General organization of
sensory systems
• Conduction of the physical
stimulus to the receptors
• Sensory receptors
• Sensory nerves
• Central nervous system
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
Bipolar receptor cells (taste)
Initiation of nerve impulses
• Occurs at the first node of Ranvier
Two different types of receptors, with bipolar nerve fibers
Sensory transduction (mechanoreceptor in a muscle)
Central nervous system
HUMAN
Mouse
Chick
Auditory nervous system
Ascending auditory pathways
P rimary
audit ory cort ex
Midline
Ret icular
format ion
Vent ral
t halamus
MGB
Inferior
colliculus
Cochlea
AN
LL
Cochlear
nucleus
From: Møller, 2005
Two different ascending sensory
pathways have been identified:
• The classical systems
• The non-classical systems
Classical auditory
pathways
Non-classical
auditory pathways
From: Møller: Sensory Systems, 2003
Non-classical auditory
pathways
Receive input from the
somatosensory system
Use the dorsal part of the
MGB
From: Møller, 2005
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
Visual system
Classical ascending pathways Non-classical ascending pathways
A
B
External
eye muscles
Midline
P rimary
visual cortex
Extrastriate
cortex
P rimary
visual cortex
Other regions
of the CNS
T halamus
P ulvinar
T halamus
Superior
colliculus
T halamus
LGN
LGN
P retectal
nucleus
Light
reflexes
Retinal
ganglion cells
Retinal
ganglion cells
SCN and
hypothalamus
From: Møller, 2005
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
Somatosensory pathways
Classical pathways
Non-classical pathways
Pain pathways
Limbic system
Association cortex
SII
"WHAT"
SI cortex
"WHERE"
Thalamus
dorsal
Thalamus
ventral
Medial
lemniscus
Reticular formation
AROUSAL
Anterior
lateral tract
Fig 3.3
From: Møller, 2005
Cerebral
cortex
Thalamus
Trigeminal
ganglion
Midbrain
Motor
nuclei
RF
Brainstem
Spinalcord
Processing after primary sensory
cortices
• Integration of input from different sensory
systems occurs in association cortices
• Parallel processing
• Stream segregation
The
neocortex
has six
layers
Simplified
diagram of the
connections to
and from the
different layers
of the cerebral
cortex
From: Møller: Sensory Systems, 2002
Maps
Tonotopic
Somatotopic
SURFACE VIEW
LOWER
BODY IS
REPRESENTED
NEAR THE
MIDLINE
Tonotopic organization in the CN of a cat, as an
example of tonotopic organization in the auditory
system
Tonotopic organization in
the CN of a cat, as an
example of tonotopic
organization in the auditory
system
Parallel processing
Stream segregation
Parallel processing:
Cochlear nucleus
Function of sensory nervous
systems
• Processing of sensory input at the
peripheral level
– Convergence (spatial integration)
– Interplay between inhibition and excitation
Spatial integration:
Receptive field of a
dorsal column nucleus cell
Convergence of input to a secondary neuron
Lateral inhibition
Central processing of sensory
information
• Each stage enhances or suppress specific
Information
Parallel processing:
The same information is
processed in different structures
Stream segregation:
Different kinds of information is
processed in different structures
(“What” and “Where”)
Processing after primary sensory
cortices
• Integration of input from different sensory
systems occurs in association cortices
Stream segregation
Cortical circuitry
Dorsal stream
“where”
Ventral stream
“what”
From: Møller: Sensory Systems, 2003
Sensory information can reach
other regions than sensory
regions
Motor systems
Memory
Emotional brain (limbic system)
Two different
routes to the
Amygdala from a
sensory system
Arousal
and
plasticity
Cortex
AAF
"High Route"
AI
Association
cortices
Polymodal
association
cortex
Other cortical
areas
AII
Nucleus
basalis
Amygdala
"Low Route"
AL
Dorsal
medial
MGB
ICX ICC
DC
ABL
ACE
Thalamus
Endocrine
Ventral
MGB
Autonomic
Behavioral
From: Møller, 2005
Connections from a sensory system to the amygdala
“the high route”
From: Møller: Sensory Systems, 2003
Connections from a sensory system to the amygdala
“the low route”
From: Møller: Sensory Systems, 2003
Connections from the amygdala
From: Møller: Sensory Systems, 2003
Hypoactive sensory disorders
• Loss of sensitivity
– Hearing loss
– Poor vision
– Numbness
– Loss of vestibular (balance) function
-10
0
10
20
30
40
50
60
70
80
90
100
110
0.125
0.25
0.5 0.75 1 1.5
Frequency in kHz
2
3
4
6
8
NOISE IMMISSION LEVEL
HEARING LEVEL AT 4 kHz
Genetic, epigenetic and environmental
Causes (and a stochastic component ?)
Courtesy of M. Charles Liberman
Age-related
hearing loss
Normal variations in hearing loss of
70 year old individuals
Number
100
4000 Hz. Women
Number
100
90
90
80
80
70
70
Left Ear
Right Ear
60
50
50
40
30
30
20
20
10
10
0
0-9
20-29
40-49
60-69
80-89
10-19
30-39
50-59
70-79
90-99 dB
Left Ear
Right Ear
60
40
0
4000 Hz. Men
0-9
20-29
40-49
60-69
80-89
10-19
30-39
50-59
70-79
90-99 dB
Variations in speech discrimination in
70 year old individuals
%
100
%
100
Men 70 years old
N=179
N=197
75
75
50
50
Left Ear
Right Ear
25
0
Women 70 years old
Left Ear
Right Ear
25
•92% 76-88%<76% <48%
deaf
0
•92% 76-88%<76% <48%
deaf
Hearing loss in Ménière's disease
-10
0
10
20
30
40
50
60
70
80
90
100
110
0.125
0.25
0.5 0.75 1 1.5
Frequency in kHz
2
3
4
6
8
A
Effect of surgical injuries
to the auditory nerve:
-10
0
I
10
20
30
40
50
60
70
80
90
Large decrease in speech
discrimination
II
100
110
0.125
0.25
0.5 0.75 1 1.5
2
3
4
6
8
Frequency in kHz
I: Pre-op Discr.=96% AS
II: 5 days post-op Discr.=0% AS
B
-10
0
10
I
20
30
40
50
60
70
80
II
90
100
110
0.125
0.25
0.5 0.75 1 1.5
2
Frequency in kHz
I: Pre-op Discr.=80% AS
II: 7 days post-op Discr.=30% AS
3
4
6
8
Hyperactive sensory disorders
•
•
•
•
•
Tinnitus
Paresthesia
Phosphenes
Phantom sensations
Central neuropathic pain
Subjective and objective tinnitus
• Different forms of tinnitus have very
different effects on an individual’s life
Similarities between chronic
pain and severe tinnitus
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 person’s entire life in major ways
Patients’ own perception varies between mild,
moderate and severe (disabling)
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
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
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)
Severe tinnitus affects a
person’s entire life in major
ways
• Prevents or disturbs sleep
• Interferes with or prevents
Intellectual work
• Often accompanied by altered
perception of sound
Severe pain affects a person’s
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
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)
Little is known about the cause
of subjective tinnitus
• Noise exposure
• Ototoxic antibiotic
• Acoustic tumors
The sympathetic nervous
system is involved in some
forms of severe tinnitus
Some forms of tinnitus
can be cured by sympathectomy
Deprivation of sound can cause
changes in neural processing such
as change in temporal integration
• Expression of neural plasticity
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
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
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
Non-classical auditory
pathways
Receive input from the
somatosensory system
Use the dorsal part of the
MGB
From: Møller, 2005
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
Connections between spinal
C2 segment and the dorsal
cochlear nucleus
Can explain why electrical
stimulation of the skin behind the
ears can modulate tinnitus
Symptoms and signs of
neuropathic pain
and severe tinnitus
• Strong emotional components
• Depression
• High risk of suicide
Severe tinnitus is often associated
with affective (mood) disorders
• Depression
• Phonophobia
The amygdala is involved in fear
and other mood disorders
Connections from the auditory
system to the amygdala
• Cortical-cortical connections
(the “high route”)
• Subcortical connections
(the “low route”)
Arousal
and
plasticity
Cortex
AAF
"High Route"
AI
Association
cortices
Polymodal
association
cortex
Other cortical
areas
AII
Nucleus
basalis
Amygdala
"Low Route"
AL
Dorsal
medial
MGB
ABL
ACE
Thalamus
Endocrine
Ventral
MGB
Autonomic
Behavioral
ICX ICC
DC
Fig 3.7
From: Møller, 2005
CONCLUSION
ACTIVATION OF NON-CLASSICAL
ASCENDING SENSORY PATHWAYS
CAN CAUSE SYMPTOMS AND SIGNS
OF SEVERAL DISEASES
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
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)
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
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