Transcript Somatosensory Areas Oct 2007Handouts.pps

SOMATOSENSORY SYSTEM
DR SYED SHAHID HABIB
MBBS DSDM FCPS
Assistant Professor
Dept. of Physiology
College of Medicine & KKUH
OBJECTIVES
TO STUDY AND UNDERSTAND
COMPONENTS
SENSORY PATHWAYS
FUNCTIONS
RECEPTORS
SENSORY TRACTS
SOMATOSENSORY CORTEX
CLINICAL APPLICATION
OF SOMATOSENSORY SYSTEM
FUNCTIONS
• collection of sensory input
• integration
• motor output
FUNCTIONS
• Soma= body
• Sensory = sensation
• It is a system which gives
informations about sensory stimuli
through skin and the body adjusts its
responses through the motor
system.
COMPONENTS
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Receptors
Peripheral nerves
Spinal cord
Tracts
Thalamus
Thalamocortical projection
Somatosensory cortex
3rd order
Thalamus
Sensory Cortex
SI & SII
2nd order
Tracts
Medullary Nuclei
Dorsal Horn
Of
Spinal cord
1st order
Peripheral
Nerve
Receptor
Stimulus
LOCALIZATION &
PERCEPTION
OF SENSATION
RECEPTORS
•
These are sensors made up of nerve
endings (dendrites) present in the skin.
• They are:
1. Rapid adapting or phasic receptors eg
meissner’s corpuscles(touch), pacinian
corpuscles(vibration)
2. Slowly adapting or tonic eg ruffini’s
(pressure ,skin stretch ) krause’s end
bulbs,and Merkel’s disks.
3. Bare nerve endings for pain sensation
SOMATIC PORTION OF THE SENSORY SYSTEM
Information enters the central
nervous system through
peripheral nerves and is
conducted immediately to
multiple sensory areas in
(1) the spinal cord at all levels; (2)
the reticular substance of the
medulla, pons, and
mesencephalon of the brain;
(3) the cerebellum;
(4) the thalamus; and
(5) areas of the cerebral cortex
SENSORY TRACTS
1. Dorsal Column Tracts
Or Lemniscal system
Fasiculi Gracilis and Cuneatus
Functions: Touch (fine), localization of stimulus,
position sensation & from the joints, pressure
Vibration & streognosis.
2. Anterolateral System
Ventral & lateral spinothalamic tracts.
Functions: Pain sensation, thermal sensation, tickle &
itching, crude touch and sexual sensation.
DORSAL COLUMN-MEDIAL LEMNISCAL SYSTEM
1. Touch sensations requiring a high degree of
localization of the stimulus
2. Touch sensations requiring transmission of fine
gradations of intensity
3. Phasic sensations, such as vibratory sensations
4. Sensations that signal movement against the skin
5. Position sensations from the joints
6. Pressure sensations having to do with fine degrees
of judgment of pressure intensity
In this pathway through
the brain stem, each
medial lemniscus is
joined by additional
fibers from the sensory
nuclei of the trigeminal
nerve; these fibers
subserve the same
sensory functions for
the head that the dorsal
column fibers subserve
for the body.
Spatial Orientation of the Nerve Fibers in the Dorsal
Column-Medial Lemniscal System
• is maintained
throughout. For instance,
in the dorsal columns of
the spinal cord, the fibers
from the lower parts of the
body lie toward the center
of the cord, whereas those
that enter the cord at
progressively higher
segmental levels form
successive layers laterally.
Spatial Orientation of the Nerve Fibers in the Dorsal
Column-Medial Lemniscal System (cont.)
In the thalamus, the tail
end of the body
represented by the
most lateral portions of
the ventrobasal
complex and the head
and face represented
by the medial areas of
the complex.
ANTEROLATERAL SYSTEM
1. Pain
2. Thermal sensations, including both warmth and
cold sensations
3. Crude touch and pressure sensations capable
only of crude localizing ability on the surface of
the body
4. Tickle and itch sensations
5. Sexual sensations
S S L T
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S L T C
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C T L S
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SOMATOSENSORY CORTEX
Somatosensory area I is so much more extensive and so
much more important than somatosensory area II that in
popular usage, the term "somatosensory cortex" almost
always means area I.
CEREBRAL CORTEX
1. The incoming sensory signal excites neuronal layer IV first;
then the signal spreads toward the surface of the cortex and
also toward deeper layers.
2. Layers I and II receive diffuse, nonspecific input signals
from lower brain centers that facilitate specific regions of
the cortex. This input mainly controls the overall level of
excitability of the respective regions stimulated.
3. The neurons in layers II and III send axons to related
portions of the cerebral cortex on the opposite side of the
brain through the corpus callosum.
4. The neurons in layers V and VI send axons to the deeper
parts of the nervous system. For eg to basal ganglia and
thalamus
Somatosensory Cortex (Cont.)
Cortex is made up of six layers. The incoming
sensory signal excites neuronal layer IV first and
then the signal spreads both towards the surface
of the cortex and towards the deeper layer.
Functionally the neurons of the somatosensory
cortex are arranged in vertical columns.
Each of these columns severs a single specific
sensory modality.
Receives sensory information exclusively from the
opposite side of the body.
SOMATOSENSORY CORTEX
a map of the
human cerebral
cortex, that is
divided into about
50 distinct areas
called
Brodmann's areas
based on
histological
structural
differences.
Areas 1, 2, and 3, which constitute PRIMARY
SOMATOSENSORY AREA I, 40 is SECONDARY
SOMATOSENSORY AREA II and areas 5 and 7, which
constitute the SOMATOSENSORY ASSOCIATION AREA.
what we know about somatic
sensation appears to be explained by
the functions of somatosensory area I.
• somatosensory area II, although roughly, the face
is represented anteriorly, the arms centrally, and
the legs posteriorly.
• It is known that signals enter this area from the
brain stem, transmitted upward from both sides
of the body.
• many signals come secondarily from
somatosensory area I as well as from other
sensory areas of the brain, even from the visual
and auditory areas
• removal of parts of somatosensory area II has no
apparent effect on the response of neurons in
somatosensory area I.
Functions of Somatosensory Area I
1.
2.
3.
4.
5.
The person is unable to localize discretely the different
sensations in the different parts of the body. However, he
or she can localize these sensations crudely, such as to a
particular hand, to a major level of the body trunk, or to
one of the legs. Thus, it is clear that the brain stem,
thalamus, or parts of the cerebral cortex not normally
considered to be concerned with somatic sensations can
perform some degree of localization.
The person is unable to judge critical degrees of
pressure against the body.
The person is unable to judge the weights of objects.
The person is unable to judge shapes or forms of
objects. This is called astereognosis.
The person is unable to judge texture of materials
because this type of judgment depends on highly critical
sensations caused by movement of the fingers over the
surface to be judged.
SOMATOSENSORY CORTEX (AREA SII)
• Present in the wall of the sylvian fissure.
• The localization is poor as compared to SI.
• Face is represented anteriorly, the arm
centrally and the leg posteriorly.
• Ablation of SI results in deficits in sensory
processing in SII where as ablation of SII
has no gross effect on the processing in
SI.
SOMATOSENSORY ASSOCIATION AREAS
• Situated in Brodmann’s area 5 & 7 of the central
cortex located in the parietal cortex behind SI
area.
• It plays an important role in translating the
sensory information that enters the
somatosensory areas.
• When damaged it loses the ability to recognize
complex objects on the opposite side of the body.
e.g. Amorphosynthesis, Apraxia and sensory
inattention.
Representation of the different areas of the
body in somatosensory area I of the cortex
Representation of the different areas of the
body in somatosensory area I of the cortex
LESIONS OF THE SENSORY PATHWAYS
Parietal cortex lesions
Thalamic lesions
Pontine lesions
Spinal cord compression
Spinal cord lesions
Spinal root lesions