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Somatosensory and Remaining
Sensory Systems
Ch. 7 (cont’d)
Outline
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Organization of the Somatosensory System
Somatosensory Cortex
The Olfactory System
The Gustatory System
Selective Attention
Organization of The
Somatosensory System
The Somatosensory Subsystems
• The somatosensory system comprises three
subsystems:
– exteroceptive cutaneous system
– proprioception system (monitors body
position)
– interoceptive system (monitors conditions
within the body such as blood pressure)
Somatosensory Cortex
• The various thalamic nuclei receiving
somatosensory information project to
– Primary sensory cortex in the postcentral
gyrus
– Secondary somatosensory cortex
– Posterior parietal cortex
Somatosensory Cortex
• The primary somatosensory cortex is
organized somatotopically; it is known as
the somatosensory homunculus
• Instead of one “homunculus”, the primary
cortex is comprises four parallel
somatotopically organized strips, each
sensitive to a different kind of
somatosensory input
Somatosensory
Homunculus
Somatosensory Cortex
• Large lesions to parietal cortex occasionally
produce somatosensory agnosias
– Astereognosia - loss of ability to recognize
objects by touch in absence of of defects in
somatosensation
– Asomatognosia - is a loss of the ability to
recognize parts of one’s own body
Pain and
descending analgesia circuit
• Stimulation of Periaqueductal grey
produced analgesia in rats
• Receptors for opiates analgesics in PAG,
suggesting body produces its own opiates
(endorphins)
Pain and
descending analgesia circuit
• DA circuit goes from PAG to raphe nucleus
and then to dorsal columns to dorsal horns
of spinal cord
• Phantom limb pain - chronic severe pain
that is experienced by about half of
amputees
The Olfactory System
• Smell is an olfactory system response to airborne
chemicals; there are about 1000 different olfactory
receptors, each having its own special receptor protein
• Although there appears to be no organization of receptors
at the level of the olfactory mucosa, all receptors having
the same receptor protein seem to project to the same area
of olfactory (odotopic mapping)
The Olfactory System
• Transduction of olfactory stimuli occurs in
olfactory receptors located in the olfactory mucosa
of the upper nasal cavity
• Projections to various parts of the limbic system
(which is responsible for the emotional perception
of odorants) and to the medial dorsal nucleus of
the thalamus
• The DMN eventually passes the olfactory
information on to the orbitofrontal cortex where
the odor is consciously perceived
The Gustatory System
• Taste operates in tandem with smell; gustatory receptors
are called taste buds
• There are four primary tastes: sweet, sour, bitter, and
salty; however, these perceptions do not match up nicely
with four simple gustatory receptors
• Many flavors cannot be recreated by combinations of the
primary flavors
• Some flavors seem to activate taste neurons by altering
neural activity by activity on ion channels directly rather
through a receptor
Selective Attention
• Allows us to consciously perceive just a
fraction of what we unconsciously sense
• May be top-down or bottom-up
• may be focused by internal cognitive
processes (endogenous attention; topdown) or by external events (exogenous
attention; bottom-up)
Selective Attention
• Change blindness is classic example of the
effects of selective attention; it can be
demonstrated by alternately showing
subjects two pictures that are identical in
every aspect but one. If there is a brief delay
between each picture, subjects often take a
long time to spot what becomes an obvious
difference between the pictures once they
are attending to the right location
Selective Attention
• Different parts of the brain mediate
attention to different types of stimuli; for
example faces activate the ventral visual
pathway while the position of that face
activates the dorsal visual pathway
Forebrain Structures of the
Sensorimotor System
Ch. 8
Outline
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Principles of Sensorimotor Function
Posterior Parietal Association Cortex
Dorsolateral Prefrontal Association Cortex
Secondary Motor Cortex
Primary Motor Cortex
Cerebellum and Basal Ganglia
Hierarchical Organization
• The sensorimotor system is organized like a
large effective company; the “president”
(associated cortex) issues general
commands and lower levels (motor neurons
and muscles) take care of details; the
advantage of this hierarchical arrangement
is that higher levels are left free to focus on
the complex functions
Motor Output is Guided
by Sensory Input
• Like a large company, the sensorimotor
system carefully monitors the external
world and the consequences of its own
actions, and it acts accordingly; only
ballistic movements (brief, all-or-none,
high speed movements) are not guided by
sensory feedback
Learning Changes the Locus of
Sensorimotor Control
• As a new company develops, more and
more tasks become part of the routine and
are taken over by lower levels of the
organization; the same thing happens in the
sensorimotor system; after much practice
lower levels perform well-learned tasks
with little higher involvement
Posterior Parietal
Association Cortex
• Before an effective response can be
initiated, the sensorimotor system must
know the positions of various parts of the
body and of objects in he external world;
current thinking is that the posterior
parietal cortex performs this function
Posterior Parietal
Association Cortex
• The posterior parietal cortex receives input from visual,
auditory, and somatosensory systems ( that is why it is
considered to be associated cortex) most of its output goes
to secondary motor cortices
• In addition to disrupting the accuracy of movements, large
lesions of posterior cortex can produce apraxia and
contralateral neglect
Apraxia
• Apraxia is the inability to perform
movements when requested to do so ( in the
absence of simple sensory or motor deficits,
motivational deficits, or intellectual deficits)
for example an apraxic patient may have
difficulty demonstrating hammering
movements when asked to do so but be
perfectly capable of spontaneously
hammering a nail
Apraxia
• Apraxia is almost always associated with
left hemisphere damage, but its symptoms
are always bilateral
• Right parietal damage often produces
deficits on the WAIS block-design subtest;
this is referred to as constructional apraxia
Apraxia
• Other type of apraxia called object apraxia
- patient uses wrong object for certain
programmed movements (like brushing
teeth with a comb instead of toothbrush)
• (in class video)
Contralateral Neglect
• Patients with contralateral neglect fail to respond to visual,
auditory, and somatosensory stimuli from the contralateral
half of the body
• Contralateral neglect is usually produced by very large
right parietal lesions
• Patients with contralateral neglect may shave only the right
half of their face, eat food from only the right half of their
plate, put only their right leg in their pants
Dorsolateral Prefrontal
Association Cortex
• Projections to this area are from the
posterior parietal cortex; this area in turn
projects to parts of the secondary motor
cortex, the primary cortex, and to the
frontal eye field
Dorsolateral Prefrontal
Association Cortex
• Research on nonhuman primates has
suggested that the prefrontal association
cortex is involved in assessment of external
stimuli and the initiation of responses to
them; neurons here may be activated by
characteristics of an object, its location, or
by the response that the object elicits
Dorsolateral Prefrontal
Association Cortex
• Further research shows that the motor
neurons firing the earliest (prior to a motor
task) are located in the dorsolateral
prefrontal cortex, indicating that this area
may be key in decisions regarding
voluntary response initiation
Secondary Motor Cortex
• There are three areas of secondary motor
cortex: the premotor cortex, the
supplementary motor area, and the
cingulate motor areas. They all send
information to primary motor cortex; all
receive input from primary motor cortex; all
are interconnected with one another; and all
send axons to the motor circuits of
brainstem
Secondary Motor Cortex
• Functionally, each of these areas produces
complex movements when stimulated; are
activated both before and during voluntary
movements; and are active when either side
of the body is involved in a movement
Secondary Motor Cortex
• Premotor cortex neurons often respond to
both visual and touch stimuli; it appears to
encode spatial relations of external cues and
program movements guided by these cues
Secondary Motor Cortex
• Much of the supplementary motor area
(SMA) is in the longitudinal fissure
• The cingulate motor cortex lies on the
cingulate gyrus, just below the SMA
• Secondary motor cortex is involved in the
planning, programming and generation of
complex motor sequences
Primary Motor Cortex
• Primary motor cortex is in the precentral
gyrus of the frontal lobe; it is
somatotopically organized
• The motor homunculus has a
disproportionate representation of hands
and mouth; in fact, two different areas of
each primary cortex control the contralateral
hand
Motor
homunculus
Primary Motor Cortex
• Neurons in primary motor cortex seem to
code for a preferred direction of movement;
they fire most just before and during the
movement; they fire most when the
movement is in the preferred direction and
less as the direction deviates from the
preferred one
Primary Motor Cortex
• Lesions of primary motor cortex produce
contralateral asterognosia; they reduce the
speed and force of contralateral movements,
and they make it difficult to move one body
part independently of others (They do not
produce paralysis)
Cerebellum and Basal Ganglia
• Both are important subcortical
sensorimotor structures, but neither
participates directly in the transmission of
signals to the spinal cord
• Their role seems to be to integrate and
coordinate the activity of structures at
various levels of the sensorimotor system
Cerebellum
• The cerebellum constitutes 10% of the
brain’s mass, but it contains over half the
brain’s neurons; it is organized
systematically in lobes
• It receives inputs from primary and
secondary motor cortex, from brainstem
motor nuclei and from somatosensory and
vestibular systems
Cerebellum
• It is thought to correct deviations from intended
movements
• effects of diffuse cerebellar damage include loss of the
ability to precisely control movement, to adjust motor
output to changing conditions, to maintain steady postures,
exhibit good locomotion, to maintain balance, to speak
clearly, and to control eye movements
Cerebellum
• Long-recognized role in motor learning,
more recently appreciated for a role in the
fine-tuning and learning of nonmotor
cognitive responses
Basal Ganglia
• The basal ganglia are part of a loop that
receives information from various parts of
the cortex and transmits it back to motor
cortices via the thalamus
Basal Ganglia
• Basal ganglia are involved in sequencing of movements,
like the cerebellum, its role has recently been expanded to
include a variety of nonmotor cognitive tasks
• Basal ganglia function compromised in patients with
Parkinson’s Disease (due to loss of dopamine from
substantia nigra) and Huntington’s Disease (due to loss of
cells in basal ganglia)