Sensory and motor systems
Text: selections from Chapters 8 & 10
Though broadly trained, my Ph.D. and my first job (Assistant Professor)
were in psychology. It was only in 1979 that I moved to biology. Long ago,
I learned of certain brain studies after transecting the spinal cord in
animals: the encephale isole, French for the isolated brain. This is much
like the preparatioon the Nobelist Sherrington used for studies of spinal
reflexes. I came to view studies above the transection as psychology and
below as physiology. Thus, to me, the somatosensory system and the motor
systems seem more in the jurisdiction of a physiology course than special
senses such as vision & audition.
Spinal cord has white matter (myelinated tracts)
Gray matter (cells and synapses)
Dorsal root ganglion has cells for sensory input
Ventral root has output (motor) axons
Unit of behavior is reflex
Sensory information goes to brain
Motor output comes from brain
and that is what this outline is about
Touch (somesthesis) and motor representations in the cerebral cortex
"This is your brain. This (colorful diagram) is your brain in BL A260
Brain, central sulcus with post-central gyrus (somatosensory projection)
and pre-central gyrus (motor area)
Many other aspects of "localization of function" for cerebral
cortex are shown here:
auditory area, visual area, Broca's area (speech)
Note also the cerebellum, an area devoted to motor coordination.
General and historical
A very compelling sense, from the pain of a tooth ache to the ecstasy of
There has been an emphasis on submodalities (qualities such as pain vs.
hot), where modalities refers to different senses like vision and audition
von Frey (around 1900) - punctate sensitivity - touch forearm with pencil,
sometimes feels cold, sometimes feel pressure.
This approach overemphasized correlation of histoloogical receptor type
with sensory experience.
It fit in well with Muller's (mid-1800's) "doctrine of specific nerve
energies" - in which, if the ears were made to feed in through the
optic nerve, sounds would be experienced as visual sensations because the
quality comes from the nervous system not the physics of the stimulus.
The present view of receptors and axons depends more on nerve type and adaptation,
and the central projection (axon type [A myelinated, C unmyelinated] pathway
[dorsal columns = lemniscal vs anterolateral = spinothalamic) is critical.
Receptors in the skin
The different types of receptors (in general, free nerve endings and encapsulated):
Free nerve endings
for pain, temperature and crude touch
Pacinian corpuscle - rapid adaptation
Lowenstein - peel to show layers make rapid adaptation
very sensitive, very large receptive field (area which, if stimulated, will
affect the receptor [or higher order sensory nerve]
vibration - 250 - 300 Hz
here is a Pacinian corpuscle Pacinian
corpuscle from our histology course
Meisner's corpuscles are fast but not as fast as Pacinian
encapsulation is with Schwann cell layers
most common receptors of fingers, palms and soles
smaller receptive field
"feeling" - active touch - would use fast as finger moves across
Merkel's disks are slow and have a small receptive field and are for light
finger tips, lips and genitals
static discrimination of shape
Not in diagram
Ruffini slow - large receptive field -
sensitive to stretching in deep skin, ligaments and tendons
Krauss in lips and genitals (dry vs mucous skin)
warm and cold
a person can feel a difference of 0.01oC
relation to body temperature
(cold have additional peak at high temp - paradoxical cold -
pins and needles)
cold related to menthol
hot related to capsaicin
some mediators of pain are in sting venoms
Also tissue damage substances: serotonin (platelets), prostaglandins, leucotrienes,
histamine from mast cells, substance P , bradykinin from blood borne precursor
enzyme from injury, receptor is chemoreceptor
nociceptors are in many places, but not in brain, hence brain surgery under
local anesthesia used im mapping studies in humans by Penfield
input into spinal cord
(does not include face & head which enter via cranial nerves [trigeminal])
LEFT SIDE OF FIGURE
Lemnicsal system is for localized touch.
Lower limbs are handled medially in fasciculus gracilis.
Upper limbs are lateral in fasciculus cuneatus.
First nucleus is in lower medulla
There is a cross-over, medial lemniscus and then the next nucleus is in
In projection to the brain, there is processing - lateral inhibition to
sharpen spatial localization
If you tap your forearm, there are big waves but you feel localized touch.
RIGHT SIDE OF FIGURE
spinothalamic for pain and temperature
with synapse and decussation at entry point.
There are separate tracts in spinal cord.
The lateral portion is for pain and temperature.
The ventral (anterior) part is for gross tactile sense.
Hence the nomenclature "anterolateral."
Touch can inhibit inhibit pain (a hard touch to a door knob makes an electric
shock less annoying)
A half spinal cord injury would cause contralateral loss of spinothalamic
below injury and ipailateral loss of lemniscal.
Brown-Sequard syndrome include motor (ipsilateral impairment)
referred pain for viscera is interseting
notably, blatter stretch receptors localize pain to genitals
heart attack in neck and left arm
(from intro book)
Just to show you that the material in your next transparency can also be
found in the freshman biology book.
RIGHT SIDE OF FIGURE
Penfield - homunculus
LEFT SIDE OF FIGURE
topographic map of motor cortex- compare with corresponding sensory homumculus
work by neurosurgeon Penfield, note relative "magnifications"
toes (thought to curl with sexual excitement) in motor cortex across from
genital projection in postcentral gyrus.)
Pyramidal system with corticospinal tract
Corticospinal tract Pyramidal motor system (75-90% crosses) 10 to the 6th
named because it goes through pyramids on ventral medulla
(though it might have been named from pyramidal shaped neurons in layer
V incl. Betz cells)
lateral and anterior pathways
Initiation of voluntary motor movements
output for face and upper body via facial nerve (and trigeminal, vagus,
Basal ganglia (nuclei)
Not just motor cortex, but huge parts of cortex feed to basal ganglia (and
Extrapyramidal (because it lies outside the pyramids)
caudate + putamen = striatum (striated because strands of internal capsule
make it look striated)
putamen + globus pallidus = lentiform nucleus [lens shaped] (see
sheep brain horizontal section)
inputs to basal nuclei
substantia nigra (nigrostriatal dopamine system)
(also in medial forebrain bundle [through lateral hypothalamus] is the mesolimbic
outputs from basal ganglia
The globus pallidus is a relay nucleus for the caudate and putamen and so
is the subthalamus.
To VA/VL complex of thalamus to motor cortex
Parkinson's see synapse
Huntington's (1872) disease (chorea) choreoathetosis
Dominant late onset - many interesting genetic counseling issues. The Folk
singer Woodie Guthrie died of Huntingtons. There is a big family tree derived
from Venezuela near lake Maracaibo
On post-mortem, degeneration of putamen and caudate is observed.
It is on short arm of chromosome 4
1983 and since: cloning -CAG repeat (polyglutamine repeat), 15-34 (normal)
-> 42-66 (Huntington's)
Other trinucleotide repeat diseases: fragile X syndrome, myotonic dystrophy,
sometimes they get worse from generation to generation (anticipation)
in some ways, Huntingtons is the opposite of Parkinsons in that circuit
has thalamus increasing excitation to cortex.
Dysmetria (cannot approach target), ataxia, intentional tremor if cerebellar
cerebellum highly developed in electric fish
cerebellum is involved in rhythmic activity and plasticity
An additional decussation makes it so that cerebellum controls the ipsilateral
side of the body.
Questions from 2004 - 2011 exams that relate to this outline
Why would it be useful to have rapidly adapting touch receptors, that rapid
adaptation resulting in vibration reception at 250-300 Hz?
for active feeling touch of a textured surface
Your fingertips, tongue, and lips are very sensitive for fine touch. For
instance a small thing stuck between your teeth feels bigger to your tongue
than it looks when you floss it out. How is this difference (your legs,
back and arms are not as sensitive) represented on the postcentral gyrus?
bigger areas for lips, fingertips and tongue
For motor function, describe the function of either (1) tie internal capsule,
or (2) the nigrostriatal tract.
internal capsule has axons from precentral gyrus in corticospinal tract,
nigrostriatal tract sends dopamine from substantia nigra to striatum
A cell body in the precentral gyrus sends an axon through a decussation
in the medulla oblongata. Where does that axon make its synapse?
onto the spinal motor neuron in the ventral horn of the spinal cord gray
"An enzyme at the site of injury converts a precursor into bradykinin."
What does this tell us about the somatosensory system?
nociceptors are actually chemoreceptors
A free nerve ending or an encapsulated touch receptor like a Pacinian corpuscle
that mediates fine touch has an axon that comes into the central nervous
system. Where does this cell have its synaptic termination?
way up in the medulla
How did they learn which parts of the body project to which parts of the
gently stimulate the gyrus in a patient under local anesthesia ahd ask where
(s)he feels a tingle
"If half of your spinal cord were lesioned, say as a result of an accident,
you would have an ipsilateral loss of sensation mediated by the lemniscal
system below the site of the injury." What is the situation (side of
the body relative to which half is cut) for pain and temperature sensation
below the site of the injury?
The corticospinal tract would be involved in arm and leg movements. In what
way is this situation different for the face?
In terms of the extrapyramidal motor system of the brain, answer either
(1) where does dopamine come from? or (2) Where dies dopamane go to?
substantia nigra, striatum
"The thalamus is a motor relay." How can that be true when, in
fact, the tract from the precentral gyrus to the spinal motor neuron does
not have a synapse in the thalamus?
basal ganglia and cerebellum feed back to postcentral gyrus through thalamus
What would happen to the coding sequence of the mutant allele for Huntington's
chorea from one generation to the next?
there would be more CAG's coding for more glutmines
A neurosurgeon applies a gentle electrical stimulus to the postcentral gyrus
of an awake patient under local anesthesia. What does the subject say (or
(s)he feels something on a certain area of the body
A somatosensory cell has a synapse in the dorsal (and lateral) part of the
spinal cord. Where does the postsynaptic cell make its synapse in the brain?
Thalamus (spino-thalamic division of the somatosensory input)
The basal nuclei collect information from all over the brain and send it
to (where?) to achieve better coordination of motor movements.
Motor cortex (via thalamus)
In the 1800s, a famous French physician studied stroke patients. What conclusions
did he make upon examining their brains in autopsy that led to the naming
of a brain area after him?
Broca found a small area on only one side of the brain that mediated the
motor aspect of speech
Axons for one sensory system are in the dorsal columns. Answer either (1)
Where are these dorsal columns? Or (2) What is being perceived by this sensory
Dorsal white matter of spinal cord, fine touch
Cells with sensory information make their synapses in the postcentral gyrus.
Where are the cell bodies of these cells?
Cells in the precentral gyrus innervate muscles in the face via cranial
nerves, while the tracts are localized (where?) to reach the motor neurons
for the part of the body below the neck.
Anterior and lateral parts of the white matter of the spinal cord
"Huntingtons was found to be a triplet repeat disease." What does
One DNA triplet (CAG) coding for glutamine expands to too many copies
Basal nuclei (caudate, putamen, and globus pallidus), part of the extrapyramidal
motor system, output through the thalamus to where?
the precentral gyrus
Faciculus cuneatus and fasciculus gracilis are part of what system?
lemniscal (for fine touch)
The gene (or gene product) of the gene that is abnormal in Huntington's
chorea has a variable number of what?
Where is the decussation (cross over) of the lemniscal system for localized
A cell in the precentral gyrus whose axon is part of the corticospinal tract
makes its first synapse on what cell?
spinal motor neuron
What is the function of the postcentral gyrus?
primary somatosensory projection
What is mediated by the spinothalamic (anterolateral) system that decussates
at the level it enters the dorsal root?
pain and temperature
What are cells in the dorsal root ganglion used for?
somatosensory input to spinal cord
What property do the layers of encapsulation of a Pacinian corpuscle confer?
Between the tract called the medial lemniscus and the postcentral gyrus,
there is a synapse in what famous "relay station?"
"The left half of the brain is for the right side of the body and vice-versa."
Is this conventional wisdom true for pain and temperature as well as for
yes, both, though crossings are at different levels
What is the function of the precentral gyrus?
voluntary motor initiation, motor cortex
Why is the anterolateral system so named (where is it?) and what is it used
location between anteriot (ventral) and lateral in spinal cord white matter,
pain and temperature
Why is Huntington's disease sometimes called "chorea?"
chorea refers to jerky movements
Relate the expressions "triplet repeat" and "polyglutamine."
extra CAG's (nucleotide sequence) code for a string of glutamines (amino
Where does a cell in the precentral gyrus make its first synapse?
Spinal motor neuron
In terms of localization of function of the cerebral cortex, what is the
function of the postcentral gyrus?
How would the protein product of the Huntington's disease gene compare for
someone without vs. with the disease?
more glutamines (amino acids) in a row with because of CAG triplet repeat
The output of the motor cortex (precentral gyrus, pyramidal system) goes
all the way to the spinal motor neuron. By contrast, where do basal nuclei
(extrapyramidal system) feed to?
thalamus, motor cortex
"That area of the skin, if deformed, that affects the response of one
specific Pacinian corpuscle." What is this called?
Serotonin is a mediator for nociceptors. Name another.
prostaglandins, leukotrienes, histamine, substance P, bradykinin
The spinothalamic system for pain and temperature is also called the anterolateral
system. Why (does it have this other name)?
tracts are in ventral (anterior) and lateral parts of white matter in spinal
Between the pain receptor and the lateral spinothalamic tract, where is
the first synapse?
in dorsal horn of spinal cord gray matter
Face and hand occupy about half the map of the motor cortex (precentral
gyrus). How was this map determined?
stimulate brain of awake surgery patient and see where movement is
Earlier this semester, you learned that the substantia nigra does not make
enough dopamine in Parkinson's disease. How does the substantia nigra feed
into the motor system?
connects to striatum in extrapyramidal motor system
Voluntary motor movements for the face go out through cranial nerves. By
contrast, what tract carries motor output from the motor cortex (precentral
gyrus) to the lower part of the body?
Where is the cell body for the somatosensory receptor cell?
right outside the dorsal root of the spinal cord, dorsal root ganglion
What is the function of the postcentral gyrus?
Why would it be useful to have a touch receptor that is sensitive for vibration?
for active feeling
Where is the first synapse in the lemniscal input?
in the medulla (fasciculi gracilis and cuneatus)
What does it signify that the hand is as large as the arm on the motor cortex?
magnification where motor movements are more dextrous
In comparison with the extrapyramidal system, what is corticospinal spinal
Where does the globus pallidus feed to?
to thalamus to motor cortex
The dopamine system is called "nigrostrital." Give the real names
of the areas for which it got this name.
substantia nigra, striatum
"Nociceptors are chemoreceptors." Explain.
chemicals at the site of injury like bradykinin stimulate them
Where is the first synapse in the anterolateral input?
right in the dorsal horn
Why would half the children of a Huntington's victim be expected to get
Huntington's disease is called a "triplet repeat disease." Triplets
Why does a textbook have a lateral view of the cerebral cortex which has
different locations colored differently?
to emphasize localization of function, for instance sensory areas for different
"A rapidly adapting pressure receptor is useful for active touch."
As you feel a textured surface, that receptor is vibrated
For the anterolateral system, a cell in the dorsal horn of the gray matter
of the spinal cord makes its synapse (where in the brain?).
(Refer to the previous question.) "This anterolateral system ultimately
projects to the same part of the cerebral cortex as another system. Answer
either (1) What is this part of the cortex called? Or (2) What is this other
system? (name, location in spinal cord, OR function will suffice).
(1) motor cortex = precentral gyrus (2) lemniscal carried in dorsal columns
mediating fine touch
"The corticospinal tract is for voluntary motor output." What
about the face?
Cranial nerve 5 trigeminal
"Huntington's disease shows 'anticipation,' getting worse from generation
to generation." What changes (molecularly)?
Nucleotide triplet CAG (that codes for glutamine
WHY (note, I am just asking why) would a half spinal cord lesion affect
senses mediated by spinothalamic vs lemniscal systems below the injury differently?
Where they cross over is different, spinothalamic below the lesion, lemniscal
How was the map of the postcentral gyrus obtained?
Gently electrically stimulate and ask the person where (s)he feels it
"The thalamus is a relay (more than a relay!) for sensory and motor
systems." And yet the corticospinal tract went right past the thalamus
without making a synapse. In what system, then does the thalamus have motor
Extrapyramidal - basal ganglia feed back to motor cortex via thalamus
What is the use of having a receptor like the Pacinian corpuscle that is
responding to vibration at 250 to 300 Hz?
in active touch (feeling), a textured surface will stimulate a given receptor
Explain the mechanism of nociceptor function in terms of bradykinin.
it is a chemoreceptor responsive to signals indicating tissue damage like
A receptor mediating fine touch discrimination comes into the spinal cord
at the dorsal root ganglion. Where does it make its first synapse?
in the medulla
For the lemniscal system, answer either (1) Why is it called the lemniscal
system? Or (2) What specific portion of somatosensory function does it mediate?
there is a tract in the brain called the medial lemniscus, fine touch discrimination
In brain surgery under local anesthesia, how would the subject respond to
gentle electrical stimulation to any given spot on the postcentral gyrus?
(s)he would indicate that (s)he felt something (relating to the map)
"The decussation of the pyramidal tract" Answer either (1) At
what level is it? Or (2) What is it called (as it descends after it decussates,
please do not repeat "pyramidal tract")?
medulla where pyramids are seen, lateral corticospinal tract
"The motor impairment from a half spinal cord injury would be ipsilateral."
after the decussation, tract is on the same side of the body as the muscles
Motor output to the face is not in tracts in the spinal cord. What nerves
or tracts send voluntary motor output to the face?
facial nerve (and some other cranial nerves)
Name a structure in the extrapyramidal system that is functionally between
the substantia nigra and the thalamus.
caudate, putamen, globus pallidus, striatum, lentiform nucleus
In Huntington's chorea, what would change (molecularly) if the disorder
were worse one generation than in the previous generation?
the triplet (CAG) that codes for glutamine
A receptor mediating pain or burning sensation comes into the spinal cord
at (what?) ganglion?
dorsal root ganglion
What is it that is anterior and lateral that gave the anterolateral system
tracts in the white matter of the spinal cord
For the basal ganglia (nuclei) explain the naming of either (1) the lentiform
nucleus or (2) the striatum.
lens shaped in horizontal secton, looks striated from branches of internal
"In a sense, Huntington's is the opposite of Parkinson's." How
hyperkinesia vs hypokinesia
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