Sensory and motor systems

Text: selections from Chapters 8 & 10

Personal reflection

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.

Review

Fig. 8.28
Spinal cord has white matter (myelinated tracts)
and
Gray matter (cells and synapses)
Dorsal root ganglion has cells for sensory input
Ventral root has output (motor) axons
Unit of behavior is reflex
In addition,
Sensory information goes to brain
and
Motor output comes from brain
and that is what this outline is about

SENSORY

Touch (somesthesis) and motor representations in the cerebral cortex

Fig. 8.6
"This is your brain. This (colorful diagram) is your brain in BL A260 class"
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 an orgasm
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

Fig. 10.4
Cutaneous receptors
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 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
textured surface

Merkel's disks are slow and have a small receptive field and are for light touch
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

Nociceptors
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

Spinal cord

Fig. 8.24
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.
ipsilateral projection
First nucleus is in lower medulla
There is a cross-over, medial lemniscus and then the next nucleus is in the thalamus.

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

Brain

TRANSPARENCY (from intro book)
Just to show you that the material in your next transparency can also be found in the freshman biology book.

Fig. 8.7

RIGHT SIDE OF FIGURE
sensory magnifications
Penfield - homunculus

MOTOR

Motor systems

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.)

Fig. 8.25
Pyramidal system with corticospinal tract
Corticospinal tract Pyramidal motor system (75-90% crosses) 10 to the 6th axons
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, accessory, hypoglossal)

Basal ganglia (nuclei)

Fig. 8.11
Not just motor cortex, but huge parts of cortex feed to basal ganglia (and cerebellum).
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)

Fig. 8.21
inputs to basal nuclei
substantia nigra (nigrostriatal dopamine system)
(also in medial forebrain bundle [through lateral hypothalamus] is the mesolimbic dopamine system.)

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

Disorders

Parkinson's see synapse lecture

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, and others
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.

Cerebellum

Dysmetria (cannot approach target), ataxia, intentional tremor if cerebellar damage
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 - 2008 exams that relate to this outline

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?

CAG (glutamine)

Where is the decussation (cross over) of the lemniscal system for localized touch?

in medulla

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?

rapid adaptation

Between the tract called the medial lemniscus and the postcentral gyrus, there is a synapse in what famous "relay station?"

thalamus

"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 fine touch?

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 for?

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 acid)

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?

somatosensory projection

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?

receptive field

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 cord

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?

corticospinal, pyramidal

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?

somatosensory projection

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 tract called?

pyramidal system

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 the disease?

autosomal dominant

Huntington's disease is called a "triplet repeat disease." Triplets of what?

nucleotides

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 modalities

"A rapidly adapting pressure receptor is useful for active touch." Explain.

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?).

thalamus

(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 above

62. How was the map of the postcentral gyrus obtained?

Gently electrically stimulate and ask the person where (s)he feels it

63. "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 synapses?

Extrapyramidal - basal ganglia feed back to motor cortex via thalamus

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this page was last revised 8/3/09