Purves et al., Chapter 9 (the somatic sensory system) and Chapter10 (pain)
General and historical
A very compelling sense, from the pain of a tooth ache to the ecstasy of
considered in domain of "Physiology"
(vision and audition are more in the realm of psychology)
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 (turn of the century) - 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 and axons
Tables 9.1 p. 193 & 9.2 p. 195
Much information here - I will emphasize different sizes of myelinated (A)
axons, alpha biggest and delta is smallest, and unmyelinated (C) axons.
Fig. 9.5 p. 194
Skin (glabrous, there is also hairy)
The different types of receptors (in general, free nerve endings and encapsulated):
Free nerve endings
for pain, temperature and crude touch
the axons are C fibers (unmyelinated) and A delta, also slow
Landmark paper: MMendelson & WR Loewenstein, Mechanisms of receptor
adaptation, Science 144, 554, 1964 (see also J NIH Res., vol 8, 41-45, 1996.
Here is the work attributed mostly to Loewenstein in which he shows that
the Pacinian corpuscle is rapidly adapting because of the layers surrounding
the nerve ending (by dissecting off these layers).
Also, there is an electrical adaptation preventing continued spikes after
Pacinian corpuscle - rapid adaptation
A beta axons
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
A beta axons
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
A beta axons
static discrimination of shape
Ruffini slow - large receptive field -
sensitive to stretching in deep skin, ligaments and tendons
A beta axons
also Krauss in lips and genitals (dry vs mucous skin)
Fig. 9.7A p. 197
muscle spindles (nuclear bag fibers)
muscle spindle tension presets readiness for reflex, gamma motor neurons
to intrafusal fibers
Ia sensory axon
also Golgi tendon organs Ib afferents
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
Personal reflection My interest in Drosophila vision started
with an undergraduate project in 1968. In graduate school, I learned that
several scientists had isolated mutants with abnormal vision. DJCosens and
AManning (Nature 224, 285-287, 1969) published "Abnormal electroretinogram
from a Drosophila mutant." I met Cosens one and only one time
in 1978 and asked him "How did you find that mutant?" He told
me it had abnormal mating. By about that time, Baruch Minke, a leader in
that work who I met in 1974, had named the mutant trp (transient receptor
potential) on the basis of its ERG. It is amazing, with hindsight, what
happens when somebody decides to breed that fly and study its progeny.
Recent progress on determining channel properties
C. Seydel, How neurons know that it's cold outside, Science 295, 1451-1452,
D.E.Clapham, Hot and cold trp ion channels, Science 295, 2228-2229, 2002
cold related to menthol
Fig Chapter 10 Box A p. 212
hot related to capsaicin
Fig Chapter 10 Box A p. 212
Both involve VR-1 channel with homology to transient receptor potential
(trp) originally discovered in Drosophila because of difficulty in using
visual cues in mating and found not to have sustained photoreceptor potentials.
Fig. 10.2 p. 211
Pain is faster in A delta fibers than in C fibers
A delta mechano and mechano-thermal, and C fiber polymodal
Fig. 10.7 p. 221
Some mediators of pain are in bee and wasp sting venoms (serotonin, histamine,
Also tissue damage substances (Table 9.1): , serotonin (platelets), prostaglandins,
Histamine from mast cells, substance P
Bradykinin from blood borne precursor - enzyme from injury
Fig. 10.7 p. 221
In summary, nociceptor is really a chemoreceptor
Nociceptors are in many places, but not in brain, hence brain surgery under
local anesthesia used in mapping studies in humans by Penfield.
Fig. 9.8A p. 199
input into spinal cord
Fig. Box 9A p. 191
segmental organization of spinal cord - the dorsal root ganglion where input
translates into dermatomes - which place is innervated
herpes zoster "shingles" reactivated virus - localized to one
Fig. 9.8B p. 199
face & head enter via trigeminal nerve
Lower limbs are handled medially in gracile tract.
Upper limbs are lateral in cuneate tract.
First nucleus is in lower medulla
There is a cross-over, and then the next nucleus is in the thalamus.
This lemnicsal system is evolutionarily "new" (reptiles and above)
and is for localized touch.
In projection to the brain, there is processing - lateral inhibition to
sharpen spatial localization.
(This is the first mention of lateral inhibition, a fundamental mechanism
of sensory processing.)
If you tap your forearm, there are big waves but you feel localized touch.
Fig. 10.6A p. 219
spinothalamic 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."
Sharp pain can inhibit inhibit worse pain (example: a hard touch to a door
knob makes an electric shock less annoying)
"neospinothalamic" (more recently evolved) A-delta
"paleospinothalamic" (more ancient) C fibers
A small injury to the former can lead to intractable pain, so "psychosurgery"
can be helpful.
Dull pain (paleospinothalamic, C fiber) has more diffuse projection (see
below) and thus is less localized.
Fig. 10.4 p. 215
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)
Fig. Box B, Chap 10 p. 214
referred pain for viscera is interseting
heart attack in neck and left arm
notably, bladder stretch receptors localize pain to genitals
Fig. Box C Chapter 10 pp. 215-216
Interestingly, visceral pain goes in dorsal columns.
Very useful since midline myelotomy for palliative treatment in terminal
and painful cancer.
Fig. 9.8B p. 199
sensation from face - trigeminal
Cell is in trigeminal ganglion and first synapse is in a nucleus at the
The diving reflex,
that we study in undergraduate physiology lab, is mediated by the trigeminal
sensory input. There are 3
branches that can be individually manipulated (pick nerve # V, of course).
Fig. 10.6B p. 219
pain from face - trigeminal
Thalamus and cortex
Fig. 9.10 p. 202
VPL of thalamus to Postcentral gyrus- S1 = areas 1, 2, 3a & 3b
arranged in columns - a vertical electrode penetration same submodality
each S1 nerve responds to only one receptor type
Fig Box B Chapter 9 p. 203
In sensory map of cortex, all cells as electrode penetrates vertically are
from one area (Mountcastle)
(a) Ocular dominance coumns for vision (Hubel and Wiesel) Nobel
(d) Woolsey - (box) "barrels" from vibrissae (whiskers)
Fig. 9.3 p. 192
two point threshold
2 mm fingertips, 30 arm, 70 back
this relates to the cortical projection (next:)
Fig. 9.11 p. p. 202
Penfield - homunculus
(from intro book)
Here is the more traditional way those data are presented
Box D, Chapter 10 p. 222
Phantom limbs and phantom pain
hand maps on face - => plasticity, in that there is a rearrangement in
postcentral gyrus and hand is near face
now thought to be multiple maps not just association area
=> parallel rather than serial processing
Fig 10.8 A p. 225
Pain modulation includes an efferent system
periaqueductal grey (PAG) enkephalin
Fig 10.8 B p. 225
There are "microcircuits" in the dorsal (posterior) horn of spinal
all sensory input uses glutamate
pain also uses substance P
capsaicin causes release of substance P
enkephalin from Substantia Gelatinosa interneuron - presynaptic
(of course, opiates are narcotic analgesics)
stimulate - cause analgesia
connect to Raphe
itch - only skin and mucous - opiates not suppress
Exam questions from 2005 - 2012 relating to this outline
After C-type sensory afferents arrive in the spinal cord, the projection
of this information ought to ascend in what portion of the spinal cord?
Why would you expect a Pacinian corpuscle to have a large receptive field?
since it is deep, deflection of a large area of skin would stimulate it
For either (1) Group I and II afferent axons or (2) Gamma motor neurons,
state what is feeding into it (for 1) or what it feeds to (for (2).
stretch receptors, intrafusal muscles
Capsaicin stimulates the VR-1 receptor. Answer either (1) What would be
the "normal" (biologically relevant) stimulus? (2) What related
Drosophila mutant was discovered much earlier? or (3) Say something about
the molecular structure of this receptor.
heat, transient receptor potential, channel
What is the difference between the gracile and cuneate tracts?
gracile is information from lower limbs, cuneate from upper
Why isn't the face included in the diagram of dermatomes?
trigeminal cranial nerve input is separate from segmented spinal cord input
In terms of axon type, what is the difference in the neospinothalamic tract
vs the paleospinothalamic tract?
neo A delta, paleo C
Most pain from the lower body travels in the anterolateral tracts. For the
exception, answer either (1) What kind of pain travels elsewhere? (2) Where
does it travel?
viceral pain, dorsal columns
The two point discrimination threshold is 3 mm for the index finger and
45 mm for the calf. The tongue was not in the textbook figure, but you should
be able to give me that number here.
probably about 3 like the finger
The trigeminothalamic tract carries what particular type of information?
touch, pain, etc from the face
Your coverage in this course gave you only one neuron that releases eckephalin.
Where is it located?
dorsal horn, substantia gelatinosa
Stimulation of 9 square mm of skin affects one Merkl disk; by contrast,
stimulation of 60 square mm of skin affects one Ruffini end organ. Thus
the Ruffini has a larger (what is the expression?) than the Merkl.
Feature detection is the expression for the processing of sensory input
so that reduced information is passed along to the next higher level in
the nervous system. Drawing an analogy to the visual system, I argued that
it is easy to locate a gentle tap to the forearm even though all the flesh
that jiggles is actually stimulated. What is the expression for the type
of neural interaction that mediates this feature detection?
The afferent from a nociceptor has its cell body in the dorsal root ganglion.
Where is the first synapse? (Answer both: location plus which side, using
the appropriate term to answer which side.)
dorsal horn gray matter ipsilateral
Ia, II and A-beta are among the afferent axon types. Give me either [(1)
the designation or (2) a specific function] of a slower afferent axon type.
A-delta, C, pain, temperature
For proprioception and the stretch reflex, give the specific name of either
[(1) one of the two types of intrafusal muscle fibers, or (2) the fusimotor
nuclear chain or nuclear bag fiber, gamma motor neuron
A VR-1 receptor is a ligand-gated channel for which capsaicin is the ligand.
Answer either: (1) What is the more natural stimulus that affects this channel?
Or (2) What was the original member of this channel family called when it
was discovered to be deficient in a Drosophila visual mutant?
warm, transient receptor potential
There is a synapse in the gracile or cuneate nucleus. Answer either (1)
Where is the "beginning" of the cell that makes the synapse? Or
(2) Name one type of receptor that sends information in on this pathway.
touch, in the skin, Merkl, Meisner, Ruffini, Pacinian, stretch receptor
The anterolateral system is for pain. There is a notable exception (in terms
of spinal tract location). Answer either (1) What type of pain is carried
in this exceptional tract? Or (2) Where in the spinal cord is it carried?
viceral, dorsal columns at midline
Tell me a place where the 2-point discrimination threshold for fine touch
is less than 5 mm.
fingertips, lips, tongue
Name a transmitter used in a microcircuit in the dorsal (posterior) horn
gray matter (substantia gelatinosa).
glutamate, enkephalin, substance P
Regarding receptive field, answer either (1) Why is the Pacinian corpuscle's
receptive field the largest? Or (2) What units would you use to describe
the receptive field of a touch receptor?
deeper in the skin, a greater area of deformation would stimulate it; square
Under what normal circumstance would your receptors that are specifically
responsive to vibration be stimulated?
while running your fingers over a textured surface (active touch)
Gamma motor neurons (the fusimotor system) connects to (answer either) (1)
What? Or (2) For what purpose?
intrafusal muscle, preset the stretch of the stretch receptor
For capsaicin, answer either (1) What does the receptor look like (molecularly)?
(2) What would be the more normal way to stimulate this molecule? or (3)Tell
where (or how) this type of receptor was first found.
a channel, heat, transient receptor potential phenotype of Drosophila led
to mating defect
"In conclusion, a pain receptor is a chemoreceptor for (name a chemical)."
histamine, acetylcholine, serotonin, bradykinin
In the lemniscal system, a sensory receptor in the lower part of the body
makes its first synapse (where)?.
In the sense of touch, neural processing takes place to sharpen spatial
localization of the stimulus. What kind of neural processing?
As described by dermatomes, sensory receptors in the legs input to sacral
and (what?) parts of the spinal cord.
For the touch input from the face that travels in the medial lemniscus,
answer either (1) What is the cranial nerve for input? Or (2) What is the
specific part of the thalamus for synapses?
trigeminal (V), VPM
A cell in the dorsal horn of the gray matter of the spinal cord carries
pain and temperature information to (where is the first synapse in?) the
You were introduced to the term "neospinothalamic" whose input
is from A-delta fibers. In comparison, answer either (1) What slower fibers
have input for the more evolutionary ancient system? Or (2) What is this
more ancient system called?
C fibers, paleospinothalamic
In general, pain from the lower body goes in the anterolateral system with
(what exception?). [Note, the question still applies to the lower part of
the body, i.e., not the face.]
from the viscera, via dorsal columns
Heart rate and blood pressure are changed substantially when a person dives
into water. These changes can be mediated by fairly minimal stimulation
with water. Answer either (1) What is that minimal part of the body that
needs to be stimulated? Or (2) What is the nerve that carries that information
to the brain?
face, trigeminal (V)
Compare the two-point discrimination threshold for the tongue vs for the
way smaller for tongue
Where did the Raphe nucleus get the sensory information to feed out to the
cell in the dorsal horn of the gray matter of the spinal cord?
any of these will do: Somatosensory cortex -> amygdala and hypothalamus
-> midbrain periaqueductal gray
For what specific aspect of mechanosensation would the rapid adaptation
of Meissner's corpuscles be useful?
active (feeling) touch (of a textured surface)
Answer either (1) gamma motor neurons innervate what specific type of cell?
Or (2) what is the function of this activation?
intrafusal muscle fibers, preset the stretch of the stretch receptor
Capsaicin activates a channel that normally functions to detect (what?).
"First pain" and "second pain." What is the axonal difference?
A delta (small, myelinated) vs C (small, unmyelinated
What is the function of the cell that is stimulated by bradykinin?
The head of a neurosurgery patient is opened to expose the brain using only
local anesthetic. Answer either: (1) How do they get away with that? Or
(2) Why would they do it that way?
no pain receptors in the brain, to make certain they are not messing with
a really critical function
There is a cell body in the dorsal root ganglion for touch (not pain) input.
Where are the "beginning" and also the synaptic terminals of this
beginning - the receptor itself, end - dorsal column (cuneate and gracile)
In what way are the functions of the cuneate and gracile nuclei different?
cuneate upper body, gracile lower body
Before it reaches the thalamus, but after it enters the central nervous
system, in what way does pain and temperature information from the face
differ in its pathway from mechanosensory informationfrom the face?
Strangely, for pain, the pathway first descends (from the pons to the medulla)
The mean two-point discrimination threshold for the fingers is less than
5 for the fingers and more than 45 for the calf. Units?
The VPL of the thalamus connects to the somatosensory cortex. What does
VPL stand for?
ventral posterior lateral
The Raphe nucleus and the reticular formation feed to a cell in the dorsal
horn in the gray matter of the spinal cord for what purpose?
to modulate pain where it inputs
What does "trp" stand for when applied to channels?
transient receptor potential
A C (nociceptive) fiber synapses in the dorsal horn, and the post-synaptic
cell ascends in what part of the spinal cord?
In the Brown-Sequard syndrome, where is there reduced sensation of two-point
discrimination after a hemisection of the spinal cord?
ipsilateral below lesion
Narcotic analgesics would affect interneurons using what peptide in the
substantia gelatinosa of the spinal cord?
In contrast to the anterolateral system for somatic pain, where does visceral
in center of dorsal column
What aspect of neural organization explains why the irritation of shingles
(Herpes zoster) might be restricted to a small area in the body?
What is the function of bradykinin in sensory reception?
mediator of pain at receptor
A pathway from amygdala and hypothalamus through periaqueductal gray to
dorsal horn modulates what sensation?
Discriminative touch for the face comes into the brain by what nerve?
What aspect of Pacinian corpuscle function did Lowenstein demonstrate by
peeling off layers of the encapsulation?
it is phasic, i.e. responds transiently
Where does a sensory receptor for discriminative touch make its first synapse?
gracile or cuneate nucleus in lower medulla
Tell me a part of the body where the two point discrimination threshold,
measured in mm, is very low. (Pay close attention that "low" refers
What is it called when you feel a heart attack in your arm?
What sensory receptor has inputs via Group I and II afferent axons?
muscle stretch receptor
What is the difference in information carried in gracile vs cuneate tracts
in the dorsal columns?
lower vs upper parts of body
Why are the hands and face grossly enlarged in the sensory homunculus?
because of increased somatosensory "magnification" (low two point
Translate "midline myelotomy is a paliative neurosurgical intervention
for cancer patients whose pain is otherwise unmanageable."
for visceral pain, tract is in dorsal columns, and cutting myelinated fibers
will decrease suffering in terminal patients
A C nociceptive fiber makes its excitatory connection in the dorsal horn
to the cell whose axon is in the contralateral anterolateral system. How
does an enkephalin-containing local neuron mediate descending influence?
the interneuron inhibits via a presynaptic connection to the excitatory
Why would a Pacinian corpuscle have a larger receptive field than a Merkel's
being deeper, deformation of a larger skin area would stimulate it
In talking about proprioception, the muscle spindle and the reflex arc,
several different myelinated nerve axons were shown. What are the sensory
Capsaicin gates the VR-1 channel that is normally used for what type of
"In summary, the nociceptor is really a chemoreceptor." Name a
serotonin, prostaglandins, leukotrienes, histamine, substance P, bradykinin
Where (medial vs lateral) do axons from upper body input travel (relative
to those from lower body) in the dorsal columns?
Lower limbs are handled medially in gracile tract. Upper limbs are lateral
in cuneate tract.
There is somatosensory input from 7 cervical, 12 thoracic, 5 lumbar, and
4 sacral dermatomes. Why doesn't the face input through one of these?
It comes via the trigeminal
"A hemisection of the spinal cord leads to a contralateral loss of
spinothalamic input from below the injury." This is in contrast with
what other loss of what other system?
ipsilateral of lemniscal
About as much of the sensory homunculus is devoted to the lips as to the
legs. Make a statement about two-point discrimination threshold that relates
to this point.
lips, tongue fingers have a 1-2 mm two point threshold, legs and back are
In what way is the periaqueductal gray relevant in the somatosensory system?
part of efferent system to modulate afferent input
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