Touch (somesthesis)

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 an orgasm
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 stimulus onset.

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 textured surface

Merkel's disks are slow and have a small receptive field and are for light touch
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
Proprioceptors -
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 and needles")

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, 2002.
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
Nociceptors
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, acetylcholine).
Also tissue damage substances (Table 9.1): , serotonin (platelets), prostaglandins, leukotrienes,
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.

Input

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 is
translates into dermatomes - which place is innervated
herpes zoster "shingles" reactivated virus - localized to one sensory ganglion

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.
ipsilateral projection
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)
Jargon -
"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 mid-pons level.

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 1981
(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
sensory magnifications
Penfield - homunculus

TRANSPARENCY (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

Higher areas
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 cord
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?

anterolateral

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.

receptive field

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?

lateral inhibition

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 efferent axon].

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 mm

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)?.

gracile nucleus

In the sense of touch, neural processing takes place to sharpen spatial localization of the stimulus. What kind of neural processing?

lateral inhibition

As described by dermatomes, sensory receptors in the legs input to sacral and (what?) parts of the spinal cord.

lumbar

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 brain.

VPL (thalamus)

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 forearm.

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?).

heat

"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?

pain

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 cell?

beginning - the receptor itself, end - dorsal column (cuneate and gracile) nuclei

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?

mm

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?

antero-lateral system

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?

endorphin

In contrast to the anterolateral system for somatic pain, where does visceral pain ascend?

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?

dermatomes

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?

pain

Discriminative touch for the face comes into the brain by what nerve?

trigeminal (V)

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 to mm.)

finger tips

What is it called when you feel a heart attack in your arm?

referred pain

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 threshold)

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 synapse

Why would a Pacinian corpuscle have a larger receptive field than a Merkel's disk.

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 axons called?

IA

Capsaicin gates the VR-1 channel that is normally used for what type of stimulation?

warm

"In summary, the nociceptor is really a chemoreceptor." Name a chemical.

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 way bigger

In what way is the periaqueductal gray relevant in the somatosensory system?

part of efferent system to modulate afferent input


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This page was last updated 11/15/12