Touch (somesthesis)

Purves et al., Chapter 9 (the somatic sensory system) and Chapter10 (pain)

Washington University Medical School's Neuroscience Tutorial has good coverage on this topic:
Basic Somatosensory Pathway
Somatosensation from the Body
Somatosensation from the Face

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 & 9.2
Much information here (did they forget to fill in conduction velocities?) - I will emphasize different sizes of myelinated (A) axons, alpha biggest and delta is smallest, and unmyelinated (C) axons.

Fig. 9.5
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
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
hot related to capsaicin

Fig Chapter 10 Box A
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
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
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
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
input into spinal cord

Fig. Box 9A
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
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
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
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
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
Interestingly, visceral pain goes in dorsal columns.
Very useful since midline myelotomy for palliative treatment in terminal and painful cancer.

Fig. 9.8B
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.

Fig. 10.6B
pain from face - trigeminal

Thalamus and cortex

Fig. 9.10
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
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. 9.8
two point threshold
2 mm fingertips, 30 arm, 70 back
this relates to the cortical projection (next:)

Fig. 9.11
sensory magnifications
Penfield - homunculus

Box D, Chapter 10
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
Pain modulation includes an efferent system
periaqueductal grey (PAG) enkephalin

Fig 10.8 B
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

Fig 9.7 C
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 - 2007 relating to this outline

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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