Notes � Neuro II, somatic sensory system (object manipulation + surface feature analysis)
Greg Detre
Tuesday, 09 October, 2001
Dr John Iles, St Hughes
Essay title
How does the somatic
sensory system contribute to object manipulation and surface feature analysis?
Reading list
Notes � Carpenter (1995), Neurophysiology
Notes � Kandel, Schwarz and Jessell, Essentials of Neural science
amputated limb stuff
cortex columnar organisation
monkey pictures
3a/3b � 1, 2
aphasias
Notes � Kandel & Schwarz, Principles of neural science, ch 26 on Touch
The somatic sensory system is concerned with four major modalities:
discriminative touch (recognising the size, shape and texture of
objects and their movement across the skin)
proprioception (the sense of static position and movement of limbs and body)
nociception (the signalling of tissue damage, often perceived as pain)
temperature sense ( warmth and cold).
Most aspects of touch are carried by the dorsal column-medial lemniscal
system.
The skin and underlying tissue contain four types of receptors:
the
superficial skin has:
rapidly
adapting Meissner�s corpuscles
slowly
adapting Merkel�s cells
(both
respond to touch)
deeper
tissue contains the
rapidly
adapting pacinian corpuscles, which respond to vibrations
slowly
adapting Ruffini�s corpuscles, which respond to rapid indentation of the skin
These are:
innervated
by peripheral axons of nerve cells in the dorsal root ganglia
their
central branches ascend in the dorsal columns and synapse with second-order
neurons in the dorsal column nuclei
then
cross the midline in the medulla and ascend through the brain stem on the
contralateral side as the medial lemniscus
in
the thalamus they synapse on third-order cells in the ventral posterior medial
and ventral posterior lateral nuclei
the
third-order neurons in the thalamus send axons to the primary somatic sensory
cortex (S-I) in the post-central gyrus of the parietal lobe, subdivided into
Brodmann�s areas 1, 2, 3a and 3b
most
thalamic fibers terminate in 3a and 3b
the
cells in 3a and 3b project to 1 and 2
thalamic
neurons also send a sparse projection directly to 1 and 2, and to the adjacent secondary
somatic sensory cortex (S-II)
S-II
is also innervated by neurons from each of the four areas of S-I
(Mishkin
found that these S-I projections are required for the perceptual function of
S-II, e.g. removing the neural connections in S-I representing the hand area completely
prevents stimuli applied to the skin of the hand from activating neurons in
S-II)
some thalamic neurons project to the posterior parietal cortex
(Brodmann�s areas 5 and 7), which also receives input from S-I
Notes � Kaas & Collins (2001), The organisation of sensory cortex
Notes � Johnson (2001), The roles and functions of cutaneous mechanoreceptors
The results support the idea that each of the four mechanoreceptive
afferent systems innervating the hand serves a distinctly different perceptual
function, and that tactile perception can be understood as the sum of these
functions.
The four cutaneous mechanoreceptive afferent neuron types that
innervate the glabrous skin comprise slowly-adapting type 1 (SA1) afferents
that end in Merkel cells, rapidly adapting (RA)afferents that end in Meissner
corpuscles, Pacinian (PC) afferents that end in PC corpuscles, and slowly
adapting type 2 (SA2) afferents that are thought to terminate in Ruffini
corpuscles. Each of these neuron types responds to cutaneous motion and
deformation in a different way. The mechanosensitive transducers reside in the
unmyelinated endings of the afferent fibers. The receptors� selectivity seems
to be due as much to the receptor structure that surrounds each of these
endings as to the transducer itself.
The Merkel cell has the simplest structure; it is a special cell type
in the basal layer of the epidermis that enfolds the unmyelinated ending of the
SA1 afferent fiber. The SA1 receptor is selectively sensitive to a particular
component of the local stress-strain field,which makes it sensitive to edges,
corners,and curvature;it is not known whether this selectivity is due to the
Merkel cell or to the transducer mechanism within the afferent terminal.
Meissner corpuscles are\ relatively large cell assemblies in the dermal ridges
that lie just beneath the epidermis. They comprise cell layers that cushion and
enfold the large leaf-like endings of two to six RA afferent fibers. This
pillow-like arrangement appears to act as a filter that protects the
velocity-sensitive endings from static skin deformation. PC corpuscles reside
in the dermis and deeper tissues. The PC corpuscle is a large,layered
onion-like structure with as many as 70 layers,enclosing a single nerve ending
that is sensitive to deformation in the nanometer range. The layers function as
a series of mechanical filters to protect the extremely sensitive receptor from
the very large, low-frequency stresses and strains of ordinary manual labor.
The Ruffini corpuscle, which is located in the connective tissue of the dermis,
is a relatively large spindle shaped structure tied into the local collagen
matrix. It is, in this way, similar to the Golgi tendon organ in muscle. Its
association with connective tissue makes it selectively sensitive to skin
stretch. Each of these receptor types and its role in perception is discussed
below.
Merkel SA1 (slowly-adapting) afferents
They give us the sensation of texture(???) and curvature when we move our fingers over an object � they have the highest spatial resolution, aren�t much affected by contact force, and are at least 10 times more sensitive to dynamic stimuli (at up to 80mm/s)
Meissner RA (rapidly-adapting) afferents
These are even more sensitive to dynamic stimulation than Merkel SA1s, and because they respond to stimuli over their entire receptive fields, they resolve spatial detail poorly.
Johansson has shown that as we lift and manipulate an object there are
frequent microscopic slips between the object and the skin, and that the skin
motion associated with these slips evokes reflexive increases in grip force � A
complication is that the required grip forces depend on factors like surface
coefficient of friction, as well as the object's weight. The evidence from
microneurographic recordings in humans as they lift and manipulate objects and
controlled psychophysical/neurophysiological experiments is that RA afferents
provide the signals that are critical for grip control
Pacinian afferents
These are remarkably sensitive (sometimes responding to 10nm of skin motion). However, they have extremely low spatial resolution, perhaps even having the entire hand as a receptive field. Also, they intensely filter low-frequency stimuli that would otherwise overwhelm them. Third, they respond to stimuli less than 100-150Hz with a phase-locked, Poisson distribution.
The pacinian population produces a high-fidelity neural image of transient
and vibratory stimuli transmitted to the hand by objects held in the hand. An
important consequence of this is the perception of distant events through
transmitted vibrations when we grasp an object in the hand (Hunt). They are
also responsible for perceiving events at the working surface of a tool/probe
that we are skilled with as though our fingers were present.
RA afferents are about 2 orders of magnitude less sensitive than pacinian corpuscles. That they pick up vibrations from distant events was discovered by accident when spontaneous discharges were linked to ambient vibrations in the laboratory.
If it were not for the steep filtering provided by the multi-layered
fluid-filled corpuscles, the sensitive receptor within would be over-whelmed by
the deformations produced by these forces. If the extrapolation to low
frequencies illustrated in the paper is accurate, a peak-to-peak motion of 1cm
at 2Hz would not activate the PC system.
Ruffini SA2 afferents
SA2 afferents innervate the skin less densely than either SA1 or RA afferents.
SA2 receptive fields are about five times larger, six times less
sensitive to skin indentation but 2-4 times more sensitive to skin stretch than
SA1 afferents. The SA2 population transmits a neural image of skin stretch to
the CNS with relatively little interference from objects held in the hand.
They have never been observed in neurophysiological studies of
mechanoreceptors in the monkey hand, and so have been studied less extensively
than the other afferent types.
Two important roles have been identified for SA2 afferents:
1. perception of the direction of object motion/force when producing skin stretch (although Gardner and Sklar have shown with vibrating pins that activate only RA and Pacinian afferents that motion and motion direction are still discriminated effectively without SA2 afferent input).
2. the SA2 afferents assist (along with muscle spindles and possibly joint afferents) in the perception of hand shape and finger position through the pattern of skin stretch produced by each hand and finger conformation - simply stretching this skin, activating SA2 afferents strongly (and SA1 afferents more weakly) produces the illusion of finger flexion, as does tendon vibration
The much greater sensitivity to stretch than indentation suggests that
the SA2 receptor is sensitive to horizontal tensile strain. This and the SA2
receptor's deep location seems to shield SA2 afferents from the confounding
effects of the indentation produced by an object, leaving it free to signal the
object's direction of motion and hand conformation.
Conclusion
There is accumulated evidence for a sharp division of function among the four cutaneous afferent systems that innervate the human hand:
SA1 - spatial structure of objects and surfaces that is the basis of form and texture perception
RA - motion signals from the whole hand, critical for grip control and information about the motion of objects contacting the skin
Pacinian - vibrations transmitted to the hand from objects contacting or grasped in the hand (helping us to perceive distant events through held objects)
SA2 - skin stretch over the whole hand, possibly for hand conformation from the dorsal SA2 image (and ventral when the hand is empty), and ventral (when the hand is full) for the direction of motion of objects moving across the skin surface and about the direction of forces exerted on the hand
Because the computational problems inherent in processing information
for form and texture perception (SA1) have little in common with the problems
inherent in processing information about motion and motion direction (RA and
SA2 functions), the distinctively different functions identified for the four
cutaneous mechanoreceptive afferent systems suggest the existence of distinct
and separate central systems for processing the information provided by each of
the primary afferent groups. For example, area 3b of primary somatosensory
cortex seems to contain neurons highly selective for spatial form and to
preserve spatian information at high scanning velocities. On the other hand,
these 3b neurons are no more sensitive to motion or motion direction than are
primary afferents, suggesting that the processes underlying motion perception
lie elsewhere.
One problem for experimenters is that all four afferent systems are so
sensitive that almost all suprathreshold stimuli activate all four systems. In
order to find out more about the central pathways for each of the systems, we
need to learn how to more selectively stimulate the afferent systems. Then we
can understand the operations that underlie the perceptual functions each of
the systems.
Notes - Bierman et al. (1998), Interaction of finger representation in the human first somatosensory cortex
too hard
Notes - Proske et al. (2000), The
role of muscle receptors in the detection of movements
abstract doesn�t look relevant
Notes - Killackey (1995), The formation of a cortical somatotopic map (in rodents)
The somatosensory cortical map in the rat has been demonstrated by a
number of anatomical techniques, including the Nissl stain, histochemistory of
succinic dehydrogenase, cytochrome oxidase and AChE and immunocytochemistory of
5-HT
The distribution of thalamocortical afferent terminals arising in the
ventral posterior nucleus forms this same map
During development, the subcortical and cortical maps emerge
sequentially, beginning at the periphery and ending in the cortex. Peripheral
damage during a perinatal period (extending from approximately six days before
birth to four days after) alters both subcortical and cortical maps. This has
been taken as strong evidence that the action of the periphery through thalamic
afferents has a major organising influence on the somatosensory cortical map.
Later peripheral damage produces local map changes without affecting the overall
boundaries.
Further evidence that the formation of of a vibrissae-related pattern
is extrinsic to the neocortex is the demonstration that the visual cortex of
the neonatal rat, when transplanted to the region of the somatosensory cortex,
is capable of supporting ingrowth of thalamocortical afferents, and teh
expression of a vibrissae-related pattern (Schlagger & O'Leary, 1994).
Notes - Zhang et al. (2001), Functional characteristics of the parallel SI- and SII-projecting neurons of the thalamic ventral posterior nucleus in the marmoset � abstract only
The functional organization of the primate somatosensory system at
thalamocortical levels has been a matter of controversy, in particular, over
the extent to which the primary and secondary somatosensory cortical areas, SI
and SII, are organized in parallel or serial neural networks for the processing
of tactile information
There appeared to be no systematic differences in functional capacities
between SI- and SII-projecting neurons of each of these three classes, based on
receptive field characteristics, on the form of stimulus-response relations,
and on measures derived from these relations. These measures included threshold
and responsiveness values, bandwidths of vibrational sensitivity, and the
capacity for responding to cutaneous vibrotactile stimuli with phase-locked,
temporally patterned impulse activity. The analysis indicates that
low-threshold, high-acuity tactile information is conveyed directly to both SI
and SII from overlapping regions within the thalamic VP nucleus. This direct
confirmation of a parallel functional projection to both SI and SII in the
marmoset is consistent with our separate studies at the cortical level that
demonstrate first, that tactile responsiveness in SII largely survives the SI
inactivation and second, that SI responsiveness is largely independent of SII.
It therefore reinforces the evidence that SI and SII occupy a hierarchically
equivalent network for tactile processing.
Notes � Huffman & Krubitzer (2001), Thalamo-cortical connections of areas 3a and M1 in marmoset monkeys
In this study we examine the thalamic connections of
electrophysiologically defined regions in area 3a and architectonically defined
primary motor cortex (M1). Our studies demonstrate that area 3a receives input
from nuclei associated with the somatosensory system:� the superior, inferior, and lateral divisions of the ventral
posterior complex (VPs, VPi, and VPl, respectively). Surprisingly, area 3a
receives the majority of its input from thalamic nuclei associated with the
motor system, posterior division of the ventral lateral nucleus of the thalamus
(VL), the mediodorsal nucleus (MD), and intralaminar nuclei including the
central lateral nucleus (CL) and the centre median nucleus (CM). In addition,
sparse but consistent projections to area 3a are from the anterior pulvinar
(Pla). Projections from the thalamus to the cortex immediately rostral to area
3a, in the architectonically defined M1, are predominantly from VL, VA, CL, and
MD. There is a conspicuous absence of inputs from the nuclei associated with
processing somatic inputs (VP complex). Our results indicate that area 3a is
much like a motor area, in part because of its substantial connections with
motor nuclei of the thalamus and motor areas of the neocortex (Huffman et al.
[2000] Soc. Neurosci. Abstr. 25:1116). The indirect input from the cerebellum
and basal ganglia via the ventral lateral nucleus of the thalamus supports its
role in proprioception. Furthermore, the presence of input from somatosensory
thalamic nuclei suggests that it plays an important role in somatosensory and
motor integration.
Notes � web, �Sect 2: Somatosensory system�
S-I
3a stimulation from muscle afferents
3b stimulation from Merkel's disc and Ruffini organs (slow adapt)
1 stimulation from Meissner's and pacinian corpuscles (fast adapt)
2 stimulation from joint afferents
S-II
Receives input from the primary somatosensory cortex;
Functions in stimulus recognition;
Sends output to the Association cortex
Somatosensory Association Cortex
Brodmann's areas 5 & 7
Located on posterior parietal cortex
Integrates sensory input from the thalamus and the secondary somatosensory cortex.
Functions in perception of body parts in relation to one another, body parts in relation to the external environment, relation of objects in the environment, spatial memory, and attention.
Discarded
In considering the mechanisms involved in object manipulation and surface feature analysis, I intend to focus on discriminative touch, althoguh
Questions
at what point is information from both sides of the body integrated???
what does he mean by the somatic sensory system �contributing� to object manipulation and surface feature analysis???
isn�t object manipulation a motor thing??? yes, to some extent
is kinesthesia involved??? does proprioception include it???
kinesthesia - The sense of muscular effort that accompanies a voluntary motion of the body. Also, the sense or faculty by which such sensations are perceived. So kin�hetic (-tk) a., belonging to kin�hesis; also, involving or utilizing kin�hesis; kin�hetically adv.
Kandel & Schwarz � Principles
is that all there is to the somatic sensory system??? what about the somatic association cortex??? where does it go after S-II??? what does the posterior parietal cortex do???
where is vision integrated with somatosensory information for object manipulation (and indeed surface analysis)???
is the striatum vision??? is that where it happens???
where do layers 5 and 6 come into the equation???
do humans have 3 types of hair receptor???
which bits of the somatosensory cortex are somatotopic???
Kaas & Collins � organisation of sensory cortex
CO-dense and CO-light
macrovibrissae �
large whiskers (sometimes (macro) vibrissae)
morphological map
caudal/rostral
Killackey et al. � The formation
of a cortical somatotopic map
perinatal - Of or pertaining
to the period comprising the latter part of f
tal life and the early postnatal
period (commonly taken as ending either one week or four weeks after birth: see
quots.).
why are there so many (5)
topographic maps in the rat � surely it would want to do processing at a higher
level than the ordered sensory input after just one map???
Johnson � The roles and functions of cutaneous mechanoreceptors
cutaneous � a) Of, pertaining to, or affecting, the cutis or skin b) External, superficial.
is a nm 1/1000th of 1/1000th of a millimetre???
db/decade and third order derivations
are Pacinian corpuscles rapidly adapting???
is there a difference between rapidly and completely adapting???
what are high-pass filters???
what does it mean to say that �The SA2 population transmits a neural image of skin stretch to the CNS with relatively little interference from objects held in the hand�???
what does this mean??? �Third, they respond to stimuli less than 100-150Hz with a phase-locked, Poisson distribution.�
horizontal tensile strain = shear???
Proske et al. (2000), The role of muscle receptors in the detection of movements
muscles� thixotropic property - The property of certain gels of becoming fluid when agitated and of reverting back to a gel when left to stand (OED)
fascicle - a bunch, bundle
Zhang et al. (2001), Functional characteristics of the parallel SI- and SII-projecting neurons of the thalamic ventral posterior nucleus in the marmoset
the antidromic collision technique
Excerpts