Somatosensory System
Touch - mechanoreception
Limb posture movements and forces – (kinesthesis) proprioception
(Haptics – object recognition through touch + proprioception, usually
with the hand)
Readings: Wolfe et al Ch 13
Supplementary reading: Kandel et al Chs 22-23
http://www.youtube.com/watch?v=FKxyJfE831Q
Somatosensory perception and action tightly linked.
The somatosensory pathways of the brain integrate information from thousands of
sensors in each hand, transforming it to a form suitable for cognition.
ie force on a tool is located at the contact point of the tool by monitoring the
vibrations and forces produced by those distant conditions.
Also complex object recognition from spatio-temporal signatures.
Sensory information is extracted for the purpose of motor control as well as cognition,
and different kinds of information are extracted for those purposes. We can, for
example, shift our attention from the baseball’s shape to its location in the hand to
readjust our grip for an effective throw. This selective attention to different aspects of
the sensory information is brought about by cortical mechanisms.
Need to distinguish between active and passive touch:
Motor deficits such as weakness, stiffness, or clumsiness may result from sensory loss,
which is why passive sensory testing is important in the neurological examination.
Four different kinds of mechanoreceptor
Confocal micrograph
+ Hair
follicle
afferents
20-50
corpuscles/afferent
??
RA1
Slipping objects
High density
SA1
PC (RA2)
Form/braille
Transmitted vibration
Texture-hardness
textures
roughness
SA2
Stretch?
Most
Sensitive
to
deformatio
Reaction to slippage: 65 msec. Of this, 45 msec is peripheral
nerve and motor response time. ie 20msec for central
modulation.
Reaction time to apply force in response to shape of grasped
object - 100 msec.
Compare with vision. (Saccadic reaction time = 200 msec)
Dorsal root ganglion cells serve a dual role
of transduction and information transmission.
Multiple fibers from the axon branch to
form a large receptive field.
Receptive fields in the skin overlap.
Large variation in two-point threshold across the body surface
The Spatial Resolution Varies Because the Density of Mechanoreceptors Varies
RA mechanoreceptor responds to sinusoidal mechanical stimuli with a single action potential for each cycle.
The lowest stimulus intensity that evokes one action potential per cycle of the sinusoidal stimulus is called
the receptor's “tuning threshold.”
Merkel: 5-15 Hz
Meissner: 20-50 Hz
Pacinian: 60-400 Hz
Tuning thresholds
for vibration
RA’s – lower thresholds
25 Hz vibratory stimulus
Compare with
vision
Minimal detectable amplitude of vibration at the fingertip, as a function of the
vibratory frequency
merkel
meissner
Pacinian
Variation in receptive field size for different receptor types
Compare with vision
merkel
meissner
ruffini
pacinian
Stimulus intensity in encoded by spike rate
Neural response
Perceived magnitude (pressure)
Kinesthetic receptors for Proprioception
A. The muscle spindle is located within skeletal muscle and is excited by stretch of the muscle.
It consists of a bundle of thin (intrafusal) muscle fibers entwined by a pair of sensory axons, and is
also innervated by several motor axons (not shown) that produce contraction of the intrafusal
muscle fibers. Stretch-sensitive ion channels in the sensory nerve terminals are linked to the
cytoskeleton by the protein spectrin
B. The depolarizing receptor potential recorded in a group la fiber innervating the
muscle spindle (upper record) is proportional to both the velocity and amplitude of
muscle stretch parallel to the myofilaments (lower record). When stretch is maintained
at a fixed length, the receptor potential decays to a lower value.
Although the receptor potential and firing rates of the sensory axons are proportional to
muscle length, these responses can be modulated by higher centers in the brain that regulate
contraction of intrafusal muscles. In this manner the spindle afferents are able to signal the
amplitude and speed of internally generated voluntary movements as well as passive limb
displacement by external forces.
Proprioceptors in muscle spindles and tendons
Proprioceptors
Ia, Ib, and II
Golgi tendon organs, located between skeletal muscle and tendons, measure the
forces generated by muscle contraction. Although these receptors play an important
role in reflex circuits modulating muscle force, they appear to contribute little to
conscious sensations of muscle activity. Psychophysical experiments in which
muscles are fatigued or partially paralyzed have shown that perceived muscle force
is mainly related to centrally generated effort rather than to actual muscle force.
Joint receptors play little if any role in postural sensations of joint angle. Instead, the
perception of the angle of proximal joints such as the elbow or knee depends on
afferent signals from muscle spindle receptors and efferent motor commands.
Likewise, conscious sensations of finger position and hand shape depend on stretch
receptors in the skin as well as muscle spindles and possibly joint receptors.
All four mechanoreceptors detect
hand contact with the object but
each monitors a different aspect of
the action as the task progresses.
SA1 fibers encode the grip force
and SA2 fibers the hand posture.
RA1 fibers encode the rate of force
application and movement of the
hand on the object. RA2 fibers
sense vibrations in the object with
each movement: at hand contact,
lift-off, table contact, and release
of grasp.
dorsal column-medial lemniscal system
for tactile sensations and proprioception
and
anterolateral system for pain
and temperature
The fibers of the medial lemniscus, (tactile and proprioceptive signals), terminate in the
ventral posterior nucleus of the thalamus. The medial zone of the nucleus receives
trigeminal nerve fibers from the head and face. The lateral zone receives fibers from the
dorsal column nuclei; these inputs are arranged somatotopically, with the forelimb
medial and the trunk and legs lateral.
The ventral posterior nucleus— called nucleus ventralis caudalis in humans— has
traditionally been thought of as a single nucleus in which the fibers carrying cutaneous
signals terminate in a large central and caudal region while those conveying
proprioceptive information terminate in a dorsal and rostral region.
More recently, Jon Kaas and his colleagues have argued that the two regions represent
separate nuclei: the ventral posterior nucleus proper, which receives the cutaneous
information conveyed by medial lemniscal and trigeminal axons, and the ventral
posterior superior nucleus, which processes proprioceptive information. These nuclei
send their outputs to different subregions of the cerebral cortex. The ventral posterior
nucleus transmits cutaneous information primarily to area 3b of the primary
somatosensory cortex, whereas the ventral posterior superior nucleus conveys
proprioceptive information principally to area 3a in the postcentral gyrus.
Serial + parallel processing
Proprioceptive input
PR=parieto-rostroventral
Ventral path to SII and from there to the caudal insula,
to areas of the temporal lobe, and to premotor and
prefrontal cortical areas. Eventual convergence of
somatosensory, auditory, and visual information in the
medial temporal lobe is viewed as the path toward
shape and form processing. This ventral path is vital for
inserting new information into declarative memory and
accessing established memories for comparison with
ongoing events.
Dorsal path to superior parietal lobule, providing
areas in that lobule with somesthetic information
for control of voluntary movements, selective
attention, and information about how to perform
different tasks, sometimes known as the “how”
pathway.
(PET scan), allow neurologists to image the somatotopic functioning of the cortex in individual patients. While these imaging me
microelectrode maps made in animals, they are useful diagnostic tools in clinical neurology.
Columnal organization of S1
Figure 23-7 Each region of the somatic sensory cortex receives inputs from primarily one type of receptor.
thods are less precise tha
Tactile information from the fingertips is used faster than can be readily
explained by rate codes.
(Johansson & Birznieks NN 2004)
Reaction to slippage: 65 msec - 45 msec is peripheral nerve and motor response time.
Applied forces in response to shape of grasped object - 100 msec.
Excitatory and inhibitory zones of receptive fields of neurons in area 3b. The excitability of
cortical neurons varies over time and as a function of the stimulus location on the skin. Peak
excitation occurs in the middle of the receptive field 15 to 25 ms following brief taps on the
skin. Inhibition occurs shortly thereafter and is strongest at 45 ms. Delayed inhibition allows
each stimulus to be perceived as a distinct event when delivered at rates lower than 25 Hz (cf
vision). Each map indicates the intensity of excitation (red) and inhibition (blue) produced
over 10 ms periods following brief taps to a small patch of skin on the fingertip. Cortical
neurons vary in the relative strength and spatial location of excitatory and inhibitory fields.
Receptive fields of neurons in S1 are larger than those of the sensory afferents. Each of the hand figurines
shows the receptive field of an individual neuron in areas 3b, 1, 2, and 5, based on recordings in alert
monkeys. Colored regions indicate the region where light touch elicits action potentials. Neurons at
later stages of cortical processing (areas 1 and 2) have larger receptive fields and more specialized inputs
than neurons in area 3b. The neuron from area 2 is directionally sensitive to motion toward the fingertips.
Neurons in area 5 often have symmetric bilateral receptive fields at mirror image locations on the
contralateral and ipsilateral hand.
Ablation of area 3b: general losses
Ablation of area 1: loss of texture discrim
Ablation of area 2: loss of 3D form discrim
Direction sensitivity emerges in
Area 1 and 2, not present in 3b
Note overrepresentation of
Hand, etc and
Underrepresentation of
Torso
Cortical magnification
Similar to vision.
PR=parieto-rostroventral
Task dependent responses in S-ii but not S-I
In an elegant study Ranulfo Romo and his colleagues compared responses of
neurons in S-I, S-II, and various regions of the frontal lobe of monkeys while the
animals performed a two-alternative forced-choice task . The animals were
rewarded if they correctly recognized which of two vibratory stimuli was higher
in frequency. Neurons in S-I faithfully represented the vibratory cycles each
stimulus, firing a brief burst in response to each. In contrast, S-II neurons
responded to the first stimulus with spike trains proportional to its frequency but
responded to the second stimulus with a signal that combined the frequencies of
both stimuli.
Thus the same vibratory stimulus could evoke different firing rates in S-II,
depending on whether the preceding stimulus was higher or lower in frequency.
During reaching and grasping, neural activity in the posterior parietal cortex coincides
with activation of neurons in motor and premotor areas of the frontal cortex and
precedes activity in S-I. There is strong evidence that area 5 receives convergent central
and peripheral signals that allow it to compare central motor commands with
peripheral sensory feedback during reaching and grasping behaviors. Sensory feedback
from the hand to the posterior parietal cortex is used to confirm the goal of the planned
action, thereby reinforcing a previously learned skill or correcting those plans when
errors occur.
Predicting the sensory consequences of hand actions is an important component of
active touch. For example, when we view an object and reach for it, we predict how
heavy it should be and how it should feel in the hand; we use such predictions to initiate
grasping. Corollary discharge from motor areas to somatosensory regions of the cortex
may play a key role in active touch. It provides posterior parietal neurons with
information on intended actions, allowing these neurons to compare planned and actual
neural responses to tactile stimuli.
Orientation tuning in SII
receptive fields. Finger pads
have similar tuning.
Results from lateral inhib,
not composition of inputs as
in vision
Ablation of SII:
Loss of discrim of shape and
texture
Doesn’t affect SI responses
Ablation of SI silences SII
Responses modulated by attention
Two patients with lesions to the
anterior parietal lobe show severe
impairment in both sets of tactile
tests but only moderate
impairment in the motor tasks.
Three patients with posterior
parietal lesions show only minor
deficits in simple somatosensory
tests but severe impairment in
complex tests of stereognosis and
form. Motor deficits are greater in
skilled tasks.
.
normal
1. Touch, discrimination
2. Shape recognition
3. Grip force
4. Skilled movement
Eg peg in slot.
Four patients with combined lesions to anterior and posterior parietal cortex show severe impairment in all tests.
Interestingly, the patient who showed the least impairment in this group (patient f) suffered brain damage at birth; the
developing brain was able to compensate for the loss of major somatosensory areas. Lesions in the other patients resulted
from strokes later in life.
Effect of lesion in area 2 - loss of ability to coordinate fingers, plus sensory deficits.
Somatotopic organization of SI
End of lecture
Mechanoreceptors Differ in Morphology and Skin Location
RA
SA1
SAII
Receptive fields in area 3b of S1
Spatial characteristics of stimuli depend on
activity across population of receptors.
Figure 22-8 The firing patterns of mechanoreceptors in the superficial layers of the skin encode the texture of obj
Figure 21-12 Inhibition of selected projection neurons in a sensory relay nucleus enhances the contrast b
inhibitory pathways in the circuitry of the dorsal column nuclei, the first relay in the system for touch. The projection (
thalamus. They receive excitatory input from touch receptor axons traveling in the dorsal columns. These afferent fibe
Velocity invariance of cell’s
responses.