Download Lect7_somatosensory_2012

Lecture 7
Somatosensory System
stimulus
receptors
neural pathways
plasticity
perception
"Body senses"
Cutaneous: detection of stimuli on skin
Kinesthetic: detection of internal conditions: body position & movement
receptors in joints, tendons & muscles
Cutaneous:
Stimulus:
pressure on skin
temperature
mechanoreceptors -- sensitive to physical distortion
Sensitivity: skin is sensitive to bumps that are only .006 mm high, .04mm
wide (that’s 6 X 40 microns!)
Types: (16.2)
Pressure/Light touch/vibration
Pacinian corpuscle -deep in dermis of skin
large: can be 2mm long X 1 mm in diameter
onion-like
large receptive fields - entire finger or 1/2 of palm
rapidly adapting
vibration - 200-300 Hz = best frequency
Ruffini endings
small Pacinians
large receptive fields
slowly adapting
Meissner's corpuscles
1/10 size of Pacinians
much smaller rf's
rapidly adapting
vibration - 50 Hz
Merkel's disks
nerve terminal + flattened epithelial cell which makes synapse onto
nerve terminals. Epithelial cell may be mechanically sensitive.
small rf's
slowly adapting
Krause end bulbs
junctions of dry skin & mucous membrane
Hair follicle receptors
Nociception: (pain)
Free nerve endings
Thermoreceptors
cold receptors
tuned for low skin temperatures (~25 deg C)
warm receptors
tuned for warm skin temperatures (~43 deg C)
Proprioception
Latin: proprius = "ones' own"
Sensation of one's own body's position & movement
Receptors=
muscle spindles
measure length & rate of muscle stretch
Golgi tendon organs
gauge force via. tension in tendon
Misc. receptors in joint connective tissue
Most = rapidly adapting => better for moving joint, worse for resting
position
Central Somatosensory Pathways:
peripheral nerves => dorsal root ganglion
ascend in spinal cord
sorted by submodality:
dorsal column/medial lemniscal pathway
touch & vibration, limb proprioception
spinothalamic pathway
pain & temperature
=> Nucleus VP of thalamus (ventral posterior)
=> S1 - primary somatic sensory cortex
Neurons have receptive fields on the body surface
somatotopy
cortical magnification (16.4)
fingers, face, lips, genitals >> back, elbow, leg, etc.
(analogous to magnification of fovea in visual system; also
sometimes see magnification for auditory frequencies; esp. bats - doppler shift)
magnification = proportional to density of peripheral receptors
Columns =
1 class of sensory receptors / column
Perception:
Lots of receptor types, but no clean correspondence between receptor
type & perception
Localization of tactile stimuli
accuracy of ability to localize stimuli varies with location on the body
surface
2 point threshold = measure of localization ability
touch skin with 1 point or 2 points - can you tell the difference?
if 2 points are close, can't tell the difference between 1 and 2
separation varies with location on body surface
e.g. thumb: 4 mm
leg = about 48 mm
Thresholds differ for males & females 16.7, 16.8
Pain
most pain producing stimuli = potentially damaging
e.g. intense thermal, sound, light, chemical & electrical stimulation
pain = important for survival
patients with insensitivity to pain incur serious skin injuries
e.g. leprosy (Hansen's disease), though not specific to pain, results
in a loss of pain and other tactile input. Injuries to extremities => gangrene,
necrosis
Some patients have anomalous disorders that make them unable to feel
pain, though other skin senses = intact
outcome => failure to protect body from injury usually leads to early
death
e.g. failure to shift weight, roll over at night => joint decay =>
infection => death
Masking
pain can be masked by simultaneous intense stimulation , not
painful
e.g. rubbing, scratching an itch
Phantom limb story:
When an area of cortex ceases to be responsive to input from one
place and becomes responsive to input from another place on the body, there
must be corresponding changes in subsequent areas -- the "LABEL" on the line
must be changed every where, or it doesn't make sense.
patients with amputation of limbs often continue to be able to "feel"
their limbs. Turns out, input from other regions of the body surface are
interpreted as being on the phantom limb. (Ramachandran story), can mask pain
in the phantom limb
Neural basis of perception
Involvement of S1 inperception:
Map of body surface is plastic:
Merzenich experiments
-map hand region of cortex
-amputate or denervate 1 digit
-remap cortex => area previously corresponding to that finger now
= responsive to input from neighboring fingers
Map = subject to modification based on experience
- map hand region
train monkey on tactile discrimination task
- remap hand region
trained finger => expanded cortical representation
= possible basis for perceptual learning?
microstimulation in S1:
Romo et al:
trained monkeys to perform a flutter frequency discrimination
task
then, replaced one flutter stimulus with microstimulation in
S1 at appropriate somatotopic location
=> monkeys could compare stimulation & mechanical stimuli
& perform correctly
i.e. appropriate stimulation in S1 = sufficiently realistic that it
can substitute for an actual tactile stimulus!