SAI
=Merkel cell
- just under epidermis
- moderately low threshold
- irregular discharge
- sensitive to edges and curvature
- continue to fire when pressure is held (slowly adapting behavior)
SAII
=Ruffini ending
- in dermis
- high threshold to indentation
- regular discharge
- sensitive to lateral skin stretch (and direction)
FAI
Meissners corpusles
- just under epidermis in peaks
- low threshold, sensitive to low-frequency vibration
- don’t continue to fire w/stimulation (fast-adapting behavior)
- sensitive to velocity and motion across skin (not how hard the indent is)
FAII
=pacinian corpuscle
- deep in dermis
- very low threshold, sensitive to high frequency vibration
- don’t continue to fire w/stimulation
- sensitive to acceleration of indent
receptor fields
=region of skin that will elicit a response in a particular neuron
type I: small receptor fields, merkels have specific borders and more obscure for ruffini
type II: large receptor fields, distinct borders for messner’s and more obscure for pacinian
which receptors are more concentrated in the fingertips?
SAI and FAI merkel and meissner
how do SAI’s code for shape and size?
curvature?
- for smaller probe, there’s a higher discharge in fewer receptors
- for larger probe, there’s a lower discharge in more receptors (less velocity specific)
- curvature is determined based on how many and which are firing (ie as a population code)
what are some functional roles for each receptor?
- SAI-edges, curvature, texture
- SAII-skin stretch
- FAI-motion detection, grip control
- FAII-feeling through objects, fine textures
which receptors respond to accidental slips while holding an object?
SA1 and FA2
FA1 controls grip-ie there’s more activity for slippery objects
what is the role of skin receptors in the feet in standing?
- sensitive to contact pressures and potential changes in pressure distribution
- evidence: stim of plantar skin increases sway, cooling/ischemic block of leg increases sway
where was multi-unit activity predominately evoked in the foot?
-medial or lateral aspect
which receptor types were present in the foot sole?
which had the lowest thresholds?
receptor fields?
- all four
- SAI and SAII had similar sized receptor fields, type Is had 4–6 hotspots
- FAII had lowest thresholds and obscure fields, SAII had highest (single hotspot)
were the SAII receptors sensitive to direction?
was there background activity?
no background activity
all sixteen demonstrated preferential skin strain axis
does movement at a joint evoke a particular response from cutaneous afferents in the skin overlying it?
-yes, bending the toes elicited a response in FAIs, SAIs, and SAIIs
name differences in receptors specific to the foot
- large percentage of rapidly adapting receptors
- higher activation thresholds than hand (skin thickness?) + much larger receptor fields
- absence of background activity (is sometimes present in SIIs)
kinesthesis
-orientation in space and perception of body movement involving consciousness
proprioception
reflexive perception of body movement and orientation in space involving
- sense of position and movement of the limbs
- sense of tension or force
- sense of effort or heaviness
- sense of body image and posture
evidence of somatosensory role in proprioception and kinesthesis
- removal of afferent inputs causes reduced control of movement-eg GL and IW
- muscle spindles: vibration leads to perception of lengthening and a dissociation b/t velocity and length (position)–>ie muscle spindles play a conscious role
examples of muscle vibration illusions
a. vibrate both achilles w/subject against wall and they think they’re falling forward
b. vibrate biceps with finger on nose and nose seems to grow out to keep in contact with “extending” arm
joint receptors in proprioception/kinesthesis
- there is little if any evidence of decreased joint position sense after replacement
- receptors get better with increased sensitivity (swelling) and worse with anesthetic
- better for faster movements, ie may play some role
cutaneous receptors
SAII found to code for finger position (and velocity weakly)
a. stretching the skin can cause illusions of finger moving (numb skin) (stretch=contraction, release=extension)
b. functional knee braces, which heighten skin stretch, can increase proprioceptive awareness around the joint
- also the purpose behind compression garments
what is sense of effort?
a signal of central origin associated with level of perceived force
what were subjects asked to do to test sense of effort?
match effort and position of one hand with the other
- hand passively moved to different targets and either just matched or matched with 20% MVC
- people were generally good at matching effort without vision
- then they paralyzed the hand with a cuff and people thought they were making flexion/extension efforts despite no EMG activity + thought position of hand was extended when making extension efforts and vice versa
- this was magnitude sensitive-if they were making a greater effort, there was a greater perceived error in position (shown in a couple subjects)
reflex
automatic, stereotyped motor response to sensory stimulus
- from stimulation of peripheral receptors
- involves sensory neuron, interneuron, motor neuron
