3 parts of the somatosensory system
cutaneous senses
proprioception
kinesthesis
cutaneous senses
responsible for perceptions like touch and pain
usually caused by stimulation of the skin
proprioception
the ability to sense the position of the body and limbs
kinesthesis
the ability to sense the movement of the body and limbs
epidermis
outer layer of the skin
layer of dead cells
dermis
below the epidermis
mechanoreceptors
receptors that respond to mechanical stimulation like pressure, stretching, and vibration
Two categories of the mechanoreceptors
slowly adapting receptors - prolonged firing to continued pressure
rapidly adapting receptors - bursts of firing just at the onset and offset of pressure
two receptors close to the skin surface near the epideris
have small receptive fields
merkel receptor (SA1)
Meissner corpuscle (RA1)
cutaneous receptive field
area of skin that causes neurons to fire when stimulated
type of perception associated with:
Merkel receptor
Meissner corpuscle
fine details
controlling handgrip
Neuron firing patters of SA vs RA receptors
SA: continuously through pressure
RA: right at the beginning........right at the end
Two mechanoreceptors deeper in the skin
Large receptive fields
Ruffini cylinder (SA2)
Pacinian corpuscle (RA2 or PC)
Perception associated with:
ruffini cylindar
pacinian corpuscle
stretching of skin
rapid vibrations and fine texture
what is the travel issue involved with perception and skin receptors
receptors are distributed all over the body
signals must travel all the way to the brain before touch is perceived
dorsal root
bundles on each of the 31 segments of the spinal cord
conduct signals from the skin to the cord
two pathways of spinal cord touch signals
medial lemniscal pathway
spinothalamic pathway
medial lemniscal pathway
large fibers carry signals relating to sensing the position of limbs and percieving touch
high speed
spinothalamic pathway
smaller fibers
carry signals relating to temperature and pain
where do the two spinal pathways lead
thalamus
most synapse in ventrolateral nucleus, some in other talamic nuclei
where do signals go after the thalamus
somatosensory receiving area (S1) in parietal lobe
possibly also to secondary somatosensory cortex (S2)
How is the somatosensory cortex organized
Into maps that correspond to locations on the body
homunculus
the body map in S1
example of touch magnification factor
areas like the fingers have larger areas in the brain dedicated to them (relative to their size)
neural plasticity monkey experiment
trained the monkies to use their finger in a certain way, eventually the area for that finger expanded in the cortex
Plasticity in musician
players of stringed instruments have larger cortical areas for the hand they use for the bow
tactile acuity
the ability to detect details on the skin
what determines the way we perceive details on the skin
receptor properties
where is there a high density of merkel receptors
why is this significant
fingertips
they are the part of the body most sensitive to detail
What is a physical feature that determines tactile acuity?
Spacing of receptors
(although they are the same on all fingers when some are better than others)
what is the cortical mechanism of tactile acuity
all based on receptive fields
areas better with detail have smaller fields that don't overlap
the ones that are worse have more overlapping fields - less seperation in cortex - less likely to feel something as two seperate points
surface texture
physical texture of a surface created by peaks and valleys
Katz on perception of texture
based on both spatial cues and temporal cues
spatial cues
provided by large surface elements (bumps and grooves)
can be felt when skin moves across and when it is just pressed down
temporal cues
info via vibrations caused by moving skin over a surface
fine texture
not detected unless fingers move
duplex theory of texture perception
the Katz idea that there are two types of receptors involved in texture perception
which receptor responds better to low frequencies and which one to high
low- Meissner corpuscle
high- Pacinian corpuscle
haptic perception
perception where 3d objects are explored with the fingers and hand
3 systems used during haptic perception
sensory system
motor system
cognitive system
passive vs active touch perceptions
passive - you feel the stimulation of the skin
active - you experience the object you are touching
exploratory procedures (EPs)
the movements of you hands and fingers while identifying 3d objects through touch
4 EPs
lateral motion
pressure
enclosure
contour following
what happens to neurons as you begin to move from mechanoreceptors towards the brain
they become more specialized
what kind of receptive fields do neurons in the ventral posterior nucleus (in the thalamus) have
center-surround
What are receptive fields like for touch in the cortex
some center-surround, some even more specialized (orientations, movement in certain direction, etc)
Specific touch neurons
specific orientations
movement in specific directions
grasping specific objects
while receptor stimulation will trigger a response, what other things can influence the size of response
attention
thinking
other perciever actions
3 types of pain
inflammatory
neuropathic
nociceptive
inflammatory pain
caused by damage to tissue or inflammation of joints or by tumor cells
neuropathic pain
caused by lesions or other damage to nervous system
nociceptive pain
caused by activation of nociceptors
nociceptors
receptors specialized to respond to tissue damage or potential damage
different kinds respond to different stimuli (heat, chemical, pressure, cold, etc)
early model of pain
direct pathway model
What are some reasons that contradict the direct pathway pain model
sometimes people dont percieve pain from serious wounds
phantom limb pain
gate control model of pain
Inhibitory messages sent to transmission cells from L-fibers (non-painful stim) and central control fibers (cognitive)
Excitatory messages sent to transmission cells from S-fibers (pain)
where are gate control cells
the dorsal horn of the spinal cord that extends from dorsal root
4 types of cognitive influences on pain
expectation
shifting attention
content of emotional distraction
hypnosis
pain matrix
all the areas of the brain that are involved in pain perception together
hypothalamus, amygdala, thalamus
S1, anterior cingulate cortex, prefrontal cortex, and insula
what is the multimodal nature of pain
it is sensory and emotional
What is the anterior cingulate cortex for in terms of percieving pain?
determining the affective component of unpleasantness
unpleasentness can change even when intensity remains the same
a neurotransmitter that acts on the same receptors as opiods
endorphines (endogenous morphine)
naloxine
blocks opiods by attatching to receptor sites of heroin and endorphins
why does naloxone decrease the effectiveness of placebos
placebos cause the release of endorphins
where are placebo related endorphins released in the nervous system?
the specific locations patients expect pain to be reduced
brain activates the pathways to those regions