Topic Test 4 - Part 3A (touch) Flashcards
motor function is divided into vision, vestibular and somatosensory. Describe somatosensory
- distributed all over body
- responds to many kinds of stimuli
- subgrouped into 4 senses:
>touch
>pain
>temp
>proprioception
most somatosensory receptors are _
mechanoreceptors
- receptive to physical distortion
*measure physical distortion, stretch, vibration, pressure
what are the 4 primary receptors in skin
- pacinian corpuscles
- meissner’s corpuscles
- ruffini endings
- merkel’s disks
the primary receptors in the skin vary in terms of
- receptive field (large vs small)
- adaptation (rapid vs. slow)
list the mechanoreceptors from superficial to deep
- merkel’s disk
- meissner’s corpuscle
- ruffini’s ending
- pacinian corpuscle
describe pacinian corpuscles
- largest and deepest mechanoreceptor in skin
- get compressed and detect pressure and vibration
>large receptive field
>rapid adapting
*lots of these in the skin
describe rapid adapting in the context of pacinian corpuscles
- react quickly to inital contact, but not sustained contact
- best at detecting finer textures and high frequency vibrations
describe meissner’s corpuscles
- small receptors in upper dermis: common in fingers
- detect fine touch and pressure
> small receptive field
> rapid adapting
describe rapid adating in the context of meissner’s corpuscles
- react quickly to initial contact, but not sustained contact
- best at detecting heavier textures and lower frequency vibrations
from the rapid adapting mechanoreceptors, which are better at which frequency?
- meissner’s corpuscles better at low frequency (heavier texture, contours/waves are further apart)
- pacinian corpuscles better at high frequency (finer texture, contours/waves are closer together)
give some examples of actions that are interpreted more by meissner’s corpuscles/pacinian corpuscles
meissner’s: moving fingers across keys on a keyboard
pacinian: moving hand across the surface of smooth table
describe ruffini endings
- large receptors in the dermis layer
- detect stretch and deformation
> large receptive field
> slow adapting
describe slow adapting in the context of ruffini endings
- react at sustained deformations
- best at detecting grip/position
ex. pick up water bottle and holding it, it’s job is to continue telliing the brain that there is something still in contact with hand
describe merkel’s disks
- small receptors in epidermis, common in fingers
- detect fine touch and pressure
- small receptive field
- slow adapting
describe slow adapting in the contet of merkel’s disks
- react to sustained deformations
- best at static discrimination of shapes/textures
what mechanoreceptor has a small receptive field and rapid adaptation?
meissner’s corpuscle
what mechanoreceptor has a large receptive field and rapid adaptation?
pacinian corpuscle
what mechanoreceptor has a small receptive field and slow adaptation?
merkel’s disk
what mechanoreceptor has a large receptive field and slow adaptation?
ruffini’s ending
what does the firing rate look like in different mechanoreceptors?
- pacinian: not a lot aside from some axons firing at the beginning and subseqeuent change later
- slow adapting (merkel’s and ruffini’s), see consistent firing (merkel’s more close together at the beginning)
- meissner’s: a lot at beginning, then nothing till end
for each example, name the mechanoreceptor likely responsible:
a) catching football
b) reaching into bag for pen
c) holding hand
a) pacinian, maybe also meissner’s if using more fingers
b) meissner’s (rapid adapting, finger tip)
c) initial contact could be rapid like pacinian, slow adapting when you hold a long time like ruffini
describe 2 point discrimination
- sensitivity to discriminate small points varies greatly across the body
-more sensitive in important places (finger>forearm>calf, more neurons devoted to fingers)
ex. if 2 points pressing into index finger, can tell there are 2 contact points
two point discrimination is accomplished by:
- greater density of mechanoreceptors
- smaller field size (2 Ms - meissner/merkel)
- greater brain tissue devoted to these areas
what’s the argument for barefoot running/walking?
- ability to improve sensory info coming from the feet
- if we are running or walking, will get a lot of info coming from rapid adapters
- if we have minimalist or barefoot shoes, getting more info from these signals that can help improve contact, movement etc
the path to the brain begins with an axon… describe primary afferent axon
- enters spinal cord at dorsal root
- cell bodies lie in dorsal root ganglion (pseudo-unipolar neurons)
- 4 types of primary afferent axons
*first order neuron: sensor to spinal cord
what are the 4 types of primary afferent axons?
Which mediates touch?
- Aa, AB, Ad, C axons
- AB mediates touch
list the types of primary afferent axons from smallest to largest. Which are myelinated?
c<Ab<AB<Aa
- all As are myelinated (larger + myelination = faster)
- Cs are NOT myelinated (smaller and slower, just a little faster than walking speed/1.1 m/s)
*proprioception/Aa has almost double speed of touch info AB, need muscles to react quickly
what are each primary afferent axon responsible for?
C - temperature, pain, itch
Ab - pain, temp
AB- mechanoreceptors of skin
Aa - proprioceptors of skeletal muscle
the path to the brain continues into the spinal cord, describe
two AB branches:
- directly ascending the spinal cord to the brain
- synapses with second-order sensory neuron (for reflexes)
*most second-order sensory neurons lie in the dorsal horn
*sending info from pacinian corpuscle, going to first order AB axon, going up towards the brain, will synapse in spinal cord for reflexes but just thinking of coming in and going straight up towards the brain
what pathway is involved in dorsal column - touch info
dorsal column-medial lemniscal pathway (DCML)
describe the order of dorsal column-medial lemniscal pathway (DCML)
1) ascending branch goes up the dorsal column
2) synapse on the dorsal column nuclei in the medulla
3) dorsal column nuclei axons decussate and ascend the medial lemniscus
4) synapse in the VP nucleus of the thalamus
5) neurons in the VP nucleus project to somatosensory cortex
describe ipsilateral and contralateral
ipsilateral = same side
contralateral = opposite side
- if you have sensation on right hand, it will stay ipsilateral up until the point that it crosses over in the medulla and then we can say that signal/info is contralateral (on the left side now)
another way of describing the dcml pathway (not on notes*)
- info coming in, some contact causing some mechanical deformation, up dorsal column (white matter in dorsal part of SC), don’t have synapse until medulla, cross over to opposite side and go to medial lemniscus, vp nucleus goes to post central gyrus
describe the dcml pathway in context of neurons/synapse points
- 3 neuron pathway with 3 synapse points required to reach S1 (first order, 2nd, 3rd)
- these exist for a reason other than simply passing along info (editing/regulating info)
- we can assume info is altered at each synapse
- adjacent inputs can be inhibited to enhance tactile stimuli
describe where the 1st, 2nd and 3rd order neurons in the DCML pathway are going
1st: (from spinal cord i think?) sensory info coming in
2nd: in the medulla, goes up to thalamus
3rd: goes up to cortex
describe the 3 neurons and 3 synapses in the dorsal column - touch info (dcml)
where is the first synapse?
3 neurons
- first order, second order, 3rd order
3 synapses
- 1st to 2nd, 2nd to 3rd, 3rd to cortex
*medulla is first synapse (info before unedited)
what are dermatomes used for
diagram - the distribution/mapping of spinal nerves (segmental organization of spinal cord)
*paint a picture of where those neurons branch out and cover areas of our body
* if we have issues, this is where we will see patterns
*helpful for understanding where we might have pain/motor dysfunction or different sensation
pacinian corpuscles in hand will connect to which area of the spine based on dermatomes?
c8
describe herniated disks
- most common in 30s-50s (~2% of adults)
- most common in the lower back (l4/5 & l5/s1 - 95% of cases *disk between them)
> pain - back and leg (glutes, thigh, calf, even foot)
> numbness or tingling
> weakness
what are the 4 different types of disk issues?
- degenerative disk
- bulging disk
- herniated disk
- thinning disk
define degenerative disk
starts to lose some of its nice cushy structure, sometimes micro tears, starts to lose some of the fluid
describe bulging disk
some kind of protrusion out the back but not fully herniated/slipped (don’t have the entire nucleus of the disk coming out)
describe herniated/slipped disk
have a bunch of disk coming out, slips out the back and sits on the nerve root (potentially on sensory and motor info, will compress nerve root and can be really painful)
how are herniated disks diagnosed?
physical exam, imaging and/or even nerve tests
what kind of symptoms might you see for l4/l5 slip?**
some pain in back, but if sitting on that nerve can potentially disrupt signals for entire dermatome
- pain can go into side of lega and top of foot
- ppl will complain of calk pain/foot tingling/muscle weakness
describe the treatment for herniated disc
1) rest, physical therapy, pain medications
- 85% resolve in 8-12 weeks
2) surgical - discectomy/microdiscectomy
- conservative failed to resolve
- progressive/debilitating pain, numness, weakness (*chronic)
*injections
compare discectomies and microdiscectomies*
discectomy = taking whole disk out and fuse them
microdiscectomy = piece of disk tissue is sitting on nerve root, go in and snip out extra piece that is sitting on nerve root
*trying to get rid of disk tissue that hasn’t come back in
what injections can be done for herniated discs?**
- nerve block (doesn’t really change anything, blocks a nerve for short amount of time)
- corticosteroid (reduce inflammation and hopefully give it a bit of space/time for the disk to go back in)
describe lateral inhibition
- inhibit adjacent inputs to enhance tactile sensitivity
- increases contrast to allow for more precise/finer location of sensation
why is lateral inhibition important?**
- mechanoreceptors have diff areas they cover, with a bit of overlap that helps identify where contact is
- neighbouring neurons help us pinpoint better
- want to be specific about where contact is
- central signal is higher and neighbouring are much lower
*interneurons act in inhibitory way
describe sensory gating
- corticothalamic feedback influences sensory processing (feedback loop)
- cortex helps to filter irrelevant or repetitive information
- “feel what you want to feel”
- however, these complex pathways remain unclear
- mayb be related to many cognitive disorders (eg. schizophrenia)
*thalamus<->cortex in DCML pathway
what are 2 examples where sensory gating does not work as it should?
ADHD and schizophrenia
schizophrenia: massive inability to control sensory info coming in
ADHD: cortex should tell thalamus to focus/ignore other sensory info but you can’t focus (minor)
what area of the brain is the primary somatosensory cortex?
3b
how do we know 3b is the primary input site for the primary somatosensory cortex?
1) receives input from VP nucleus
2) highly responsive to somatosensory input
3) damage impairs sensation
4) electrical stimulus creates sensations
list the areas adjacent to 3b
somatosensory 3a
- dense thalamus input, but more body position
somatosensory 1&2
- receives info from 3b
- generally related to texture, size and shape
who was wilder penfield
- canadian neurosurgeon
- laid the groundwork for what sensory info relates to what area
-in seizures, trying to connect to area of the brain
*cortical somatotopy - nowadays, treated pharmacologically and removed only when really severe
what is a representation of cortical somatotopy?
homunculus
describe the posterior parietal cortex
- allows for processing of basic sensory info and integration with other senses
- posterior parietal cortex 5
- posterior parietal cortex 7
**basic tactile info coming in, integrating into more complex things
describe the role of posterior parietal cortex (area 5)
sensory integration for the planning and organization of movement
describe the role of posterior parietal cortex 7
sensory integration for object recognition and spatial relationships
