Week 11 : sense of touch Flashcards
1
Q
Sense(s) of touch…
A
- different sensations that arise from the skin are extremely broad
- object features are often combined with tactile sensations that arise internally (vestibular & proprioceptive) to gain full understanding of objects & how to interact them
- cues incredibly useful when other sensory info isn’t available (searching for object in purse e.g.)
2
Q
Haptic exploration…
A
- the use of our hands to define an object by its features
- powerful way to explore environment & form predictions about behaviour & use of novel object
- can give us… texture (lateral motion), weight (unsupported holding), hardness (pressure), shape & volume (contour following & enclosure), temperature (static contact) & contour following
- integrates info from mechanoreceptors, proprioceptors, thermoreceptors & maybe nociceptors
- key in reading braille
3
Q
Skin
A
- 9 pounds
- sense organ of touch
- touch receptors exist everywhere on the surface of the skin, not necessarily in a uniform way tho (more on fingers & lips, less upper back & legs/arms)
4
Q
Sensory cells within the skin… (2)
A
- epidermis = outer layer, consists mainly of dead cells that function to protect the underlying tissue… avascular (gets oxygen from air instead of blood stream)… keep out pathogens & keep in fluids
- dermis = contains oil & sweat glands, vasculature, hair cells a variety of sensory cell subtypes
5
Q
mechanoreceptors
A
- sensory receptors in the skin that transduce physical movement on the skin into neural signals
- touch perception
- stimulation of skin activates one or more of the 4 types of mechanoreceptors (SAI, SAII, FAI, FAII)
- these mechanorlecpeors induce a neural signal sent to the spinal column then the brain
- by combining the outputs our skin can encode texture, patterns of stimulation, amount of pressure & when the touch occurred, etc.
6
Q
SAI
A
- Merkel cells = have sustained response to continued pressure, giving max response to steady pressure
- slow adapting
- small receptive field
- high spatial resolution
- respond best to vibrations at low frequencies
- important for perception of pattern and texture in touch
- upper dermis
- produce steady stream of neural response when skin is deformed
- responsible for 2-point threshold
- important when we need fine manual control, esp without visual feedback
- highest density on fingertips & lips
7
Q
FAI
A
- Meissner corpuscle endings
- fast adapting
- small receptive field
- high spatial resolution
- respond well to low-frequency vibrations
- upper dermis
- respond vigorously when the skin is first touched, then again when the stimulus ends
- esp good at detecting ‘slip’
8
Q
SAII
A
- Ruffini endings = have a sustained response t continued pressure, so maximum response to steady pressure
- slow adapting
- good at stretching from side to side (crucial for object grasping)
- larger receptive fields
- more vital for detecting presence of light touch than pinpointing where it occurs (low spatial resolution)
- in lower dermis
9
Q
FAII
A
- Pacinian corpuscle endings
- fast adapting
- large receptive field
- low spatial resolution
- respond well to high-frequency vibrations
- lower dermis
- respond vigorously when the skin is first touched, then again when the stimulus ends(onset & offset)
- used when we use fine motor control (fine texture)
10
Q
Two-touch threshold response
A
- distribution of the receptor subtypes are highly variable by body area… so our sensitively to different tactile sensations is also highly variable
- two-touch threshold demonstrates this
- The smallest separation between the two points that can be distinguished reliably as two separate points of pressure is taken as the threshold for that area of the body
- these thresholds vary across the body and even in one part (e.g. finger tip way more sensitive than palm)
11
Q
Thermoreception
A
- to survive we must keep our internal temperature fairly constant
- the ability to sense changes in temperature on the skin is called thermoreception
- passive = cold air against face / active = touch someones face to see fi they have fever
12
Q
Thermoreceptors
A
- sensory receptors in the skin that signal info about the temperature as measured on the skin
- free nerve endings in the upper dermis
- respond to a range of skin temperatures from 17c-43c and take action to move to wamer/cooler environment if our internal temperature is threatened
- our skin maintains a surface temperature between 30c-36c (here our thermoreceptors are mostly inactive)
- can also asses temperature of objects placed against skin
13
Q
Cold fibres vs warm fibres
A
- they respond preferentially to temperatures that are colder/warmer than resting body temp
- at regular body temp, both maintain some level of output
- temperatures below body temp result in increased output from cold fibres & decreased from warm fibres (vice versa)
- more interested in changes in temp & the output is reduced with prolonged exposure to other temp
- warm fibres have secondary peak when exposed to very low temps (paradox heat experience) super cold temp = burning pain
14
Q
Pain…
A
- the perception & unpleasant experience of actual/threatened tissue damage
- result of activation of receptors in our skin & elsewhere, as well as unpleasant subjective feeling associated w it
15
Q
Nociceptive pain…
A
- the pain that develops from tissue damage that causes nociceptors in the skin to fire
- occurs from direct trauma to the skin
16
Q
Nociceptors…
A
- free nerve endings in skin (receptors) that cause us to feel pain when activated
- found in dermis & epidermis
- 2 main types…
1. a-delta fibers = myelinated / conduct signal rapidly / respond to both heat and pressure / stinging feeling of pain & experience when first injured
2. c-fibers = nonmyelinated / much slower / respond to pressure, extreme degree & toxic chemicals / chronic experience of throbbing pain & a bit delayed
17
Q
pain reflex
A
- signal generated by pain-sensitive receptors travel via ascending neural pathways to cortex…
- but also secondary pathway that bypasses brain to reflex away from bad stimuli…
- touch hot object… signal from pain receptors travels via afferent projection to spinal cord… targets interneuron… interneuron targets efferent motor neuron that innervates a muscle in the area of sensation… this contract muscle to pull affected area away from hot object
- all within a fraction of a second
18
Q
Gate theory of pain control
A
- Nociceptors In the skin detect damage/trauma at the skin and transmit that information up the spinothalamic pathway
- there is also a downward pathway… that can inhibit the flow of info upward from the pain receptors… then the experience of pain is reduced/inhibited
- pathway… message from pain receptors in skin stimulates neurons & relay to brain… brain responds to this pain signal by releasing endorphins that send inhibitory input back down to the neurons… this reduce the signals generated by those neurons & pain to brain
19
Q
why is gate theory of pain control important to us?
A
- the ability to down regulate pain is useful in managing situations in which pain is experienced but where there Is no real threat to survival (e.g. working out/dentist trip)
- BUT not being able to perceive pain has crazy bad circumstances (ppl with mutation to the gene SCN9A)
20
Q
closed loop…
A
somatosensory & motor systems are often illustrated in and conceived of as 2 halves of a closed loop… info travels via ascending or afferent pathways to the brain, where it is interpreted by the somatosensory cortex & used to generate appropriate commands in the motor cortex …. them send back down via descending/efferent pathways to generate muscle activity… we will focus on ascending path
21
Q
neural pathways outside the brain…
A
- nerve endings send axon into a nerve bundle where its joined by many other axons from adjacent nerve fibres
- this enters the spinal cord in the dorsal root ganglion… cells enter the spinal cord in the dorsal root
- once in spinal column, sensory info divided into 2 distinct pathways, which travel up the spinal column to the brain
22
Q
Dorsal column-medial leminiscal pathway
A
- carries info from tactile (mechanoreceptors) and from the proprioceptors (muscle position)
- travels on dorsal side (back) of spinal column
- makes a synapse in the medulla of the brain, where it crosses over to the contralateral side (in the brain stem it crosses)
- then, ascends into the brain as the medial lemniscus fibre bundle to the ventral posterior nucleus of the thalamus
- last to somatosensory cortex
23
Q
Spinothalamic pathway
A
- carries info from the pain (nociceptors) and thermoreceptors (temp)
- cross over to contralateral side within spinal column (spinal synapse allows for reflexes before brain)
- then, pass through medulla on way to ventral posterior nucleus of the thalamus via spinothalamic tract
- last to somatosensory cortex & insular cortices
24
Q
the differences and consequences of the paths
A
- the spinothalamic pathway crosses the body at the spinal cord and the dorsal column medial laminisco pathway doesn’t until the brain stem
- this causes weird things if we have spinal cord damage
- we would loose tactile + proprioceptive sensation from the side of the body on which the damage occurs, and loss of pain and temp sensation on opposite side
25
somatosensory cortex
- located in parietal lobe of cerebral cortex
- all of the sensory info from the cell subtypes project to the primary somatosensory cortex (S1 located posterior to central sulcus) from the ventral posterior nucleus of the thalamus
- maintains a somatotopic map of the body
- can be subdivided into 4 functionally separate subregions...
- at the bottom of S1 is S2 (secondary) & this area responds to variety of sensations that arise from both sides of body (ventral 'what')
26
somatosensory cortex 4 regions
1. area 1 = mechanoreceptors
2. area 2 = proprioceptors
3. area 3A = nociceptors
4. area 3B = nociceptors & mechanoreceptors
1/3B = skin / 2/3A = deep bone & muscle sensation
27
temperature info takes a slightly different pathway...
- Thermoreceptors send axons up the spinothalamic pathway that travel to the ventral posterior nucleus of the thalamus
- From there to S1
- Then goes to areas of the frontal lobe, including the insular cortex and the anterior cingulate cortex
28
Somatosensory homunculus
- Wilder Penfield 70 years ago
- illustration of important concepts (somatotopic map)
- it has an orderly organization & neighbouring areas of the body are represented by neighbouring areas of cortex
- the size of the region of cortex that represents a given body part is not related to the size of the body part, but the importance of input from that region (majority hand & face)
29
Modern homunculus
- mapping mainly held up over time
- but developments have allowed it to be even more detailed (down to individual finger)
- thumb has largest representation & lots of overlap with pinky+ring finger which makes sense
- when index finger removed, area of brain normally responding to that digit became sensitized to stimulation of the tomb & middle finger
- this plasticity also demonstrated by enhanced foot/hand representations in soccer players & musicians
30
Perception of itch...
- pruriceptors
- respond mostly to chemical irritants on skin instead of tissue damage
- send axons up spinothalamic tract & synapse in the somatosensory cortex
- function of itching... (1) scratch to remove irritant (2) itching causes us to scratch, which then induces actual, minor, tissue damage & induces an autoimmune response
31
Proprioception
- gives us our awareness of how our bodies are positioned
- perception of the movements & position of our limbs
- afferent fibers carry this info to the brain (somatosensory cortex)
- all consumption affects this system
32
3 different kinds of sensory receptors in proprioceptive system give us info...
1. muscle spindles = muscle cells that have receptors connected to them that sense info about muscle length (flexed/extended/) & therefore muscle action
2. joint receptors = receptors found in each joint that sense info about the angle of the joint from the pressure applied to the bones in the joint
3. Golgi tendon receptors = receptors in the tendons that measure the force of a muscle's contraction
33
proprioception & reflexes...
- proprioceptive receptors are important part of reflex circuitry that allows us to quickly correct limb position to maintain balance/correct unexpected deviations
- like the kicking motion of a reflex test
- like pain receptors, synapse in spinal cord on innerneurons & motor neurons to correct
34
Vestibular system
- responsible for spatial orientation & acceleration of the head (motion)
- info from here is used by brain to generate motor commands that allow us to maintain balance, stabilize head & body during movement & maintain posture
- located in semicircular canals (rotations) & otolith organs (acceleration) adjacent to the inner ear
- same nerve that carries auditory info to brain carries vestibular info also
35
semicircular canals
- 3 tubes aligned perpendicular (right angled) & each encode movement of head in one direction
- each canal is filled with liquid called endolymph
- when head moves in any direction, endolymph lags behind cuz gravity & inertia cause relative movement in opposite direction
- this relative motion bends cilia on the hair cells & triggers neural response
- By comparing the movement in each canal, the brain can make inferences about rotations to the left or right (yaw), front or behind (roll) and up or down (pitch)
- At the bottom of each canal is a chamber called the ampulla, which contains a structure called the crista
- Inside the crista are the hair cells with stereocilia, the receptors
36
otolith 2 organs...
1. urticle = encode horizontal movement (like walking along a sidewalk)
2. saccule = encode vertical movement (bouncing on tramp)
- in both cases... movement causes a shift of small crystals called otoliths which lie atop a gelatinous material into which the hair cells & their stereocilia are embedded
- these shifts displace the gelatinous which applies a force to the hair cells which depolarize to signal movement
37
vestibular brain
- nerve fibers from the semicircular canals & otolith come together in vestibular nerve then synapses in brain stem area called the vestibular complex
- vestibular complex projects to several areas, including the parietal insular vestibular cortex, located in the parietal lobe
- This area of the brain is thought to maintain a representation of head angle, critical for maintaining balance
38
Vertigo
- caused by disruption to the arrangement of otoliths within the urticle & saccule
- such that the vestibular organ signals head movement when the eyes say a person is not moving
- this gives a chronic feeling of sea sickness
39
Tactile agnosia...
- inability to identify objects by touch (like haptic perception)
- Neurological condition caused by damage to the somatosensory areas of the parietal lobe
- problem of identification, not perception
- usually only one hand effected
