Week 11 : sense of touch

Question 1

Sense(s) of touch…

Answer 1

• 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.)

Question 2

Haptic exploration…

Answer 2

• 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

Question 3

Skin

Answer 3

• 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)

Question 4

Sensory cells within the skin… (2)

Answer 4

1. 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
2. dermis = contains oil & sweat glands, vasculature, hair cells a variety of sensory cell subtypes

Question 5

mechanoreceptors

Answer 5

• 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.

Question 6

SAI

Answer 6

• 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

Question 7

FAI

Answer 7

• 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’

Question 8

SAII

Answer 8

• 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

Question 9

FAII

Answer 9

• 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)

Question 10

Two-touch threshold response

Answer 10

• 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)

Question 11

Thermoreception

Answer 11

• 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

Question 12

Thermoreceptors

Answer 12

• 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

Question 13

Cold fibres vs warm fibres

Answer 13

• 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

Question 14

Pain…

Answer 14

• 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

Question 15

Nociceptive pain…

Answer 15

• the pain that develops from tissue damage that causes nociceptors in the skin to fire
• occurs from direct trauma to the skin

Question 16

Nociceptors…

Answer 16

• 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

Question 17

pain reflex

Answer 17

• 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

Question 18

Gate theory of pain control

Answer 18

• 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

Question 19

why is gate theory of pain control important to us?

Answer 19

• 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)

Question 20

closed loop…

Answer 20

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

Question 21

neural pathways outside the brain…

Answer 21

• 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

Question 22

Dorsal column-medial leminiscal pathway

Answer 22

• 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

Question 23

Spinothalamic pathway

Answer 23

• 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

Question 24

the differences and consequences of the paths

Answer 24

• 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

Question 25

somatosensory cortex

Answer 25

• 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’)

Question 26

somatosensory cortex 4 regions

Answer 26

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

Question 27

temperature info takes a slightly different pathway…

Answer 27

• 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

Question 28

Somatosensory homunculus

Answer 28

• 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)

Question 29

Modern homunculus

Answer 29

• 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

Question 30

Perception of itch…

Answer 30

• 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

Question 31

Proprioception

Answer 31

• 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

Question 32

3 different kinds of sensory receptors in proprioceptive system give us info…

Answer 32

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

Question 33

proprioception & reflexes…

Answer 33

• 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

Question 34

Vestibular system

Answer 34

• 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

Question 35

semicircular canals

Answer 35

• 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

Question 36

otolith 2 organs…

Answer 36

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

Question 37

vestibular brain

Answer 37

• 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

Question 38

Vertigo

Answer 38

• 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

Question 39

Tactile agnosia…

Answer 39

• 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