Week 4 - Somatosensation

Question 1

What is Somaesthesis

Answer 1

Soma = body
Aesthesis = to feel/perceive

Somaethesis = the bodily senses
Including
- touch
- proprioception
- kinaesthesia
- pain
- itch
- tickle
- heat

Question 2

Somatosensation is a …

Answer 2

Near sense (in contrast with olfaction and vision)
is classed as a low sense by Plato and Aristotle (in that it is foundational)

Question 3

Touch is useful for

Answer 3

• Body information; posture, position, movement, pain
• Recognition and use of objects; food, tools, surfaces and other animals (or humans)
• Communication; conveying messages and social cues
• Proper development - body growth (we need it from an early age)

Question 4

Ian Waterman

Answer 4

Is a case study showing that we can survive without touch, but that this is incredibly difficult

• At age 19, Ian had a rare neurological illness (like a flu-like viral infection).
• His immune system attacked his own sensory nerves, resulting in a loss of sensation below the neck. This was gradual onset but permanent. Ian retained his sense of pain and heat
• ian appeared drunk, with slurred speech, no sense of position, movement or touch below the neck
• Ian learnt to to control movement and walk again after 18 months using visual cues - however, this requires immense concentration and any lapse would result in a fall

Whilst Ian has shown it is possible in certain cases to survive without touch, he has had to alter his life, avoiding crowds and planning excertions

Question 5

Classifying the submodalities of somatosensation

Answer 5

1. Cutaneous Sensation = a sensation (as of warmth, cold, contact, or pain) aroused by stimulation of end organs in the skin.
   - Pressure and Vibration (by mechanoreceptors)
   - Temperature (thermoreception)
   - Pain (nociception)
2. Proprioception
3. Kinaesthesia
4. Pain and Itch

Question 6

4 Criteria for Sense Classification

Answer 6

1. Specialised to receive a particular stimulus (eg. has specific receptors)
2. Performs signal transduction
3. Relays the neural signal to the brain by a certain pathway
4. Has it’s own cortical region for processing

Question 7

What are the touch stimuli for cutaneous touch

Answer 7

Physical stimuli are required for cutaneous touch sensation
1. Mechanical compression (detected by a mechanoreceptor)
2. Vibration (detected by a mechanoreceptor)
3. Thermal Energy Transfer (detected by thermoreceptors)
(Chemesthesis could apply here too aka chemical sensation for pain/nociception)

Question 8

Pathway of Somatosensation (for non-painful cutaneous sensation)

Answer 8

1. Receptors in the skin (Pacinian, Ruffini, Meissner corpuscle, Merkel discs)
2. Peripheral nerves or cranial nerve 5 (trigeminal)
3. Spinal cord or cranial nerve for in the face
4. Medial lemniscal tract (for cutaneous touch) or spinothalamic tract (for pain and temp)
5. Brainstem
6. Thalamus
7. Primary Somatosensory Cortex

Question 9

What are the decussation points in the lemniscal and spinothalamic tracts?

Answer 9

Lemniscal tract carries signals for non-painful cutaneous information, crosses over in the brainstem

Spinothalamic tract carries pain and temp info and crosses over in the spinal cord (dorsal portion)

Question 10

What sensory nerves are used to carry afferent non-painful cutaneous sensation?

Answer 10

All peripheral nerves (remember the dermatome which shows a point to point mapping system of peripheral nerves to spinal cord) and the cranial nerve 5 (trigeminal) for sensation in the face

Question 11

How come Ian Waterman’s infection didn’t result in loss of sensation from the face

Answer 11

He didn’t lose his trigeminal nerve

Question 12

Sense organ for touch (non-painful cutaneous sensation)

Answer 12

= the skin
- touch uses a variety of receptors located in the skin at different depths
- these receptors are specialised for pressure, stretch and vibration
- touch receptors are mechanoreceptors
including;
- Meissner’s Corcpuscles
- Pacinian Corpuscles
- Ruffini’s Corpuscles
- Merkel’s Discs

Things to note;
Skin receptors are;
1. Subsurface (unlike chemoreceptors, aren’t on the surface of the sense organ)
2. Are oriented vertically and/or horizontally
3. Have different activation thresholds
4. have different firing rates

Question 13

Why is there such variation in the types of touch receptors (for non-painful cutaneous touch)?

Answer 13

To account for all the different submodalities of touch

Question 14

Pacinian Corpuscles

Answer 14

‘squishy or cushion like’
- Are deep (in dermis) and horizontal in orientation
- easily deformed structure
- low threshold (fire easily)
- rapidly adapting
- responds to dynamic pressure but not static/steady pressure
- Responds to sudden stimuli (like a poke or tickle)

Question 15

Meissner’s Corpuscles

Answer 15

• In epidermis
• Are shallow (in epidermis) and vertical in orientation
• only in ‘glaborous’ –> non-hairy skin
• Mechanically deformed by light touch
• Very low threshold
• Rapidly adapting
• Respond esily to dynamic, moderate stimulation
• respond to textures

Question 16

Ruffini’s Corpuscles

Answer 16

intermediate depth (in dermis)
- horizontal orientation (moderate surface area)
- Mechanically deformed by stretch
- slow response rate (2-3Hz) allows them to respond to stable, low frequency, stimulation (stretch)
- Not rapidly adapting
Not well understood.. likely have a role in non-tactile signalling like proprioception as they also respond to movement of joints

Question 17

Merkel Discs

Answer 17

• Shallow with moderate surface area
• Just under the epidermis
• Mechanically deformed by pressure, but not as easily as pacinian corpuscles
• Slow response rate
• slow adapting
• respond to static pressure/touch
• Good for static discrimination on shapes and edges (eg. when holding an object)

Question 18

Signal Transduction of Non-painful Cutaneous Sensations

Answer 18

Mechanoreceptors are activated under the skin, this causes signal transduction in sensory nerves (peripheral nerves) whose axon propogates to the spinal cord (long axon)

In the case of the face, touch elicits signals in the trigeminal nerve travelling to the brainstem

Question 19

Types of Axons in Somatosensory System (for non-painful cutaneous touch)

Answer 19

Different kinds of axonal fibres behave in different ways due to differences in their diameter, myelination state and conduction velocity.

Fibers of the A group; Consist of A-alpha, a-beta, a-delta, and a-gamma (the type is determined by the info they carry and the tissues they innervate)
- have large diameter (so low resistence)
- Are myelinated (high insulation)
- high conduction velocity

Question 20

a-beta fibres

Answer 20

• have large diameter (so low resistence)
• Are myelinated (high insulation)
• high conduction velocity
  (as with all group a fibres)
• are intermediate in size (6-12 um)
• have a conduction velocity between 33- 75 m.s)

All 4 mechanoreceptors used in non-painful cutenous somatosensation use a-beta fibres

Question 21

Reflex Arcs in somatosensation

Answer 21

Sensory fibres in somatosensation carry information from a defined region of the body to the spinal cord (dermatome) and feed sensory signals into the dorsal spinal cord. from there, they can either ascend to the brain or activate a reflex arc

In reflex arcs, the sensory neuron (carrying afferent info from the skin) synapses onto an interneuron in the sc. (relay neuron) and then innervates a motor neuron (allowing efferent info to exit the sc to the muscles of the body)

This is a simple sensorimotor loop eg. withdrawal reflex

Question 22

Outline the Medial Lemniscal Ascending Pathway in terms of ordered neurons

Answer 22

This path carries non-painful info

Sensory neuron (first order neuron) –> Brainstem (second order neuron) –> thalamus (third order neuron) –> Somatosensory cortex

Question 23

Somatosensory Cortex and its role in Non-Painful Cutaneous Touch Sensation

Answer 23

The somatosensory cortex;
= a long strip of cortex extending (roughly) from ear to ear

• Has 2 major subdivisions;
  1. The primary somatosensory cortex (SI) receives input from the thalamus and is broken down into Brodmann’s 3a, 3b, 1 and 2 (also called the post-central gyrus)
  2. The secondary somatosensory cortex (SII) which received inputs form the thalamus and primary somatosensory cortex
• is somatopically mapped (Penfield’s sensory homunculus) where neighbouring regions of the body map correspond to neighbouring regions of the somatosensory cortex (generally)

Question 24

Wilder Penfield and the ‘Montreal Procedure’

Answer 24

Penfield and his colleagues published their findings about a method of surgery that would become known as “the Montreal procedure” in 1952. The procedure enabled surgeons to operate on the brains of epileptic patients and destroy the cells where seizures originated without impacting other important regions of the cortex.
The Montreal procedure was curated by having Penfield electrically stimulate regions of the somatosensory cortex over the course of his career allowing him to map the sensory homunculus

Question 25

Somatotopic organisation of the somatosensory Cortex

Answer 25

Refers to the point to point mapping of the body to the somatosensory cortex.
In general, neighbouring regions of the body map correspond to neighbouring regions of the sensory cortex. The amount of cortical space allocated to a body region in the somatosensory cortex also corresponds (or is proportional) to the represented area’s sensitivity

Question 26

Plasticity of the Somatosensory Cortex

Answer 26

The somatosensory cortex and somatotopic organisation of this region shows functional plasticity.
This is because the primary somatosensory cortex representations can change depending on the amount of sensory input being received.

Eg. Digit fusion experiments in monkeys (saw a reduction in cortical space allocated to the affected digits)
eg. Attended tactile practise and training in monkeys (ie. grasping tasks which trained monkeys and increased the cortical space allocated to digits used in these tasks)
eg. string musician studies in humans (elbert et al., 1995) saw increased cortical representation of the digits in the somatosensory cortex
eg. Phantom limb pain after amputation
– lower arm amputation leads to somatosensory cortex reorganisation (cortical lip/cheek representation moves to former hand area and there is greater representation of the upper arm). In these studies the magnitude of phantom pain is strongly correlated to the amount of re-organisation of the cortex (these findings notably are correlation not causation. Some others believe the pain comes from decreased connectivity between the primary somatosensory cortex and other brain regions - eg. makin et al., 2013)

The plasticity of the somatosensory cortex shows central (cortical) not peripheral changes to our sensory capacity

Question 27

Digit fusion experiments as an example of somatosensory plasticity

Answer 27

Eg. Digit fusion experiments in monkeys (saw a reduction in cortical space allocated to the affected digits)

Question 28

Tactile practise in monkeys as evidence for somatosensory plasticity

Answer 28

eg. Attended tactile practise and training in monkeys (ie. grasping tasks which trained monkeys and increased the cortical space allocated to digits used in these tasks)

Question 29

String musician studies as evidence for the plasticity of the somatosensory cortex

Answer 29

eg. string musician studies in humans (elbert et al., 1995) saw increased cortical representation of the digits in the somatosensory cortex

Question 30

Phantom limb pain after amputation as an example of somatosensory plasticity

Answer 30

eg. Phantom limb pain after amputation
– lower arm amputation leads to somatosensory cortex reorganisation (cortical lip/cheek representation moves to former hand area and there is greater representation of the upper arm). In these studies the magnitude of phantom pain is strongly correlated to the amount of re-organisation of the cortex (these findings notably are correlation not causation. Some others believe the pain comes from decreased connectivity between the primary somatosensory cortex and other brain regions - eg. makin et al., 2013)

Question 31

Somatosensory plasticity is an example of..

Answer 31

central (not peripheral) changes to sensation

Question 32

Kinaesthesia and Proprioception

Answer 32

= the ability to sense the movement and position of our own body

Question 33

How do kinaesthesia and proprioceptive sensory processes differ from other forms of sensation?

Answer 33

• sensation of this sort arises from inside the body - aka requires a physical stimulus from the internal environment to trigger sensation (eg. mechanical stretch, tension in muscles etc)
• The sensory info from these modalities goes large un-noticed (aka are automatic/subconscious) however, is essential for;
• posture
• large scale movement (walking/reaching)
• small scale movement (haptic exploration and use)
• Turning and counterbalance (orienting and balance)

Question 34

What is Kinaesthesia and Proprioception essential for?

Answer 34

The sensory info from these modalities goes large un-noticed (aka are automatic/subconscious) however, is essential for;
- posture
- large scale movement (walking/reaching)
- small scale movement (haptic exploration and use)
- Turning and counterbalance (orienting and balance)

Question 35

Sensory Receptors used in Kinaesthesia and Proprioception

Answer 35

1) Muscle spindles (proprioceptors) muscle length
= a bunch of 4 - 8 muscle fibres surrounded by connective tissue and sensory nerve endings.
- respond to changes in muscle length
- are found in high densities in the hand, neck, ocular muscles and in low densities in large muscles that generate coarse movement

2) Golgi Tendon Organs; muscle tension
= Are similar to muscle spindles in structure but are located in tendons
- respond to changes in muscle tension
- eg. would activate during a bicep curl due to tension in the bicep muscle

3) Join Receptor Neurons; joint movement
=free nerve endings in joints and respond to joint movement (little is known about these)

All afferent sensory neurons from these receptors travel to the spinal cord

Question 36

Sensory Fibres for P/K Sensation

Answer 36

a-alpha axonal fibres carry electrical signals from the muscle spindles, golgi tendon organs and joint receptor neurons to higher centres.

Fibres of the A group;
- have a large diameter
- are myelinated
- have a high conduction velocity

a-alpha fibres have;
- the thickest diameter of the a group (13-20um)
- are myelinated
- Have the fastest conduction velocity o the a group (80 - 120m/s)

This structure relates to function as we need rapid propagation of kinaesthesia and proprioceptive signals to allow for millisecond by millisecond adjustments

Question 37

Pathway to the brain for Kinaesthetic and Proprioceptive signals

Answer 37

1. Sensory receptors are activated allow for signal transduction to occur (these are the muscle spindles, golgi tendon organs and joint receptor neurons)
2. The neural signal is relayed towards the brain along a-alpha neuronal fibres along the dorsal column (or medial lemniscal tract) or the spinocerebellar tract)
3. At the cortex, P/K information goes to the primary somatosensory cortex and motor cortex (to make rapid adjustments to body position and movements etc) and the parietal cortex and cerebellum for processing also

Question 38

Spinocerebellar Tract

Answer 38

Carries proprioceptive and Kinaesthetic information to the cerebellum.
It follows the same path as the dorsal column until the brainstem at which point information is carried to the cerebellum instead of the thalamus

Question 39

Quotes from Naito et al., 2002 and Hagura et al., 2009 on the cortical processing of proprioceptive and kinaesthetic information

Answer 39

Naito et al., 2002; “I can feel my hand moving; a new role of the primary motor cortex in somatic perception of limb movement”

Hagura et al, 2009; “Visuokinestethic perception of hand movement is mediated by cerebro-cerebellar interaction between the left cerebellum and right parietal cortex

Question 40

What are the 4 areas used in cortical processing of proprioceptive and kinaesthetic information

Answer 40

1. Motor cortex
2. Somatosensory cortex
3. Parietal cortex
4. Cerebellum

Question 41

Role of the Posterior Parietal Cx in Kinaesthesia and Proprioception

Answer 41

The posterior parietal cortex received projections from the primary somatosensory cortex (SI) and receives prjections from visual and auditory cortices, thalamus and hippocampus.
This region is believed to be an association cortex, involved in integrating and processing multisensory associations. Helps to greater the Gestalt (aka the perceptual whole)

This is important as movement planning required information coded in various coordinate systems to be unified. This includes info on body position and current movement (P/K sensory data). The somatosensory cortex is not well placed to do this (due to relatively little visual/auditory input) so the posterior parietal cortex is involved.

Question 42

Functions of Touch aka the online body scan

Answer 42

The online body scan consists of cutaneous, proprioceptive and kinaesthetic information which is processed together to generate a continual map of where the body is in space

It considers the following;
- posture
- locomotion
- limb movement
- internal stretch
- internal/external pain
- and assess if something is touching us

The parietal cortex is an important site in terms of online processing of bodily information (as it integrates and creates a unified percept).

Question 43

Polymodal Integration

Answer 43

= how information from the different sensory modalities (such as sight, sound, touch, smell, self-motion, and taste) may be integrated by the nervous system.

The parietal cortex is key in this. It coordinates multiple inputs and assesses if SI and SII information aligns with inputs from the motor cortex, if this compares to proprioceptive, vestibular and visual info too.
Information from the various sense regions of the brain needs to match for coherent perception, failures in this process can impair fluid movement, fine motor control and normal body awareness

Question 44

What is the outcome of successful Polymodal Integration

Answer 44

= Fluid movement, fine motor control and normal body awareness

Question 45

What happens when there are issues in polymodal integration or a discrepancy between incoming sensory information

Answer 45

A mismatch in sensory information results in the parietal cortex ‘filling in the blank’. Bizarre perceptions can occur as a result

eg. - rubber hand illusion
- phantom limbs
- somatoparaphrenia (= idea that a portion of your body doesn’t belong to you. Often occurs in people who are paralysed and is thought to involve damage to the fronto-temporal-parietal network)

Question 46

Functions of Touch

Answer 46

1. Body Info - online body scan
2. Identification and Use of Things
3. Communciation
4. development

Question 47

Functions of touch aka identification and use of things

Answer 47

• This function of touch relies on haptics (aka GR. to touch) which are informed by cutaneous touch, P/K information and sensorimotor integration

This function of touch can help us identify and use objects, people, and is involved in mother-infant recognition, haptic face perception (knowing someone’s face by feel) and male/female identification

Question 48

What’s the absolute threshold for perceiving the size of an with a given texture? (Skedung et al., 2013)

Answer 48

~10nm
Shows the fine sense of touch in the fingertouch

Question 49

Lederman and Klatzky (1987) ; The 6 basic exploratory hand movements

Answer 49

Suggests from 6 exploratory hand movements we can extract 7 properties of a given object.

The 6 movements are;
1. Lateral motion (to detect texture with the Meissner Corpuscles)
2. Unsupporting holding (to detect weight)
3. Pressure (to detect hardness with pacinian corpuscles)
4. Enclosure (to detect global shape and volume with Merkel discs)
5. Static contact (to detect temperature with thermoreceptors)
6. Contour following (to detect global shape and exact shape)

Properties.
1. Texture
2. Weight
3. Hardness
4. Volume
5. Global Shape
6. Temperature
7. Exact Shape

Question 50

Haptic Technology - robotics

Answer 50

Explores how we can replicate the bodies receptors in technology and robotics

eg. Humanoid Robot HRP-2; was the first humanoid robot to incorporate haptic exploration in grasping tasks.
Can be used to;
- build furniture
- assist old people
- detect suspect presence (can detect different surfaces)

Question 51

Haptic Technology - Touchscreens

Answer 51

= A touchscreen with added touch features
- Tesla Touch for example can simulate the feel of textures and materials by emitting electrical impulses to trigger vibration or friction
- conversely technology can perform different functions by detecting different pressure of touch etc

Question 52

Thomas Et al., 2021 Prostethic limbs and Haptics

Answer 52

Thomas et al build prostethic limbs with haptic feedback (vibration) to improve the ease of device use.
It would send vibrations into a users arm to indicate when the clasp on the prosthetic was touching an object and grasping objects at different intensities to create the perception of pressure.
This is a less mentally taxing experience for the user as they aren’t completely reliant on visual feedback alone

Question 53

Haptic Illusions; The fishbone illusion (geometry of a shape)

Answer 53

• asked participants to feel concave bumps on a plastic surface whilst blindfolded then have them pick in a mcq what texture they felt
• Participants tended to believe they’d felt a texture with convex bumps rather than concave ones.

A rough explanation;
- in the task, participants experience complex distortions of the skin (friction, pressure, changes in geometry etc) –> Therefore are receiving multiple bottom up signals
- due to its ambiguity the brain makes sense of the texture with top down influences (memory, expectation etc)
The final percept is a best guess on what participants expect to feel

Question 54

The velvet hand illusion (Yokosaka et al., 2021)

Answer 54

Has participants whilst blindfolded rub their hands together with a series of vertical wires sandwiched in between.
Participants are asked to describe the sensation and tend to report it as being velvety

As the intensity of the illusory sensation increased, tactile sensation is reported as softer, wetter, warmer, more favourable. Also, when the illusion is strong, sensation is reported as being similar to leather and fabrics rather than wire

A rough explanation;
- in the task, participants experience complex distortions of the skin (friction, pressure, changes in geometry etc) –> Therefore are receiving multiple bottom up signals
- due to its ambiguity the brain makes sense of the texture with top down influences (memory, expectation etc)
The final percept is a best guess on what participants expect to feel

Question 55

Functions of Touch - Communication

Answer 55

touch can signal attention, friend, foe lover etc.
We use it in semantic communication (eg. braille) and social communication

It is believed the earliest form of communication was a combination of touch and vision

Tactile communication is present even when words aren’t

Attribute important components of intersonal relationships throughout life (eg. social customs/politeness, love and intimacy, social dominance, friendship etc
- contact patterns infleucned by these factors and others (male/female, familiarity, age and culture etc )

Question 56

Functions of Touch - development

Answer 56

Touch deprivation is a form of sensory deprivation and is detrimental to development in many species.

In rats, severe endocrine imbalance from early tactile deprivation can occur which impacts development

Human infants that are regularly held/stroked exhibit better body weights, more activity and more curiosity
- massage aids in premature babies can help them gain weight
- skin to skin promotes bonding/attachment and may assist in bringing in a milk supply and getting better sleep

Question 57

How does touch exert positive effects? Yu et al., 2022

Answer 57

Social touch- like tactile stimulation activates oxytocin-producing neurons of the hypothalamus in mice. Activation of these neurons promotes social interaction and conditions place preference.
Oxytocin is the bonding hormone and is important in bonding (ie. sex and mother/child bonding)
(oxytocin may be reinforcing in this context)