W12 - Touch and Pain ✅

Question 1

How does the skin transmit sensory signal to brain?

Answer 1

• Skin is the largest organ in the body
• 2 types of skin: glabrous (palms of hands and feet) & hairy
• Sensitive to 4 kinds of sensations: touch, pain, body sense and temperature

Sensory processing:
1. stimulus contact with skin
2. receptor in skin fires
3. signal travels via spinal cord to brain
4. signal reach somatosensory cortex on opposite side of the body

Question 2

What are the types of tactile receptors?

Answer 2

1. Merkel’s disc
   - lie right on the epidermis (top outer part)
   - sensitive to fine details (e.g. braille letters)
2. Meissner corpuscle
   - oval receptor on the epidermis
   - detect flutter (e.g. object slipping through
3. Ruffini organ
   - middle of the dermis (middle layer)
   - can sense stretching of skin
4. Paccinian corpuscle
   - large oval receptor in dermis AND on top of subcutaneous (fat) layer
   - detect vibration and fine texture

Question 3

Why do we have multiple receptor types?

Answer 3

• Many receptors = detect many types of
  information
• A single stimulus can activate many
  different receptor systems

Question 4

What is meant by receptive field?

Answer 4

The area of skin that a particular cell receives information about

Paccinian corpuscles have larger receptive fields than Meissner’s corpuscles
-> difference in structures, specialised functions and the need to capture broader range of stimuli

Question 5

How can we detect the threshold of touch perception?

Answer 5

• Using 2-point discrimination across the body
• The smallest separation of 2 separate but adjacent points of stimulation on the skin that just produces two distinct impressions of touch
• More extreme thresholds for fingertips, lips and toes
• Higher threshold = more receptors present

Question 6

What is the difference between active versus passive touch?

Answer 6

1. Active touch: active exploration of the environment -> have advantages
   - more parts of body contact object
   - search for the distinct feature of object for recognition
   - engage using kinesthetic senses
2. Passive touch: body is stationary

Question 7

What cues do we look for when perceiving texture of an object?

Answer 7

1. Spatial cues: bump and grooves (when finger is stationary or moving)
2. Temporal cues: move finger across surface

Question 8

What is the double dissociation found in touch perception? (+ what & where processing)

Answer 8

1. Tactile agnosia: cannot identify objects by
   touch, but no problems with spatial processing (can recognise object but can’t describe them)
2. Tactile extinction BUT without problems in object recognition

Brain imaging during object processing:
- WHAT: primary and secondary somatosensory cortex (sensory processing
- WHERE: superior parietal area (responsible for spatial perception & memory)

Question 9

What are some examples of touch illusions?

Answer 9

1. Aristotle’s illusion: when crossing your finger and holding an object in-between, feels like you are holding 2 objects
   -> brain failed to take into account crossing fingers
2. Cutaneous rabbit: when making widely separated two-finger taps along your arm in a rhythmic motion, it feels like rhythmic continuous taps even in the gap regions that you don’t tap

Question 10

What are the two types of pain?

Answer 10

Nocieceptor - receptor for pain (detect extreme stimuli or signal from damaged cells)

1. A delta fibres - fast sharp pain (e.g. pin prick, pinches, extreme temp)
2. C fibres - slow dull pain

-> Same stimulus can activate both systems (e.g. intial sharp pain when being hit, then dull stinging pain afterwards)

Question 11

What factors can affect the level of pain we feel?

Answer 11

1. Affected by a person’s mental state (if scared, pain feels worse and vice versa)
2. Occur in the absence of stimulation (e.g. phantom limb pain)
3. Attention (the more you pay attention to a pain inducing stimulus, the more it hurts)
4. Gating of pain in spinal cord - influence degree to which painful information reaches the brain -> non-painful tactile inputs (e.g. massage) and top-down input (e.g. expectations)

Question 12

What is proprioception + example?

Answer 12

1. Body awareness without relying on vision (spatial) and it relies on:

• Signals from muscles (e.g. tension, joint,..)
• Vestibular system (balance)
• Tactile receptors (touch)
• Kinesthesis - movement of the limbs in space

2. Example for proprioception damage - Ian Waterman

• Lost proprioception, kinesthesis and touch due to viral infection
• Gradually regain movement by compensating with visual information
• Unable to move when dark
• Retain low C fibres (unmyelinated fibres - responsible for feeling dull pain)