Touch

Question 1

What are the characteristics of touch perception?

Answer 1

• part of somatosensory system
• provides information about the nature of surfaces and objects in the world that are in direct contact with the organism (the skin). It is vital for survival in that it informs is of potentially useful and harmful stimuli
• can evoke many sensations including mechanical pressure (contact, vibration, roughness, etc.), temperature, itch and pain
• Receptors in the muscles and joints also encode the postures, locations and movements of the body (‘proprioception’). Important for active touch

Question 2

how is touch distinct from the other 4 sense modalities?

Answer 2

• Receptors for touch are varied and distributed throughout the entire body unlike those of other sensory modalities (e.g. hearing) which are confined to specific, localised structures
• Perceptual apparatus (receptors, neural pathways, etc.) mediating touch responds to many different types of stimulation and the quality of the sensations produced is extremely diverse

Question 3

what are the 7 types of receptor in the skin?

Answer 3

• Meissner’s corpuscle (pressure)
• Basket cell (pressure)
• Merkel’s disc (pressure)
• Free nerve ending (pain, temperature & pressure)
• Kraus end bulb (temperature?)
• Ruffini ending (pressure)
• Pacinian corpuscle (pressure)

Question 4

What are the Characteristics of Cutaneous Touch Receptors?

Answer 4

• the different sensory qualities are mediated by different specialised receptors within the skin layers
• Mechanoreceptors are touch receptors that respond to pressure or indentation of the skin
• The smooth, hairless portions of skin (‘glabrous skin’) found on the palms, fingers, soles of the feet, etc. contain 4 main types of mechanoreceptors
  Pacinian corpuscles, Meissner corpuscles, Merkel discs and Ruffini endings (‘corpuscular’)
  ~ 17 000 mechanoreceptors on the hairless skin on the hand (Johansson & Vallbo, 1983)
• A given cutaneous pressure sensation may thus arise from activation of several different specialised mechanoreceptors rather than a single one
• Unclear if a particular type of receptor exclusively evokes a specific touch sensation.

Question 5

what do the 4 types of mechanoreceptors do?

Answer 5

• send information to the brain via afferent (‘touch’) fibers that can be classified according to their properties

Question 6

what are the temporal properties of Mechanoreceptors/Afferents for Touch

Answer 6

slowly adapting fibres respond continuously to a persistent tactile stimulus and rapidly adapting fibers respond only to the onset and termination (i.e. a changing) of a stimulus

Question 7

what are the spatial properties of Mechanoreceptors/Afferents for Touch?

Answer 7

Receptive fields in the skin have a concentric organisation (e.g. excitatory centre and inhibitory surround). Some fibers have large receptive fields and others small receptive fields

Question 8

Pacinian Corpuscle

Answer 8

Adaptational property: Rapid
Receptive field size: Large
Location in skin: Subcutaneous
Sensation evoked: Vibration

Question 9

Meissner corpuscle

Answer 9

Adaptational property: Rapid
Receptive field size: Small
Location in skin:Superficial
Sensation evoked: Flutter

Question 10

Merkel Discs

Answer 10

Adaptational property: Slow
Receptive field size: Small
Location in skin:Superficial
Sensation evoked: Pressure

Question 11

Ruffini ending

Answer 11

Adaptational property: Slow
Receptive field size: Large
Location in skin: Subcutaneous
Sensation evoked: Buzz-like

Question 12

sensitivity of the Pacinian Corpuscle

Answer 12

• Vibration sensitivity is U-shaped function of stimulus frequency
• Sensitivity greatest at ~ 250 Hz (threshold ~ 0.0001 mm)
• Crosses show thresholds for skin with no Pacinian corpuscles
• Rapid changing tactile stimulation encoded by Pacinian corpuscles
• At lowest frequencies detection mediated by other receptors (e.g. Meissner corpuscles, Merkel discs)

Question 13

Are all regions of the skin surface equally sensitive?

Answer 13

• The receptors in our skin are not distributed in a uniform way around our bodies
• Absolute thresholds for passive touch measured by recording minimum force necessary to detect fine nylon filament on skin
• Two-point localisation threshold (smallest separation between 2 points that are just resolvable) also varies across body surface

Question 14

what is the Ascending Pathways for Somatosensory Information?

Answer 14

• Synapse with neurones in ‘lemniscal pathway’
• Axons project to the brain stem, cross the midline, and then to the thalamus on the contralateral (opposite) side of the brain
• Thalamic neurones send axons to somatosensory cortex in the parietal lobe
• Information about touch (mainly from corpuscular mechanoreceptors in skin) is carried by large, fast, myelinated afferent fibers that enter the dorsal side of spinal cord
• Information about pain and temperature is carried by a second pathway (‘spinothalamic tract’)

Question 15

Where in the Brain is Incoming Touch Information Processed?

Answer 15

Somatosensory Cortex

Question 16

Which Regions of the Body Receive the most Cortex?

Answer 16

the body parts that are most important for survival of that species

Question 17

What Information is Gained from Active Touch?

Answer 17

• When we touch something our bodies actively explore the objects present in our immediate environment
• Information from the skin’s touch receptors is combined with information from ‘proprioceptors’ in the muscles and joints which encode the postures, locations and movements of the body
• When we actively explore the world with touch, we thus perceive objects through a combination of tactile and proprioceptive information

Question 18

Braille: Tactile Shape Discrimination Using Active Touch

Answer 18

• Our ability to discriminate and recognize complex kinds of information using active touch alone is utilised by the Braille reading system
• Using the fingers it is much easier to read text written in Braille letters than embossed (raised) Roman letters (Roman letters easily confused).
• Loomis (1981) showed that is due to the limited ability of the skin to resolve fine spatial details (mechanical properties of skin cause ‘blurring’)

Question 19

why is pain a difficult sensory experience to study?

Answer 19

• subjective experience
• Difficult to assess the quality (‘sharp’, ‘dull’, ‘burning’, ‘itch’, etc.) and intensity of pain
• Often impossible to identify a specific evoking stimulus for pain (individual differences)
• Psychological factors (e.g. expectation, attitude, attention, suggestion, emotional state, cognitive processes) may greatly influence the intensity and quality of the pain experienced

Question 20

what are nociceptors?

Answer 20

the receptors which when stimulated cause pain
- usually free nerve endings that are found almost everywhere within hairy and glabrous skin
- most common type of skin receptors
- also found found in muscles, tendons, joints and other internal organs
- found wherever ‘pain spots’ are located on skin
Neural messages from nociceptors are carried by specific afferent (‘pain’) fibres that travel from the skin to the spinothalamic tract of the spinal cord

Question 21

what is gate-control theory?

Answer 21

• theory of pain
• Fast ‘touch’ fibres and slow ‘pain’ fibres connect with substantia gelatinosa (SG) and transmission cells (T cells) in spinal cord
• T cells send pain information to the brain
• SG acts as “gate” to allow or inhibit T cells

Activity in fast fibers tends to close the gate (touch but no pain) and slow fibers open the gate (pain)