Touch and Pain Flashcards
What is skin?
The largest sense organ in the body
What is the area and weight of skin?
- Area = 1.8 m^2
- Weight = 5 kg
What are the two types of skin?
- Glabrous – palms of hands and feet
- Hair – everywhere else in the body
What is the physiology of skin senses?
- Stimulus contacts the skin
- Receptor in skin fires
- Signal travels to the brain via the spinal cord
- Signal reaches somatosensory cortex on opposite side of the body
What are some skin sensations?
- Touch (mechanical stimuli)
- Pain
- Body sense (proprioception)
- Temperature
What are the different types of tactile receptors?
- Merkel’s disc
- Messner corpuscle
- Ruffini organ (deeper in the skin)
- Paccinian corpuscle (deeper in the skin)
What do Merkel’s disc receptors detect?
Fine details (e.g. braille)
What do Meissner corpuscle receptors detect?
Flutter (e.g. object slipping through fingers)
What do Ruffini organs detect?
Stretching, e.g. due to picking up something
What do Paccinian corpsucle receptors detect?
Vibration, fine texture e.g. using a tool
Why do we have multiple receptor types?
Many receptors = detect many types of information (c.f. rods and cones)
What can a single stimulus activate?
Many different receptor systems
What is the receptive field?
- The area of skin that a particular cell receives information about
- Paccinian corpuscles have larger receptive fields than Meissner’s corpuscles
What is the two point threshold and what affects it?
- Two point threshold – the smallest separation of 2 separate but adjacent points of stimulation on the skin that just produces two distinct impressions of touch
- Finger tip 2mm
- Arm 3.5 cm
- Number of receptors (how close together they are) affects to point threshold (closer the receptors then the lower the threshold)
- Amount of space in primary somatosensory cortex is much larger for certain body parts (leads to lower two point threshold)
Give features of the fingertip
- More receptors
- ‘fovea’ of the skin
- Acuity can change with experience e.g. Braille readers, musicians
What is active touch and what is passive touch?
- Active touch: active exploration of environment
- Passive touch: body is stationary (something lands on your skin)
What are advantages of active touch?
- More parts of body contact object
- You can search for the most diagnostic parts of objects to feel
- Kinaesthetic senses are also engaged
What cues do you use when perceiving texture?
- Spatial cues – bumps and grooves, when finger is stationery or moving
- Temporal cues – only when move finger across surface
Paccinican corpuscles – adaption to high frequencies impairs performance (when they are taken out impairs our performance in perceiving texture)
Can perceive texture via a tool (e.g. a pen)
What is the double dissociation for what and where processing?
Tactile agnosia: cannot identify objects by touch, but no problems with spatial processing
Tactile extinction without problems in object recognition
When patients felt for objects when being scanned what areas of the brain were activated when they described what the activity was and where the activity was?
- What object was
Activity in primary and secondary somatosensory cortex - Where object was
Activity in superior parietal areas (known for processing space – auditory and visual stimuli too)
What are top-down influences on touch?
- Must update as move body position
- Emotional effect – the same sensation may be pleasurable or unpleasant
- Expectation, surprise – tickling
Why can’t you tickle yourself?
- Can predict consequences of one’s own action
- Same touch rated as more ticklish when produced by experimenter rather than self
What was the old view on pain?
That it was overstimulation of any system
Now what do we know we have that detects pain?
Nocieceptors - receptors for pain
What are the two types of pain?
- A delta fibres – fast pain (sharp): pin prick, pinches, extreme temperature
- C fibres – slow pain (dull) – many types of pain (mild stimulation can be pleasurable) – unmyelinated
- Same stimulus can activate both systems
Pain is more than just receptor activity. What else is pain affected by and when else does it occur?
- Be affected by a person’s mental state
E.g. battlefield analgesia (not notice pain on the battlefield) - Occur in the absence of stimulation
E.g. phantom limb pain - Be affected by attention
What does gating of pain in the spinal cord influence?
Influences degree to which painful information reaches the brain
What can pain be reduced by?
- Non-painful tactile inputs (massage, rubbing)
- Top-down input (expectations etc.)
What is a phantom limb?
- After a limb is amputated, patient may feel a ‘phantom limb’ in its place
- There is no external stimulus, but patient still perceives the ‘limb’
What is proprioception?
- Where your body is in space
Signals from muscles - Also can use other modalities – vestibular system, tactile receptors etc.
- Also kinesthesis – movement of limbs in space
What happened with the man who lost his body?
- IW lost most proprioception, kinesthesis and touch due to viral infection at age 19
- Learned, over 3 years, to compensate using only visual information
- Unable to move if it is dark
- Lost fast myelinated fibres – retained slow C fibres
What receptor is used for light sensations of pleasant touch?
CT (C tactile)
