Somatosensation

Question 1

What is touch useful for?

Answer 1

1) Body info - posture, position, movement & pain

2) Recognise & use objects

3) Communication

4) Body growth

Question 2

Who is Ian Waterman?

Answer 2

Had a neurological illness that prevented sensation of touch / position / movement below the neck, but no loss of sensation of pain / heat.

Slurred speech, drunken movements.

Autoimmune response attacked his own sensory nerves

Spent 18 months learning to control movement

Question 3

What falls under cutaneous sensation?

Answer 3

• Pressure
• Vibration
• Temperature
• Pain

Question 4

What are some non-cutaneous sensation?

Answer 4

Proprioception, kinaesthesia and itch.

Question 5

How does the somatosensory system work for non-painful cutaneous sensation?

Answer 5

Skin receptors -> peripheral nerves (cranial nerve 5) -> cross over at the brain stem to go to either spinothalamic tract (pain & temp) or leminiscal tract (cutaneous touch) -> thalamus -> primary somatosensory cortex (SI)

Question 6

What nerves does touch use?

Answer 6

All peripheral nerves + cranial nerve 5 (trigeminal)

Question 7

What kind of receptors are touch receptors, and what are they specialised for?

Answer 7

Mechanoreceptors - pressure, stretch & vibration

Question 8

What are the four kinds of touch receptors?

Answer 8

1) Meissner’s corpuscle

2) Pacinian corpuscle

3) Ruffini’s corpuscle

4) Merkel’s disk

Question 9

When is the pacinian corpuscle activated?

Answer 9

When pain is activated; sensitive to high vibration stuff / rapidly happening things)

Question 10

What are some unique features of non-painful cutaneous sensation?

Answer 10

1) Sub-surface
2) Either oriented vertically / horizontally
3) Diff activation thresholds
4) Diff activity rates

Question 11

Describe pacinian corpuscles.

Answer 11

• Deep, horizontal orientation
• Easily deformed; responds to dynamic but not steady pressure
• Low threshold, rapidly adapting (respond easily but transiently to stimuli)
• Responds to sudden stimuli like tickles / pokes

Question 12

Describe Meissner’s corpuscles.

Answer 12

• Shallow, vertical orientation
• Only in hairless, smooth skin (glabrous - 40% of sensation in hand)
• Mechanically deformed by light touch
• Very low threshold, shallow position, rapid adaptation, easy response to dynamic, moderate stimuli (different textures)

Question 13

Describe Ruffini’s corpuscles.

Answer 13

• Intermediate depth, horizontal orientation (moderate surface area)
• Mechanically deformed by stretch
• Slow response rate (2-3 Hz) allows response to stable, low frequency stimulation (stretching / joint movement)
• could help in non-tactile signalling (proprioception)

Question 14

Describe Merkel’s disks.

Answer 14

• Shallow, moderate surface area
• Mechanically deformed by pressure (not as easy as PCs)
• slow response & adaptation
• respond to static pressure / touch
• static discrimination of shapes & edges (holding an object)

Question 15

What happens when mechanoreceptors are activated?

Answer 15

Electric signal transduction to sensory neurons under the skin.

Question 16

What factors contribute to different behaviours in nerve fibers / axons?

Answer 16

Diameter, conduction velocity and myelination state.

Question 17

How do fibers differ?

Answer 17

Information they carry and the tissues they innervate

Question 18

What are the characteristics of fibers in “A” group?

Answer 18

Large diameter and greater myelination = higher conduction velocity

Question 19

What type of A fibres are the four mentioned mechanoreceptor types?

Answer 19

A-beta fibres.

Question 20

Describe the 4 mechanoreceptor fibres in terms of diameter, conduction velocity and myelination

Answer 20

Intermediate size (6-12 micrometers), myelinated & fast velocity (33-75 metres/s)

Question 21

How does each nerve act in response to non-painful cutaneous sensation?

Answer 21

Carry signals from defined region of the body (dermatome)

Feed sensory signals to dorsal spinal cord –> either to brain via ascending pathway or just activate a reflex arc

Question 22

How does the reflex arc work within the spinal cord?

Answer 22

Sensory neuron (afferent info from skin) –> interneuron (relay neuron) –> motor neuron (efferent info to muscles)

• sensorimotor loop

Question 23

What pathway does non-painful information take?

Answer 23

Medial-Leminiscal Pathway:

sensory neuron –> brainstem –> thalamus –> somatosensory cortex

Question 24

What is the somatosensory cortex?

Answer 24

Long, thin strip of cortex extending roughly from ear to ear.

2 major subdivisions:
SI:
- receives input from thalamus
- divided further in Brodmann’s 3a, 3b, 1 & 2

SII (secondary somatosensory cortex):
- receives input from thalamus & SI

Question 25

What are internal sensations essential for?

Answer 25

- Posture - Large scale movement (walking / reaching) - Small scale movements (haptic exploration & use) - Turning & counterbalance (orienting / balancing)

Question 26

What contributes to the feel of a movement, versus position of a movement?

Answer 26

Feel: kinaesthesis Position: Proprioception

Question 27

What are the three types of sensory receptor neurons?

Answer 27

1) Muscle spindles (proprioceptors): - bunch of 4-8 muscle fibres surrounded by connective tissue & sensory nerve endings - respond to change in muscle length - high density in hand, neck, ocular muscles - low density in large muscles that generate coarse movement 2) Golgi tendon organs: - similar but located in tendon; respond to change in muscle tension 3) Joint receptor neurons: - free nerve endings in joints; respond to join movement