Somatosensory physiology

Question 1

What are the 4 main classes of receptors in the somatosensory system?

Answer 1

1) Tactile (innocuous - not painful)
2) Nociceptive
3) Proprioceptive
4) Thermal

Question 2

What are the 4 types of innocuous tactile receptors?

Answer 2

1) Merkel
2) Ruffini
3) Meissener
4) Pacinian

Question 3

What are the 3 types of proprioceptive receptors?

Answer 3

1) Muscle (spindle) - in muscles
2) Golgi tendon receptors - in joints
3) Some inputs from cutaneous mechanoreceptors

Question 4

How are thermal sensations mediated?

Answer 4

Mediated by thermoreceptors localized to discrete zones that exhibit hot and cold sensitivity

Question 5

By what 3 receptors are nociceptive (painful) sensations mediated?

Answer 5

1) Mechanical nociceptors
2) Thermal nociceptors
3) Polymodal nociceptors

Question 6

What are the 5 types of sensory receptors in the skin, where is each located?

Answer 6

1) Meissener’s corpuscle - Just beneath epidermis (between papillae)
2) Merkel’s discs - Just beneath the epidermis (aligned with papillae)
3) Pacinian corpuscles - in the subcutaneous tissue
4) Ruffini’s corpuscles - deep in the dermis

Question 7

What is the morphology of Meissener’s corpuscles?

Answer 7

Looping axon terminals that intertwine supporting cells

Question 8

What is the morphology of Merkel discs?

Answer 8

Dome structure atop axon terminals

Question 9

What is the morphology of pacinian corpuscles?

Answer 9

Sensory axon surrounding by fluid filled capsule, onion shaped appearance

Question 10

What is the morphology of Ruffini endings?

Answer 10

Nerve terminals intertwined with collagen fibrils

Question 11

What is the morphology of nociceptors?

Answer 11

Free nerve endings that penetrate epithelial cells, no morphological specialization

Question 12

Which type of cutaneous sensory receptor has the smallest receptive field?

Answer 12

Merkel discs

Question 13

Which type of sensory receptor has the largest receptive field?

Answer 13

Pacinian corpuscles

Question 14

Which cutaneous sensory receptors are rapidly adapting?

Answer 14

Meissener’s corpuscles and pacinian corpuscles

Question 15

Which cutaneous sensory receptors are slow adapting?

Answer 15

Merkel’s discs and Ruffini endings

Question 16

Which 2 types of cutaneous sensory receptors have a small receptive field?

Answer 16

Meisseners and merkels discs

Question 17

Which 2 types of cutaneous sensory receptors have a large receptive field?

Answer 17

Pacinian corpuscles and Ruffini endings

Question 18

What is the role of Merkel cells in perception?

Answer 18

Detect pressure, form and texture

Question 19

What is the role of Meissener’s corpuscles in perception?

Answer 19

Detect flutter and motion

Question 20

What is the role of Ruffini corpuscles in perception?

Answer 20

Detect skin stretch

Question 21

What is the role of Pacinian corpuscles in perception?

Answer 21

Detect vibration (250-350 Hz)

Question 22

Which type of cutaneous sensory receptor is involved in reading braille?

Answer 22

Merkel afferents

Question 23

In terms of 2 point discriminatory thresholds across the body, what is sensitivity correlated with?

Answer 23

Density of sensory innervation

Question 24

What are the 2 areas of high sensitivity (small 2 point discriminatory threshold)?

Answer 24

1) Fingertips

2) face

Question 25

What are the 2 areas of low sensitivity (large 2 point discriminatory threshold)?

Answer 25

1) Torso | 2) limbs

Question 26

What are the 4 'types' of temperatures that different thermoreceptors respond to?

Answer 26

1) Cold pain 2) Cold 3) Warm 4) Hot pain

Question 27

Temperatures ranging from noxious heat to noxious cold activate several members of which family of receptors?

Answer 27

The Trp family of receptors

Question 28

Which receptor does menthol activate?

Answer 28

Trpm8

Question 29

Which receptor does capsaicin activate?

Answer 29

Trpv1

Question 30

How does capsaicin produce the pungent burning sensation in the mouth?

Answer 30

``` Belongs to a group of chemicals known as vanilloids Acts on thermal receptors (class of nociceptor) VR1 is expressed in cultured cells Capsaicin causes an inward Ca2+ current which is dose dependent according to 'hotness of the pepper' - this receptor is also activated by heat ```

Question 31

What proprioceptor is found in muscles, what does it do?

Answer 31

Muscle spindles | Provide sensory feedback from muscle fibres on body position and movement

Question 32

What proprioceptor is found in joints?

Answer 32

Golgi tendon organ | Regulate muscle tension or force of contracting and prevent muscle overloading

Question 33

Conduction velocity of a nerve fibre is positively correlated with what?

Answer 33

Axon diameter

Question 34

Large diameter, rapidly conducting afferents are associated with which type of somatosensory receptor?

Answer 34

Low threshold mechanoreceptors

Question 35

Small diameter, slow conducting afferents are associated with what 2 types of somatosensory receptor?

Answer 35

1) Nociceptors | 2) Thermoreceptors

Question 36

How can the Herpes Zoster Virus enable us to visualise dermatomes?

Answer 36

Virus lurks in primary sensory neurones then can reactivate as shingles - get inflammation and blistering on skin of area of affected dorsal root/sensory neuron

Question 37

Innocuous sensitivities are carried through the CNS by which pathway?

Answer 37

Dorsal column pathway

Question 38

Noxious and thermal sensitivities are carried through the CNS by what pathway?

Answer 38

Spinothalamic pathway

Question 39

What are Brodmann areas of the brain?

Answer 39

47 areas within the cerebral cortex which subserve different physiological functions

Question 40

Where does the secondary somatosensory cortex lie?

Answer 40

Adjacent to the primary somatosensory cortex (S1) along the lateral sulcus

Question 41

What is the function of the secondary somatosensory cortex?

Answer 41

Plays a key role in sensory and motor integration Receives corpus callosum inputs to form a 'joined-up' body image Build info from multiple body areas and form a body image

Question 42

Where are the association areas localized to, what is there function?

Answer 42

Localised to the posterior parietal cortex | Play a role in integration of multimodal senses

Question 43

Damage to the posterior parietal cortex (ie the association areas) leads to what 2 unusual neurological disorders?

Answer 43

1) Astereoagnosia - the inability to identify objects on basis of touch alone - would only be able to confirm what it was by using other sense eg. vision 2) Neglect syndrome - body part or visual field is disregarded

Question 44

After passing through the ventrobasal complex in the thalamus and to the post central gyrus what areas does somatosensory info then pass to?

Answer 44

Passes to the posterior parietal (secondary association area) then from there to: 1) Motor system 2) Polysensory association areas 3) Limbic system (learning and memory)