Somatic sensation

Question 1

Merkels cells

Answer 1

Slowly adapting type 1 afferents
Smallest receptive field so best for reading braille
close to epidermis
Has narrow linear dynamic range to encode magnitude with frequency

Question 2

Meissner’s corpuscle

Answer 2

Rapidly adapting type 1 afferent
Responds best to low frequency vibration; slip between object
Close to epidermis
Glabrous skin

Question 3

Pacinian corpuscle

Answer 3

Rapidly adapting type 2 afferent
Responds best to high freq (300Hz) vibration
Deeper in dermis

Question 4

Ruffini endings

Answer 4

Slowly adapting type 2 afferents
Deeper in skin
Respond to skin stretch; good for hand position and object motion perception

Question 5

Accessory structure

Answer 5

Structure not directly involved in the transduction of a stimulus but that plays a role in conduction or analysis e/g lamellae of Pacinian corpuscle; confers property of being rapidly adapting

Question 6

Phase locking

Answer 6

Nerve always fires at same part of the AP

Allows additional recruitment to encode magnitude of a stimulus rather than frequency of discharge

Question 7

Free nerve endings

Answer 7

Primarily respond to temperature and nociceptive stimuli

Question 8

Paradoxical cold

Answer 8

A heat stimulus on a cold spot will feel cold due to labelled line code and lack of comparison with other receptors because this is a discrete stimulus

Question 9

C tactile afferents

Answer 9

Longer lasting response to brushing of skin

Correlates with pleasantness of the stimulus

Question 10

ABeta fibres

Answer 10

Largest, shortest latency
Carry mechanoreception; proprioception and touch
First affected by hypoxia, last by anaesthetic

Question 11

Adelta fibres

Answer 11

Responsible for sharp stabbing pain

Medium

Question 12

C fibres

Answer 12

Unmyelinated, smallest
Responsible for warm burning pain/itching pain
First affected by anaesthetic and last by hypoxia

Question 13

Anterolateral system/spinothalamic tract

Answer 13

Major ascending nociceptive pathway
Axons from neurons in layers 1,5,6,7
Ascending in contralateral, anterolateral white matter
Crosses midline straight after leaving dorsal horn

Question 14

Dorsal column/medial lemniscal system

Answer 14

Mainly AAlpha and ABeta
Fibres reach the DCN (gracile for lower limb, cuneate for upper)
Then cross midline and travel contra laterally to thalamus via medial meniscal tract

Question 15

Brown sequard syndrome

Answer 15

Hemisection through spinal column

• Ipsilateral loss of tactile and limb position sense
• Contralateral loss of pain/temp

Question 16

Syringomyela

Answer 16

Loss of middle of spinal cord so Adelta and C are selectively abolished
Bilateral loss of pain and temp

Question 17

Posterior column syndrome

Answer 17

Dorsal aspect of spinal cord carrying Abeta fibres means bilateral loss of limb position and tactile sense

Question 18

Role of lateral inhibition

Answer 18

Receptive fields have excitatory and inhibitory regions
Stimulating OFF surround can turn off the output from the excitatory centre

Good for sharpening two point discrimination

Question 19

Cells in posterior part of ventromedial nucleus

Answer 19

From lamina 1

Neurons here respond best to noxious heat stimuli above 43 degrees

Question 20

Somatosensory cortex

Answer 20

Primary input is layer 4
Layers 5/6 do descending info to BG, SC, brainstem
Layers 2/3 to other parts of cortex

Question 21

Homunculus

Answer 21

Map of the somatosensory inputs to the cortex

Question 22

Area 2 of somatosensory cortex

Answer 22

Directional sensitivity
Via 3 sets of 2 receptive fields where one is excitatory and one is inhibitory
Responds best to stimulus moving through excitatory region first and then inhibitory

Question 23

What areas are active during active touch

Answer 23

Primary somatosensory cortex (this is only one active in passive touch)
Primary motor cortex
ACC (anterior cingulate cortex)

Question 24

Role of context

Answer 24

Attention enhances responses of enurons

Memory also plays a role e.g bigger output if this stimulus is larger than previous one

Question 25

Cortical remapping

Answer 25

After limb lost, the area on the cortex that represents that body part is taken over by the neighbouring area So touching another area gives sensation in amputated limb

Question 26

Pain

Answer 26

Unpleasant sensory and emotional experience associated with actual or potential tissue damage

Question 27

Dorsal horn lamina with wide dynamic range neurons

Answer 27

Lamina 5 | Also has input from Abeta, Adelta, C

Question 28

Referred pain

Answer 28

Pain perceived at a distance from affected organ due to visceral-somatic convergence on second order neurons

Question 29

Cortical representations of pain

Answer 29

Primary somatosensory cortex ACC Insula (homeostasis)

Question 30

Gate control theory of pain

Answer 30

Input from Abeta fibres can active inhibitory interneurons to inhibit transmission cell in lamina 5 to decrease pain felt

Question 31

Areas for descending control of pain

Answer 31

Periaqueductal gray in midbrain (has map within it for applying to endogenous opioid system to areas) Raphe Magnus nuclei in the rostral medulla Nucleus paragigantocellularis in rostral medulla

Question 32

Sensitising agents that don't directly excite the nociceptor

Answer 32

PGE2 | Nerve growth factor

Question 33

Allodynia

Answer 33

Previously painless stimuli now painful

Question 34

Hyperalgesia

Answer 34

Previously painful stimuli now more painful

Question 35

How does PGE2 act to sensitise the nerve

Answer 35

Acts via EP4 receptors which are Gs coupled so get increase in adenylyl cyclase activity, more cAMP, more PKA, phsophrylates Nav channels to decrease threshold

Question 36

Neuropathic pain

Answer 36

When peripheral nerve damage leads to pain that outlasts the injury itself

Question 37

Neurogenic inflammation

Answer 37

Nociceptive nerve terminals release substance P and CGRP when stimulated causing vasodilation and increased permeability

Question 38

Mui opioid receptor

Answer 38

Gi coupled Betagamma subunit activates GIRK channels to cause K+ exit and hyper polarisation (presynaptically this inhibits neurotransmitter release) Decreased cAMP so less PKA, counteracting effects of PGE2 on Nav channels

Question 39

Treating neuropathic pain

Answer 39

Analgesics: NSAIDs, opioids Antidepressants: serotonin-NA-reuptake inhibitor (NA is involved in descending modulation of pain) Gabapentin: decreases Cav expression NaV blockers like lidocaine