Somesthesis/Nociception

Question 1

Two major groups of tactile receptors

Answer 1

• Slowly adapting: respond to enduring stimulus

- Rapidly adapting: only respond at onset and sometimes the termination of a long lasting stimulus

Question 2

Principle underlying two-point discrimination

Answer 2

• Relationship between receptor density and receptive field size
• Greater discrimination abilities: high density of receptors, with small receptive fields
• Fingertips have greatest capabilities for 2 pt discrimination

Question 3

Cells involved in texture discrimination

Answer 3

• Merkel’s: slowly adapting
• Meissner’s corpuscles: rapidly adapting
• Pacinian corpuscles: rapidly adapting

Question 4

Two major ascending somatosensory pathways

Answer 4

• Dorsal column-lemniscal: epicritic

- Anterolateral: protopathic

Question 5

Changes in receptive field in second order receptor neurons (somatosensory)

Answer 5

• Larger receptive fields due to convergent input with overlapping fields
• More complex with both excitatory and inhibitory regions

Question 6

VPL projects to

Answer 6

-Medial and superior portions of the postcentral gyrus

Question 7

VPM projects to

Answer 7

-Lateral portion of the postcentral gyrus

Question 8

Primary somatosensory Brodmann’s areas

Answer 8

• 3a, 3b, 1, 2

- 3a and 3b get thalamic projects that are then sent to 1 and 2

Question 9

Jacksonian seizure

Answer 9

• Begins in fingers, spreads to hand, up the arm, down across shoulder, into the back, down the ipsilateral leg
• Explained by the sensory projection onto the brain

Question 10

Nociceptors structure

Answer 10

-Free nerve endings, no specialized structures

Question 11

Thermal/mechanical nociceptors

Answer 11

• Small diameter
• Thinly myelinated
• Adelta fibers
• 5-30 m/s conduction
• Sensations of sharp, pricking pain

Question 12

Polymodal nociceptors

Answer 12

• Activated by a variety of high-intensity mechanical, chemical, and hot or cold stimuli
• Small diameter
• Unmyelinated
• C fibers
• 0.5-2m/s conduction

Question 13

When do nociceptors begin to discharge?

Answer 13

Only when the stimulus is intense enough to cause damage

Question 14

Early and late pain sensations

Answer 14

• Early: tingling and then sharp pain caused by activated Adelta fibers
• Late: duller, long lasting pain caused by C fiber activation

Question 15

TrpV1 receptor

Answer 15

• Vanilloid receptor found in both Adelta and C fibers
• Activated by: capsaicin, heat, acids, anandamide
• Action potential Na voltage gated channels

Question 16

Hyperalgesia

Answer 16

-Increased sensitivity to pain in and around the damaged tissue

Question 17

Cause of hyperalgesia

Answer 17

• Sensitization of nociceptors by various substances
• Bradykinin, histamine, prostaglandin release enhances responsiveness of nociceptive endings
• Electrical activity releases chemicals, like substance P, that cause vasodilation, swelling, and histamine release by mast cells

Question 18

Brachial plexus avulsion

Answer 18

• Patients feel a burning pain in the dermatomes corresponding to the denervated area
• Pain is thought to result from hyperactivity of dorsal horn neurons in the deafferented region of the cord

Question 19

Nociceptive fiber termination in the dorsal horn

Answer 19

-Lamina 1 and 2 mostly, some Adelta in lamina 5

Question 20

Lamina 1 dorsal horn neurons

Answer 20

• Pain information
• Excited solely by Adelta and C fibers
• Nociceptive specific

Question 21

Lamina V dorsal horn neurons

Answer 21

• Input from mechanoreceptors and nociceptors
• Wide dynamic range neurons
• Respond to somatosensory and noxious stimuli, much larger receptive fields

Question 22

Somatotopy of nociceptive input into the somatosensory cortex

Answer 22

-There is none!!!

Question 23

Descending inhibition of spinothalamic tract

Answer 23

-Mediated by activation of enkephalin interneurons in the dorsal horn

Question 24

Opiate action of nociceptive and dorsal horn neurons

Answer 24

• Nociceptive afferent have opiate receptors on their terminals and dendrites
• Opiates inhibit release of glutamate, substance P, and other transmitters
• NT release suppressed by activation of opiate receptors on the sensory neurons
• Can also act postsynaptically
• BOTH presynaptic and postsynaptic actions!!!