Somatosensation

Question 1

What is somatosensation? WHat are the 3 subdivisions of somatosensation?

Answer 1

the process that conveys information regarding the body surface and its interaction with the environment.

• mechanoreception (disciminative touch)
• Thermosensation
• Nociception

Question 2

What are the receptors for the following sensations:

steady pressure

stretch

flutter

vibration

Answer 2

steady pressure = Merkel’s disc

stretch = Ruffini

flutter = Meissner’s corpuscles

vibration = Pacinian corpuscles

Question 3

What two touch receptors adapt slowly and what two adapt rapidly?

Answer 3

slowly = Merkel’s and Ruffini

rapidly = Meissner’s and Pacinian

Question 4

Of the rapidly adapting touch receptors, which one has a large receptive field and which has a small receptive field?

Answer 4

large - Pacinian

small - Meissners

Question 5

What are the thermoreceptors? WHere are they located? Do they have small or large receptive fields? Rapid or slow adapters?

Answer 5

They are free nerve endings in the superficial layers of the skin

small receptive fields

rapid adaptation

Question 6

What do the cold thermoreceptive proteins detect?

How about the warm theroreceptors?

Answer 6

Cold: 5-35 degrees, over 45 degrees and menthol

warm: warmer temperatures and capsacin

Question 7

Is there a secondary messenger involved in thermoreception?

Answer 7

No – the receptor proteins are coupled directly to channels so no secondary messenger is needed

Question 8

How are mechanoreceptors activated?

Answer 8

There is a structural protein attached to the channel and when the membrane is deformed, the channel is pulled open directly

Question 9

What will activate nociceptors?

Answer 9

stimulit hat actually open the channel:

capsaicin, heat over 45 degrees C, and protons

Question 10

What will cause the threshold for the nociceptor channels to lower? What is this called?

Answer 10

Serotonin, Bradykinin, and prostaglandins

this is sensitization

Question 11

How do NSAIDs work for pain directly at the receptor level?

Answer 11

They inhibit the prostaglandins, so you don’t get the sensitization of the channels - making them less likely to fire

Question 12

What is hyperalgesia and what is allodynia?

Answer 12

hyperalgesia - basically sensitization; experiencing pain from a light stimuli like a feather brush

allodynia is experiencing pain when there is no stimulus at all - it’s perception of pain without sensation of nociception

Question 13

What are TRP channels?

Answer 13

they are nonspecific cation channels similar to voltage gated channels (and nicotinic receptors) because they let in Na+ K+ and Ca2+ (although Na+ goes more than the others)

The transducer is directly connected to the channel

Implicated in many sensory processes including somatosensitizations

Question 14

What is point acuity and how is it tested?

Answer 14

Point acuity is how well you localize a point on your skin, tested by the Two-Point Discrimination Test

It’s the physical distance where you perceive two stimuli instead of one

Better on fingertips than on back

Question 15

How can be divide somatosensory nerve fibres into subcategories?

Answer 15

conduction velocity - determined by diameter and degree of myelination

Question 16

What are the 3 subgroups of somatosensory nerve fibers?

Answer 16

A - beta fibers (large myelinated fibers - mechanoreception)

A - gamma fibers (fast pain, mechanoreception, cold)

C fibers (mechanoreception, thermoreception, slow pain)

Question 17

What nerve fibers are associated with fast pain and which are associated with slow pain?

Answer 17

fast = A-gamma

slow = C

Question 18

What are the 3 types of pain?

Answer 18

1. acute nociceptive pain
2. Inflammatory pain
3. Neuropathic pain

Question 19

Describe acute nociceptive pain?

Answer 19

It’s divided into fast and slow

fast is sharp/pricking pain that is well localized

slow pain is achy/dull pain that isn’t well localized

Question 20

What are the two potential causes of inflammatory pain?

Answer 20

damage to the receptors themsevles or sensitization to receptors due to adjacent damaged cells

Question 21

What is the general cause of neuropathic pain?

Answer 21

reorgnization of the pathways such that you don’t need a stimulus to perceive pain

Question 22

What are two characteristics of chronic pain syndromes and what types of pain are they more often associated with?

Answer 22

hyperalgesia and allodynia

associated with inflammatory and neuropathic pain

Question 23

What is referred pain?

Answer 23

It’s when activation of nociceptors in the viscera is perceived as a somatosensory stimulation - like when heart attacks hurt in the left arm

It’s related to the labelled line theory

Question 24

Describe the gate theory of pain

Answer 24

Inhibitory interneurons regulate the transmission of ascending nociceptive information at the elvel of the second order neuron, allowing modulation of the signal - either increasing or decreasing it depending on the situation

Question 25

Under normal circumstances without pain, what is the status of the inhibitory interneuron?

Answer 25

Under normal conditions, the inhibitory interneuron - the gate - is contantly active and thus closed.

It constitutively secretes glycine onto the second order neuron such that no signals go through the nociceptive pathway

Question 26

In the event of a painful stimulus, what happens to the inthibitory neuron and basic circuit?

Answer 26

1. The glutamate from the first order neuron acts on metabotropic glutamate receptors on the inhibitory interneuon and shuts it off, thus opening the gate
2. The same glutamate that is released from the first order neuron and acts on AMPA receptors of the second order neuron, activating it
3. Because the gate is open and no glycine is being released onto the second order neuron, the signal passes through

Question 27

How does the inhibitory interneuron (the gate) get turned back on after a pinaful stimulus?

What is the significance of this clinically?

Answer 27

The mechanoreceptors will reactivate the inhibitory interneuron and close the gate

This is why rubbing a bumped elbow makes it feel better and why TENS electrical stimulation works - it stimulates the mechanoreceptor fibers

Question 28

What effect do opioid drugs have on the basic circuit in terms of the gate theory of pain?

Answer 28

Opioids modify the synapse such that the inhibitory interneuron is more active

they do this by acting on opioid receptors located on the cell body of the interneuorn - the gate

keeps the gate closed