IC3- Touch and pain receptors

Question 1

When the touch receptors (afferents Aβ) are stimulated, which pathway do the first order neurons travel through next?

Answer 1

• Touch signals travel through the dorsal columns of the spinal cord
• First-order neurons ascend the dorsal columns without crossing over immediately

Question 2

Where does crossing over happen for the touch pathway? Elaborate

Answer 2

1. In the dorsal column pathway, first order neurons synpase with the second order neurons in the medulla (haven’t crossover yet!!)
2. Afterwards, second-order neurons then CROSS OVER to the contralateral side before ascending to the thalamus

Question 3

In the dorsal column pathway, where does the second neuron synapse with the third neuron?

Where does it travel to after?

Answer 3

In the thalamus.

Then afterwards it travels to the somatosensory cortex

Question 4

When the pain receptors (afferents Aδ and C) are stimulated, which pathway do the pain signals travel through?

Where does the first order neuron synapse with the second order neuron?

Answer 4

First-order neurons synapse with second-order neurons in the dorsal horn of the spinal cord

Crossing over happens in the spinal cord itself, and pain signals then travel through the ventrolateral/ anterolateral quadrant (spinothalamic tract) of the spinal cord.

Question 5

In the anterolateral quadrant (spinothalamic tract) pathway, where does the second neuron synapse with the third neuron?

Where does it travel to after?

Answer 5

At the thalamus

Then afterwards it travels to the somatosensory cortex

Question 6

Which pathway do the pain and touch receptors travel through?

Answer 6

Touch pathway = dorsal column pathway
Pain pathway = spinothalamic pathway

Question 7

What are the 4 principles that underlie sensory processing?

Answer 7

1. Action potential as signal to relay information
2. Signal travels along topographic lines
3. The signal travels along labeled line
4. The signal along the relay encode for properties of the stimuli, such as –

Question 8

Explain what it means for signals to travel along topographic lines

Answer 8

It means that different populations of afferent relay information to different regions in the somatosensory cortex.

There is regular organisation.

Question 10

What is the significance of such regular organisation via topography lines?

Answer 10

This regular organisation makes it easy for the brain to interpret the signals

Question 11

According to the somatosensory homunculus, which parts of the body are more medial and lateral?

Answer 11

Lower part of the body (feet, hands) are more medial.

The upper part of the body (face) is more lateral.

Question 12

Explain the somatosensory processing principle of ‘the signal travels along labeled line’

Answer 12

There are different types of receptors to pick up only one type of specific stimuli, and the signals travel along separate pathways to the CNS.

Eg. Touch and pain pathways are separate.

Touch pathway = dorsal column pathway
Pain pathway = spinothalamic pathway

Question 13

Labelling occurs via different receptors, where different signals travel through different pathways. What are some examples of specific receptors? (5)

Answer 13

• Vision (photoreceptors)
• Chemoreceptors
• Osmoreceptors
• Thermoreceptors
• Nociceptors (stimulated by tissue damaging stimuli)

Question 14

Increasing the rate of electrical stimulation of the same receptor (eg touch receptor) will evoke sensation of pain. T/F?

Answer 14

False.

According to the label line concept, increasing the rate of electrical stimulation will only increase magnitude of the same sensation which is touch!!!

Pain sensation is a different label line altogether.

Question 15

In the principles of somatosensory processing, how does the signal along the relay encode for properties of the stimuli?

Answer 15

Eg. Intensity

The more intense the stimulus, the more the number of action potentials per unit time (frequency) and more intense will be the sensation

Question 16

Define hyperalgesia

Answer 16

Increased pain to a given noxious stimulus

Question 17

Define allodynia

Answer 17

Pain to a normally non-painful stimulus (e.g. pain to a
stimulus normally sensed as touch)

Question 18

What are the possible mechanisms that can lead to touch allodynia? (3)

Answer 18

1. Degeneration of inhibitory neuron
2. The nature of the inhibition is affected such that the inhibitory neuron excites the spinothalamic neuron post-injury
3. Sensitization of nociceptors due to the release of variety of chemicals at damaged site → lowers threshold to excite, no. of TRV1 receptors increase.

In turn, the excitable nociceptors affect 2nd order spinothalamic (ST) neuron by release of chemicals glutamate, Substance P and CGRP, making it more excitable.

Such a ST neuron might be more responsive to touch stimulus (hence non-painful stimulus might activate the nociceptors this time round)

Question 19

When there is injury, what are the chemicals released which can sensitize the nociceptors? (recognise, no need to memorise all)

Answer 19

CGRP, Substance P, histamine, bradykinin, 5-HT, PG

Question 20

With regards to touch allodynia, which medications can help in pain relief?

Include their MOAs

Answer 20

• Aspirin (COX inhibitor → blocks PG synthesis)
• Acetaminophen (reduction of the COX pathway in CNS and antagonism of TRPV receptors esp spinal cord)
• Ubrogepant (migraine)

Question 21

Explain the general gist of the modulation of pain

Answer 21

Pain can be enhanced or suppressed according to the individual’s state

E.g. stress (about something else) can cause the pain to be suppressed

anxiety causes pain to be enhanced

Question 22

When the nociceptors are hyperexcitable, how do they lead to sensitization of the second order neuron?

Answer 22

They release glutamate which binds to the NMDAR and AMPAR receptors on the dendritic spines → ↑ in no. of AMPAR receptors on ST neuron + lower threshold to excite AMPAR receptors

This leads to sensitization of the second order neuron

Question 23

What are the 2 ways of pain modulation?

Answer 23

• Segmental modulation (gate theory)
• Descending (PAG)

Question 24

Elaborate on the segmental modulation (gate theory)

Answer 24

Stimulation of large diameter afferents (for touch) excite inhibitory interneuron that in turn decreases transmission of pain signal in spinal cord.

(more information: the theory suggests that when touch sensations are actively transmitted, they compete with and inhibit the transmission of pain signals, effectively closing the gate and dampening the pain experience.)

Question 25

Which part of the brain is involved in pain modulation (descending way)?

Answer 25

Midbrain PAG (periaqueductal gray)

Question 26

Explain the route of pain modulation through the periaqueductal gray

Answer 26

Stress/ morphine/ expectation → midbrain (PAG) → excites neurons in medulla (nucleus raphe magnus) → excites neurons in spinal cord (interneuron) → **inhibit transfer of signal to ST neuron → blocks sensation of pain

Question 27

Elaborate more on how during pain modulation, the transfer of signal from the first order neuron and second order ST neuron is inhibited (talk about the neurotransmitter involved

Answer 27

E (endogenous opioid peptides) releases enkephalin at the synapse which inhibits the transfer of signal

Question 28

What are the types of movements generated? (3)

Answer 28

1. Reflexes
2. Rhythmic motor patterns (require voluntary initiation and termination)
3. Voluntary

Question 29

Which part of the brain controls voluntary movement?

Answer 29

Cortex

Question 30

Which part of the brain controls reflexes and rhythmic motor patterns?

Answer 30

Brain stem

Question 31

What happens if the efferent neurons are damaged?

Answer 31

• Paralysis (no motor behaviour)

Question 32

For reflexes and rhythmic motor patterns, how is the signal transmitted to the motor efferent neurons?

Answer 32

Neurons in brainstem synapse with lower motor neurons in the ventral horn of the spinal cord

Question 33

For voluntary movement, how is the signal transmitted to the motor efferent neurons?

Answer 33

Signal travels form neuron in cortex down the corticospinal pathway to synapse with lower motor neurons in the ventral horn of the spinal cord

Question 34

Function of the cerebellum related to signalling?

Answer 34

Indirectly affects movement by adjusting the output of the efferent via cortex and brainstem → makes the movements more ‘smooth’

Question 35

Since the cerebellum helps to make our movements more ‘smooth’, what happens if there is a lesion in the cerebellum? (3)

Answer 35

• Lesion disrupt coordination of limb and eye movement
• Impair balance
• Decrease muscle tone

Question 36

What are the 2 key roles of the basal ganglia?

Answer 36

• Initiation
• Selection of motor program

Question 37

What happens if there is degeneration of dopamine neurons in the basal ganglia? (ie. what disease?)

Answer 37

Parkinson’s disease (marked by tremor, rigidity and bradykinesia)

Question 37

What are the effects of damage to the basal ganglia? (2)

Answer 37

1. Disorder of movement
   - Tremor at rest
   - Rapid flicking movements or chorea
   - Violent flailing movement or ballism
   - Slow writhing/ twisting
   movements (athetosis/dystonia)
   - Bradykinesia
2. Disorder of posture
   - Rigidity

(parkinson’s disease marked by tremor, rigidity and bradykinesia)