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PHYS - NEURO > 12. SOMATOSENSATION II > Flashcards

12. SOMATOSENSATION II Flashcards

1
Q

What would the effect of a lesion on the left side of the spinal cord be?

A
  • Loss of pain & temperature sensations from the left side
  • Left side C fibres enter the dorsal root of the spinal cord which synapses with second order neurones
  • It the dessicates (crosses over) to opposite side of the spinal cord
  • Ascends to anterolateral tract, but disrupted by lesion
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2
Q

What would be the effect of a lesion on the right side of the spinal cord?

A
  • Loss of tactile sensation on the right side of the body

- A beta afferent fibres enter the spinal cord, ascends through white matter (dorsal column) into DCLM blocked by lesion

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3
Q

What is the dual aspect model for pain?

A
  • Pain has two components:
    1. SENSORY (discriminative)
  • location, duration, intensity, quality
    2. AFFECTIVE (motivational)
  • unpleasantness, effect on mood, arousal or behavior
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4
Q

What do nociceptors detect?

A
  • Nociceptors detect noxious stimuli
  • Detection & localisation of pain is dependent on nociceptors
  • Specific classses of sensory afferents are nociceptors
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5
Q

Which sensory afferents are nociceptors?

A
  • A delta & C fibres

- But not all nociceptors are A delta & C fibres

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6
Q

How are nociceptors specialised?

A
  • Nociceptors will only respond to stimuli that is noxious
  • Meaning the stimuli has reached the threshold where it can cause extreme pain or damage
  • For example, nociceptors won’t respond to mild temperature, only noxious temperature
  • They only begin reacting once the threshold for noxious stimuli is reached
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7
Q

What two aspects of pain do A delta & C fibres contribute to?

A
  • A delta & C fibres are involved in first & second pain

- First pain is the immediate pain response after exposure to a stimuli, the second pain is the maintained pain

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8
Q

How are A delta fibres specialised for first pain?

A
  • A delta fibres are moderately fast conducting & thinly myelinated
  • This makes them specialised to detecting first pain, as they can produce immediate sensation of pain
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9
Q

How are C fibres specialised for second pain?

A
  • C fibres are unmyelinated & slow conducting

- The slow conduction allows them to be responsible for a slow wave of pain that is maintained or constant

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10
Q

What do tactile mechanoreceptors respond to & how?

A
  • Tactile mechanoreceptors respond to stretch
  • Tactile mechanoreceptors have mechanically sensitive ion channels
  • Upon stretch or a mechanical stimulus. depolarization occurs producing an action potential
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11
Q

Which receptor is involved in the transduction of heat?

A
  • TRPV1 receptor is involved in the transduction of heat

- Also known as the vaniloid receptor

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12
Q

What sensory afferents is the TRPV1 receptor found on?

A
  • A delta & C fibres
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13
Q

What does the TRPV1 receptor do?

A
  • TRPV1 is an ion channel that allows the influx of Na+ & Ca2+, leading to depolarisation
  • It can respond to heat, chemicals like acid
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14
Q

How does capsaicin result in the sensation of heat?

A
  • Capsaicin is an ingredient found in chilly peppers

- It is an agonist to the TRPV1 receptors, so when this receptor is activated it produces the sensation of heat

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15
Q

What maintains nociceptor activity after injury (tissue damage)?

A
  • Inflammatory mediators produced due to the inflammatory response
  • Maintains sensitivity of the sensory afferents (C fibres)
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16
Q

How is nociceptor activity maintained after injury by inflammation?

A
  • Tissue damage stimulates the inflammatory response
  • Inflammatory response produces inflammatory molecules, activates A delta & C fibres
  • Inflammatory mediators/signalling molecules keep the relevant ion channels on the afferents, open to maintain depolarisation
  • Even after the noxious stimuli is removed, the nociceptor action potential firing will be removed
  • Collateral branches of C fibres will produce Substance P which keeps the inflammatory cascade going, which maintains the sensitivity of the sensory afferents
17
Q

What are the two consequences of maintained nociceptor activity after injury?

A
  1. Hyperalgesia - hypersensitivity in areas that normally produce pain due to hypersensitivity of C fibres (Substance P production)
  2. Allodynia - normally innocuous stimuli causes pain (wouldn’t usually)
18
Q

What is referred pain?

A
  • Referred pain is when pain is perceived at a location that is not the site of painful origin or stimuli
19
Q

Why does referred pain occur?

A
  • Dorsal horn interneurones receive input from A delta & C fibres. They can also receive input from visceral afferents
  • The visceral afferents converge onto the same neurone as the sensory afferents. The higher centres/brain interprets this as the pain coming from the sensory afferents, so from the body surface/tissue rather than the viscera
  • Visceral afferents converging onto same neurone as sensory afferents -> somatic pain
20
Q

Describe the cortical representation of STT & DCML?

A
  • Axons from the STT & DCML don’t converge onto the same thalamic neurones, the pathways are parallel but maintained separate
  • The primary somatosensorry cortex is needed for teh localisation of pain
  • STT = pain, temperature, coarse/crude touch
  • DCLM = discriminative touch, proprioception & vibration
  • However, stimulating the primary somatosensory cortex results in referred tactile not painful sensations
21
Q

What are the two systems involved in pain that are branches of the STT?

A
  1. LATERAL SYSTEM
  2. MEDIAL SYSTEM
    - Both are branches of the STT which diverge at the level of the thalamus
    - STT = pain, temperature, coarse touch
22
Q

What is the lateral system of pain processsing?

A
  • Sub-division of STT
  • The lateral system is involved in the sensory-discriminative
  • Responds to pain & temperature
  • Nociceptive & tactile afferents
  • Projects to primary & secondary somatosensory cortex (S1 &S2) via somatosensory thalamic nuclei
  • Via the ventral posterior nucleus
  • KEY PROJECTIONS: VENTRAL POSTERIOR NUCLEUS & S1, S2
23
Q

What is the medial system of pain processing?

A
  • Sub-division of STT
  • Affective-motivational model
  • Projects to different cortical areas via MID LINE THALAMIC NUCLEI
  • Projects to ANTERIOR CINGULATE CORTEX & INSULAR CORTEX via mid line thalamic nuclei
  • Also projects to cortical regions of limbic system: hypothalmus, amygdala, PAQ
24
Q

How can opioids help with pain modulation?

A
  • Opiate receptors are located in areas such as the brain stem & spinal cord
  • E.g enkephalin is an endogenous opioid which can bind to opiate receptors on the pre-synaptic terminal of C-fibres. This can decrease transmission of pain by c fibres via the STT
25
Q

What are some examples of endogenous opioids?

A
  • Encephalins

- Endorphins

26
Q

What two drug classes can be used to treat pain caused by tissue damage?

A
  1. NSAIDS

2. OPIATE DRUGS

27
Q

What type of drug can be used to treat chronic pain?

A
  • Chronic pain lasts for longer than 3 months & can be a result of nerve damage from prior injury
  • Antidepressants such as amitryptigline which should be given at a lower dosage & in the absence of clinical depression
28
Q

What is phantom limb syndrome?

A
  • A condition where patients experience sensation in a limb that is no longer present. Can also occur in the breasts, genitals or teeth
  • Reported to affect amputees who are resistant to treatment manifesting as chronic pain
29
Q

What causes phantom limb syndrome?

A
  • Cortical reorganisation of the somatosensory cortex
  • Somatosensory cortex continues to receive signals from the nerve endings/afferents that originally supplied inputs & outputs of the missing limb
30
Q

What surgery is carried out for individuals unresponsive to treatment?

A
  • Anterior cingulotomy - places lesions on targetted regions of the anterior cingulate cortex on both sides
  • Used as a last resort
  • Anterior cingulate cortex is part of medial system/branch of STT involved in pain

PHYS - NEURO (18 decks)

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