12. SOMATOSENSATION II Flashcards
What would the effect of a lesion on the left side of the spinal cord be?
- 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
What would be the effect of a lesion on the right side of the spinal cord?
- 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
What is the dual aspect model for pain?
- Pain has two components:
1. SENSORY (discriminative) - location, duration, intensity, quality
2. AFFECTIVE (motivational) - unpleasantness, effect on mood, arousal or behavior
What do nociceptors detect?
- Nociceptors detect noxious stimuli
- Detection & localisation of pain is dependent on nociceptors
- Specific classses of sensory afferents are nociceptors
Which sensory afferents are nociceptors?
- A delta & C fibres
- But not all nociceptors are A delta & C fibres
How are nociceptors specialised?
- 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
What two aspects of pain do A delta & C fibres contribute to?
- 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
How are A delta fibres specialised for first pain?
- 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
How are C fibres specialised for second pain?
- 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
What do tactile mechanoreceptors respond to & how?
- Tactile mechanoreceptors respond to stretch
- Tactile mechanoreceptors have mechanically sensitive ion channels
- Upon stretch or a mechanical stimulus. depolarization occurs producing an action potential
Which receptor is involved in the transduction of heat?
- TRPV1 receptor is involved in the transduction of heat
- Also known as the vaniloid receptor
What sensory afferents is the TRPV1 receptor found on?
- A delta & C fibres
What does the TRPV1 receptor do?
- TRPV1 is an ion channel that allows the influx of Na+ & Ca2+, leading to depolarisation
- It can respond to heat, chemicals like acid
How does capsaicin result in the sensation of heat?
- 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
What maintains nociceptor activity after injury (tissue damage)?
- Inflammatory mediators produced due to the inflammatory response
- Maintains sensitivity of the sensory afferents (C fibres)
How is nociceptor activity maintained after injury by inflammation?
- 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
What are the two consequences of maintained nociceptor activity after injury?
- Hyperalgesia - hypersensitivity in areas that normally produce pain due to hypersensitivity of C fibres (Substance P production)
- Allodynia - normally innocuous stimuli causes pain (wouldn’t usually)
What is referred pain?
- Referred pain is when pain is perceived at a location that is not the site of painful origin or stimuli
Why does referred pain occur?
- 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
Describe the cortical representation of STT & DCML?
- 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
What are the two systems involved in pain that are branches of the STT?
- LATERAL SYSTEM
- MEDIAL SYSTEM
- Both are branches of the STT which diverge at the level of the thalamus
- STT = pain, temperature, coarse touch
What is the lateral system of pain processsing?
- 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
What is the medial system of pain processing?
- 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
How can opioids help with pain modulation?
- 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
What are some examples of endogenous opioids?
- Encephalins
- Endorphins
What two drug classes can be used to treat pain caused by tissue damage?
- NSAIDS
2. OPIATE DRUGS
What type of drug can be used to treat chronic pain?
- 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
What is phantom limb syndrome?
- 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
What causes phantom limb syndrome?
- 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
What surgery is carried out for individuals unresponsive to treatment?
- 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
