Session 4: Sensory Pathways Flashcards
18.10.2019
Name 4 somatosensory modalities
- mechanical
- thermal
- proprioceptive
- nociceptive
What are the qualities of A-beta fibres?
- large diameter
- myelinated
- fast signal transmission
- innocuous mechanical stimulation
- i.e. mechanoreceptors in skin
What are the qualities of A-delta fibres?
- smaller diameter
- myelinated
- slower signal transmission
- noxious mechanical and thermal stimuli
- i.e. pain, temperature
What are the qualities of C-fibres?
- small diameter
- unmyelinated
- slow signal transmission
- noxious mechanical, thermal and chemical stimulation
- i.e. temperature, pain, itch
What are the different types of sensory nerve endings?
- free: thermoreceptors and nociceptors
- enclosed: mechanoreceptors
=> individual axons of sensory nerves have modified terminals
Define: Sensory receptor
a transducer that converts energy from the environment into neuronal APs.
What are the different types of receptors?
- mechanoreceptors (4 different ones)
- nociceptors
- thermoreceptor
What are the 4 types of mechanoreceptors?
- Meissner corpuscle
- merkel cells
- Pacinian corpuscle
- Ruffini endings
What so Meissner Corpuscles sense?
Fine discriminative touch, low frequency vibration
What do Merkel cells sense?
Light touch and superficial pressure
What do pacinian corpuscles sense?
Detects deep pressure, high frequency vibration and tickling
What do Ruffini endings sense?
Continuous pressure or touch and stretch
Stimulus threshold
‘’A threshold is the point of intensity at which the person can just detect the presence of a stimulus 50% of the time (absolute threshold)’’
Thermoreceptors
- Aδ- and C-fibres
- Free nerve endings
- Transient receptor potential (TRP) ion channels
- 4 heat activated:
TRPV1-4 - 2 cold activated
TRPM8
TRPA1
- 4 heat activated:
Stimulus intensity
Increased stimulus strength and duration = increased neurotransmitter release = greater intensity
What does a modality refer to?
The type of information encoded
What do different mechanoreceptors differ in?
- location
- structure
- size
- what they encode
Receptive field
the region on the skin which causes activation of a single sensory neuron when activated
Adaptation: tonic receptors
- Detect continuous stimulus strength
- Continue to transmit impulses to the brain as long the stimulus is present
- Keeps the brain constantly informed of the status of the body
- e.g. Merkel cells: Slowly adapt allowing for superficial pressure and fine touch to be perceived.
Tonic receptors - do not adapt or adapt very slowly
Adaptation: phasic receptors
- Detect a change in stimulus strength
- Transmit an impulse at the start and the end of the stimulus
- e.g. when a change is taking place: The pacinian receptor -> Sudden pressure excites receptor
Transmits a signal again when pressure is released
What are the properties of small and large receptive fields?
- Small receptive fields allow for the detection of fine detail over a small area ->Precise perception
- Large receptive fields allow the cell to detect changes over a wider area (less precise perception)
What are the receptive fields on the fingers like?
The fingers have many densely packed mechanoreceptors with small receptive fields
What is two-point discrimination?
- Minimum distance at which two points are perceived as separate
- Related to the size of the receptive field
e.g. hand detects 2 points at 4mm distance. Back: 40 mm
C5 dermatone
clavicle
C6 dermatome
thumbs
T4 dermatome
nipples
T10 dermatome
umbilicus
Where are cell bodies of the body and face found respectively?
Cell bodies are in the dorsal root ganglia (body) and trigeminal ganglia (face)
What two groups can neurones in the dorsal horn be divided into?
- Those with axons that project to the brain (projection neurons)
- Those with axons that remain in the spinal cord (interneurons)
What is lateral inhibition?
- sometimes receptive fields overlap -> difficult to distinguish between 2 similar locations
- in the dorsal horn there are inhibitory interneurones between sensory neurones
- most intense stimulus comes through
=> to prevent overlap of receptive fields and to facilitate pinpoint accuracy in localisation of the stimulus
What are the two ascending pathways?
- the dorsal column system (touch + vibration)
- the spinothalamic tract (pain, temperature, crude touch)
Dorsal column system
- innocuous mechanical stimuli: fine discriminative touch, vibration)
- A-beta fibres enter via the dorsal horn and enter ascending dorsal column pathway.
What are the 2 ascending dorsal column pathways?
- fascilicus gracilis (lower limb and below T6)
- fascilicus cuneatus (upper limb and above T6)
-> both ipsilateral
What are the different neurones in the dorsal column system?
- 1st order neurone: first synapse in the gracile nucleus / cuneate nucleus (medulla) -> travels ipsilaterally
- 2nd order neurone: decussate in the caudal medulla and forms the contralateral medial lemniscus tract; terminate in the ventral posterior lateral nucleus of the thalamus (VPL)
- 3rd order neurone: from the VPL, project to the somatosensory cortex
Somatosensory homunculus
- Size of somatotopic areas is proportional to density of sensory receptors in that body region
- pain and temperature localisation is not as precise
The spinothalamic (anterolateral) pathway
- Pain and temperature sensations ascend within the lateral spinothalamic tract
- Crude touch ascends within the anterior spinothalamic tract
What fibres are for crude and for fine touch?
- Crude touch is mediated by Adelta fibres (Free nerve ending)
- Fine touch is mediated by Abeta fibres (Meissner’s corpuscles)
What are the different neurones in the spinothalamic tract?
- 1st oder neurone: terminates in the dorsal horn
- 2nd order neurone: decussates immediately forming the spinothalamic tract; terminates in the ventral posterior lateral nucleus (VPL) of the thalamus
- 3rd order neurone: projects into the somatosensory cortex
VPL - topographic representation of the body
There is a topographic representation of the body in the VPL (lower extremities are lateral)
What are the key differences in the dorsal column system and the spinothalamic tract?
Dorsal Colums:
- decussates at the medulla
- light touch, vibration, two point discrimination
Spinothalamic:
- decussates immediately
- pain, temperature, coarse touch
What warm temperature is perceived as painful?
> 43 degrees C
What would happen in an anterior spinal cord lesion?
- Spinothalamic tract damage causes pain and temperature loss below the level of the lesion (bilateral)
- Retained light touch, vibration and 2-point discrimination due to intact dorsal columns
(Blocked anterior spinal artery causes ischemic damage to the anterior part of the spinal cord)
What is pain?
- An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.
- there are social, emotional and learning components to pain
Which fibres mediate sharp, intense or first pain? What are the 2 types of nociceptors?)
A-delta
(Nociceptors:
- Type 1: noxious mechanical
- Type 2: noxious heat )
Which fibres mediate dull, aching or second pain?
C-fibres
- Noxious thermal, mechanical and chemical stimuli (polymodal)
Pain pathways - what are the 2 components?
- Lateral spinothalamic tract
(sensory component) - Spinoreticular tract
(emotional component)
What are nociceptors?
- a sensory receptor for painful stimuli
What receptors do e.g. Wasabi and Caspaicin bind to?
Thermoreceptors (Wasabi: cold; Caspaicin: hot)
interesting fact: capsaicin stays bound
Pain Matrix in the brain
- fMRI have shown a cerebral signature for pain
- Cortex:
SI
SII
Insula cortex
Anterior cingulate cortex
- Prefrontal cortex
- Amygdala
- Cerebellum
- Brainstem
There is no specific place for pain in the brain (in the US sometimes fMRIs are used to see if someone had pain in court - dangerous path to take)
Gate control theory
- Inhibition of primary afferent inputs before they are transmitted to the brain through ascending pathways
- e.g. when you want to touch something that hurts
- stimulates A-beta fibres which block C-fibers
also: strong emotions can inhibit pain, also hypnosis
What is an example of emotional inhibition of pain?
- ‘’75% of recently wounded soldiers with various significant injuries such as bone fractures and abdominal wounds reported pain that
was insufficiently severe to require the use of analgesics - strong emotions can inhibit pain
What is chronic pain?
> 3 months
Descending control pathways of pain inhibition
- strong emotions can inhibit pain
(- you touch the body part that hurts?) - Periaqueductal grey (PAG)
- placebo can activate descending inhibition
- Facilitation and inhibition of nociceptive processing in the dorsal horn
Monoamines
- Serotonin
- Noradrenaline
PAG
- periaqueductal grey
- a key area in the descending control pathway
- can either be facilitative or inhibitory
- monoamines play a role
- Serotonin and noradrenaline are inhibitory -> they can reduce function of the C-fibres
How do opioids work?
Opiods inhibit action at the RVM and at the PAG (preaqueductal grey)
Nociceptive pain examples
- noxious stimulation of a nociceptor (somatic or viscera)
- arthritis
- fractures
- burns
- headache
( - skin, muscles, ligaments, bone, joints, viscera)
Neuropathic pain examples
- lesion or disease of the somatosensory system
- sciatica
- diabetic
- trauma
- chemotherapy
- post-surgical
examples of mixed (nociceptive and neuropathic pain)
- osteoarthritis
- lower back pain
How many people in the UK have chronic pain?
28 million
Allodynia
pain due to a stimulus that does not normally provoke pain
Hyperalgesia
increased pain from a stimulus that normally provokes pain
- Primary - Secondary
Peripheral sensitisation
- tissue damage: inflammatory substances released
- these chemicals (NT, glutamate, serotonin, ATP, adenosine peptides, bradykinin, cytokines, chemokines, lipids, proteases)
- these chemicals modulate the excitability of nociceptors making them more sensitive
=> decreases threshold to peripheral stimuli at the site of injury
Central sensitisation
- C fiber sensitivisation in the periphery
- Adjacent Aδ-fibre sensitisation in the dorsal horn
=> decreases threshold to peripheral stimuli at an adjacent site to the injury
Diagnosis of neuropathic pain
- symptoms have to be consistent with the site of injury
- use questionnaire as help
- quantitative sensory testing (QST)
What are the 3 neuropathic pain clusters?
- sensory loss
- thermal hyperalgesia
- mechanical hyperalgesia
What drugs can target descending control pathways for pain relief?
- opiods
- antidepressants (TCA, SNRI, SSRI)
-> big difference in analgesic effect in the different antidepressants
Descending control pathway in chronic pain - facilitation and inhibition
Noradrenaline - protective, inhibitory
Serotonin - harmful, facilitative
Conditioned pain modulation
- specific pain types at a different location can increase threshold or decrease painfulness
transcranial direct current stimulation
- non invasive primary motor cortex stimulation
- activation of endogenous analgesic systems in the brain (PAG, anterior cingulate cortex)
- analgesic mechanisms unclear
How many different thermoreceptors are there in humans?
6
- 4 heat activated: TRPV1-4
- 2 cold activated (TRPM8, TRPA1)
What is the difference between phasic and tonic receptors?
- A tonic receptor is a sensory receptor that adapts slowly to a stimulus and continues to produce action potentials over the duration of the stimulus.
- In this way it conveys information about the duration of the stimulus. Some tonic receptors are permanently active and indicate a background level. Examples of such tonic receptors are pain receptors, joint capsule, and muscle spindle.
- A phasic receptor is a sensory receptor that adapts rapidly to a stimulus. The response of the cell diminishes very quickly and then stops.
- It does not provide information on the duration of the stimulus
- instead some of them convey information on rapid changes in stimulus intensity and rate.
- An example of a phasic receptor is the Pacinian corpuscle.
VPL
- ventral posterior lateral nucleus of the thalamus
- 2nd order neurones coming from the medulla (after crossing over) of the dorsal column terminate there
- 3rd order neurones of the dc begin there and travel to the ss-cortex
- topographic representation of the body in the VPL (lower extremities are lateral)
Where to 2nd order neurones of the dorsal column decussate?
- at the caudal medulla
- then travel up the contralateral medial leminiscus tract
QST
quantitative sensory testing
- e.g. you can test 2 point discrimination
Which fibres mediate which type of pain?
- Aδ fibers mediate sharp, intense or first pain
Type 1: noxious mechanical
Type 2: noxious heat - C-fibres mediate dull, aching or second pain
Noxious thermal, mechanical and chemical stimuli (polymodal)
Which NT is involved in the transmission of pain in the spinal cord?
glutamate
What is the emotional component of the pain pathway?
- spinoreticular tract
- has 4 levels of neurones
Is pain objective or subjective?
- Pain is a subjective process
- only the patient can tell you something is painful
- ## ouch pain threshold is highly variable
RVM and PAG
RVM: rostral venteromedial medulla
PAG: periaqueductal gery
-> both areas that are affected by opioids to cause pain inhibition.
What are the types of pain sensitisation?
Peripheral (decreases threshold to peripheral stimuli ar the site of injury)
Central (decreases the threshold to peripheral stimuli at an adjacent site to injury; expansion of receptive field; spontaneous pain).
Primary and secondary hyperalgesia
- Primary hyperalgesia describes pain sensitivity that occurs directly in the damaged tissues.
- Secondary hyperalgesia describes pain sensitivity that occurs in surrounding undamaged tissues
