Neuro 7 - Sensory pathways Flashcards
What are sensory modalities?
Types of stimulus
Mechanoreceptors of skin are which sensory fibre?
A-beta (v fast)
Pain / temperature fibres are which sensory fibre
A-delta (fast-ish)
Slow pain/temperature/itch fibres are which sensory fibre?
C fibres
How are axons adapted to sensing different stimuli?
They have different modified nerve endings/terminals
What is the absolute threshold?
Stimulus strength which produces a positive response 50% of the time (i.e. is detected 50% of the time)
Increasing stimulus strength and duration does what?
Increases neurotransmitter release causing greater intensity
Describe TRP channels
They are free nerve endings with high thermal sensitivity - temperature changes activates TRP channels
How many heat activated TRP channels are there?
4
TRPV1-4
How many cold activated TRP channels are there?
2
TRPM8
TRPA1
Name the 4 mechanoreceptors
- Meissners corpuscle
- Merkel cells
- Pacinian corpuscle
- Ruffini endings
What type of touch are Meissners corpuscles for?
Fine discriminative touch
What type of touch are Merkel cells used for?
Light touch and superficial pressure
What type of touch are Pacinian corpuscles used for?
Detecting deep pressure, vibration and tickling
What type of touch are Ruffini endings for?
Continuous pressure / touch and stretch
Describe tonic receptors
Detect continuous stimulus strength - transmits impulses for stimulus duration
Give an example of a tonic receptor
Merkel cells
Do tonic receptors adapt?
No, or they adapt very slowly.
Keeps brain constantly informed of body status
Describe phasic receptors
These detect changes in stimulus strength - fire impulses at start and end of stimulus (i.e. when a change is taking place)
Give an example of a phasic receptor?
Pacinian corpuscle
What is a receptive field?
Region on the skin which activates one single sensory neurone when stimulated
If you want fine detail, how big should the receptive field be?
Small
Describe two point discrimination
Minimum distance at which 2 points are perceieved as separate - related to size of receptive field
What type of sensory fibres are nociceptors?
A-delta fibres
A-delta fibres are myelinated. There are 2 types - describe
Type 1: A-delta mechanoheat receptors (noxious mechanical/thermal stimuli)
Type 2: A-delta mechanoreceptors (noxious mechanical stimuli)
Which type of sensory fibre is unmyelinated?
C fibres
What modalities do C fibres respond to?
Thermal, mechanical and chemical stimuli
They are polymodal
In the sensory pathway, where are cell bodies present?
DRG and Trigeminal ganglia (face)
How many rexed laminae are there?
7
Where do mechanical stimuli terminate (A-beta and A-alpha) in the dorsal horn?
3-6 rexed laminae
Deep
Where do pain and temperature (A-delta and C fibres) stimuli terminate in the dorsal horn?
Rexed laminae 1-2
Superficial
What is the main excitatory neurotransmitter of the dorsal horn?
Glutamate
How can overlapping receptive fields be overcome?
Via lateral inhibition - facilitates pinpoint accuracy in localising stimulus.
Thus facilitates enhanced sensory perception
What mediates lateral inhibition?
Interneurons in the dorsal horn of SC
S1 = Primary somatosensory cortex. Where is it located?
Postcentral gyrus
S2 = Secondary somatosensory cortex. Where is it located?
Parietal operculum
What is the posterior parietal cortex needed for?
Spacial awareness of body
When innocuous mechanical stimuli enter the ascending dorsal column pathway (via A-beta) , what are the differences between lower limbs and upper limbs?
(AP1)
Below T6 = lower limb = travels ipsilaterally along gracile tract
Above T6 = lower limb = travels ipsilaterally along cuneate tract
Where do first order neurone terminate?
AP1
In the medulla
Within the medulla, gracile tract fibres have their first synapse where?
(AP1)
In the gracile nucleus
Within the medulla, cuneate tract fibres have their first synapse where?
(AP1)
In the cuneate nucleus
What do the second order neurons do?
AP1
They decussate in the caudal medulla, forming the contralateral medial lemniscus tract
Where do second order neurons terminate?
AP1
In the Ventral Posterior Lateral Nucleus (VPL) of the nucleus
Where does sensory information from lower limbs terminate in relation to the vPL
(AP1)
More laterally
What do 3rd order neurons do?
AP1
3rd order neurons convey information from VPL to Somatosensory cortex
What is meant by somatosensory humunculus
AP1
Area whereby size of somatotopic areas is proportional to density of sensory receptors in that body region
What is AP1?
Ascending pathway 1 - innocuous mechanical stimuli
AP2 =?
Ascending pathway 2 - pain/temperature/crude touch
The spinathalamic pathway involves 2 aspects which are?
AKA anterolateral pathway
“Lateral” aspect of spinothalamic tract is where pain/temperature sensations ascend
“Anterior” aspect of spinothalamic tract is where crude touch sensations ascend
In AP2, where do the primary afferent axons terminate?
What happens next?
Primary afferent axons (first order) terminate upon entering SC (in the dorsal horn)
Second order neurons decussate immediately and form spinothalamic tract
Where do second order neurons terminate?
AP2
Ventral posterior lateral nucleus (VPL)
Lower extremities more lateral
What are key differences between dorsal column and spinothalamic tracts?
Dorsal column = decussates in medulla
ST tract = decussates in SC
Dorsal column = light touch, vibration, 2 point discrimination
ST = Pain, temperature, coarse touch
There is an emotional component to pain, where does the signal from the SC go to?
The parabrachial area
How can we test the integrity of the ascending sensory pathways?
Quantitative sensory testing (QST)
If someone gets an anterior SC lesion, they cannot feel pain and temperature below the level of the lesion. What can they feel?
Light touch and vibration below the level of the lesion
What are the 6 different types of pain?
- Nociceptive (usually acute)
- Muscle
- Somatic
- Visceral
- Referred
- Neuropathic
What is neuropathic pain?
Pain caused by a lesion/disease of the somatosensory nervous system
Do analgesic drugs (e.g. opiates) have a good relieving effect for neuropathic pain?
No, poor response to analgesics
What is the most common form of neuropathic pain?
Radicular low back pain (sciatica)
What is allodynia?
Pain caused by a stimulus that usually does not cause pain
What is hyperalgesia?
Increased pain from a stimulus that normally provokes pain
What is paraesthesia?
Abnormal sensation (whether spontaneous or evoked)
How can synaptic plasticity cause chronic pain?
or central sensitisation
NMDA receptor activation
causes Ca2+ mediated synaptic plasticity
meaning increased synaptic strength which reduces inhibitory influences on dorsal horn neurons
Constant NMDA activation therefore causes chronic pain development
Name some neuropathic phenotypes
Heat hyperalgesia Cold hyperalgesia Mechanical hyperalgesia Mechanical allodynia Loss of sensation
What are the 2 inhibitory pathways for descending control of nociception?
- PAG-RVM axis (brainstem) which uses 5-HT (aka serotonin) as its inhibitory monoamine
- Locus cereleus (pons), inhibitory via noradrenaline
How do endogenous opioids increase descending inhibition?
- PAG-RVM contains lots of miu-opioid receptors
- Endogenous opioids inhibit glutamate release (less spinothalamic neurones activated)
Opioids part of endogenous analgesic system
Aside from opioids, what other class of drugs may have some efficacy in neuropathic pain?
Anti-depressants
Why may SNRIs (class of antidepressants) enhance descending NA inhibition?
Stimulate NA and inhibitory influences within the synapse
What is conditioned pain modulation used for?
To measure the level of descending control in patients
