Somatsosensory Pathway Flashcards
Define a somatosensory modality and give examples.
A modality is a type of stimulus (hot, pain, touch) they have specialised receptors that transmit information through specific anatomical pathways to the brain.
Eg mechanoreceptors: touch, pressure, vibration, propioception
Thermoreceotor : temperature
Nociceptor: pain
Describe the 3 sensory fibres
- Mechanoreceptor of the skin (Ab) -very fast, large, myelinated and transmit mechanical stimulation
- Pain and temperature receptors (Ad) - slightly fast, myelinated and transmit fast pain and temperature signals
- Itch, pain and temperature (c-fibres) - slow, transducing, unmyelinated and transmit slower and achy pain.
Together these sensory fibres make up peripheral fibres
Describe the role and purpose of each type of nerve ending.
Individual axons within the sensory fibres have modified terminals which are specific to different modalities. The modified nerve endings are really close to skin.
C-fibres are unmyelinated and have specialised, free nerve ending that are selfie for heat.
The Ab mechanoreceptor fibres have encapsulated nerve endings.
What is the activation thereshold?
Sensory receptors are transducers that convert energy from the environment into neuronal action potential. The absolute thereshold is the level of stimulus (strength) enquires to produce a one of detection 50% of the time. The nerve endings becomes depolarised (if strong enough you get action potential)
Describe the structures of a thermoreceptor that elicit a response
Thermoreceptors have free nerve endings with high thermal sensitivity. Changes in temperature activate a family of transient receptor potential (TRP) ion channels.
- 4 heat activated TRP channels (TRPV- 1-4) from noxious heat to tepid heat
- 2 cold activated TRP channels (TRPM 8 and TRPA 1).
On the nerve endings there is a combination of different receptor types - specific to different temperature.
Describe the structure of mechanoreceptors that relate to its function
Mechanoreceptors are specific to different types of touch by adaptation (how skin transmits signal and how the brain interprets them)
- Adaptation-tonic receptor: detects continuous stimulus strength, do not adapt or adapt very slowly, they continue to transmit impulses to the brain as long as the stimulus is present. They keep brain constantly informed of the status of the body (eg merkel cells allows for fine touch)
- Adaptation-phasic receptor: detect a change in stimulus strength, they transmit an impulse at the start and at the end of the stimulus (when change is taken place). Phasic receptors adapt quickly (eg pacinan receptor transmitting pressure)
Describe the two point discrimination theory
The minimum distance at which two points are perceived as separate. This is related to the site of receptive field, the smaller the field, the more densely packed the neurones are, so more likely to identify two points of touchy
Describe the different types of nociceptors
Ad fibres mediate sharp, intense pain, myelinated and fast. Type 1 are noxious and thermal stimuli. Type 2 purely noxious stimuli.
C-fibres mediate dull, persistent or second pain, unmyelinated, respond to thermal and chemical stimuli, including inflammatory mediators. Polymodal and respond to all modalities.
Describe the route of different nerves in dorsal horn
When the nerve enters the dorsal horn it terminates in rexed laminae
- Innocuous mechanical stimuli (Ab and Aa) terminate in lamina III-VIII
- pain and temperature stimuli (Ad and Cxfibres) terminate in lamina I-II
Describe lateral inhibition
This process prevents the overlap of two receptive fields therefore making it easier to distinguish between two stimulus locations. This is mediated by inter neurones within the dorsal horns of spinal cord.
Inhibit pain response by stimulating Ab fibres (if hurts rub)
Describe the ascending pathway (dorsal column)
Transmits innocuous mechanical stimuli (fine discrimination touch and vibrations). Ab fibres enter via dorsal horn and enter dorsal column pathways, information conveyed from the lower limbs (below T6) travel ipsilateral along the gracile tract. Information conveyed from the upper limbs travel ipsilateral along the cunaet tract.
First order neurones terminate in the medulla, topographic representation of the body in Brainstem
Second Order neurones cross in caudal medulla: decusate in the caudal medulla, forms the conralateral medial leminiscus tract. The axons of the second order neurones terminate in the VPL nucleus of the thalamus.
Third order neurones from the VPL terminate in the somatosensory cortex. The size of the somatotropic areas are proportional to density of sensory receptors in that body regions.
Describe the ascending pathway (spinothalamic)
Anterior (crude touch) and lateral (pain and temperature)
First order terminate in dorsal horn
Second order neurones decussate immediately in spinal cord and terminate in medulla and terminate in VPL
Explain the differences between the 2 ascending pathways
Dorsal column : transmits light, touc and vibration and 2 point discrimination and crosses in brain stem the medulla.
Spinothalamic: transmit pain, temperature, coarse touch and cross in spinal cord.
Describe the emotional component of pain pathway
Pain travels through to spine via the parabrochial are in the Brainstem then to limbic system. This is called spino-reticular system
How do you test the integrity of the ascending pathways
- QST - SENSITIVITY TESTING
- EPT - ELECTRICAL CURRENT
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