Somatosensory pathways Flashcards
What is a modality?
Type of stimulus
What modalities are sensed by a mechanoreceptor?
Touch, pressure, vibration, proprioception
Give an example of a type of mechanoreceptor?
Meissner’s corpuscle
Describe the function of TRP channels
= free nerve endings with thermal sensitivity, activated at different temperatures
Recall the 3 main classifications of sensory nerves
A-beta
A-delta
C
Recall what each class of sensory fibre is responsible for sensing
A-beta = innocuous mechanical stimuli A-delta = pain, temperature C = slower, achey pain; temperature
Compare the structures of each class of sensory fibres
A-beta = large, myeinated A-delta = medium, myelinated C = narrow, unmyelinayed
Define “stimulus threshold”
Level of tsimulus that produces a response 50% of the time
Define “adaptation”
change over time in the responsiveness of the sensory system to a constant stimulus
Recall the 2 different classes of receptors with respect to adaptation, and explain the difference
Tonic: detect continuous signal strength
Phasic: detect a change in signal strength
Give an example of a receptor that demonstrates tonic activity
Merkel cells
Describe the function of tonic receptors
Continue to transmit to the brain as long as the stimulus is present
Describe the function of phasic receptors
Transmit a signal at the beginning and end of a stimulus
Define “receptive field”
The region of skin which causes activation of a single sensory neuron when activated
Explain the significance of receptive field size
The smaller the receptive field, the more precise and fine the sensitivity due to higher neuron density
Explain the function of lateral inhibition
Since receptive fields may overlap, stimuli may seem more perfuse than they really are. LI aids localisation of the stimulus via interbeurons in the dorsal horn
Recall the 2 types of nocioceptor
1 - A-mechanoheat receptors
2 - A-mechanoreceptors
How are fibres entering the dorsal horn organised?
Rexed laminae - layers of grey matter
What sort of sensory fibres are capable of sensing pain and temperature, and in which rexed laminae do they terminate?
A-delta and C: I and II
What sort of sensory fibres are capable of innocuous mechanical stimuli, and in which rexed laminae do they terminate?
A-beta and A-alpha: III-VI
Describe the 2 roles of interneurons in the spinal cord
- Connect between laminae
2. Connect adjacent peripheral inputs (for lateral inhibition)
Where do A-beta fibres enter the spinal cord?
Dorsal horn
What are the dorsal columns involved in sensing?
Fine discriminative touch and vibration
At which vertebral level is the upper and lower-limb spinal nerve division?
T6
Describe the 2 tracts in the dorsal columns
- Gracile tract: information from lower limbs; medial
2. Cuneate tract: information from upper limbs; lateral
What is a fasciculus?
Bundle of axons
Compare the termination of 1st, 2nd and 3rd order neurons between the dorsal columns and the spinothalamic tract
1st Order: Dorsal columns = medulla Spinothalamic tract = immediately in spinal cord Both: 2nd - VPL nucleus in thalamus 3rd - SI
What is the medial lemniscus?
Section of dorsal column that runs between medulla and VPL
Compare the point at which the ascending pathway changes side between the dorsal columns and the spinothalamic tract
Dorsal column: medulla
Spinothalamic tract: immediate in spinal cord
What is the spinothalamic tract involved in sensing?
Light touch, vibration, 2-point discrimination
Describe the ascending path of the trigeminal nerve
- Synapses with 2nd order neuron at trigeminal cranial nucleus
- Relays through the VPM nucleus to cross midline
- Joins medial end of medial lemniscus
- –> SI
Via what tract do pain pathways interact with the limbic system
Spinoreticular tract
Recall the functions of SI, SII and the posterior parietal cortex
Information is first processed in SI before entering SII/ PPC
