Somatosensory System FINAL Flashcards
Mechanoreceptors function
Detect touch and proprioception
Sensory neuron types fastest to slowest and uses of each
ABeta- Innoucuous mechanical stimulation
ADelta- Noxious mechanical and thermal stimulation
C fibres- Noxious mechanical, thermal and chemical stimulation
Features of ABeta,ADelta and Cfibres that affect its speed
ABeta- Largest diameter +myelinated
ADelta- smaller diameter +myelinated
C fibres - smallest diamater and not myelinated
Sensory Receptor:
Transducer that converts energy from environment into neuronal action potential
Receptors with Free endings
Nocioceptor and thermoceptors
Receptors with enclosed endings
Mechanoreceptors
Thermoreceptors are what type of fibres
Express what on their neurons
Adelta and C fibres
Express TRP -Transient Receptor Potential ion channels
Heat activated TRP
Cold activated TRP
TRP1,2,3,4
TRPM8, TRPA1
Mechanoreceptors are what sort of fibres
AB fibres
5 types of mechanoreceptors and functions
1) Meissner’s corpuscle
2) Merkel Cells
3) Pacinian Corpuscle
4) Ruffini endings
5) Hair follicle receptors
1) Meissner’s-Discrimnative touch
2) Merkel Cells-light and superficial pressure
3) Pacinian corpuscle-High frequency vibration
4) Ruffini endings-continous pressure/touch and stretch
5) Hair follicle receptors: light touch
Nociceptors are what fibres
Adelta and C fibres like thermoceptors
Stimulus threshold:
Point of intensity at which person can JUST detect the presence of a stimulus 50% of the time– absolute threshold
Stimulus Intensity
Increased stimulus strength and duration=increased neurotransmitter release=greater intensity
> encoded by how quickly neurons fire
Adaption: what are the 2 types
What is sensory adaption
1) Tonic receptors
2) Phasic Receptors
Change with time in responsiveness of a sensory receptor to a stimulus
Mechanism of tonic receptors
E.g of tonic receptor
1) Detect continuous stimulus strength
2) Fire quickly at point of stimulation and slow down after
3) Continue to transmit impulses to brain as long as stimulus is present
4) Keeps brain constantly informed
Merkel cell
–>Slowly adapt, allowing for superficial pressure and fine touch to be percieved
Mechanism of phasic receptors
E.g
1) Detect change in stimulus strength
2) Transmit impulse at start and end of stimulus
Pacinian Corpuscle
- Sudden pressure excites receptor
- Transmits signal again when pressure is released
Receptive fields
Region of skin which causes activation of a SINGLE sensory neuron when activated
Small receptive fields
Precise perception- allow for detection of fine detail over small area
Large receptive fields
Less precise, allow cell to detect changes over a wider area
Two point discrimnation
min distance at which 2 points are perceived as separate
->related to size of receptive fields
Where are the dermatomes C5, T4
c5- Clavicle, t4= nipple
Where are cell bodies found in body and in the face
In body–dorsal root ganglia
In face- Trigeminal ganglia
Info from upper and lower bodies received at what level of spine
Upper= Cervical
Lower=Lumbar
Where are gracilis and cuneatus fasciulus located
In spinal cord
Where are secondary (internal arcuate fibres) from and reach where
From gracilis and cuneatus in spinal cord to thalamus
Secondary neurons cross over where and form
Cross over at medulla
Medial leminiscus tract
Secondary neurons axons synapse on neurons in
Ventral nuclear group
Tertiary neurons from where to where
Thalamus to primary motor cortex
Somatosensory function from face: Enters brain at
Where do primary neurons synapse
Pons
Trigeminal nucleus
2 main types of dorsal horn neurons
1) Projection neurons-Neurons with axons that project to brain
2) Interneurons- Neurons with axons that remain in spinal cord
Lateral inhibition is
what is the purpose
How is it mediated
Difference between adjacent inputs is enhanced by lateral inhibtion
Prevents overlap of receptive field
Facilitates
-pinpoint accuracy in localisation
-enhanced sensory perecption
inhibitory interneurons with dorsal horn of of spinal cord
Describe the ascending pathway of touch proprioception
1) Innocuous mechanical stimuli: -fine discriminatory touch or vibration
2) ABeta fibres enter via dorsal horn and enter ascending dorsal column pathways
3) Info from limbs travel ipsilaterally long respective dorsal column tracts (gracilis/cuneatus)
4) Fibres in gracile tract have their first synapse in the gracile nucleus –same for cuneatus
5) 1st order neurons terminate in medulla
6) 2nd order axons decussate in medulla at caudal medulla and form contralateral medial lemniscus tract
7) 2nd order neurons terminate in thalamus at ventral posterior lateral nucleus (VPL)
8) 3rd order neurons from VPL project to somatosensory cortex of post central gyrus of parietal cortex
Describe the ascending pathway of pain, temperature and crude touch
1)Pain and temperature sensations ascend within Lateral spinothalamic tract
Crude touch ascends within the anterior spinothalamic tract
2)Primary afferent axons terminate upon entering spinal cord
Secondary neurons decussate immediately in spinal cord and form spinothalamic tract
3) Secondary neurons terminate in VPL of thalamus
Dorsal column Vs Spinothalamic tracts
Dorsal column:
- Light touch
- Vibration
- 2 point discriminiation
Spinothalamic tracts:
- Pain
- Temp
- Crude/Coarse touch
Meissner’s corpuscle type of fibre
Abeta
Crude touch uses what fibre
Adelta
Nociceptors
Are what types of fibres and each mediate
ADelta and Cfibres
ADelta
Sharp, Intense first pain
CFibres
Dull, aching or second pain
Describe the 2 types of Adelta fibres of nociceptors
Type 1
-Noxious mechanical
Type 2
-Noxious heat
Cfibres
Noxious thermal, mechanical and chemical stimulus
Type of neurotransmitter released from sensory afferents in response to noxious stimuli
Glutamate –> Excitatory
Pain pathway:
Sensory component through which tract?
Lateral spinothalamic tract
Pain pathway:
Emotional component through which tract?
Spinoreticular tract
What is the gate control theory
Common behaviour that shows this
Inhibition of primary afferent inputs before transmitted to brain through ascending pathways
Instinctive rubbing– stimulates ABeta which blocks C fibres of pain to brain
Descending control pathways for pain can be inhibited by? potential
Strong emotions can inhibit pain
-Electrical analgesia Descend from somatosensory cortex
P
Key area in descending control pathway
PAG- periaqueductal grey
Nociceptive processing in dorsal horn can be affected by
Facilitation and inhibition
Monoamines that inhibit activity in spinothalamic tract and function of efferent C fibres
Serotonin–facilitate pain
Noradrenaline –inhibit pain
Nociceptive pain
e.g
Neuropathic pain:
e.g
Mixed
Noxious stimulation of a nociceptor (somatic or viscera)
e.g
Lesion/ disease or somatosensory system
e.g sciatica, diabetic, trauma
Ostearthritis
Tissue damage results in release of
Result of release
Neurotransmitters Peptides Lipids Protease Cytokines Chemines
Produce inflammation, initiate healing, make nociceptors more sensitive by modulating their excitability
Peripheral sensitisation
Central Sensitisation
Reduce threshold to peripheral stimuli at site of injury
Reduce threshold to peripheral stimuli at an adjacent site to injury
- Expansion of receptive field
- Spontaneous pain
Allodynia
Pain due to a stimulus that does not normally provoke pain
Hyperalgesia
Primary
Secondary
Increased pain from stimulus that normally provokes pain
Primary- at site of stimulus
Secondary- around site of stimulus
