Somatosensory system Flashcards
What are the functions of the somatosensory pathways? What does it generally consider?
Ability to interpret bodily sensation
Mechanical, thermal, proprioceptive, nociceptive (anything negative)
Consists of skin, tissue, joints, nerves, spinal cord and brain
What are the different somatosensory modilities?
Mechanical, Thermal, Proprioceptive, Nociceptive
Modalities mean each of these different nerves
What do the terminals of sensory nerves look like?
Modifed nerve endings - free nerve endings-thermoreceptors and nociceptors
Enclosed nerve ending-mechanorecptors
What are the 3 main classes of sensory neurons
Aalpha-motor
Abeta-smaller than Aa but faster than res, mechanoreceptors of skin (large and fast), myelinated increase speed (m/sec)
Reaches CNS before pain
Agamma-nociceptors and mechanical stimuli (smaller than Ab)
C-Unmyelinated, smaller- slower-pain temperature, itch
Define receptor. What types of thermoreceptors are they?
Receptors are transducers that convert energy from the environement to Action potential
Themroreceptors-Agamma, C fibres-sensitive via membrane. Very sensitive, but not evenly distributed
Free nerve endings
Temp ability depends of what receptors (TRP ion channels)-4heat activated ion channels (TRPV1-4), 2 cold activated-TRPM8/A1 (activated by wasabi)
MEchanoreceptors
Define the 5 main mechanoreceptors
Meissners corpuscules (fine discriminative, low frequency vibration)
Merkel cells-light souch, superficial oressure
Pacinian corpuscule-deep pressure, high frequency vibration
Ruffini endings-continous pressure and stretch
ALL are Abeta type fibres
Define stimulus threshold
Point of intensity where a person can detect the stimulus 50% of the time (like microfilaments-which thickness)
Define stimulus intensity
Define by how much a neuron fires above its threshold-with longer/stronger timulus, more AP leading to increased sensrory
What are tonic receptors?
Detect continous stimulus strength- but adapt very slowly/dont adapt
fire all the time of the stimulus-keeps brain informed
MERKEL cells are a type of that
What is adaptation?
Detection of continous stimulus strength-your body wont keep telling you about it if nothing changes (like a hand on a table for long)-only stimlus at start and end
What are Phasic receptors?
They detect change in stimulus strengtgh-only fire when there is a change-adapt fast
Pacinian recepot-fire start and end of pressure, not in between
What is a receptive field>
Region of the skin which activates only 1 sensory neuron
Different sizes in places
Upper arm have large ones-so cant differciate too much
Fingers have timy ones, which allows very precise
Back is super large-reduced precision
What is two point discrimination?
Minimun distance at which 2 points are considered diferent
Usually dependent on the size of the receptive field
Vary wildly between body-hand is 4/5mm, but back/calf is over 45mm
What are somatosensory dermatomes?
area of skin innvervated by a sigle spinal nerve
Each nerve has a ventral and dorsal root
Provide a mapping system to see where sensory is going
C5-clavicule, C6 (6 shooter)-thumb. T4 is nipples, T10 is belly body
Where are the cell bodies for neurons in body and face?
body dorsal root ganglia in body, and trigeminal ganglia for face
Legs go up gracile fasciculutus, and arms go up the cuneate fasciculutus -cross over in the medulla to thalamus
(gracile more medial than cuneate)
Trigeminal go directly via pons to trigeminal nucleus, then to thalamus
What are the two neuron types in the dorsal horn>
Those that prokect to brain (projection neurons)-
And interneurons that remain in spinal cord
Thermo and noci (Adelta/C fibre-are more superficial)
Ab are deeper in the dorsal horn
What is lateral inhbition?
Explain how receptive fields can overlap with other-difficult to distinguish
In the dorsal horn, inhbitory interneuron stop from adjacent neurons from being activated too, increasing perception
Describe the touch and proprioception tract
Ab fibres via dorsal horn to dorsal columjn pathays
Via fasciculutus gracilis(under T6)/Cuneatus (above T6)
On the SAME SIDE (ipsilateral) and cross over (dessecate) in medulla
Do fine discriminative touch and vibration
1st order neuron to medulla each synapse within cuneate/gracilis nuclei in medulla
2nd order go to caudal medulla -form contralateal lemniscus tract
2nd order neurons teperminate in ventral posteror lateral nuctleus of thalamis (VPL)
3rd order neurons terminate in somatosensory cortex-size of area in brain depends on densirty of fibres from that area (hand massive)
Describe the ascending pathway for pain, temperature and crude touch
Carried in SPINOTHALAMIC pathway (pain/temp-lateral, Crude touch in the anterior)
CROSS OVER at the same level as they enter (immediate), then go up to thalamus/somatosensory cortex
1st order terminate in spinal cord/dorsal horn
2nd orderneuron cross over and to thalamus to ventral posterio lateral nucleus (VPL)
What is the clincal relevance of 2 point discrimination test
Called psychophysical assessment-depends on patients a lot
Tests the integrity of acending-can test sensation. pain, discrimation (dorsal column)
In anterior spinal artery damge-lose spinothalamuc tract below lesion-BUT dorsal part is intact (light touch, vibration, 2point discrimation)
Define pain rohgly
Unpleasent sensory and emotional experience associated with actual or potential tissue damage
Describe Nociceptors types and roles
Adelta fibres-type 1 noxious mechanical, noxious heat-fast sharp pain
C fibres mediate the dull/aching pain
What is the neurotransmitter used in pain processing?
Glutamate released as an answer to the stimuli, to the dorsal root then can go to several levels-physical and emotion/learning
How is pain carried to the brain
Sensroy is carried via lateral spinothalamic tract, but spinoreticular tract carries the emotion component
In the brain, involves incredible amounts of part (frontal, amygdala, cerebellumn brainstem, etc
Explain how nociceptor input can be gated by CNS
Gate control theory-Inhbition of primary afferent inputs before they are transmitted to brain
Like rubbing elbow after hitting it-activating Ab fibers to activate inhbitory neurons, stopping C Fibre info from passing to brain
Also descending control pathways-strong emotions can inhbit pain-involved periaqueductal grey-facilitation and inhbition of nociceptive precessing. Use serotonin/noradrenaline (opiods similar)
Placebo seems to do that
What are the main mechanisms of chronic pain?
Pain that last over 3 month-
Can be nociceptive pain-skin, msucle, ligaments, etc (arthirits) or neuropathic pain (sciatica, trauma, diabete)c
Also mixed pain-lower back pain, osteoarthiritis
What is peripheral sensitisation?
As an area is damaged, create a inflmatory soup-create inflamation, but also modulate nociceptors or the area
make it more sensitive (reduce threshold but peripherally and centrally (via plasticity of projection neuron of dorsal horn)
Define Allodynia and hyperalgesia
Allodynia-Pain due to stimulus that doesnt normally provoke pain
Hyperalgesia-increased pain from normally painful stimuli (primary-at the area of damge, secondary-around) -pretty much reduce stimulus threshold needed
How do you diagnose neuropathic pain?
Very filtered-has to have neurological damage. Symptoms have to match. Have to have sensory lose or gain
Usually start with questionnaires, but not diagnostic
What are the 3 main neuropathic pain cluster types
Sensory loss, Thermal hyperalgesia and mechanical hyperalgesia
