Sensory pathways Flashcards
What are the four somatosensory modalities?
Modality: Difference experience of somatosensory stimmulus
- Touch
- Temperature
- Proprioception
- Nocioceptive
What is temperature perception?
Which fibres transport this information?
information about temperature conveyed by free nerve ending receptors:
A(delta) fibres
- noxious mechanical and thermal stimulation
- Myelinated
- Thinner
–> Medium conduction speed (5-30 m/s)
C-fibres
- noxious mechanical, thermal and chemical stimulation
- non-myelinated
- thin
–> slow conduction speed (0.5-2m/s)
What is Touch perception?
Perception of light mecahincal stimmulation, coveyed by Mechanoreceptors (Aß-fibres)
- Enclosed nerve endings
- Fast condtuction
What kind of Receptors are Aß fibres?
What to they convey?
- Mechanoreceptors of skin –> innocious mechanical stimmulation
- Myelinated
- Thick
- –> fast conduction (35-75m/s)
What are A(Delta) fibres? What is their function?
A(delta) fibres
- noxious mechanical and thermal stimulation –>sharp first pain
- Myelinated
- Thinner
- –> Medium conduction speed (5-30 m/s)
What are C-fibres?
What is their function?
- noxious mechanical, thermal and chemical stimulation –> secound slow, dull pain
- non-myelinated
- thin
- –> slow conduction speed (0.5-2m/s)
What is a Stimmulus threshold?
’A threshold is the point of intensity at which the person can just detect the presence of a stimulus 50% of the time (absolute threshold)
What is a Stimulus intensity?
Increased stimulus strength and duration = increased neurotransmitter release = greater intensity
What is adaptation?
reversiblen, vorübergehenden Anpassung der Empfindlichkeit eines Sinnessystems, Sinnesorgans oder eines Rezeptors an Änderungen der Reizintensität
Two mechanisms/receptors:
- Tonic receptors:
- don’t adapt or only very slowly
- convey persistent Stimulus to brain –> keep it updated about about status of the body
- e.g. Merkel cells
- Phasic receptors
- detect a change in stimulus strength
- transmit an impulse at start and end of a stimulus i.e. when something has changed
What is a receptive field?
Part of skin that following a stimulus only activates a single sensory neuron
–> is very different in different parts of the brain
What is two-point discrimination?
The smallest distance at which 2 sensory stimmuli are percieved as separate
- very much dependant on receptive field (–> need activation of 2 receptive fields for this)
What is Lateral inhibition?
What is it mediated by?
- Receptive fields can overlap.
- This would make it difficult to locate stimuli.
- HOWEVER, lateral inhibition prevents this overlap.
Lateral inhibition enhances the difference between adjacent inputs.
It is mediated by inhibitory interneurons within the dorsal horn of the spinal cord.
What is a projection neuron?
Neurons which axons project to the brain
What is a interneuron?
Neuron which axonal projection remains within the spinal chord
Which nociceptors are there?
Which type of information do they transmit?
- A(delta) fibres: mediate sharp, intenst first pain
- Type 1
- noxious mechanical
- Type2
- noxicous thermal
- C-fibres: dull, aching secound pain
* Noxious thermal, mechanical and chemical stimuli (polymodal)
Where does nociceptive processing in the spinal chord take plece?
Which neurotransmitter is involved in this?
It takes place in the more lateral/superficial part of the dorsal horn
- Glutamate is used as excitory neurotransmitter
What is the gate-controll theory?
Sensory impulses can be inhibited before they are transmitted to the brain through ascending pathways –> Change in pain felt
-
Central pathway
- opioid lecture: activation of NRM via NRPG leading to decreased pain conduction at central horn
-
Peripheral inhibition pathway (rubbing of area)
- Stimulation of touch receptors in the same area as the pain sensation will lead to increased activity of the non-nociceptive touch neurones meaning that there is increased activation of the inhibitory interneurone and hence reducing the activity going up the spinothalamic tract.
–> Inhibition of A(delta) transmission due to Aß stimmulation
How could decending pathways influence pain perception?
Neurotransmitters e.g. Noradrenaline or Serotonin could Facilitation and inhibition of nociceptive processing in the dorsal horn
Explain the Dorsal-colum pathway
Conveys Fine discrimiation, touch etc. from upper (fascigulus cuneatus) and lower limbs (fascculus gracilis) via Aß fibres
- st. order neurons enter spinal chord and ascend via the dorsall column pathways and synapse in medulla (in gracile nucleus and cuneate nucleus)
- nd order neurons start in dorsal pahtways of medulla, decussate and continue via the medial lemniscus pathway into the ventral posterior lateral nucleus in thalamus (has represenations: lower limbs = more lateral)
- rd order neurons start there and enter into the somatosensory cortex
Explain the Spinothalamic (anterolateral pathway)
Conveys pain, temperature, crude touch etc. via A(delta) fibres + C fibres
- st order neurons: enter the spinal chord and immediatly synapse at posterior horn
- nd order neurons decussate and ascend anteriorly and fom the spinothalamic tract
- synapse in ventral posterior lateral nucleus of thalamus (topographic representation –> lower extremities are lateral)
- rd order neurons go into somatosensory cortex
How would a patient present with an aterior spinal chord lesion?
No problem in dorsal column pathways
- i.e. normal two point discrimination, touch senstion etc
BUT:
- Problem in spinothalamic tract –> no pain and temperature sensation bilaterally
How are the different sensory fibres in the dorsal horn organised?
The Pain fibres (Adelta+ C) are more superficiall
Mechano (Aß) are deeper
Ich which part of the thalamus do the spinothalamic and the dorsal colum pathways terminate?
They terminate in the ventral posterior lateral nucleus
It also has a topographic representation with lower limbs being more lateral
What is pain?
Which pathways are involved?
The physical end emotional response to a potentially damaging stimulus
Sensory component of pain:
- Transported by the spinothalamic tract
Emotional component of pain:
- Transported by the Spinoreticular tract
What is nocioceptive pain?
noxious stimulation of a nociceptor (normal)
- arthritis, fractures, burns, headaches etc.
What is neuropathic pain?
= lesion or disease of the somatosensory system e.g. sciatica or post-surgical
- not nocioceptor transmits pain but the damage to a nerve is painful
•Explain how nociceptive input can be gated by peripheral and central mechanisms
Peripheral mechanisms: Gate control theory:
- firing of a e.g. C fibre can be inhibited by anohter primary ascending signal (e.g. Aß fibre fiereing) before it reaches the spinal chord
Central mechanism: Descending control pathway
- In dorsal column (where nocioceptive processing would normally uccur with glutamate) at synapse other neurotransmitters can influence processing e.g. Serotonin, endorphins, noradrenaline etc. (Picuture)
What is hyperalgesia?
What are different forms of hyperalgesia?
Hyperalgesia: increased pain from a stimulus that normally provokes pain (normally due to hypersensitivity to nocioceptors)
- Primary e.g. in area of inflammation
- Secondary: increased pain sensitivity outside of the area of injury or inflammation). Secondary hyperalgesia is due to central neuron sensitization and requires continuous nociceptor input from the zone of primary hyperalgesia for its maintenance
What is the most common cause of peripheral neuropathy is
Diabetis
(in stocks and gloves pattern)
