Ascending Neural Pathways Flashcards
which ascending tracts carries info from the lower body extremities?
fasciculus gracilis
which ascending tracts carries info from the upper body extremities?
fasciculus cuneatus
which tracts carry proprioceptive info from muscle spindles (dorsal) and golgi organs (ventral)?
dorsal and ventral spinocerebellar
what tract carries pain?
spinothalamic
what does fasciculus gracilis tract carry?
info from lower limb extremities
what does fasciculus cuneatus carry?
info from upper limb extremities
what do dorsal and ventral spinocerebellar tracts carry?
proprioceptive info from muscle spindles (dorsal) and golgi organs (ventral)
what does the spinothalamic tract carry?
principally pain
what are the 3 principal ascending tract systems in the spinal cord?
- dorsal colum-medial lemniscal pathway
- spinothalamic pathway
- spinocerebellar pathway
what pattern do conscious sensory pathways generally follow?
- first order neuron terminates in spinal cord or brainstem
- second order neuron projects to thalamus
- third order neuron projects to primary sensory cortex
what is the dorsal column-medial lemniscal pathway for?
mechanical stimuli:
- conscious proprioception
- discriminative touch
- vibration
- presssure
what sort of fibres is the axon of the medial lemniscal pathway composed of?
large diameter, fast Aβ fibres
in the medial lemniscal pathway, 1st order neurons ascend ipsilaterally in dorsal columns known as?
- fasciculus gracilis (enters up to T6)
- fasciculus cuneatus (enters above T6)
what happens after gracile and cuneatus nuclei synapse in brainstem with 2nd order neurons?
- they decussate (cross over)
- form the medial lemniscus
- project to VPL (ventral posterolateral lobe) of thalamus
what do pathological processes in the medial lemniscal pathway cause?
gait ataxia - brain is deprived of info about position of feet
what do lesions on the cervical cord cause?
upper extremity ataxia
what is gait ataxia?
abnormal, uncoordinated movements due to brain deprived of info about position of feet
what causes upper extremity ataxia?
lesions on cervical cord
what does dorsal column disease cause?
paraesthesia in distal parts of extremities
what is paraesthesia usually the result of?
ectopic discharge in damaged dorsal colum axons:
- may be present before any abnormalities are detectable
what is the spinothalamic pathway for?
nocioceptive stimuli
- also pathway for mechanical, chemical and thermal detection of course or non-discriminating touch
- pain
- temperature
the axons of the spinothalamic pathways are composed of what type of fibres?
- smaller diameter
- slow Aδ or C fibres
- no specialised sensory fibres
- myelinated fibres: quick to cross
the fibres of the spinothalamic pathway are myelinated - what does this mean?
they are ‘quick to cross’
what are features of Aδ fibres?
detect:
- non discriminative touch
- heat.cold
- sharp pain
what are features of C fibres?
detect:
- dull aching pain
- itch (histamine sensitive)
what happens in the spinothalamic pathway?
- 1st order neurons synapse with 2nd order in dorsal horn
- they ducussate and ascend in lateral or ventral spinothalamic tract
- synapse at thalamus with 3rd order neurons
- project to sensory cortex
what are features of the spinothalamic LATERAL route?
mixture of Aδ and C fibres carrying info about pain and temp
what are features of the spinothalamic ANTERIOR/VENTRAL route?
only C fibres carrying info about coarse, non discriminating touch via mechanosensitive fibres
what do lesions in the lateral spinothalamic tract cause?
- decreased perception of pain and temperature on contralateral side of body
- paraesthesia (shooting pain)
what is the role of nocioceptors?
detects noxious stimuli using free nerve endings and relays signals to brain
what are features of polymodal nociceptors?
- un-myelinated C fibres
- mechanical
- chemical
- thermal
what are features of mechanical nocioceptors?
- lightly myelinated Aδ fibres
- sensitive to high pressure
what are features of chemical nocioceptors?
- lightly myelinated Aδ fibres
- sensitive to histamine, gives itch feeling
what are features of thermal nocioceptors?
- lightly myelinated Aδ fibres
- sensitive to high/low temp
what type of fibres conduct pricking/stabbing pain?
Aδ fibres
- fast
- arrive first in CNS (fast conduction velocity)
what type of fibres conduct burning/aching pain?
C fibres
- slower pain
- arrive later in CNS (slow conduction velocity)
what does substance P do?
modulates the peripheral neuronal response to noxious stimuli
when is SP released and from where?
- when stimulated in fast succession
- from pain neurons
what does capsaicin do in the body?
- mimics chemicals produced at sites of tissue damage
- activates Trp1 channel in nocioceptor neurons
- opens to allow Ca2+ and Na+ into cell
- causes depolarisation and AP
what is referred pain?
pain felt in a part of the body other than its actual source
how does referred pain happen?
- signals of noxious and normal cutaneous stimuli enter spinal cord at same point
- cross talk between modalities in dorsal horn
- signals from viscera get picked up by ascending nerve fibres mapped cortically to dermis
what is the role of the spinocerebellar pathway?
- takes info about body’s position and movements to cerebellum for integration
- 2 principal routes: ant. and post.
- used in cerebellum for control of posture and co-ordination of movement
how many neurons does each pathway of spinocerebellar (ventral or dorsal tract) have?
- 2 along its length (1st and 2nd order)
- located in dorsal horn of spinal cord
- terminate in vermis of cerebellum
how do spinocerebellar dorsal tract neurons ascend and where do they enter?
- ipselaterally
- enter cerebellum via inferior peduncle
how do spinocerebellar ventral tract neurons ascend and where do they enter?
- contralaterally
- enter cerebellum via superior peduncle
what is friedreich’s ataxia?
inherited disease where spinocerebellar tract becomes increasingly ineffective
what causes friedreich’s ataxia?
mutliple repeates of a gene for the protein ‘frataxin’ which is responsible for iron metabolism in mitochondria
what does friedreich’s ataxia result in?
- progressively uncoordinated arm and leg movements
- wide based ‘reeling’ gait
- intention tremor
