Ascending Neural Pathways Flashcards
What are the 3 principal ascending tract systems of the spinal cord?
- dorsal column - medial lemniscus
- spinothalamic
- spinocerebellar
What tract carries information from the lower body extremities?
fasciculus gracilis (up to T6)
Where does fasciculus cuneate carry information from?
upper body (from above T6)
Where does the first order neuron/first afferent terminate?
the spinal cord or brainstem
Where does the second order neuron project to?
the thalamus
Where does the third order neuron project to?
primary sensory cortex
What is the dorsal column - medial lemniscal pathway for?
MECHANICAL STIMULI
- conscious proprioception
- discriminative touch
- vibration
- pressure
What fibres is the medial lemniscal pathway composed of?
large diameter, fast A-beta fibres
In the dorsal columns, how do the 1st order neurons ascend?
ipsilaterally
What are the steps in the medial lemniscal pathway?
- 1st order neurons ascend dorsal colum
- at gracile and cuneate nuclei, they synapse with 2nd order neurons
- they then decussate and form medial lemniscus
- project to VPL of thalamus
What pathological process occurs when the brain is deprived of information about the position of the feet?
gait ataxia
What can lesions in the cervical cord cause?
upper extremity ataxia
What does paraesthesia usually result from?
ectopic discharge in damaged dorsal column axons
What is the spinothalamic pathway for?
NOCICEPTIVE STIMULI
- mech/chem/therm detection of coarse or non-discriminating touch
- pain
- temperature
What sort of fibres are spinothalamic pathways composed of?
+ smaller diameter
+ A-delta or C fibres
+ no specialised sensory endings
+ myelinated fibres (quick to cross)
What do A-delta fibres detect?
+ non-discriminative touch (strong stimuli that does not damage skin)
+ heat/cold
+ sharp pain
What do C fibres detect?
+ dull, aching pain
+ itch (histamine sensitive)
+ thermal
+ mechanical
What are the steps in the spinothalamic pathway?
- 1st and 2nd order neurons synapse in dorsal horn
- decussate then ascend lateral OR ventral tracts
- synapse with 3rd order neurons at thalamus
- project to sensory cortex
What are the features of the spinothalamic LATERAL route?
+ mix of A-delta and C fibres
+ carries info about:
- pain/heat and cold (A-delta)
- dull pain/itch (C)
What are the features of the spinothalamic VENTRAL/ANT. route?
only C fibres carrying info about coarse/non-discriminating touch via mechanosensitive fibres
What are the 4 types of nociceptor?
+ polymdal
+ mechanical
+ chemical
+ thermal
What are the features of pricking/stabbing pain?
+ fast
+ conducted along A-delta fibres
+ arrives first in CNS (fast conduction velocity)
What are the features of burning/aching pain?
+ slower
+ conducted along C fibres
+ arrives later in CNS (slow conduction velocity)
What is the role of substance P?
+ modulates the peripheral neuronal response to noxious stimuli
+ released from pain neurons when stimulated in fast succession
+ increase sensitivity to noxious stimuli
What is referred pain?
+ signals of noxious and normal cutaneous stimuli enter spinal cord at same point
+ cross talk in dorsal horn between modalities common
+ signals from viscera picked up by ascending nerve fibres mapped cortically to dermis
What is the role of the spinocerebellar pathway?
PROPRIOCEPTIVE INFO
takes info about body’s position and movements to cerebellum for integration (for control of posture and co-ordination of movement)
What are the two principal routes for info in the spinocerebellar pathway?
+ ventral/anterior route
+ dorsal/posterior route
Where is information carried from in the spinocerebellar pathway?
Dorsal route: from muscle spindles (muscle tension)
Ventral route: from golgi organs (proprioception) and touch receptors
How many neurons does each pathway contain along its length (1st and 2nd order)
only 2 neurons
Where are the 2nd order cell bodies located?
dorsal horn of spinal cord, and terminate in vermis of cerebellum
How do dorsal tract neurons ascend?
ipsilaterally
How do dorsal tract neurons enter the cerebellum?
via the inferior peduncle
How do ventral tract neurons ascend?
contralaterally
How do ventral tract neurons enter the cerebellum?
via the superior peduncle
What is Friedreich’s ataxia
Inherited disease where spinocerebellar tract becomes increasingly ineffective
What causes Friedreich’s ataxia
multiple repeats of gene for protein (Frataxin) 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
