Somatosensory Pathways Flashcards
What is a dermatome?
An area of skin that is supplied by a single spinal nerve’s sensory root
What foramen do the spinal nerves use to leave the vertebral column and how does that vary depending on the level you’re at?
C1-C7, exit above the corresponding vertebra
C8 and low come out below the corresponding vertebra
Why does cutting one spinal nerve usually not result in compete sensory loss of its dermatome?
the dermatomes overlap
In general, where are the cell bodies of primary sensory neurons located? Do the fibers stay ipsilateral or go contralateral?
the cell bodies are out in peripheral ganglia and stay ipsilateral
Sensory pathways to the cerebral cortex involve as least how many neurons? Which one crosses the midline?
3 nuerons
the 2nd crosses the midline
Where do second order neurons project to? From there where does the third neuron go
to the thalamus
to the cortex
What forms of sensation are carried in the dorsal column/medial lemniscus pathway?
fine touch
vibration
joint position
What sorts of fibers do you find in the dorsal column/medial lemniscal pathway?
A-beta firbers: large myelinated
Describe the pathway thorugh the dorsal column?
Primary neuron cell body in the DRG,
fibers enter spinal cord through dorsal horn
fibers ascend in the dorsal columns (either fasciculus gracilis or fasciculus cuneatus)
synapse in the caudal medulla in either the gracilis nucleus or cuneatus nucleus
2nd neuron crosses the midline to the medial lemniscus
fibers ascend to the VPL of the thalamus
goes to the postcentral gyrus
While most fibers in in the dorsal column pathway ascend, what are the two places local branches synapse within the dorsla horn?
onto Lamina V (to donate some touch sensation to the spinothalamic tract)
onto interneurons to contribute to motor reflexes
What percentage of fibers actually reach the nuclei?
only about 25% - most are lost as collaterals along the way
At what level of the spinal cord dow the fasciculus cuneatus start to appear?
T6 and above
Describe convergence.
How is it related to receptive fields?
multiple primary neurons synapse onto only one second order neuron cel body in the dorsal column nuclei
this means that second order neurons have much larger receptive fields than primary neurons and the recptive fields overlap extensively
What is the “point of convergence”?
an area where stimulation will activate multiple 2nd order neurons thorugh receptive field overlap
this is the idea of population response
What is the somatotopic organization within the medial lemniscus? In the VPL?
In the ML = lower extremities are ventral and upper extremities are dorsal (man standing on the pyramids)
In the VPL = the lower extremities are lateral and the uppe rextremities are medial (man on his side)
What is the one sense that doesn’t synapse in the thalamus?
olfaction
What information is carried by the spinothalamic tract?
pain
temperature
very crude touch
Why does the spinothalamic tract have some crude touch, but you don’t lose touch if it’s damaged?
It picks up some collaterals from the medial lemniscal tract in lamina V of the dorsal horn
you don’t lose touch because the medial lemniscal will be intact
Describe the pathway of the spinothalamic tract.
- small diameter afferents have their cell bodies int he dorsal horn
- primaries synapse onto the secondary neurons in the substantia gelatinosa of the dorsal horn
- secondary fibers cross the midline thorugh the anterior commissure and ascend int he spinothalamic tract
- they synapse onto the third order neurons in the thalamus (VPL and other places)
- These run through the internal capsule fo the somatosensory cortex
What position is the spinothalamic tract int he cord as it ascends?
ventrolateral funiculus
What’s the wishy washy deal about how the secondary axons cross?
They don’t all cross at the same level they come in, although most do
- some will ascend and cross within one or two segments
What neurotransmitters are released by the primary neuron onto the second order neuron in response to intesntly nxious stimuli? What effec twill this have over time?
glutamate and substance p
over time, this was cause changes in the second order neuron such that receptors are upregulated leading to chronic pain
What attempts to lessen the effect of the substance P and glutamate on the 2nd order neurons?
enkephalins released by interneurons from the central grey
Some colateral branches are given off as the spinothalamic tract ascends. To what two important areas and for what reasons?
- to the central gray of the brainstem so it can modulate the signal from STT neurons
- To the reticular formation to aid in arousal
Most of the fibers in the spinothalamic tract terminate at the VPL nucleus, but some that go through the reticular fiber go where?
to the central lateral cuneus, then onto the limbic emotional centers = this is where the suffering of pain comes in
Lesions of the VPL will result in what?
difficulty localizing pain
True or false: the VPL is somatotopically arrange, but the central lateral nucleus is not.
true
What will a lesion of the central lateral nucleus cause?
a block in the suffering of pain, but not the localization of pain
What is Brown Sequard Syndrome?
Also called hemiplegia
it’s a loss of sensation and motor function due to a lateral hemisection of the spinal cord
What happens in syringomyelia?
A hole (cyst) develops in the center of the cord and expands out and up/down.
can be from a CSF blockage
Ultimately it blocks the crossing STT fibers at the anterior white commissure, resulting in a BILATERAL loss of pain and temperature sensation
