Somatosensory Pathways

Question 1

What is a dermatome?

Answer 1

An area of skin that is supplied by a single spinal nerve’s sensory root

Question 2

What foramen do the spinal nerves use to leave the vertebral column and how does that vary depending on the level you’re at?

Answer 2

C1-C7, exit above the corresponding vertebra

C8 and low come out below the corresponding vertebra

Question 3

Why does cutting one spinal nerve usually not result in compete sensory loss of its dermatome?

Answer 3

the dermatomes overlap

Question 4

In general, where are the cell bodies of primary sensory neurons located? Do the fibers stay ipsilateral or go contralateral?

Answer 4

the cell bodies are out in peripheral ganglia and stay ipsilateral

Question 5

Sensory pathways to the cerebral cortex involve as least how many neurons? Which one crosses the midline?

Answer 5

3 nuerons

the 2nd crosses the midline

Question 6

Where do second order neurons project to? From there where does the third neuron go

Answer 6

to the thalamus

to the cortex

Question 7

What forms of sensation are carried in the dorsal column/medial lemniscus pathway?

Answer 7

fine touch

vibration

joint position

Question 8

What sorts of fibers do you find in the dorsal column/medial lemniscal pathway?

Answer 8

A-beta firbers: large myelinated

Question 9

Describe the pathway thorugh the dorsal column?

Answer 9

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

Question 10

While most fibers in in the dorsal column pathway ascend, what are the two places local branches synapse within the dorsla horn?

Answer 10

onto Lamina V (to donate some touch sensation to the spinothalamic tract)

onto interneurons to contribute to motor reflexes

Question 11

What percentage of fibers actually reach the nuclei?

Answer 11

only about 25% - most are lost as collaterals along the way

Question 12

At what level of the spinal cord dow the fasciculus cuneatus start to appear?

Answer 12

T6 and above

Question 13

Describe convergence.

How is it related to receptive fields?

Answer 13

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

Question 14

What is the “point of convergence”?

Answer 14

an area where stimulation will activate multiple 2nd order neurons thorugh receptive field overlap

this is the idea of population response

Question 15

What is the somatotopic organization within the medial lemniscus? In the VPL?

Answer 15

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)

Question 16

What is the one sense that doesn’t synapse in the thalamus?

Answer 16

olfaction

Question 17

What information is carried by the spinothalamic tract?

Answer 17

pain

temperature

very crude touch

Question 18

Why does the spinothalamic tract have some crude touch, but you don’t lose touch if it’s damaged?

Answer 18

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

Question 19

Describe the pathway of the spinothalamic tract.

Answer 19

1. small diameter afferents have their cell bodies int he dorsal horn
2. primaries synapse onto the secondary neurons in the substantia gelatinosa of the dorsal horn
3. secondary fibers cross the midline thorugh the anterior commissure and ascend int he spinothalamic tract
4. they synapse onto the third order neurons in the thalamus (VPL and other places)
5. These run through the internal capsule fo the somatosensory cortex

Question 20

What position is the spinothalamic tract int he cord as it ascends?

Answer 20

ventrolateral funiculus

Question 21

What’s the wishy washy deal about how the secondary axons cross?

Answer 21

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

Question 22

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?

Answer 22

glutamate and substance p

over time, this was cause changes in the second order neuron such that receptors are upregulated leading to chronic pain

Question 23

What attempts to lessen the effect of the substance P and glutamate on the 2nd order neurons?

Answer 23

enkephalins released by interneurons from the central grey

Question 24

Some colateral branches are given off as the spinothalamic tract ascends. To what two important areas and for what reasons?

Answer 24

1. to the central gray of the brainstem so it can modulate the signal from STT neurons
2. To the reticular formation to aid in arousal

Question 25

Most of the fibers in the spinothalamic tract terminate at the VPL nucleus, but some that go through the reticular fiber go where?

Answer 25

to the central lateral cuneus, then onto the limbic emotional centers = this is where the suffering of pain comes in

Question 26

Lesions of the VPL will result in what?

Answer 26

difficulty localizing pain

Question 27

True or false: the VPL is somatotopically arrange, but the central lateral nucleus is not.

Answer 27

true

Question 28

What will a lesion of the central lateral nucleus cause?

Answer 28

a block in the suffering of pain, but not the localization of pain

Question 29

What is Brown Sequard Syndrome?

Answer 29

Also called hemiplegia

it’s a loss of sensation and motor function due to a lateral hemisection of the spinal cord

Question 30

What happens in syringomyelia?

Answer 30

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