Three-Neuron Chain

Question 1

Primary or First Order neuron cell bodies are located where?

Answer 1

Dorsal Root Ganglia from S5 through C1

Question 2

First order neurons are pseudounipolar. What does pseudpunipolar mean?

Answer 2

The neuron contains an axon that has split into two branches; one branch runs to the periphery and the other to the spinal cord

Question 3

Peripheral processes (axons) up to the level of the dorsal root ganglian are what fiber type?

Answer 3

Heavily Myelinated

Type Ia, Ib, II

Question 4

What are the names of the two pathways that ascend up the Dorsal Column?

Answer 4

1. ) Fascisculus Gracilis
2. ) Fasciculus Cuneatus

Question 5

Fascisculus Gracilis is associated with what portion of the Dorsal Column?

Answer 5

Fibers originating from S5-T7

Medial portion

Question 6

Fascisculus Cuneatus is associated with what portion of the Dorsal Column?

Answer 6

Segments T6-C1

Lateral Portion

Question 7

Fascisculus Cuneatus is affiliated with sensations occuring at what area of the body?

Answer 7

Upper Trunk and Upper Limb

Question 8

Fascisculus Gracilis is affiliated with sensations occuring at what area of the body?

Answer 8

Lower trunk and Lower Limbs

Question 9

Central processes (axons) enter the spinal cord how?

Answer 9

Segmentally and ascent in either

1. ) Fasciculus gracilis
2. ) Fasciculus cuneatus

Question 10

Fasciculus gracilis and Fasciculus cuneatus fibers are added sequentially within the dorsal columns in what type of fashion?

Answer 10

Topographical Fashion

—from medial (lower extremities) to lateral (upper extremities)

Question 11

Second order neurons cell bodies are located where?

Answer 11

1. ) Nucleus Gracilis
2. ) Nucleus Cuneatus

Question 12

Where are the nuclei of Nucleus Gracilis and Nucleus Cuneatus located?

Answer 12

The level of the closed or Caudal medulla

Question 13

Where do ascending fibers of the first order neuron synapse in the Dorsal Column?

Answer 13

Nuclei of the Nucleus Gracilis and Nucleus Cuneatus

—which are located at the level of the caudal medulla

Question 14

Axons of secondary fibers form what type of fibers that cross?

Answer 14

Internal Arcuate Fibers

Question 15

Once the internal arcuate fibers cross the midline they form what?

Answer 15

The Contralateral Medial Lemniscus

Question 17

How is the Contralateral Medial Lemniscus organized?

Answer 17

Somatotopically

Question 18

Contralateral Medial Lemniscus projects where?

Answer 18

To the Ventral Posterolateral (VPL) Nucleus of the Thalamus

Question 19

Third order neuron cell bodies are located where?

Answer 19

In the Ventral Posterolateral Nucleus

(VPL)

Question 20

Where do the axons of third order neurons project?

Answer 20

Through the posterior limb of the internal capusle to the Primary Somatosensory Cortex

Question 21

How are the areas of the primary somatosensory cortex numbered and oriented?

Answer 21

3,1,2; postcentral gyrus in the parietal lobe

Question 22

Somatoptic organization is synonomus with what?

Answer 22

Somatosensory homunculus

Question 23

Areas that are most sensitive to the body, ie. index finger/tongue have the greatest represtenation in what cortex?

Answer 23

Primary somatosensory cortex

Question 24

What is the Exception to the normal “three-neuron chain” model when dealing with discrimitive touch and conscious proprioception?

Answer 24

Proprioception of the lower extremity

Question 25

Proprioception of the lower extremity involves two extra nuclei, what are they?

Answer 25

1. ) Nucleus Z (part of Nucleus Gracilis) 2. ) Clarkes Nucleus (spinal cord levels T1-L2)

Question 26

What extra nucleus is part of the rostral end of Nucleus Gracilis?

Answer 26

Nucleus Z

Question 27

What extra nucleus when dealing with proprioception of the lower extremity is affiliated with of spinal cord levels T1-L2?

Answer 27

Clarke's Nucleus

Question 28

The alternative pathway for proprioception (the exception) carries what?

Answer 28

Some of the modality of conscious proprioception for the lower extremities

Question 29

Where is the extra neuron added in reference to the "exception"

Answer 29

Between the Dorsal Root Ganglion and the Nucleus Gracilis

Question 30

**Exception question:** Where do some of the large diameter first-order neuron fibers end?

Answer 30

In the ipsilateral dorsal nucleus of Clarke

Question 31

​Exception question: A second neuron sends it axon out ipsilaterally to ascend where?

Answer 31

In the dorsolateral fasciculus ---adjacent to the Dorsal Spinocerebrellar Tract

Question 32

Where is the Dorsolateral Fasciculus located?

Answer 32

Adjacent to the Dorsal Spinocerebrellar Tract

Question 33

Exception Question: As the second order neuron sends its axon out ipsilaterally in the Dorsolateral fasciculus, where is the next synapse/ where is it headed?

Answer 33

Nucleus Z

Question 34

Exception Question: Axons of nucleus Z join what fibers?

Answer 34

Internal arcuate fibers

Question 35

Exception Question: After Axons of nucleus Z join the internal arcuate fibers they cross contralaterally to ascend as part of what?

Answer 35

**Contralateral Medial Lemniscus** **-**from here the rest of the system is the same

Question 36

What has been purposed for the reason of the "exception" aka alternative pathway in the three-neuron chain mechanism for discrimative touch and proprioception?

Answer 36

To serve as a neural pathway maintining some conscious perception of the LE if the dorsal column system is damaged due to trauma or degenerative illnesses