10.1 Somatosensory Tracts 1 (Dennis)

Question 1

What type of information is carried by the PCML pathway?

Answer 1

Two point touch

Deep Pressure

Vibration

Propriception

Crude touch

Thermal sensation

Nociception

Question 2

Where are the cell bodies for first order sensory neurons?

What type of nerves are these?

From what layer are these nerves derived?

Answer 2

Dorsal root ganglion.

Pseudounipolar nerves.

Neuroectoderm.

Question 3

Where do the afferent first order fibers of the PCML system enter the spinal cord?

Answer 3

At the medial division of the posterior root.

Question 4

Where is the switch over from fibers coming together to form the fasciculus gracilis to becoming the fasciculus cuneatus?

Answer 4

T6

Question 5

In the fasciculae gracilis and cuneatus, are the fibers from the upper extremity more lateral or more medial to the fibers from the lower extremity?

Answer 5

Upper extremity fibers are more lateral in the PCML system.

Question 6

Where do we find the nuclei gracilis and cuneatus?

Answer 6

In the posterior medulla.

Question 7

What is the specialization of the outer shell and inner core of the nuclei gracilis and cuneatus?

Answer 7

The outer shell of the nuclei gracilis and cuneatus specializes in proprioception.

The inner core specializes in sensation.

Question 8

What do we call the decussation of the PCML secondary fibers immediately after the synapse of the nucleus gracile/cuneatus?

Answer 8

Anterior arcuate fibers.

Question 9

At the level of the pons, are fibers from the nucleus gracilis medial or lateral to the fibers of the nucleus cuneatus?

Answer 9

Fibers from the nucleus gracilis are lateral in the pons.

Question 10

What nucleus in the thalamus does the PCML pathway synapse onto?

Answer 10

The ventral posteriolateral nucleus of the thalamus.

Question 11

Where do the third order neurons of the PCML system travel?

What important structure do they pass through?

What is the name of the pathway?

Answer 11

The fibers from the ventral posterolateral nucleus runs through the thalamocortical pathway, through the posterior limb of the internal capsule, and to the somatosensory cortex.

Question 12

What artery supplies the PCML in the spinal cord?

What artery supplies the PCML in the medulla (and up from there)?

Answer 12

Spinal cord = posterior spinal artery

Medulla and up = anterior spinal artery

Question 13

What artery supplies the ventral posterolateral nucleus?

What would happen if these were occluded?

Answer 13

The thalamogeniculate branches of the posterior cerebral artery.

Complete loss of discriminative touch, pressure sensation, and proprioception to the contralateral body.

Question 14

Where would lesions to the middle cerebral artery cause loss of sensation?

Where would lesions to the anterior cerebral artery cause loss of sensation?

Answer 14

The anterior cerebral artery damage would eradicate sensation from the lower extremity.

Middle cerebral artery damage would eradicate damage from the upper extremity and face.

Question 15

What symptom is associated with damage to the secondary somatosensory area?

Answer 15

Agnosia – the loss of ability to recognize a limb as part of the body.

Question 16

What are the functions of the sensory nuclei of the trigeminal nerve?

Answer 16

Main sensory nucleus (msT)

• Touch and pressure.

Spinal nucleus

• Touch
• Pressure
• Proprioceptin
• Pain
• Temperature

Mesencephalic nucleus (mes)

• Proprioception for chewing.

Question 17

What are the two divisions of the main sensory nucleus of the trigeminal nerve?

Answer 17

The main sensory nucleus has two divisions:

• Dorsomedial division
  
  • Afferents just from the oral cavity.
• Ventrolateral division
  
  • Afferents from V1, V2, and V3

Question 18

What is the purpose of the mesencephalic nucleus?

Answer 18

Primarily relays proprioceptive information related to chewing.

Question 19

What are the three parts of the spinal nucleus of the trigeminal nerve?

Answer 19

Pars caudalis,

Pars oralis,

Pars interpolaris

Question 20

What are the targets of primary afferent nerves?

Answer 20

laminae I, II, and V of posterior horn

Question 21

What is the other name for Lissauer’s tract?

Answer 21

posterolateral fasciculus

where afferents of ALS travel

ascending branches go up to brain

some descending branches terminate on interneurons in SC gray matter for reflexes

Question 22

where are 3rd order neurons of the direct spinothalamic path located?

Answer 22

VPL

Question 23

What are spinoreticular fibers?

Answer 23

branches from ALS that terminate in reticular formation of brainstem

third order neurons then project to medial thalamic nuclei

Question 24

What are spinobulbar fibers?

Answer 24

info from sC that ascends to various brainstem nuclei

Question 25

What are spinomesnecephalic fibers?

Answer 25

from ALS --\> terminate in midbrain reticular formation or transition to form spinotectal fibers --\> superior colliculus and anterior pretectum

Question 26

What is the somatotopic relationship of the ALS?

Answer 26

axons from lower levels = lateral axons from upper extremity = medial

Question 27

What is the blood supply to the ALS?

Answer 27

vasa corona and sulcal branches of the anterior spinal A

Question 28

What would occlusion of the arterial vasocorona and sulcal branches of Ant spinal A cause?

Answer 28

patchy loss of pain and thermal senses over contra side of body

Question 29

What does hemisection of the SC cause?

Answer 29

contralateral loss of pain and thermal sensations about 2 segments below lesion ipsi loss of tactile and vibratory sense

Question 30

Where are medullary ALS fibers?

Answer 30

near anterolateral surface anterior to spinal trigeminal nucleus spearated from PCMLS

Question 31

What artery supplies the ALS fibers in the medulla? What happens if a lesion occurs here?

Answer 31

PICA contra loss of discriminative touch and vibratory sense, but pain/thermal sense is WNL (one modality absent, but not the other)

Question 32

How does the spinal trigeminal nucleus run?

Answer 32

afferents enter pons --\> descend in spinal trigeminal tract, lateral to spinal nucleus --\> becomes continuous w/ Lissauers tract in upper cervical cord both nucleus and tract extend caudally to about 3rd cervical segment

Question 33

What are the parts of the spinal trigeminal tract and nucleus?

Answer 33

pars caudalis: moust caudal, from C2, C3 to obes pars oralis: most rostral, from main sensory nucleus to pontomedullary jxn pars interpolaris: in btw, in rostral medulla

Question 34

How are the spinal trigeminal tract and nucleus oriented in regard to dermatomes on face?

Answer 34

opposite: on face V1 = superior in tract: V1 is inferior, V2 middle, V3 superior

Question 35

How are fibers arranged in the pars caudalis?

Answer 35

oral fibers (most superior on face) = most superior caudal and lateral fibers = most caudal

Question 36

What is the pattern of sensory loss from a spinal trigeminal tract lesion?

Answer 36

onion-peel sensory loss more caudal lesion --\> larger the area surrounding the mouth that is spared from sensory loss more reostral --\> sensory loss starts and back of head and converges on mouth

Question 37

Where do neurons decussate in the Trigeminal tract?

Answer 37

second order axons from spinal trigeminal nucleus --\> decussate --\> ascend in contra anterior trigeminothalamic tract

Question 38

Where do nerves of the trigeminal system go once they are in the anterior trigeminothalamic tract?

Answer 38

(this is the medulla) --\> ascend to trimeminal lemniscus in midbrain --\> synapse is **contra VPM** of thalamus --\> tertiary axons to **posterior limb of internal capsule** --\> SI

Question 39

What are trigeminoreticular fibers?

Answer 39

pain fibers project bilaterally to reticular formation facilitates ascending reticular activating system (ARAS) = arousal and alertness

Question 40

What happens in a unilateral lesion of the trigeminal N?

Answer 40

ipsi anesthesia loss of jaw-jerk reflex atrophy of muscles of mastication loss of ipsi and consensual corneal reflex

Question 41

What is alternating analgesia?

Answer 41

brainstem lesions in upper medulla may destroy primary fibers in spinal trigeminal tract and secondary fibers in spinal lemniscus **ipsi** hemianalgesia of **face** and **contra** hemeanalgesia of the **body**

Question 42

What is alternating trigeminal hemiplegia?

Answer 42

unilateral destruction of trigeminal n and CST in the **pons** **ipsi trigeminal** anesthesia and paralysis **contra** **spastic hemiplegia**

Question 43

What nerves does the corneal reflex go through?

Answer 43

afferent through V --\> trigeminal ganglion --\> motor to VII