Somatosensory Tracts (Part 2)

Question 1

What types of information are relayed by the Anterolateral system?

Answer 1

Signals that can cause tissue damage (nociception, thermal, nondiscriminative touch (not face), itching)

Question 2

What is the pathway of the spinothalamic (direct) fibers?

Answer 2

Spine > thalamus > ventral posterolateral or ventral posterior inferior nuclei

Question 3

What is the pathway of the spinoreticular (indirect) fibers?

Answer 3

Spine > medulla, pons and midbrain (reticular formation)

Question 4

How is damaging stimuli detected?

Answer 4

Cutaneous nociceptors (free nerve endings) > synapse to primary neurons

Question 5

Free nerve endings:
Mechanisms of transmission
Receptor density concept

Answer 5

• Not specialized receptor cells. Sensation signals are transmitted through regular cellular mechanisms likek ligand gated ion channels, glu receptors or GPCR
• There is also a receptor gradient. More free nerve endings in hands/face and less on legs and back

Question 6

From the free nerve endings, where do the axons travel?

Answer 6

Enter the spinal cord via the lateral division of the posterior root and synapse on their targets on the laminae I, II and V of the posterior horn

Question 7

Which tract do the neurons enter after arriving at the spinal cord?
Pathway options for the axons:

Answer 7

Posterolateral fasciculus (Lissauer tract) and the fibers bifurcate and cross over to the contralateral side. 
Descend/ascend or stay on the level, synapse on interneurons for reflexes

Question 8

What is the difference between the direct and indirect pathways?

Answer 8

Passes through reticular formation and synapse to the cingulate, frontal and limbic cortex

Question 9

What type of fibers go through the spinothalamic pathways?

Answer 9

Spinoreticular fibers

Question 10

What type of info does the spinal trigeminal tract convey?

Answer 10

Crude touch, temperature and pain from face

Question 11

What are the divisions of the spinal trigeminal nucleus

Answer 11

Pars caudalis
Pars Oralis
Pars Interpolaris

Question 12

How is the face oriented in the spinal trigeminal tract and nucleus?

Answer 12

Opthalmic axons are located inferiorly while Mandibular axons are located superiorly (inverted face). We use this orientation clinically

Question 13

Pars caudalis

Answer 13

V3 fibers terminate in the obex

Question 14

What does the overlap of the trigeminal fibers at the cervical cord result in?
Caudal lesion: 
Rostral lesion (into brainstem):

Answer 14

Damage to spinal trigeminal tract results in onion-peel sensory loss

• mouth is spared
• sensory loss from back of head and converges on mouth

Question 15

Pars oralis:
Location?
What types of input?

Answer 15

Pons to superior medulla

Tactile info from central face

Question 16

Pars interpolaris:
Location?
What types of input?

Answer 16

Superior medulla to obex

Tactile info from peripheral face.

Question 17

Where do axons from the pars interpolaris go to?

Answer 17

Contralateral VPM of the thalamus > somatosensory cortex (for tactile info)
Inferior cerebellar peduncle > cerebellum

Question 18

What is the trigemino-reticulo-thalamic pathway?

Answer 18

Pain fibers from the trigeminal ganglion project to both sides of the reticular formation as trigeminoreticular fibers > ARAS system is activated (related to arousal & alertness

Question 19

*Vascular lesions on the Anterolateral system

Answer 19

Vascular supply to ALS originates from the arterial vasocorona via sulcal branches of anterior spinal artery
Lesions = patchy loss of pain, thermal, touch on contralateral side of body about 2 segments below lesion

Question 20

*Anterolateral cordotomy

Answer 20

Complete loss of pain, thermal and touch sensations

Question 21

*Spinal cord hemisection (Brown-Sequard): Contralateral

Answer 21

Loss of pain and thermal sensations all over body. Begins aout 2 segments below level of lesion (ALS damage)

Question 22

*Spinal cord hemisection: Ipsilateral

Answer 22

Loss of discriminative tactile, vibratory and proprioception all over body 2 segments below the level of lesion (posterior column damage). Motor loss with paralysis of extremities

Question 23

*Syringomyelia

Answer 23

Cysts in central gray matter that impinge the Anterior white commissure (which contains ALS fibers).
At c4-c5, bilateral loss of nondiscriminative tactile, pain and thermal sensation. Starts several segments below the lesion. Cape like distribution (shoulder to nipple level and arms - looks like a cape)

Question 24

*Herpes zoster:

Answer 24

Viral DNA lays dormant in the nervous system. Virions travel down peripheral process and produce skin irritation in the dermatome. Diminished sense in affected areas. Postherpetic neuralgia

Question 25

*Medullary syndrome:

Answer 25

Vascular lesions or tumors in the brainstem can impinge the ALS fibers and affect discriminative touch and nociception

Question 26

*Medial lesion for medullary syndrome

Answer 26

Contralateral loss of discriminative touch and vibratory sense. Dissociated sensory loss
Pain and thermal sensations are intact

Question 27

*Lateral lesion for medullary syndrome

Answer 27

Wallenberg syndrome. Usually caused by vascular lesion to PICA which supplies the territory of the anterolateral system and the spinal trigeminal nucleus/tract
Contralateral loss of pain and temp with ipsilateral loss of pain and temp over face

Question 28

*Unilateral trigeminal nerve lesion:

Answer 28

Loss of sensation in trigeminal dermatomes
Loss of jaw jerk reflex
Atrophy of muscles of mastication
Loss of ipsilateral and consensual corneal reflex

Question 29

*Alternating analgesia/hemianesthesia

Answer 29

Brainstem lesions destroy primary fibers in spinal trigeminal tract (descending tract) and secondary fibers in spinal lemniscus
Ipsilateral hemianalgesia of face and contralateral hemianalgesia of body

Question 30

*Alternating trigeminal hemiplegia:

Answer 30

Unilateral destruction of trigeminal nerve and CST in pons

Ipsilateral anesthesia, paralysis and contralateral spastic hemiplegia

Question 31

*Damage to trigeminal nerve or central nuclei:

Answer 31

Primary deficits include ipsilateral loss of pain, thermal or tactile sensations of face and scalp, oral cavity and teeth. Ipsilateral paralysis of mastication muscles
Also loss of afferent limb of corneal reflex