Somatosensory Tracts (Part 2) Flashcards
What types of information are relayed by the Anterolateral system?
Signals that can cause tissue damage (nociception, thermal, nondiscriminative touch (not face), itching)
What is the pathway of the spinothalamic (direct) fibers?
Spine > thalamus > ventral posterolateral or ventral posterior inferior nuclei
What is the pathway of the spinoreticular (indirect) fibers?
Spine > medulla, pons and midbrain (reticular formation)
How is damaging stimuli detected?
Cutaneous nociceptors (free nerve endings) > synapse to primary neurons
Free nerve endings:
Mechanisms of transmission
Receptor density concept
- 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
From the free nerve endings, where do the axons travel?
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
Which tract do the neurons enter after arriving at the spinal cord?
Pathway options for the axons:
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
What is the difference between the direct and indirect pathways?
Passes through reticular formation and synapse to the cingulate, frontal and limbic cortex
What type of fibers go through the spinothalamic pathways?
Spinoreticular fibers
What type of info does the spinal trigeminal tract convey?
Crude touch, temperature and pain from face
What are the divisions of the spinal trigeminal nucleus
Pars caudalis
Pars Oralis
Pars Interpolaris
How is the face oriented in the spinal trigeminal tract and nucleus?
Opthalmic axons are located inferiorly while Mandibular axons are located superiorly (inverted face). We use this orientation clinically
Pars caudalis
V3 fibers terminate in the obex
What does the overlap of the trigeminal fibers at the cervical cord result in? Caudal lesion: Rostral lesion (into brainstem):
Damage to spinal trigeminal tract results in onion-peel sensory loss
- mouth is spared
- sensory loss from back of head and converges on mouth
Pars oralis:
Location?
What types of input?
Pons to superior medulla
Tactile info from central face
Pars interpolaris:
Location?
What types of input?
Superior medulla to obex
Tactile info from peripheral face.
Where do axons from the pars interpolaris go to?
Contralateral VPM of the thalamus > somatosensory cortex (for tactile info)
Inferior cerebellar peduncle > cerebellum
What is the trigemino-reticulo-thalamic pathway?
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
*Vascular lesions on the Anterolateral system
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
*Anterolateral cordotomy
Complete loss of pain, thermal and touch sensations
*Spinal cord hemisection (Brown-Sequard): Contralateral
Loss of pain and thermal sensations all over body. Begins aout 2 segments below level of lesion (ALS damage)
*Spinal cord hemisection: Ipsilateral
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
*Syringomyelia
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)
*Herpes zoster:
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
*Medullary syndrome:
Vascular lesions or tumors in the brainstem can impinge the ALS fibers and affect discriminative touch and nociception
*Medial lesion for medullary syndrome
Contralateral loss of discriminative touch and vibratory sense. Dissociated sensory loss
Pain and thermal sensations are intact
*Lateral lesion for medullary syndrome
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
*Unilateral trigeminal nerve lesion:
Loss of sensation in trigeminal dermatomes
Loss of jaw jerk reflex
Atrophy of muscles of mastication
Loss of ipsilateral and consensual corneal reflex
*Alternating analgesia/hemianesthesia
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
*Alternating trigeminal hemiplegia:
Unilateral destruction of trigeminal nerve and CST in pons
Ipsilateral anesthesia, paralysis and contralateral spastic hemiplegia
*Damage to trigeminal nerve or central nuclei:
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
