9. Somatosensory Pathways Pt. 1 Flashcards
what are the two main pathways talked about in class part of the somatosensory system?
posterior column-medial lemniscal pathway
trigeminothalalmic pathway
(others= spinocerebellar and anterolateral)
What does the somatosensory system do?
transmits and analyzes touch or tactile info from external and internal locations on body and head
-discriminative touch, flutter-vibration, proprioception, crude touch, thermal sensation, nociception
What types of info does the posterior column-medial lemniscal system carry?
perception and appreciation of mechanical stimuli (size, shape, and texture discrimination; recognition of 3D shape and motion detection; conscious awareness of body position and limb movement in space)
basis of accurate localization of touch on diff. parts of body, with high fidelity and high degree of spatial/temporal resolution
What are the characteristic features of the edial lemniscal system? what kind of organization?
afferent fibers with fast conduction velocities and limited number of synaptic relays
PRECISE SOMATOTOPIC organization
What is two point discrimination?
ability to discriminate bw two stimuli simultaneously
varies widely over diff. parts of body and is related to density of peripheral nerve endings
What is a receptor density gradient?
exists bw various body parts and is based on the varying degree of innervation present
digits and perioral regions have increased density of tactile receptors
other regions like back have lower density
What is a receptive field? whats the difference bw small and large
area of skin innervated by somatic afferent fibers
small receptive fields- have HIGH receptor density
large fields have low density
What do primary afferent fibers consist of?
- peripheral process: extends from DRG (mechanoreceptor or free nerve ending)
- Central process: extends from DRG –> CNS
- Pseudounipolar cell body: in DRG
What makes up dermatomes segmental pattern?
peripheral distribution of afferent nerves arising from each spinal level
associated with fibers/pathways that convey pain/thermal information
where do the primary afferent fibers enter in spinal cord?
enter the posterior horn
enter spinal cord via medial division of posterior root and then branch (terminate in second order neurons at, above, or below)
Where does majority of pathway go for the primary afferent fibers of the PC-MLS? what is it called collectively?
largest set of branches ascends cranially and forms fasciculus gracilis or fasciculus cuneatus
collectively= posterior columns
How are fibers within the posterior columns organized?
topographically
most medial= sacral and coccygeal levels–> lumbar/thoracic levels below T6–> thoracic levels above T6–>low cervical levels–> upper cervical levels= most lateral
What forms the fasciculus gracilis?
sacral level fibers are positioned medially and fibers from more rostral levels up to T6 are added laterally; combined= FG
What forms the fasciculus cuneatus?
thoracic fibers above T6 and cervical fibers are lateral; together= FC
What happens if damage tracts in posterior column? SC lesion result?
location of damage will tell you if entire column damage, or if tumor impinging in one area growing from lateral–> medial or opposite, etc.
SC lesions result in Ipsilateral reduction or loss of discriminative, positional, and vibratory tactile sensation AT and BELOW segmental level of injury
If you damage the fasciculus cuneatus or higher levels what happens?
more damage bc lower levels are damaged
What is sensory ataxia?
loss of muscle stretch (tendon) reflexes, and proprioceptive losses from extemities due to lack of sensory input; may have wide stance, forceful
not getting back feedback from proprioceptive (not always a motor problem)
What is the pathway through the posterior column nuclei?
- fiber tracts extend rostrally
- when hit medulla, target second order neurons of the PCMLS which are located in the gracile and cuneate nuclei (in the caudal and posterior medulla)
- get contact of primary afferents to their second order neurons which will extend a little anteriorly and medially as internal arcuate fibers, where will cross midline (decussated)
- Extend north/ ascend again in structure= medial lamniscus (on opposite side)
How is pattern of posterior column nuclei maintained even further? What kind of information?
outer shells receive inputs from muscle spindles, joints, and pacinian corpuscles= body placement
core “clusters” receive inputs from rapidly- and slow adapting afferents = tactile info
What happens after medial lemniscus is reached?
It is initially medial and oriented from superior to inferior, and is relayed somatotopically in this order
however, as ML extends rostrally it rotates laterally at level of pons
Now: UE fibers lie medially and LE fibers laterally
What does the ML terminate in?
ventral posterolateral nucleus (VPL) of the thalamus
What is the blood supply at primary afferents and primary neurons? how about at medulla?
primary afferents and primary neurons fed by POSTERIOR SPINAL A.
at medulla: ANTERIOR SPINAL A.
What happens in a midbrain PC lesion?
damage at brainstem levels lead to deficits in discriminative touch, vibratory, and positional sensibilities over contralateral side of body (above decussation ?)
What does a right side midbrain lesion produce?
left-sided loss of proprioception and discriminative touch but NO loss of any other modality (bc damage specific to ML in the midbrain)
