Ascending Sensory Systems

Question 1

cutaneous receptors and their fnctns

Answer 1

1. pacinian corpuscle: vibration
2. meissner corpuscle: discriminative touch
3. ruffini ending: pressure
4. ending around hairs: touch
5. merkel endings: discriminative touch, fine touch, texture
6. free nerve endings: pain, temp, itch, touch

Question 2

general types of fibers in ascending/descending paths

Answer 2

1. long ascending fibers going to thalamus, cerebellum or various BS nuclei
2. long, descening fibers going from cerebral cortex or various BS nuclei to SC grey matter
3. short propriospinal fibers interconnecting different SC levels (help coordinate flexor refelxes)

Question 3

which funiculi are ascending fibers found in?

Answer 3

all three

Question 4

where are propriospinal fibers found?

Answer 4

surrounding the SC grey matter

Question 5

what do somatosensory receptors do?

Answer 5

detect mechanical, chemical, or thermal changes

Question 6

structure of somatosensory receptors

Answer 6

• all are pseudounipolar neurons
• cell body in DRG or CN ganglion
• central CNS process
• peripheral process w/ an ending in skin, m, or joint

Question 7

fasciculus cuneatus

Answer 7

extends to cuneate tubercle, which is site of nucleus cuneatus

Question 8

fasciculus gracilus

Answer 8

extends to the gracile tubercle, which is site of nucleus gracilus

Question 9

caudal medulla

Answer 9

• spinothalamic tract, NG, FG, NC, FC
• structures similar to posterior horn = spinotrigeminal tract (Lissauer’s tract) and spinal nucleus of substantia gelatinosa

Question 10

rostral medulla

Answer 10

• medial lemniscus: fibers from contralateral cuneate and gracile nuclei
• spinotthalamic tract (anterior position)
• NG and NC cross as internal arcuate fibers

Question 11

caudal pons

Answer 11

• medial lemn: more oval and horizontal (info from feet = lat, cervical = medial)
• spinothalamic tract: ant lat position

Question 12

5 ascending pathways

Answer 12

1. posterior column - medial lemniscus pathway
2. spinothalamic tract (anterolateral path)
3. posterior spinocerebellar tract
4. cuneocerebellar tract
5. anterior spinocrebellar tract

Question 13

discriminative touch of med lemn path

Answer 13

• mostly ascending large myelinated primary afferents from various mechanoreceptors
• vibratory sense
• 2 pt touch

Question 14

pathway of medial lemniscus path

Answer 14

synapse in medulla, decussates forming medial lemniscus, goes up to BS, relays in lat thalamus (VPL), terminates in postcentral gyrus

Question 15

where are cell bodies for med lemn path located?

Answer 15

spinal arrerents are in ipsilateral DRGs

Question 16

how do fibers of med lemn path divide once they enter SC?

Answer 16

DRG rootlets enter cord and divide into

1. medial: heavily myelinated, large diameter, enter post column and ascend BS
2. lateral: finely myelinated and unmyelinated, small diameter fibers

Question 17

where is FC located?

Answer 17

above T6 only

Question 18

location of 2nd and 3rd order fibers in medial lemn path

Answer 18

2nd: fibers cross midline in caudal medulla and for med lemn
3rd: originate in thalamus and ascend thru internal capsule and synapse in primary somatosensory cortex in post cent gryus

Question 19

clinical relevance of med lemn path injury

Answer 19

• impaired proprioception and discriminative touch
• tested with vibrating fork or drawing on skin
• sensory info reaches brain via multiple paths so damage to 1 rarely leads to total fnctn loss

Question 20

fnctn of anterolateral path

Answer 20

• pain and temp (one of multiple paths)

- involved in awareness and localization of painful stimulus

Question 21

pathway of anterolateral path

Answer 21

fibers enter cord via lat division of dorsal root, project brs to post horn, primary fibers synapse to secondary fibers in substantia gelatinosa of post horn, 3rd order cell bodies on VPL

Question 22

how do fibers cross midline in anterloateral path?

Answer 22

diagonally and ascend with rostral inclination

Question 23

what special fibers travel in anterolat pathway?

Answer 23

1. spinomesencephalic: imp in pain control mechanisms

2. spinohypothalamic: mediate autonomic response to pain

Question 24

clinical relevance of anterolat path

Answer 24

• damage causes loss of pain and temp (can regenerate), itch and tickle (never regenerate)
• unilat injury does not impact bladder/bowel/sexual sensation bc they ascend bilaterally
• no tactile deficit bc most infor in post column

Question 25

SC info to cerebellum

Answer 25

• info from SC by cerebellum to coordinate movement
• direct: spinocerebellar tracts, 3 are well characterized
• indirect: via BS relay nuclei

Question 26

fnctn of posterior spinocerebellar tract

Answer 26

• convey unconscious proprioceptive ipsilateral leg info

- collaterals from post columns convey tactile, pressure, and proprioceptive info (synapse at Calrke’s nucleus)

Question 27

path of posterior spinocerebellar tract

Answer 27

• axons ascend ipsilateral lat funiculus forming PSCT on cord surface
• enter cerebellum via inf cerebellar peduncle
• pathway never crosses

Question 28

cuneocerebellar tract

Answer 28

• arm proprioception*
• arm afferents ascend in FC to lateral cuneate nucleus in medulla
• axons from lat cuneate nucleus collect and form this tract, enter inf cerebellar peduncle to synapse in vermis and nearby cerebellar hemispheres

Question 29

anterior spinocerebellar tract

Answer 29

• conveys more complex info to cerebellum
• origin: lat surface of ant horn at lumbar levels
• primarily concerned w/ leg bu diff from PSCT (input more complex)
• crosses midling twice, so ultimately ipsilateral