Proprioceptive and Tactile Pathways

Question 1

ventral spinothalamic pathway

Answer 1

• conveys light (passive) touch, crude tactile sensations and pressure
• Rs for this system are free nerve endings, peritrichial N endings, Merkel’s tactile disks
• Rs are innervated by the peripheral processes of pseudo unipolar neurons whose cell bodies are in the dorsal root ganglia–primary neurons

Question 2

primary neurons and axons of the ventral spinothalamic pathway

Answer 2

• central processes of the primary neurons course thru the medial division of the dorsal root–>enter the posterior funiculus of the SC–>bifurcate in the ascending and descending fibers
• primary axons (short ascending fibers) ascend 6-10 cord segments in the posterior funiculus while sending numerous collateral terminals to secondary neurons in the ipsilateral nucleus proprius
  
  • these collateral participate in the intersegmental spinal reflexes

Question 3

secondary axons of the ventral spinothalamic pathway

Answer 3

• in the nucleus proprius
• cross in the anterior white commissure and form an ascending tract called the ventral spinothalamic tract (VSTT)
  
  • in the medulla, the VSTT joins with the LSTT and spinotectal fibers to form the spinal lemniscus which terminates on the tertiary neurons in the ventral posterior lateral (VPL) nucleus of the dorsal thalamus

Question 4

tertiary axons of the ventral spinothalamic pathway

Answer 4

-project they the posterior limb of the internal capsule and corona radiate to the primary somesthetic cortex in the post central gyrus

Question 5

steps of the ventral spinothalamic pathway

Answer 5

1. primary neurons: central process bifurcates and ascends 6-10 segments in the posterior funiculus and send terminals to secondary neurons in the nucleus proprius
2. secondary neurons: in the nucleus proprius
   
   • axon decussates in the anterior white commissure and form the ventral spinothalamic tract and join with the LSTT to form the spinal lemniscus
3. spinal lemniscus terminates on tertiary neurons in the ventral posterior lateral (VPL) nucleus in the dorsal thalamus
4. tertiary neurons in the dorsal thalamus project axons to the posterior limb of the internal capsule

Question 6

unilateral lesions of the ventral spinothalamic tract (VSTT)

Answer 6

• may be difficult to lose crude touch sensations, b/c fibers ascend in both the posterior (primary fibers) and anterolateral funiculi (secondary fibers)
  
  • this separation of info provides the system with a degree of bilaterally
• deficits assoc with discrete lesions of the VSTT may be masked if the posterior column/medial lemniscal system is intact

Question 7

unilateral lesion of the spinal lemniscus

Answer 7

• results in contralateral hemianalgesia and thermal hemianesthesia
• loss of passive touch may be masked by the intact posterior column/medial lemniscal system

Question 8

parts of the posterior columns and proprioception

Answer 8

• short ascending fibers–crude tactile

- long ascending fibers–precise tactile

Question 9

posterior column/medial lemniscal system

Answer 9

• precise tactile discrimination
• highly convergent discriminatory pathway which conveys the various modalities of light touch
  
  • modalities are segregated into distinct areas throughout the entire pathway
• Rs include: free naked N endings, Pacinian corpuscles, Meissner’s corpuscles, peritrichial N endings, Neuromuscular spindles

Question 10

primary neurons of the posterior column/medial lemniscal system

Answer 10

• Rs are innervated by the peripheral processes of pseudo unipolar neurons whose cell bodies are in the the dorsal root ganglia
• neurons that comprise this pathway are the fastest and longest neurons in the sensory system
• central processes course thru the medial division of the dorsal root, enter the posterior funiculus of the SC, and bifurcate into ascending and descending fibers

Question 11

primary axons in the posterior column/medial lemniscal system

Answer 11

• once the central processes of the primary neurons enter the posterior column of the SC, they segregate to the appropriate fasciculus
  
  • lower extremity goes to the fasciculus gracilis
  • upper extremity goes to the fasciculus cuneatus
• then the primary axons from the fasciculi terminate in the corresponding nucleus gracilis and nucleus cuneatus
  
  • these send collaterals to interneurons located in the base of the dorsal horn and intermediate gray of SC and these participate in the intersegmental reflexes

Question 12

fasciculus gracilis

Answer 12

• conveys proprioception, 2 point tactile discrimination and vibratory sensations from the lower extremities and body (below dermatome of T6)
• comprised of long ascending primary axons from between the posteromedian and posterointermediate septa
• terminates in the nucleus gracilis (clavum) in the medulla

Question 13

fasciculus cuneatus

Answer 13

• conveys proprioception, 2 pt tactile discrimination and vibratory sensations from the upper extremities and body (down to T7)
• comprised of long ascending primary axons which are located in the posterointermediate and posterolateral septa
• terminates in the nucleus cuneatus of the medulla
• note present in the SC below the level of T6

Question 14

secondary neurons in the posterior column/medial lemniscal system

Answer 14

-in the medulla, secondary axons from the nucleus gracilis and cuneatus decussate as internal arcuate fibers and form the contralateral medial lemniscus (ML)

Question 15

medial lemniscus

Answer 15

• contains the somatotropin organization of its fibers throughout the brainstem
• as the ML ascends, the position of the fibers from the upper and lower extremities rotates 90 deg in relation to each other
• terminates upon tertiary neurons in the ventral posterior lateral (VPL) nucleus of the thalamus

Question 16

tertiary neurons in the posterior column/medial lemniscal system

Answer 16

-tertiary axons from the VPL nucleus project to the primary somesthetic cortex via the internal capsule and the corona radiata

Question 17

primary somesthetic cortex

Answer 17

-post central gyrus is somatotropically organized with head region adjacent to the lateral fissure, knee region at the top of the gyrus, and leg/foot regions on the paracentral lobule

Question 18

posterior column/medial lemniscal system steps

Answer 18

1. primary neurons: peripheral processes of pseudo unipolar neurons with cell bodies in the dorsal root ganglia
   
   • enter the posterior funiculus
   • bifurcate into ascending and descending fibers
   • enter the posterior column of the SC and go to the fasciculus cuneatus/gracilis
   • then terminate in the corresponding nucleus gracilis/cuneatus
2. secondary neurons: from the nuclei cuneatus/gracilis decussate as internal arcuate fibers and form the contralateral medial lemniscus
3. lemniscus: medial lemniscus terminates in ventral posterior lateral nucleus of the thalamus
4. tertiary neurons: from the ventral posterior lateral nucleus of the thalamus to the primary somesthetic cortex

Question 19

dorsal spinocerebellar tract

Answer 19

• conveys precise coordination, postural adjustments, and precise movements of the individual Ms of the lower limbs and body
• primary central processes from the C8-L3 spinal Ns terminate on cells in the nucleus dorsalis of the thoracic SC
  
  • secondary axons ascend in the ipsilateral funiculus of the SC as the dorsal spinocerebellar tract (DSCT)–ascends to the medulla and courses thru the inferior cerebellar peduncle, and terminates in the cerebellar cortex of the vermis
• proprioception and exteroceptive info from L4-S5 spinal Ns must ascend in the fasciculus gracilis before terminating in the nucleus dorsalis
  
  • nuculeus dorsalis conveys info from these levels to the cerebellar vermis via the ipsilateral DSCT

Question 20

ventral spinocerebellar tract

Answer 20

• conveys gross postural adjustments, and overall proprioception of the lower limb
• primary axons from the lumbosacral levels of the SC
  
  • terminate on the scattered cells in the base of the dorsal horn and intermediate gray
• secondary axons decussate in the anterior commissure and ascend in the contralateral ventral spinocerebellar tract
  
  • courses thru the superior cerebellar peduncle and terminates in the cerebellar cortex of the vermis

Question 21

cuneocerebellar tract (direct arcuate fibers)

Answer 21

• direct arcuate fibers convey fine coordination, postural adjustments, and precise movements of individual muscles of the upper limbs and body
• primary axons from the C1-7 levels ascend in the lateral aspect of the posterior column and terminate in the accessory cuneate nucleus
  
  • cuneocerebellar fibers (direct arcuate fibers) project from the accessory cuneate nucleus directly to the cerebellar cortex of the vermis

Question 22

precise proprioception of individual Ms

Answer 22

• lower extremity: dorsal spinocerebellar tract

- upper extremity: cuneocerebellar tract

Question 23

gross proprioception of whole limb

Answer 23

• lower extremity: ventral spinocerebellar tract

- upper extremity: rostral spinocerebellar tract

Question 24

unilateral lesions of the posterior columns

Answer 24

-complete unilateral lesions of the posterior columns result in an ipsilateral loss of proprioception, 2 pt tactile discrimination and vibratory sensations below to the level of the lesion

Question 25

lesion of the fasciculus gracilis

Answer 25

• results in ipsilateral loss of proprioception, 2 pt tactile discrimination, and vibratory sensations from the lower half of the body and the lower extremity
• partial lesions result in a sensory dermatomal deficit corresponding to the affected region of the fasciculus gracilis

Question 26

lesion of the fasciculus cuneatus

Answer 26

• results in ipsilateral loss of proprioception, 2 pt tactile discrimination and vibratory sensations from the upper hand of the body and upper extremity
• partial lesions result in sensory dermatomal deficit corresponding to the affected region of the fasciculus cuneatus

Question 27

unilateral lesions of the medial lemniscus

Answer 27

-result in a loss of proprioception, 2 pt tactile discrimination, and vibratory sensations on the opposite side of the body and limbs

Question 28

tabes dorsalis

Answer 28

• meningovascular inflammation of the blood vessels as they pierce thru the pia at the junction of the dorsal rootlets and posterior columns
• lumbosacral Ns and SC segments are most affected
• occurs during tertiary stage of syphilis and results in bilateral ischemic necrosis of posterior columns and dorsal roots at this level
• signs and symptoms:
  
  • pts have lightning pains or rheumatic pains from the lower limbs for several years–paroxysmal lancinating pains in the lower limbs of long duration
    
    • due to irritation of epicritic pain fibers in dorsal roots
  • involvement of dorsal roots in sacral region results in atonic bladder and painless retention of urine
  • locomotor ataxia–pts walk with a broad based gait slapping feet due to loss of proprioception info from the lower extremities, the slapping of the feet give vibratory cues to help pt walk
  • positive Romberg test–suggests involvement of the posterior columns and cerebellum (so pt falls when closes eyes)
  • Argyll Robertson pupils–pupils unreactive but accommodating
  • pt has swollen, distorted joints (Charcot joints) due to degeneration of the joints secondary to loss of joint sensations