Tactile And Proprioceptive Pathways

Question 1

Describe active touch

Answer 1

Associated with capacity for learning tactile discrimination
2-pt tactile discrimination, stereognosis (awareness of shape, size, and texture), proprioception or kinesthesia (dynamic position sense), vibratory sensations, and weight perception
Conveyed by posterior column/medial lemniscal system

Question 2

Describe parietal lobe and holographic sensory body

Answer 2

In the parietal lobe, the holographic sensory body is assembled, discriminated, and interpreted relative to current and predicted motor patterns, spatial orientation, and memory
Accurate initiation of a motor movement pattern in the cortical-basal ganglia circuits is dependent upon precise, 3-dimensional representation of orientation of body and limbs to itself as well as surrounding objects
Lesions of parietal lobe may disturb recognition of our own body parts (denial of body scheme), our ability to spatially discriminate objects (agnosia or cortical astereognosis), or our ability to perform certain complex learned motor activities (apraxia)

Question 3

Describe fasciculus triangularis, septomarginalis, and interfascicularis

Answer 3

Fasciculus triangularis: descending fibers of primary neurons that originate from sacral levels and participate in intersegmental reflex arcs in spinal cord
Fasciculus septomarginalis: originate from lumbar levels
Fasciculus interfascicularis: originate from cervical levels

Question 4

What does the ventral spinothalamic pathway convey?

Answer 4

Light (passive) touch, crude tactile sensations, and pressure

Question 5

Describe the primary axons of the ventral spinothalamic pathway

Answer 5

Primary axons ascend 6-10 cord segments in the posterior funiculus while sending numerous collateral terminals to secondary neurons in the ipsilateral nucleus proprius and intermediate gray

Question 6

Describe the secondary axons for the ventral spinothalamic pathway

Answer 6

Cross in anterior white commissure and form ascending tract (ventral spinothalamic tract VSTT)
In medulla VSTT joins LSTT and spinotectal fibers to form spinal lemniscus, which terminates on tertiary neurons in ventral posterior VPL nucleus of dorsal thalamus

Question 7

Describe the tertiary axons from VPL in ventral spinothalamic pathway

Answer 7

Project through posterior limb of internal capsule and corona radiata to primary somesthetic cortex (postcentral gyrus)

Question 8

Describe the crude tactile pathway

Answer 8

Modality: light (passive) touch, crude tactile, pressure
Receptors: free nerve endings, peritrichial nerve endings, Merkel’s tactile disks
Primary neuron: central process bifurcates and ascends 6-10 segments
Secondary neuron: in nucleus proprius. Axon decussates in anterior white commissure and ascends as the anterior spinothalamic tract (ASTT). ASTT combines with LSTT and spinotectal tract to form spinal lemniscus (SL)
Tertiary neuron: in VPL nucleus of thalamus. Projects an axon to primary somesthetic cortex

Question 9

Describe unilateral lesions of the VSTT

Answer 9

May be difficult to lose crude touch sensations because fibers ascend in both posterior (primary fibers) and anterolateral funiculi (secondary fibers).
This separation provides the system with a degree of bilaterality
Deficits associated with discrete lesions of the VSTT may be masked if the posterior column/medial lemniscal system is intact

Question 10

Describe unilateral lesion of spinal lemniscus

Answer 10

Results in contralateral hemianalgesia and thermal hemianesthesia
However, loss of passive touch may be masked by intact posterior column/medial lemniscal system

Question 11

What does the posterior column/medial lemniscal system convey?

Answer 11

It is a highly convergent, discriminatory pathway which conveys various modalities of active touch

Question 12

Describe the primary neuron of the posterior column/medial lemniscal system

Answer 12

Pseudounipolar neurons whose cell bodies are in the dorsal root (spinal) ganglia
Fastest and longest neurons in sensory system
Central processes course through medial division of dorsal root, enter posterior funiculus of spinal cord, and bifurcate into ascending and descending fibers

Question 13

Describe the primary axons in the PC/ML system

Answer 13

If the information is from the lower extremity, the fibers will form and ascend in fasciculus gracilis
If the information is form upper extremity, the fibers will form and ascend in the fasciculus cuneatus
Posterior column is composed of these two fasciculi and occupy posterior funiculus of spinal cord

In the medulla, primary axons of each of these fasciculi terminate in corresponding nucleus gracilis and nucleus cuneatus

Question 14

Describe fasciculus gracilis

Answer 14

Conveys proprioception, 2-point tactile discrimination and vibratory sensations from lower extremities and body (below dermatome T6)
Comprised of long ascending primary axons which are located between posteromedian and posterointermediate septa
It terminates in the medulla in the nucleus gracilis

Question 15

Describe fasciculus cuneatus

Answer 15

Conveys proprioception, 2-pt tactile discrimination, and vibratory sensations from upper extremities and body (down to T7)
Comprised of long ascending primary axons which are located between posterointermediate and posterolateral septa
In medulla, terminates in nucleus cuneatus
Fasciculus cuneatus is not present in spinal cord below the level of the sixth thoracic cord segment

Question 16

Describe secondary neurons in PC/ML pathway

Answer 16

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

Question 17

Describe the medial lemniscus (ML)

Answer 17

Maintains somatotopic organization of its fibers throughout the brainstem
As the ML ascends, position of the fibers from the upper and lower extremities rotates 90 degrees in relation to each other
In the medulla, the lower extremity fibers are anterior, and the upper extremity fibers are posterior in the ML.
In the pons, the LE are lateral, and UE are medial in ML.
In midbrain, LE are posterolateral, and UE are anteromedial.
The ML terminates upon tertiary neurons in ventral posterior lateral (VPL) nucleus of thalamus

Question 18

Describe tertiary neurons in PC/ML pathway

Answer 18

From VPL nucleus, project to primary somesthetic cortex (postcentral gyrus) via internal capsule and corona radiata

Question 19

Describe primary somesthetic cortex

Answer 19

Postcentral gyrus is somatotopically organized with head region adjacent to lateral fissure, knee region at top of gyrus, and leg and foot regions on paracentral lobule

Question 20

Describe dorsal spinocerebellar tract

Answer 20

Precise coordination, postural adjustments, and precise movements of individual muscles of lower limbs and body
Primary central processes from C8-L3 spinal nerves terminate on cells in nucleus dorsalis of thoracic spinal cord
Secondary axons ascend in ipsilateral lateral funiculus of spinal cord as teh dorsal spinocerebellar tract (DSCT)
DSCT ascends to medulla, courses through inferior cerebellar peduncle, and terminates in cerebellar cortex of vermis
Proprioception and exteroceptive info from L4-S5 spinal nerves must ascend in fasciculus gracilis before terminating in nucleus dorsalis
Nucleus dorsalis conveys information from these levels to cerebellar vermis via ipsilateral DSCT

Question 21

Describe ventral spinocerebellar tract (VSCT)

Answer 21

Gross postural adjustments and overall proprioception of lower limb
Primary axons from lumbosacral levels of spinal cord
Terminate on scattered cells in base of dorsal horn and intermediate gray
Secondary axons decussate in anterior commissure and ascend in contralateral ventral spinocerebellar tract
Courses though superior cerebellar peduncle and terminates in cerebellar cortex of vermis

Question 22

Describe cuneocerebellar tract

Answer 22

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

Question 23

Describe rostral spinocerebellar tract

Answer 23

Presumed tract conveys gross postural and limb proprioception from upper limbs
Primary axons terminate on scattered cells located in base of dorsal horn and intermediate gray areas of cervical levels of spinal cord
Secondary axons probably course in ipsilateral VSCT and enter cerebellum via both inferior and superior cerebellar peduncles
These fibers terminate in cerebellar cortex of vermis

Question 24

Summarize which unconscious cerebellar tracts do precise proprioception of individual muscles vs gross proprioception of whole limb for lower vs upper extremities

Answer 24

Lower:
Precise: dorsal spinocerebellar
Gross: ventral spinocerebellar

Upper:
Precise: cuneocerebellar
Gross: Rostral spinocerebellar

Question 25

What does a complete unilateral lesion of the posterior columns result in?

Answer 25

Ipsilateral loss of proprioception, two-point tactile discrimination and vibratory sensations below to level of lesion

Question 26

What does a lesion of fasciculus gracilis result in?

Answer 26

Ipsilateral loss of proprioception, 2-pt tactile discrimination and vibratory sensations from lower half of body and lower extremity
Partial lesions result in sensory dermatomal deficit corresponding to affected region of fasciculus gracilis

Question 27

What does a lesion of fasciculus cuneatus result in?

Answer 27

Ipsilateral loss of proprioecption, 2-pt tactile discrimination and vibratory sensations from upper half of body and upper extremity
Partial lesions result in sensory dermatomal deficit corresponding to affected region of fasciulus cuneatus

Question 28

What does a unilateral lesion of the medial lemniscus result in?

Answer 28

Loss of proprioception, 2-pt tactile discrimination and vibratory sensations on opposite side of body and limbs

Question 29

What is tabes dorsalis?

Answer 29

Meningovascular inflammation of blood vessels as they pierce through pia at juntion of dorsal rootlets and posterior columns
Lumbosacral nerves and spinal cord segments are most frequently affected
This situation occurs during tertiary (late) stage of syphilis and results in bilateral ischemic necrosis of posterior columns and dorsal roots at this level

Question 30

What are the signs and symptoms of tabes dorsalis?

Answer 30

“Lightning pains” or “rheumatic pains” from lower limbs for last several years. Paroxysmal lancinating pains in lower limbs of long duration is common and is probably due to irritation of epicritic (type A) pain fibers in dorsal roots. Other parethesias may be present
Involvement of dorsal roots in sacral region results in atonic bladder and painless retention of urine
Locomotor ataxia: patients walk with a broad-based gait with pronounced “slapping” of feet. They are dependent upon visual cues for foot placement. Loss of proprioception information from lower extremities, both for reflexes and ascending conscious and unconscious pathways, causes the gait. Vibratory sensations are often more resistant to degenerative changes which is why patient stomps feet while walking in order to obtain tactile cues
Patients demonstrate positive Romberg test which suggests involvement of posterior columns and/or cerebellum. Standing with feet together and his arms extended in front of him, patient will fall when he closes his eyes
Argyl-Robertson pupils are also present. Pupils unreactive to light but constrict during accommodation
Patient often has swollen, distorted joints (Charcot joints) due to degeneration of joints secondary to loss of joint sensations

Question 31

Describe passive touch

Answer 31

Comprises such sensory modalities as roughness, texture, form, and localization
Passive touch is conveyed by ventral spinothalamic pathway
Clinically, passive touch tends to persist even after lesions of posterior columns