Somatosensory I

Question 1

what are the two main pathways in the somatosensory system

Answer 1

posterior column - medial lemniscal pathway

trigeminothalamic pathway

Question 2

which pathway is involved in awareness of body positions and limb movement

Answer 2

posterior column - medial lemniscal pathway (PMCLS)

Question 3

what are the characteristic features afferent fibers of the PCMLS

Answer 3

fast conduction velocity,

few synaptic relays

Question 4

what system allows of to localize touch with accuracy

Answer 4

PCMLS

Question 5

what is two-point discrimination and what system is responsible for if

Answer 5

ability to discriminate between two stimuli simulataneously

PCMLS

Question 6

what aspect of the PCMLS is two-point discrimination related to?

Answer 6

peripheral nerve ending density

Question 7

what regions of the body have increased density of tactile receptors

Answer 7

hands, perioral

Question 8

do small receptive fields have high or low receptor density

Answer 8

high receptor density

Question 9

what are the three parts of primary afferent fibers

Answer 9

peripheral process (soma => periphery)
central process (soma => CNS)
pseudounipolar cell body

Question 10

where is the afferent fiber cell body located

Answer 10

dorsal root ganglia

Question 11

what information do large-diameter afferent fibers relay

Answer 11

discriminative touch
flutter vibration
proprioception

Question 12

where do large-diameter afferent fibers enter the spinal cord

Answer 12

at the medial division of the posterior (AKA dorsal) root

Question 13

what structure do the largest set of afferent fibers form superiorly?

Answer 13

posterior columns (fasciculus gracilis and cuneatus)

Question 14

what two structures make up the posterior columns

Answer 14

fasciculus gracilis

fasciculus cuneatus

Question 15

which part of the posterior column holds fibers from T6-sacrum

Answer 15

fasciculus gracile

Question 16

which part of the posterior column holds fibers from cervical-T6

Answer 16

fasciculus cuneatus

Question 17

how are afferent fibers within the posterior column organized?

Answer 17

more inferior (sacral) fibers are medial
superior fibers are lateral

Question 18

spinal cord lesions result in loss of tactile sensation where?

Answer 18

at and below segmental level of injury

Question 19

what should you suspect if a patient presents with wide stance, stomping and absent DTRs

Answer 19

sensory ataxia, secondary to a SC lesion

Question 20

where are nucleus gracile and nucleus cuneatus found

Answer 20

posterior medulla

Question 21

where do nucleus gracile and nucleus cuneatus receive input from

Answer 21

primary afferents from the ipsilateral dorsal root ganglia via its corresponding fasciculus

Question 22

what sensory information is received at the cores of nucleus gracilis and nucleus cuneatus

Answer 22

inputs from afferent fibers

Question 23

what sensory information is received at the outer shells of nucleus gracilis and nucleus cuneatus

Answer 23

inputs from muscle spindles, joints and Pacinian corpuscles

Question 24

where do second order neurons of the posterior column nuclei send their axons

Answer 24

through the midbrain, to the contralateral thalamus

Question 25

within the medial lemniscus, how are fibers from the extremities organized

Answer 25

UE fibers lie medially | LE fibers lie laterally

Question 26

where does the medial lemniscus terminate

Answer 26

ventral posterolateral nucleus

Question 27

how would a patient with damage at the brainstem present

Answer 27

deficits in discriminative touch, sensation of position and vibration of the CONTRALATERAL side

Question 28

what is the wedge-shaped group of cells in the caudal thalamus

Answer 28

ventral posterior nuclei

Question 29

what separates the ventral posterolateral nucleus (VPL) from the ventral posteromedial nucleus (VPM)

Answer 29

fibers of the arcuate lamina

Question 30

what is the blood supply to the ventral posterior nuclei

Answer 30

thalamogeniculate branches of the PCA

Question 31

how would a patient present after compromise of the thalamogeniculate branches of the PCA

Answer 31

loss of all tactile sensation of contralateral and head

Question 32

what are the inhibitory neurons that influence the firing rates of third-order neurons?

Answer 32

local circuit interneurons

Question 33

a lesion to what structure would cause tactile loss over the contralateral upper body and face

Answer 33

MCA

Question 34

a lesion to what structure would cause tactile loss over the contralateral LE

Answer 34

ACA

Question 35

what are the subdivisions of the somatosensory cortex?

Answer 35

``` Brodmann areas: 3a 3b 1 2 ```

Question 36

from what structure does the secondary somatosensory cortex receive inputs

Answer 36

ipsilateral cortex | ventral posterior inferior nucleus of the thalamus

Question 37

what are the four trigeminal nuclei

Answer 37

main sensory nucleus (msT) trigeminal motor nucleus (mT) spinal nucleus mesencephalic nucleus (mes)

Question 38

what information is carried by the main sensory nucleus

Answer 38

touch and pressure

Question 39

what information is carried by the trigeminal motor nucleus

Answer 39

muscles of mastication

Question 40

what information is carried by the spinal nucleus

Answer 40

pain and temperature

Question 41

what information is carried by the mesencephalic nucleus

Answer 41

proprioceptive afferents from the TMJ and masticatory ms

Question 42

what division of the main sensory nucleus receives afferent input from the oral cavity

Answer 42

dorsomedial division

Question 43

what division of the main sensory nucleus receives afferent input from V1, V2 and V3

Answer 43

ventrolateral division

Question 44

where do secondary fibers of the dorsomedial divison of the main sensory nucleus travel

Answer 44

ipsilaterally via the posterior trigeminothalamic tract

Question 45

where do secondary fibers of the ventrolateral divison of the main sensory nucleus travel

Answer 45

contralaterally via the anterior trigeminothalamic tract

Question 46

what is the target of the anterior and posterior trigeminothalamic tracts

Answer 46

the ventral posteromedial nucleus (VPM)

Question 47

where do third order from the VPM project

Answer 47

through the posterior limb of the internal capsule to the primary somatosensory cortex

Question 48

what are the sxs of a spinal cord lesion

Answer 48

proprioceptive defitics on one side and anestheia on the other

Question 49

what sxs would be seen in a lesion to area 1 of the SI

Answer 49

deficit in texture discrimination

Question 50

what sxs would be seen in a lesion to area 2 of the SI

Answer 50

loss of size and shape discrimination (astereognosis)

Question 51

what sxs would be seen in a lesion to area 3b of the SI

Answer 51

deficits in size, shape and texture discrimination

Question 52

lesion of the left posterior column at L1 produces what impairments?

Answer 52

loss of light touch, vibration, position sensation of the LEFT leg

Question 53

lesion of the left fasciculus cuneatus at C3 produces what impairments

Answer 53

loss of light tough, vibration and position sensation of the left arm and trunk

Question 54

afferents from what spinal levels make up the fasciculus gracilis

Answer 54

T6 and below

Question 55

afferents from what spinal levels make up the fasciculus cuneatus

Answer 55

T6 and above

Question 56

where are the 1st order neurons of the PCMLS

Answer 56

dorsal root ganglia

Question 57

where and what are the 2nd order neurons of the PCMLS

Answer 57

medulla - nuclei gracilis and cuneatus

Question 58

where and what are the 3rd order neurons of the PCMLS

Answer 58

thalamus, ventral posterior lateral (VPL) nucleus

Question 59

where do afferents from the PCMLS go after synapsing in the thalamus

Answer 59

to the primary and secondary somatosensory cortices (via corona radiata)

Question 60

from what vessels does the primary somatosensory cortex receive blood supply

Answer 60

ACA and MCA