10. Somatosensory Pathways 1

Question 1

The somatosensory pathway transmits and analyzes touch or tactile info from external locations, such as discriminative touch, vibratiion, proprioception. Transmitted via what two pathways?

Answer 1

Posterior column/medial lemniscal

trigeminothalamic pathway

Question 2

The posterior column - medial lemniscal system (PCMLS) is involed in what ?

Answer 2

perception and appreciation of stimuli (tactile/proprioception)

Question 3

PCMLS has somatotopic organization (homunculus) and allows two-point discrimination, which is?

Answer 3

ability to discriminate between two stimuli simultaneously

Question 4

Peripheral receptor density gradient exists between various parts of the body. Which areas would have more or less?

Answer 4

Digits and perioral have increase density of tactile recep
Back and (legs) have decreased density of receptors

Question 5

A receptive field is area of skin innervated by somatic afferent fibers, a small field has what?

Answer 5

high receptor density as compared to a large field with low density (back)

Question 6

Primary afferent fibers consist of pseudounipolar cell body in the posterior/dorsal root ganglion (DRG) and has what processes?

Answer 6

Peripheral process to free nerve ending
Central process from DRG to CNS
dermatomes formed due to this

Question 7

PCMLS specific afferent fibers enter the spinal cord via what?

Answer 7

medial division of poterior root and then branch

Question 8

The largest set of branches will ascend to form the what? which collective are called posterior columns?

Answer 8

Fasciculus gracilis (LE/Lower Thoracics) or fasciculus cuneatus (UE/ Upper thoracics)

Question 9

Fibers from medial to lateral within the fasciculus gracilis have sacral, then lumbar through T6. What about fasciculus cuneatus?

Answer 9

added laterally as they join the LE, thoracic levels above T6, lower cervical, upper cervical, which is the most lateral in the fasciculus cuneatus

Question 10

A SC lesion in the fasciculus cuneatus or gracilis would result in?

Answer 10

ipsilateral loss of proprioception/touch to that level and below

Question 11

What is it called when there is a loss of muscle stretch reflexes and proprioceptive loss from extremeties due to lack of sensory input?

Answer 11

Sensory Ataxia

Question 12

The second order neurons of the PCMLS, which receive input from first order neurins, can be found in what, where?

Answer 12

in the nucleus gracile and nucleus cuneatus in the posterior medulla

Question 13

From the second or neurons of the PCMLS, it sends axons to the contralateral thalamus via?

Answer 13

internal arcuate fibers

Question 14

The internal arcuate fibers of the PCMLS cross the midline as the sensory decussation and ascend as what on the opposite side?

Answer 14

Medial Lemniscus ML

Question 15

As the medial lemniscus moves rostrally, is rotates laterally in the pons, so the LE/UE fibers are located where inside?

Answer 15

UE are medial and LE are lateral

Question 16

The medial lemniscus with the UE fibers medial and LE fibers lateral moves to the thalamus where is synapses where?

Answer 16

Ventral Posterolateral Nucleus (VPL)

Question 17

What is the main blood supply to the cuneatus and gracilis fasciculus in the SC?

Answer 17

posterior spinal A

Question 18

What is the main blood supply to the medial lemniscus and pyramids of the medulla?

Answer 18

anterior spinal A

Question 19

Damage of the BS in the midbrain of the medial lemniscus would lead to what? Why?

Answer 19

CONTRALATERAL deficits of touch and proprioception because the fibers of the PCMLS crossed in the medulla via internal arcuate fibers to ascend as the ML

Question 20

The ventral posterior nuclei are located in the caudal thalamus and are comprised of what?

Answer 20

ventral posterolateral and posteromedial nucleus (VPL/VPM) which are separated by arcuate lamina

Question 21

What is the blood supply to the VPL and VPM, which if compromised can result in loss of touch and sensation over contralateral body/head?

Answer 21

thalamogeniculate branches of posterior cerebral A

Question 22

The medial lemniscus terminates in the VPL with fibers from cuneatus and gracile nuclei terminating where?

Answer 22

cuneatus medially and gracile laterally within the VPL

Question 23

The VPL for the trunk and extremities contain two populations of neurons?

Answer 23

1. third order neurons **majority

2. local circuit interneurons (inhibitory to stop firing rate of 3rd order neurons)

Question 24

From the VPL, most of the axons terminate in the third order neurons there which send axons that traverse what to where?

Answer 24

traverse the posterior limb of the internal capsule and terminate in primary/secondary somatosensory cortex

Question 25

From the posterior limb of the internal capsule, the third order neurons from the thalamus terminate in the somatosensory cortex, which is comprised of what?

Answer 25

postcentral and posterior paracentral gyri

Question 26

LE is medial, UE is medial, and face is lateral in the parital part of the brain. Which have the largest amt of receptors?

Answer 26

hands, feet, face

Question 27

If the ACA or MCA blood supply was cut to the left side of the brain, primarily at the somatosensory cortex, what would be seen?

Answer 27

the RIGHT side (cause fibers crossed) would have no touch sense or proprioception (aca is LE, mca is upper body and face)

Question 28

What brodmann areas make up the primary somatosensory cortex from posterior to anterior? (4)

Answer 28

3a, 3b, 1, 2

Question 29

The secondary somatosensory cortex lies deep in the lateral sulcus, and gets input from what?

Answer 29

ventral posterior inferior nucleus of thalamus

Question 30

parietal cortical regions receive tactile inputs including areas 5 and 7 posterior to bordmann 2 area. Lesions here lead to agnosia which is?

Answer 30

limb is not recognized as part of the patients body

Question 31

Trigeminal carries main sensory for head and motor for mastication M. Afferents extend from cell bodies to trigeminal (semilunar) ganlgion which then does what?

Answer 31

forms continous cell column that extends from spinomedullary junction to rostral levesl of midbrain

Question 32

What are the two main sensory nucleus of the trigeminal inputs that relay info on touch and proprioceptions?

Answer 32

``` Main/cheif/princple sensory nucleus (msT) : touch and pressure Mesencephalic nucleus (mes): proprioceptive from TMJ/mastication M ```

Question 33

The main sensory nucleus receives info on tacile and proprioceptive sensations. there is a dorsomedial and ventrolateral division which receive afferents from?

Answer 33

dorsomedial: oral cavity Ventrolateral: V1,V2,V3

Question 34

Afferents from trigmenal N synapse in trigeminal ganglion and send axons to the main sensory nucleus, ventro is V1-3 and dorsomedial is from oral cavity. What happens next?

Answer 34

2nd order fibers from dorsomedial division travel IPSILATERALLY via posterior trigeminothalamic tract 2nd order fibers from ventrolateral division travel CONTRALATERALLY via anterior trigeminothalamic tract

Question 35

The posterior and anterior trigeminothalamic tracts have the same target of?

Answer 35

ventral posteromedial (VPM) nucleus in thalamus (oral cavity is medial and V1-3 is lateral)

Question 36

3rd order axons/fibers from the VPM nucelus in the thalamus project to what to get to the primary somatosensory cortex?

Answer 36

posterior limb of the internal capsule

Question 37

Tactile and proprioceptive information from UE/LE is relayed via PCMLS while head and neck is via

Answer 37

main sensory nucleus of trigeminal w some PCMLS

Question 38

The mesencephalic nucleus is comprised of pseudounipolar neurons and is the only nucleus in the CNS. What does it convey?

Answer 38

unconscious proprioceptive and pressure info from muscles of oral region

Question 39

What is this reflex: afferent limb: mesencephalic trigeminal neuron whose peripheral process innervates a masseter muscle and whose central process synapses on a trigeminal motor neuron (efferent)?

Answer 39

jaw-jerk reflex

Question 40

Mesecephalic nucleus via spinal nucleus (pars interpolaris) allows proprioceptive input from jaw/chewing via?

Answer 40

supeiror cerebellar peduncle

Question 41

From main sensory nucleus some 2ndary neurons project anterior to the vermis of the cerebellum via?

Answer 41

superior cerebellar peduncle

Question 42

What is important information that is relayed via trigeminocerebellar pathways?

Answer 42

jaw placement during mastication

Question 43

Cross sensory syndrome are different deficits that depend on where fibers cross. Brainstem: UE/LE is contralateral while face is ipsilateral. SC lesions can result in?

Answer 43

proprioceptive deficits on the right but anesthesia on the left (lack pain)

Question 44

SI leasions usually include larger areas and frequenctly result in more what?

Answer 44

global deficits, as compared to brainstem/brain injuries