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Neuro test 2 > 9. Somatosensory Pathways Pt. 1 > Flashcards

9. Somatosensory Pathways Pt. 1 Flashcards

1
Q

what are the two main pathways talked about in class part of the somatosensory system?

A

posterior column-medial lemniscal pathway

trigeminothalalmic pathway

(others= spinocerebellar and anterolateral)

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2
Q

What does the somatosensory system do?

A

transmits and analyzes touch or tactile info from external and internal locations on body and head

-discriminative touch, flutter-vibration, proprioception, crude touch, thermal sensation, nociception

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3
Q

What types of info does the posterior column-medial lemniscal system carry?

A

perception and appreciation of mechanical stimuli (size, shape, and texture discrimination; recognition of 3D shape and motion detection; conscious awareness of body position and limb movement in space)

basis of accurate localization of touch on diff. parts of body, with high fidelity and high degree of spatial/temporal resolution

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4
Q

What are the characteristic features of the edial lemniscal system? what kind of organization?

A

afferent fibers with fast conduction velocities and limited number of synaptic relays

PRECISE SOMATOTOPIC organization

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5
Q

What is two point discrimination?

A

ability to discriminate bw two stimuli simultaneously

varies widely over diff. parts of body and is related to density of peripheral nerve endings

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6
Q

What is a receptor density gradient?

A

exists bw various body parts and is based on the varying degree of innervation present

digits and perioral regions have increased density of tactile receptors

other regions like back have lower density

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7
Q

What is a receptive field? whats the difference bw small and large

A

area of skin innervated by somatic afferent fibers

small receptive fields- have HIGH receptor density

large fields have low density

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8
Q

What do primary afferent fibers consist of?

A
  1. peripheral process: extends from DRG (mechanoreceptor or free nerve ending)
  2. Central process: extends from DRG –> CNS
  3. Pseudounipolar cell body: in DRG
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9
Q

What makes up dermatomes segmental pattern?

A

peripheral distribution of afferent nerves arising from each spinal level

associated with fibers/pathways that convey pain/thermal information

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10
Q

where do the primary afferent fibers enter in spinal cord?

A

enter the posterior horn

enter spinal cord via medial division of posterior root and then branch (terminate in second order neurons at, above, or below)

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11
Q

Where does majority of pathway go for the primary afferent fibers of the PC-MLS? what is it called collectively?

A

largest set of branches ascends cranially and forms fasciculus gracilis or fasciculus cuneatus

collectively= posterior columns

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12
Q

How are fibers within the posterior columns organized?

A

topographically

most medial= sacral and coccygeal levels–> lumbar/thoracic levels below T6–> thoracic levels above T6–>low cervical levels–> upper cervical levels= most lateral

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13
Q

What forms the fasciculus gracilis?

A

sacral level fibers are positioned medially and fibers from more rostral levels up to T6 are added laterally; combined= FG

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14
Q

What forms the fasciculus cuneatus?

A

thoracic fibers above T6 and cervical fibers are lateral; together= FC

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15
Q

What happens if damage tracts in posterior column? SC lesion result?

A

location of damage will tell you if entire column damage, or if tumor impinging in one area growing from lateral–> medial or opposite, etc.

SC lesions result in Ipsilateral reduction or loss of discriminative, positional, and vibratory tactile sensation AT and BELOW segmental level of injury

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16
Q

If you damage the fasciculus cuneatus or higher levels what happens?

A

more damage bc lower levels are damaged

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17
Q

What is sensory ataxia?

A

loss of muscle stretch (tendon) reflexes, and proprioceptive losses from extemities due to lack of sensory input; may have wide stance, forceful

not getting back feedback from proprioceptive (not always a motor problem)

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18
Q

What is the pathway through the posterior column nuclei?

A
  1. fiber tracts extend rostrally
  2. when hit medulla, target second order neurons of the PCMLS which are located in the gracile and cuneate nuclei (in the caudal and posterior medulla)
  3. get contact of primary afferents to their second order neurons which will extend a little anteriorly and medially as internal arcuate fibers, where will cross midline (decussated)
  4. Extend north/ ascend again in structure= medial lamniscus (on opposite side)
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19
Q

How is pattern of posterior column nuclei maintained even further? What kind of information?

A

outer shells receive inputs from muscle spindles, joints, and pacinian corpuscles= body placement

core “clusters” receive inputs from rapidly- and slow adapting afferents = tactile info

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20
Q

What happens after medial lemniscus is reached?

A

It is initially medial and oriented from superior to inferior, and is relayed somatotopically in this order

however, as ML extends rostrally it rotates laterally at level of pons

Now: UE fibers lie medially and LE fibers laterally

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21
Q

What does the ML terminate in?

A

ventral posterolateral nucleus (VPL) of the thalamus

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22
Q

What is the blood supply at primary afferents and primary neurons? how about at medulla?

A

primary afferents and primary neurons fed by POSTERIOR SPINAL A.

at medulla: ANTERIOR SPINAL A.

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23
Q

What happens in a midbrain PC lesion?

A

damage at brainstem levels lead to deficits in discriminative touch, vibratory, and positional sensibilities over contralateral side of body (above decussation ?)

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24
Q

What does a right side midbrain lesion produce?

A

left-sided loss of proprioception and discriminative touch but NO loss of any other modality (bc damage specific to ML in the midbrain)

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25
Q

What is the ventral posterior nuclei?

A

wedge-shaped cell group located in caudal thalamus

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26
Q

What comprises the ventral posterior nuclei? what is it separated by?

A

comprised of ventral posterolateral nucleus (VPL) and ventral posteriomedial nucleus (VMP)

separated by fibers of ARCUATE LAMINA

27
Q

What is the ventral posterolateral nucleus responsible for? where does it receive input from?

A

acts as gate keeper, proprioceptive and tactile info from body

receives ascending input from medial leminiscus

28
Q

What is the ventral posteromedial nucleus responsible for?

A

recieves info from the face

29
Q

What is the arterial supply for the ventral posterior nuclei? What would damage to it do?

A

thalamogeniculate arteries (branch of posterior cerebral a.)

damage- can result in loss of all tactile sensation over CONTRALATERAL body and head

30
Q

What does the ventral posterolateral nuclei do? where do fibers terminate?

A

cells found in this nucleus are going to extend up further to cortical areas of brain that will process this information and allow you to make decision (gatekeeper for sensory info)

fibers from contralateral nucleus cuneatus terminate medial to those of nucleus gracile

31
Q

What are the two populations of identified neurons in the VPL for the trunk and extremities?

A

third order neurons (majority)

local circuit interneurons

32
Q

What are the third-order neurons of the VPL?

A

large-diamter axons that traverse posterior limb of internal capsule (white matter tract)

generate thalamococortical fibers which will exit the VPL and take sensory info to primary or secondary somatosensory cortex; go to higher areas

33
Q

What are the local circuit interneurons of the VPL?

A

will be used to moderate the frequency of relaying that info from the VPL to the cortex by other third neurons (influences their firing rate); keep from being too overactive

receives excitatory corticothalamic inputs

34
Q

Where is the primary somatosensory (SI) cortex?

A

comprises postcentral gyrus and posterior paracentral gyrus

bordered by central sulcus (ant) and postcentral sulcus (posteriory)

maps out homonculus (foot to tongue pattern along M–> L axis)

35
Q

What is the relationship bw receptor density and cortical representation in the homonculus?

A

Regions with increased receptor density (hands/lips), have large amt of dedicated cortical tissue

regions with low receptor density (back) have small cortical representation

overall size of body placement is indicative of sensory information to that area

36
Q

What is the blood supply provided to the primary somatosensory cortex (SI) cortical areas?

A

middle and anterior cerebral As

37
Q

What happens with a lesion to the middle cerebral A?

A

tactile loss over contralateral upper body and face

38
Q

What happens with a lesion to the anterior cerebral A?

A

affect contralateral lower limb

39
Q

What are the subdivisions of the primary somatosensory cortex (SI)?

A

anterior to posterior: Brodmann areas 3a, 3b, 1, and 2 (look at image in notes)

40
Q

What are other cortical somatosensory regions?

A

Secondary somatosensory cortex (S2)- under parietal lobe; receives tactile inputs from 5 and 7 mostly (more for subsequent processing, not immediate decision making, registering things long-term)

parietal cortical regions- also receive tactile inputs; posterior

41
Q

What do lesions in the parietal association areas produce?

A

AGNOSIA

contralateral body parts are lost from personal body map

sensation not radically altered, but limb is not recognized as part of patients own body

42
Q

What are spinocerebellar pathways?

A

transmit proprioceptive and limited cutaneous info to cerebellum

includes info about limb position joint angles, and muscle tension/length

cerebellar input- guides control of body muscle tone, movement, and posture

43
Q

What does the trigeminal nerve relay?

A

mixed sensory (main sensory of head) and motor (muscles of mastification)

from trigeminal ganglion

44
Q

What do the trigeminal nuclei form?

A

form a continous cell column that extends from spinomedullary jxn to rostral levels of mesencephalon/midbrain

45
Q

What is the main sensory nucleus (msT) sensitive to? where is it found?

A

touch and pressure; discriminative tactile and proprioceptive sensations

found in midpons, slightly lateral to motor nucleus

46
Q

What is the trigeminal motor nucleus (mT) sensitive to?

A

muscles of mastification

47
Q

What is the spinal nucleus specific to? what direction does it extend?

A

pain and temperature

extends caudally

48
Q

What is the mesencephalic nucleus (mes) specific to? what direction does it extend?

A

proprioceptive afferents from TMJ and masticatory m. (when to chew and not bight tongue)

extends rostrally (all way to midbrain)

49
Q

What are the divisions of the main sensory nucleus? What do they provide?

A

Dorsomedial division- afferent input from oral cavity

Ventrolateral division- afferents from V1, V2, and V3

50
Q

Where will the chief sensory nucleus be in the pathway?

A

the chief main sensory nucleus will be right near entry point of nerve and it will be in posterior position

51
Q

Describe the afferent somatotopic representation in the main sensory nucleus of CN5:

A

V1= anterior (in principle sensory nucleus)

V2= in bw

V3= posterior

52
Q

What is the path of from the principal sensory nucleus–> primary somatosensory cortex:

A
  1. primary afferents will come in through principal sensory nucleus and will contact second order neurons of CN5 nucleus
  2. 2nd order fibers of dorsomedial/ventromedial divisions project ipsilaterally/contralaterally via posterior/anterior trigeminothalamic tract
  3. Both tracts target ventral posteromedial nucleus (VPM) of thalamus (somatotopic arrangement, oral cavity= medial; external face= lateral)
  4. 3rd order axons from VPM project via posterior limb of internal capsule–> primary somatosensory cortex
53
Q

Where are second order cells of the CN 5 path found?

A

in the main sensory nucleus

54
Q

Second order fibers of the dorsomedial division project where, via what?

A

2nd order fibers of dorsomedial division project IPSILATERALLY

via POSTERIOR trigeminothalamic tract

55
Q

Second order fibers of the ventrolateral division project where, via what?

A

2nd order fibers of the Ventrolateral division project CONTRALATERALLY

via ANTERIOR trigeminothalamic tract

56
Q

What information is relayed by the PCMLS and Main sensory of CN V:

A

tactile & proprioceptive info from UE and LE, and head and neck

head and neck- main sensory nucleus (V)

UE = PCMLS (cuneate fasciculus)

LE = PCMLS (cuneate gracilis)

57
Q

What is the mesencephalic nucleus comprised of? where is it he only nucleus?

A

pseudounipolar neurons; can be considered as displaced trigeminal ganglion cells

only nucleus in the CNS!

58
Q

What is the role of the mesencephalic nucleus?

A

conveys unconscious proprioceptive and pressure info from muscles of oral region!

helps with vocalizations, eating and chewing

permits conscious awareness of facial and oral proprioception via anterior/posterior trigeminothalamic tract

afferent limb of the jaw-jerk reflex (have axons that terminate in trigeminal motor nucleus)

59
Q

What is the jaw-jerk reflex?

A

stretching the masseter (downward tap on chin)–> causes it to contract bilaterally

aff limb: mesencephalic trigeminal neuron - innervates masseter muscle spindle and central process synapses on trigeminal motor neuron (efferent limb)

60
Q

What is the largest portion of the trigeminocerebellar pathway? smaller portion?

A

largest part: mesencephalic nucleus via spinal nucleus (pars interpolaris)==> jaw placement/chewing

smaller portion form main sensory nucleus==> moderate # secondary neurnos projecting to anterior vermis of cerebellum

both via superior cerebellar peduncle

61
Q

Why is getting input from cerebellum important as far as jaw placement?

A

food texture and consistency altered as chewed since info is coming in; also change demands on jaw muscles

feedback is important and thats required to go to cerebellum to go to injury site

62
Q

Cross sensory findings: what do brain lesions result in?

A

sensory deficits of trunk/extremities contralateral to lesion

sensory deficits fo face/CN ipsilateral to lesion

right face and left arm/leg LACK proprioceptive info

63
Q

Cross sensory findings: what do SC lesions result in?

A

proprioceptive deficits on right, but anesthesia on left (or vice versa)

Neuro test 2 (15 decks)

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