Sensory Pathways

Question 1

Principles of organization

Answer 1

• Pathways carrying information from neck, trunk, and limbs (DCML and STT) are separated from those carrying sensory from the face (TTT and TL)
• Pathways carrying light touch, 2 pt discrimination, vibration, and proprioception (DCML for body, TL for face) are separated from the pathways carrying pain, temperature, and crude touch (STT for body, TTT for face)

Question 2

Neuron and axon patterns in sensory pathways 1

Answer 2

• All pathways have 3 neurons (N1, N2, and N3)
• N1 is a sensory ganglion of a cranial nerve or DRG of spinal nerve. N1 fibers extend from periphery into the spinal cord (SC) and synapse onto N2 (damage to N1 axons results in ipsilateral deficit)
• N2 is located in SC (dorsal horn) or brainstem, on the same side as the origin of sensory info (N1 axons do not decussate). Axons of N2 decussate and travel contralateral to the origin of sensory info (damage to N2 axons results in contralateral deficit) and synapse in the thalamus

Question 3

Neuron and axon patterns in sensory pathways 2

Answer 3

• N3 is found in the thalamus (VPL for DCML/STT and VPM for TL/TTT). Axons of N3 project to the primary sensory cortex via the internal capsule
• The primary sensory cortex sends info tot he sensory association cortex for object recognition
• Physical deficits will be ipsilateral to the lesion if the lesion is before decussation occurs
• Deficits will be contralateral to the lesion if the lesion is after decussation occurs

Question 4

Dorsal column- medial lemniscus pathway (DCML) 1

Answer 4

• Contains light touch, 2 pt discrimination, vibration, and proprioception info from body (not face)
• N1 in DRGs, split into 2 groups: T6-S5 and T5-C1
• The axons from T6-S5 course thru the dorsal/ventral rami and enter the spinal cord via dorsal horn, then run vertically up the spinal cord thru the fasciculus gracilis
• In spinal cord segments L5-T6, there is only a fasciculus gracilis (FG): one large structure of white matter in the most dorsal medial part of the SC (dorsal column)
• The axons in these segments carry info from the lower body limbs, and lower trunk)

Question 5

Dorsal column- medial lemniscus pathway (DCML) 2

Answer 5

• The axons from T5-C1 have the same route of entry into the SC, but run vertically up thru the fasciculus cuneatus (FC)
• In segments T5-C1 there are 2 structures in the dorsal medial part of the SC, the most medial is the FG and the more lateral one is the FC
• Axons in both the FG and FC synapse onto N2 (either nucleus gracilis or nucleus cuneatus), which resides at the junction of the SC and medulla (lower medulla)

Question 6

Dorsal column- medial lemniscus pathway (DCML) 3

Answer 6

• Axons of N2 decussate immediately and form the medial lemniscus pathway, which ascends contralateral through the brain stem to reach the thalamus
• Axons of N2 synapse onto N3 in the ventral posterior lateral (VPL) nucleus of the thalamus. The axons of N3 go from the VPL thru the posterior limb of the internal capsule and terminate in the primary sensory cortex

Question 7

Sensory cortex

Answer 7

• Sensory info from the thalamus is sent to the primary sensory cortex in the postcentral gyrus
• This is organized into a homunculus (representations of each body part by an area of brain), with the larynx>face>upper limbs/trunk>lower limbs/trunk moving from lateral>medial parts of the gyrus
• Info related to intensity is sent to somatosensory area II in wall of sylvan fissure
• General sensory association cortex in superior parietal lobule receives info for object ID (stereognosis and graphesthesia)
• Superior parietal lobule projects to inferior parietal lobule (multimodal association cortex) to integrate sensory info w/ special senses

Question 8

Damage to DCML

Answer 8

• Sensory loss will be ipsilateral to lesion if damage is before decussation (anywhere in periphery or SC, axons of N1)
• Sensory loss will be contralateral to lesion if damage is after decussation (anywhere at or above lower medulla, axons of N2/3)
• Only will lose proprioception, light touch, 2 pt discrimination, vibration, and graphesthesia/stereognosis

Question 9

Spinothalamic tract (STT) 1

Answer 9

• Carries pain, temp, and crude touch (blunt object) info from body (not face)
• N1 in DRG, axons course from ventral/dorsal rami into SC and immediately synapse onto N2 (nucleus proprius), which is in the dorsal horn of the level of the sensory info
• The axons of nucleus proprius (N2) decussate 1-2 SC segments above level of entry through the white commissure (connects the 2 halves of the SC) and enter the spinothalamic tract (STT)

Question 10

Spinothalamic tract (STT) 2

Answer 10

• STT ascends thru the SC (most anterior lateral part, or funiculus, of SC) and brain stem to terminate on N3 in the VPL of the thalamus
• N3 in the VPL of the thalamus projects neurons through the posterior limb of the internal capsule (same as DCML) to terminate in the primary sensory cortex and the posterior part of the paracentral lobule
• From the primary sensory cortex, info is sent to the general sensory cortex>superior parietal lobule>multimodal association cortex>inferior parietal lobule

Question 11

Pathology of STT

Answer 11

• Sensory loss from STT will be ipsilateral to a lesion below the decussation of a pathway
• Sensory loss will be contralateral to a lesion above the crossing of a pathway
• A lesion at any given level will produce loss of STT from the contralateral side 1-2 segments below the lesion, but loss of STT axons from the ipsilateral side at the level of the lesion only
• Therefore transecting the left half of the SC at T6 will yield loss of sense of pain/temp in the dermatomes T7/8 and below on the right side, and loss of pain/temp in the T6 dermatome only on the left side
• A lesion of the white commissure will result in loss of pain and temp on both sides over multiple SC segments

Question 12

Sensory deficits from STT lesion

Answer 12

• Loss of pain sensation
• Loss of temperature sensation
• Abnormal sensations: paresthesias/dysthesias (sponaneous shooting pain, burning sensation)
• Enhanced pain: allodynia (pain evoked by minor stimuli that should not cause pain- due to regrowth of nerves)

Question 13

Deep tendon reflexes (DTR)

Answer 13

• Sensory input by neuromuscular spindles, which react to stretch, to the anterior horn of the SC and activate motor neurons that innervate the same muscle
• This monosynaptic reflex arch causes contraction of the muscle that is stretched
• This pathway is always partially active to achieve partial contraction and maintain muscle tone
• Decreased reflex indicates peripheral injury. Exaggerated reflex indicates injury to CNS

Question 14

Overview of somatosensory pathways of the head 1

Answer 14

• Carried by the trigeminal branches (V1, V2, V3), is also composed of a 3 neuron system
• N1 is in the trigeminal sensory ganglion, and depending on where the nerve was innervating it could be part of V1 (opthalmic), V2 (maxillary), or V3 (mandibular)
• These 3 branches run together as the sensory root of V, and enter the pons
• N2 is within one of the trigeminal sensory nuclei in the brainstem, where N1 will synapse
• The location of N2 depends on the type of information being carried (TL synapses in the chief sensory nucleus, but TTT synapses in the spinal nucleus of V)

Question 15

Overview of somatosensory pathways of the head 2

Answer 15

• Axons of N2 decussate immediately in the brainstem to form the sensory tract, the axons then extend up to the thalamus and N3
• N3 for the face is in the VPM nucleus of the thalamus (ventral posterior, medial), and it extends axons through the internal capsule to the face area of the primary sensory cortex
• Primary sensory cortex then transmits info to the association cortex in the inferior and superior parietal lobules

Question 16

Trigeminal sensory nuclei

Answer 16

• Broken into 3 categories
• Light touch/discrimination, vibration, and proprioception synapse at N2 in the chief sensory nucleus of V, located in the mid pons
• Pain and temp synapse at N2 in the spinal nucleus of V, which extends from the inferior end of the chief sensory nucleus to the C3 SC level
• Proprioception for muscles of mastication synapse at N2 in the mesencephalic nucleus of V, located in the midbrain superior to the chief sensory nucleus

Question 17

Trigeminal- lemniscus (TL) pathway

Answer 17

• Carries light touch/discrimination, vibration, and proprioception from face
• N1 in the trigeminal ganglion, extends axons to mid pons and synapses on N2 in the chief sensory nucleus of V
• N2 in midpons sends axons that decussate immediately and join w/ the TTT to travel up to the thalamus
• N2 terminates its axons on N3 in the VPM of the thalamus
• N3 axons from thalamus project through the posterior limb of the internal capsule to end in the face region of the primary sensory cortex
• Axons from primary sensory cortex project to sensory association cortices

Question 18

Trigeminal- thalamic tract (TTT) 1

Answer 18

• Carries pain and temp from face
• N1 in the trigeminal ganglion, extends axons thru midpons and descend as spinal tract of V to the lower end of the medulla
• These axons synapse in the lower part of the spinal nucleus of V (N2), located medial to the spinal tract of V and in the lower medulla
• N2 axons decussate immediately and ascend as the TTT
• Since the spinal nuclei of V is large and covers many levels, just know that decussation of fibers is complete by the upper medulla

Question 19

Trigeminal- thalamic tract (TTT) 2

Answer 19

• The TTT in the upper medulla is found lateral to the ML, and at this point contains only pain and temp sensations
• At the level of the mid/upper pons, the TTT is joined w/ the TL so the combined tracts contain all sensation from the contralateral side of the face (in midbrain TTT and TL run together)
• The TTT synapses on N3 in the VPM of the thalamus, which sends axons thru the posterior limb of the internal capsule to the primary sensory cortex face area

Question 20

Pathology of face sensation

Answer 20

• Lesions affecting the descending spinal tract of V will result in loss of pain and temp sensation in the ipsilateral half of the face (before decussation): lateral medullary syndrome
• Lesions of the TTT at and above the upper pons will lead to a loss of all sensation in the contralateral half of the face
• This is b/c the TTT at these levels also contains the TL, and both have decussated at this level
• Also at these levels the TTT lies close to the DCML and STT pathways, so a small lesion is likely to affect all 3 tracts
• If this is the case, all sensation (including body/limbs) will be lost to the contralateral side of the lesion
• Lesions to the TTT below the upper pons will not affect the TTT (no one knows why), the lesion must be at upper pons to affect TTT (also would affect TL)

Question 21

Location of DCML in SC/brain stem at various levels 1

Answer 21

• In lumbar region of the SC there are large ventral and dorsal horns (sensory and motor info from lower body)
• There is only a fasciculus gracilis in the lumbar (and T12-T6) region, which is located just posterior the white commissure and medial to the dorsal horn
• In the thoracic SC levels there are small ventral horns and long, thin dorsal horns. There will be only a FG in regions T12-T6, but regions T5-T1 will also contain fasciculus cuneatus just lateral to FG
• In the cervical region of the SC there are long, thin dorsal horns but large ventral horns
• All cervical levels contain both an FG and FC (FG always medial to FC: “feet on grass”), which are carrying sensory information from the ispilateral side

Question 22

Location of DCML in SC/brain stem at various levels 2

Answer 22

• The axons from N1 synapse at N2 in lower medulla in the nucleus gracilis or nucleus cuneatus
• These nuclei are in the same region of the lower medulla roughly as the dorsal columns were in the SC
• The axons from N2 then decussate immediately to form the ML, which is in the medial part of the lower medulla (along its midine, on both sides)
• Now the fibers containing info from FG are the most anterior, and fibers containing info from FC are the most posterior, this continues through the upper medulla

Question 23

Location of DCML in SC/brain stem at various levels 3

Answer 23

• In the mid and lower pons the ML rotates counter-clockwise, so the info from the feet is lateral and the info from the arms is medial
• The ML is getting pushed posterior due to the pontine fibers still positioned medial (near midline)
• In the upper pons the ML is just anterior to the superior cerebellar peduncles (more lateral than in mid/lower pons), but the fibers are arranged in the same way (upper limbs medial, lower limbs lateral)

Question 24

Location of DCML in SC/brain stem at various levels 4

Answer 24

• In the lower midbrain, the ML is anterior to the inferior colliculi, lateral to the decussation of the cerebellar peduncles, medial to the posterior-most crus cerebri, and posterior to the substantia nigra
• In the upper midbrain the ML borders the posterior-lateral side of the red nuclei, and is medial to the posterior-most substantia nigra
• In this region the sensory tracts are rotated a little more counter-clockwise: upper limbs are anterior-medial, and lower limbs are posterior-lateral

Question 25

Where is N1 located and where do axons of N1 course in DCML?

Answer 25

- N1 located in DRG | - Axons course in Fas. gracilis (lower limbs) and Fas. cuneatus (upper limbs), on ipsilateral side of N1

Question 26

Where is N2 located and what pathway do the axons for N2 run in for DCML?

Answer 26

- N2 located in Nuc. gracilis (lower limbs) and Nuc. cuneatus (upper limbs) - Axons course through medial lemniscus (ML), on contralateral side from N1

Question 27

Where is N3 located for DCML?

Answer 27

-VPL nucleus of thalamus

Question 28

Location of STT in SC and brainstem at various levels 1

Answer 28

- As N1 axons enter the dorsal horn they synapse onto N2 in the dorsal horn - N2 axons will decussate 1-2 levels above this synapse, through the white commissure - The axons then become a part of the STT, which is in the anterior lateral portion of the SC (on the anterior lateral border of the anterior horns) - The axons in STT carry information from the contralateral side of the body - The STT remains in this location up through the lower medulla (on the lateral border of the pyramid)

Question 29

Location of STT in SC and brainstem at various levels 2

Answer 29

- In the upper medulla the STT is on the lateral edge, just btwn the inferior olive (to the anterior) and the inferior cerebellar peduncle (to the posterior) - In the lower pons the STT and the ML are close together (but remain separate); the STT is lateral to the ML - At this level the STT is just medial to the middle cerebellar peduncles - In the mid pons the ML and STT are very close together (lesion will affect them both), with the STT still lateral to the ML (both just inferior to superior cerebellar peduncles)

Question 30

Location of STT in SC and brainstem at various levels 3

Answer 30

- In the upper pons the STT remains near the ML, but is now posterior to it. The STT is just lateral to the superior cerebellar peduncles - In the lower midbrain the STT is sandwiched btwn the inferior colliculi (to the posterior) and the ML (to the anterior - In the upper midbrain the STT is anterior and lateral to the superior colliculi, and on the posterior border of the lateral-most ML

Question 31

Where is the N1 located for the STT?

Answer 31

-In the DRG

Question 32

Where is the N2 located for the STT?

Answer 32

-In the nucleus proprius, the axons from which decussate 1-2 levels higher and enter the spinothalamic tract (STT)

Question 33

Where is the N3 located for the STT?

Answer 33

-VPL of thalamus

Question 34

Where is N1 and N2 located for light touch, vibration, and proprioception info from the face, what pathway does it take and where does it end?

Answer 34

- Trigeminal ganglion is N1 - Chief sensory nucleus of V is N2, decussation immediately in midpons - Uses trigeminal- lemniscus pathway (TL) - Ends in the VPM of thalamus

Question 35

Where is N1 located for pain and temp info from the face, and where do its axons travel?

Answer 35

- Trigeminal ganglion is N1 | - Axons travel in spinal tract of V down the lower medulla

Question 36

Where is N2 located for pain and temp info from the face, and where do its axons travel?

Answer 36

- N2 in spinal nucleus of V | - Axons from N2 decussate, by upper medulla, and form the trigeminal- thalamic tract (TTT)

Question 37

Where is N3 located for pain and temp info from pace?

Answer 37

-VPM in thalamus is end of the TTT

Question 38

Location of TTT/TL in brainstem at various levels 1

Answer 38

- All sensory info from CN V enters the brainstem at the level of mid pons - Light touch, vibration, proprioception fibers synapse in main sensory nucleus of V, just anterior to the superior cerebellar peduncles - These axons decussate to reach the TL (near midline), just medial and posterior to the ML in the mid pons - Axons carrying pain and temp descend ipsilateral in the spinal tract of V

Question 39

Location of TTT/TL in brainstem at various levels 2

Answer 39

- In the upper medulla, the spinal tract of V (descending) can be seen on the medial border of the inferior cerebellar peduncles - In this same level the ascending TTT tract can be seen on the medial border of the posterior-most part of the inferior olive (this is carrying info from contralateral side) - In the lower medulla, the spinal tract of V synapses onto N2 in the spinal nucleus, which is just anterior to nucleus cuneatus - Axons from N2 decussate to form the TTT, which is just lateral to the ML at the level of the lower medulla (TTT now carries info from contralateral side)

Question 40

Location of TTT/TL in brainstem at various levels 3

Answer 40

- In the mid pons the descending spinal tract of V is on the medial border of the middle cerebellar peduncles - The ascending TTT is on the posterior border of the ML (very close to STT as well) - In the mid-upper pons, the TL tract is decussating and joining the TTT - These can be seen running together (TL just medial to TTT), both on the posterior border of the ML

Question 41

Location of TTT/TL in brainstem at various levels 4

Answer 41

- In the upper pons the decussation of TL fibers is complete and the TL/TTT pathway is formed - The TTT/TL lies on the posterior border of the ML (STT pathway just lateral to ML), since all 3 pathways are so close a lesion to this area results in loss of all sensation to the face and body on the contralateral side - In the lower midbrain the TTT/TL is directly posterior to the ML, and directly medial to the STT - In the upper midbrain the TTT/TL is directly posterior to the ML and directly medial to the STT still

Question 42

Taste pathway

Answer 42

- N1 is in sensory ganglia of cranial nerves VII (anterior 2/3rds), IX (posterior 1/3rd), and X (epiglotis) - The axons from the ganglia enter the medulla and for the solitary tract, which descends in the medulla a little before synapsing on N2 - N2 is in the nucleus solitarius of the medulla, and N2 sends axons up through the central tegmental tract (CTT) to reach bilateral VPMs in the thalamus (one nucleus solitarius sends axons to both VPMs - N3 sends axons to the insula and somatosensory where taste is represented

Question 43

Identifying the solitary tract/nucleus solitatius and lesions to the taste pathway

Answer 43

- In the medulla (only section we need to ID ST/NS from), the solitary tracts are 2 very dark circles that are near the posterior border (anterior to 4th ventricle) - The nucleus solitarius is the surrounding light circle around the dark circle - They reside about equidistant from the midline and lateral sides - Very distinct due to the dark, isolated circle of white matter (ST) surrounded by gray matter (NS) - Gray matter stains yellow, white matter stains black - Since N2 projects axons to both VPMs, KOing the CTT or the ST on one side will not result in taste deficit (since the other N2 compensates by sending axons bilaterally)

Question 44

Where are N1s located for the sense of taste, and what tract do their axons form?

Answer 44

- N1s are in sensory ganglia of CN VII, IX and X | - Their axons form the solitary tract

Question 45

Where is N2 located for the sense of taste and what tract do their axons form?

Answer 45

- N2 in the nucleus solitarius | - Their axons form the CTT (central tegmental tract)

Question 46

Where is N3 located for sense of taste and where do their axons end (i.e. where is taste represented)?

Answer 46

-N3 in VPM of thalamus, sends its axons to the somatosensory cortex of the tongue plus the insula (where taste is perceived)