L13 - Anatomy of Somaesthetic Pathway

Question 1

Classification of sensations?

Answer 1

1) Conscious (cerebral cortex, e.g. motor, somatosensory):
a) Exteroception
b) Proprioception

2) Unconscious (e.g. cerebellum; balance):
a) Interoception
b) Proprioception

Question 2

Define exteroception and interoception.

Answer 2

Exteroception: sense of direct interaction with the external world as it impacts on the body (e.g. vision, smell)

Interoception: sense of the function of the major organ systems of the body and its internal state

Question 3

Difference in proprioception between conscious and unconscious sensation?

Answer 3

Conscious = mediated by dorsal column- medial lemniscal pathway

Unconscious = mediated by spinocerebellar

Question 4

All sensations lead to perception. True or False?

Answer 4

False

Depends on whether the sensation is processed by the cerebral cortex

Question 5

Organization of somatosensory system? (3 levels)

Answer 5

1. Receptorlevel: sensoryreceptors
2. Circuitlevel: processinginascendingpathways
3. Perceptuallevel: processingincorticalsensory areas

Question 6

Classify sensory receptors by distribution and by function.

Answer 6

Distribution:

1) Exteroceptors @ body surface i.e. free nerve endings
2) Proprioceptors @ muscles, joints, tendons i.e. muscle spindle, joint kinesthetic receptor
3) Interoceptors @ viscera

Function:

1) General sensory (for general senses i.e. temp. pain. vibration…)
2) Special sensory (for special senses i.e. light, taste, sound…)

Question 7

Structure of sensory neurons?

Answer 7

Pseudounipolar sensoryneuron:
twofunctionallydistinctsegments,bothfunctionasaxons;

1) oneextendstoperipheralskinormuscle
2) theothertothecentral spinalcord.

Question 8

Relate the type of axons in sensory neurons to the signal transmitted by receptors?

Answer 8

Aα = proprioceptors of skeletal muscles

Aβ = Mechanoreceptors of skin

Aδ = Pain, temp

C (unmyelinated) = temp, pain, itch

Question 9

List the major ascending pathways to the brain. Location in spinal cord?

Answer 9

1.Spinothalamic(anterolateral)pathway

2. Dorsalcolumn‐mediallemniscus(posteriorcolumn)pathway
3. Spinocerebellarpathway

Question 10

Compare the end-points of the 3 major ascending pathways?

Answer 10

Dorsal column medial lemniscus + Spinothalamic tract = transmit impulsesviathethalamus tothesensorycortexfor consciousinterpretation.

Spinocerebellar pathway = terminatesinthecerebellum,anddoesnot contributetosensoryperception

Question 11

Compare the sensory information transmitted by the DCML pathway and the Spinothalamic pathway

Answer 11

DCML:

• Discriminative touch / 2-point localization
• Pressure
• Vibratory sense
• Conscious muscle joint sense / position sense (proprioception)

Spinothalamic:

• Pain, itch
• Temperature
• Light/crude (non-discriminative) touch

Question 12

Compare the receptors of DCML and spinothalamic tract.

Answer 12

DCML:

• Merkel cells
• Pacinian corpuscles
• Muscle spindles
• Tendon organs

Spinothalamic:
Free nerve endings (e.g. nociceptors)

Question 13

Compare the Dorsal root axon between DCML and spinothalamic tract.

Answer 13

DCML = Large diameter myelinated fiber: Aα, Aβ, Aδ

Spinothalamic: Smaller diameter myelinated, unmyelinated fibers: Aδ, C (terminate in dorsal horn)

Question 14

Location of soma, route and end-point of first-order neuron in DCML and spinothalamic pathway.

Answer 14

a) cellbodiesresideinaganglion(dorsalrootor cranial)

b) conductimpulsesfromcutaneousreceptors+ proprioceptorstospinalcordorbrainstem (CNS)
»synapsewithsecond‐orderneurons or interneurons in CNS (important for reflex)

c) DCML and spinothalamic 1st order neurons end in spinal cord

Question 15

Location of soma, route and end-point of SECOND- order neuron in DCML and spinothalamic pathway.

Answer 15

a) In dorsal horn of spinal cord (spinothalamic)/ brain stem (medulla DCML)
b) Axons decussate (spinothalamic decussate at the level of entry in the spinal cord; DCML decussate in the medulla via Gracile and Cuneate nuclei)
c) Transmit to thalamus (DCML)/ cerebellum (spinothalamic) where they synapse with 3rd order neuron

Question 16

Location of soma, route and end-point of THIRD- order neuron in DCML and spinothalamic pathway.

Answer 16

a) In thalamus
b) Carry information to cerebral cortex to synapse with neurons of primary sensory cortex
c) Primary sensory cortex

This does not apply to spinocerebellar pathway

Question 17

The connection between first, second and third order neurons in the somatosensory pathway is always direct. True or False?

Answer 17

False

At the two synaptic areas in the CNS, there may be several short interneurons between the axon of the input neuron and the dendrites of the output neuron

Question 18

Summarize the 1st, 2nd, and 3rd order neurons and destination for DCML pathway.

Answer 18

• 1storderneuron:posterior/dorsalroot ganglion ipsilateral
• 2ndorderneuron:gracilenucleus& cuneatenucleus(decussate inmedulla oblongata midline)&raquo_space;> contralateral medial lemniscus
• 3rdorderneuron:ventralposterolateral (VPL)nucleusofthethalamus
• Destination:Primarysomatosensory cortex

Question 19

Difference in function of the 2 nuclei of the second order neruon in DCML?

Answer 19

 Cuneate nucleus(more lateral) : arm, upper body

 Gracile nucleus (more medial): leg, lower body

Question 20

Summarize the 1st, 2nd, and 3rd order neurons and destination for spinothalamic pathway.

Answer 20

• 1st order = dorsal horn ganglion
• 2nd order = dorsal horn of grey matter of spinal cord decussate at level of entry across the midline via ANTERIOR WHITE COMMISSURE
• 3rd order = :ventral posterolateral(VPL)nucleusofthe thalamus
• Destination:Primarysomatosensory cortex

Question 21

Compare the tracts that form the DCML and spinothalamic tract?

Answer 21

DCML = paired tractsofthedorsalwhitecolumnofthe spinalcord—cuneatefasciculusand gracilefasciculus—andthemedial lemniscus.

Spinothalamic = lateral andanteriorspinothalamictracts

Question 22

Compare the decussation points between DCML and spinothalamic pathways.

Answer 22

DCML = Gracile/ Cuneate nuclei&raquo_space; contralateral medial lemniscus across midline of medulla oblongata

Spinothalamic = Anterior white commissure of spinal cord&raquo_space; contralateral spinothalamic tracts

Question 23

Compare the bifurcation of DCML and spinothalamic pathways’ second order neurons.

Answer 23

DCML = Bifurcate after entering medial root; large fibers ascend cranially to Gracile and Cuneate fasciculus, small fibers terminate within the spinal cord for reflex

Spinothalamic = Bifurcate into rostral and caudal branches after entering medial root of spinal cord

Question 24

Spinocerebellar tract requires 3 neurons. True or False?

Answer 24

False

Only need 2, unlike DCML and spinothalamic tract

Question 25

Function of spinocerebellar pathway?

Answer 25

Convey information from muscle or tendon  stretch to the cerebellum, which uses this  information to coordinate skeletal muscle  activity. Do not contribute to conscious sensation For unconscious proprioception  primarily terminate in the ipsilateral cerebellum (not primary somatosensory cortex)

Question 26

Describe the decussation of the spinocerebellar tracts?

Answer 26

Dorsal spinocerebellar tract = Do not decussate Ventral '' = decussate TWICE Cuneocerebellar tract = Do not decussate

Question 27

Compare the proprioceptive afferents from the different spinocerebellar tracts.

Answer 27

* Dorsal spinocerebellar tract : proprioceptive  afferents from lower trunk and lower limbs,  synapse with Clarke’s column cells  * Cuneocerebellar tract: upper limbs and neck  proprioceptive information * Ventral spinocerebellar tract: afferents from  lower limbs spinal motor neurons and  interneurons

Question 28

Which cranial nerves have somatosensory components?

Answer 28

CN 5,7,9,10 Trigeminal Facial Glossopharyngeal Vagus

Question 29

Describe the 1st, 2nd, 3rd order neuron paths for cranial nerve with somatosensory components.

Answer 29

1st = sensory nucleus of the  cranial nerve of the same side 2nd = some  cross (some do not) to the  opposite side in the brainstem  and ascend to the thalamus. 3rd = (at VPM)  ascends to the somatosensory  cortex.

Question 30

Compare the sensory information relayed by cranial nerves with somatosensory components.

Answer 30

Trigeminal = Discriminative touch, proprioception (e.g. mastication) facial (VII), glossopharyngeal (IX), vagus (X) = Pain, temp

Question 31

Compare the location of 2nd order neurons of cranial nerves with somatosensory components.

Answer 31

Trigeminal = Anterior and posterior trigeminothalamic tract VII, IX, X = Caudal > relay through spinal trigeminal tract and nucleus (pars caudalis)

Question 32

Describe the processing of sensory information at the perceptual level of the somatosensory pathways.

Answer 32

 Most somatic sensory information is relayed to thalamus for processing (conscious perception)  Adaptation reduces the amount of information reaching the cerebral cortex  A small fraction (~1%) of sensory information coming in projects to cerebral cortex >> reaches our awareness

Question 33

Compare the results of damage to primary sensory neurons vs thalamic projection neurons in spinal cord / brainstem to third order neurons.

Answer 33

Thalamic projection neurons to third order neurons: loss of sensation from body below the level of lesion Primary sensory neuron: absence of sensation from region of body supplied by those nerves

Question 34

Define dermatome. What forms it?

Answer 34

- area of skin supplied by right and left dorsal roots of a single spinal segment - formed by peripheral processes of adjacent spinal nerves overlap

Question 35

What causes Wallenberg syndrome?

Answer 35

Wallenberg syndrome/  PICA syndrome/         Lateral medullary  syndrome Blood clot at posterior inferior cerebellar artery

Question 36

List the 5 nuclei/ tracts affected in wallenberg syndrome and relate to 5 clinical manifestations.

Answer 36

1. Contralateral loss of pain and  temperature sensation from the  body (spinothalamic tract) 2. Ipsilateral loss of pain and  temperature sensation from the  face (spinal trigeminal tract and  nucleus) 3. Vertigo and nystagmus (vestibular  nuclei) 4. Loss of taste from the ipsilateral  half of the tongue (solitary tract  and nucleus) 5. Hoarseness and dysphagia, decrease gag reflex  (nucleus ambiguus or roots of  CN  IX, Glossopharyngeal and X, Vagus) **Specific to PICA**

Question 37

List the 6 nuclei/ tracts affected in AICA syndrome and relate to 6 clinical manifestations.

Answer 37

1. Spinal trigeminal nucleus = Loss of pain and temp sensation in ipsilateral face 2. Vestibular nuclei = vertigo, nausea, vomiting 3. Cochlear nucleus = ipsilateral hearing impairment **specific** 4. Facial nerve = ipsilateral facial paralysis, loss of tast at ant. 2/3 tongue, decrease lacrimation and salivation **specific** 5. Inferior cerebellar peduncle = ataxia, dysmetria 6. Sympathetic fibers = ipsilateral horner

Question 38

Cause of Brown‐Séquard Syndrome and clinical symptoms if i.e. T10 level is affected?

Answer 38

Hemisection/damage of the spinal cord. hemisectionof spinal cord at T10: Ipsilateral loss of proprioceptive sensation, two‐point discrimination and  ipsilateral motor paralysis below the level of lesion (DCML) Contralateralloss of pain and temperature sensations a few levels below lesion (SPINOTHALAMIC)