L13 - Anatomy of Somaesthetic Pathway Flashcards
Classification of sensations?
1) Conscious (cerebral cortex, e.g. motor, somatosensory):
a) Exteroception
b) Proprioception
2) Unconscious (e.g. cerebellum; balance):
a) Interoception
b) Proprioception
Define exteroception and interoception.
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
Difference in proprioception between conscious and unconscious sensation?
Conscious = mediated by dorsal column- medial lemniscal pathway
Unconscious = mediated by spinocerebellar
All sensations lead to perception. True or False?
False
Depends on whether the sensation is processed by the cerebral cortex
Organization of somatosensory system? (3 levels)
- Receptorlevel: sensoryreceptors
- Circuitlevel: processinginascendingpathways
- Perceptuallevel: processingincorticalsensory areas
Classify sensory receptors by distribution and by function.
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…)
Structure of sensory neurons?
Pseudounipolar sensoryneuron:
twofunctionallydistinctsegments,bothfunctionasaxons;
1) oneextendstoperipheralskinormuscle
2) theothertothecentral spinalcord.
Relate the type of axons in sensory neurons to the signal transmitted by receptors?
Aα = proprioceptors of skeletal muscles
Aβ = Mechanoreceptors of skin
Aδ = Pain, temp
C (unmyelinated) = temp, pain, itch
List the major ascending pathways to the brain. Location in spinal cord?
1.Spinothalamic(anterolateral)pathway
- Dorsalcolumn‐mediallemniscus(posteriorcolumn)pathway
- Spinocerebellarpathway
Compare the end-points of the 3 major ascending pathways?
Dorsal column medial lemniscus + Spinothalamic tract = transmit impulsesviathethalamus tothesensorycortexfor consciousinterpretation.
Spinocerebellar pathway = terminatesinthecerebellum,anddoesnot contributetosensoryperception
Compare the sensory information transmitted by the DCML pathway and the Spinothalamic pathway
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
Compare the receptors of DCML and spinothalamic tract.
DCML:
- Merkel cells
- Pacinian corpuscles
- Muscle spindles
- Tendon organs
Spinothalamic:
Free nerve endings (e.g. nociceptors)
Compare the Dorsal root axon between DCML and spinothalamic tract.
DCML = Large diameter myelinated fiber: Aα, Aβ, Aδ
Spinothalamic: Smaller diameter myelinated, unmyelinated fibers: Aδ, C (terminate in dorsal horn)
Location of soma, route and end-point of first-order neuron in DCML and spinothalamic pathway.
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
Location of soma, route and end-point of SECOND- order neuron in DCML and spinothalamic pathway.
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
Location of soma, route and end-point of THIRD- order neuron in DCML and spinothalamic pathway.
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
The connection between first, second and third order neurons in the somatosensory pathway is always direct. True or False?
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
Summarize the 1st, 2nd, and 3rd order neurons and destination for DCML pathway.
- 1storderneuron:posterior/dorsalroot ganglion ipsilateral
- 2ndorderneuron:gracilenucleus& cuneatenucleus(decussate inmedulla oblongata midline)»_space;> contralateral medial lemniscus
- 3rdorderneuron:ventralposterolateral (VPL)nucleusofthethalamus
- Destination:Primarysomatosensory cortex
Difference in function of the 2 nuclei of the second order neruon in DCML?
Cuneate nucleus(more lateral) : arm, upper body
Gracile nucleus (more medial): leg, lower body
Summarize the 1st, 2nd, and 3rd order neurons and destination for spinothalamic pathway.
- 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
Compare the tracts that form the DCML and spinothalamic tract?
DCML = paired tractsofthedorsalwhitecolumnofthe spinalcord—cuneatefasciculusand gracilefasciculus—andthemedial lemniscus.
Spinothalamic = lateral andanteriorspinothalamictracts
Compare the decussation points between DCML and spinothalamic pathways.
DCML = Gracile/ Cuneate nuclei»_space; contralateral medial lemniscus across midline of medulla oblongata
Spinothalamic = Anterior white commissure of spinal cord»_space; contralateral spinothalamic tracts
Compare the bifurcation of DCML and spinothalamic pathways’ second order neurons.
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
Spinocerebellar tract requires 3 neurons. True or False?
False
Only need 2, unlike DCML and spinothalamic tract
Function of spinocerebellar pathway?
Conveyinformationfrommuscleortendon stretchtothecerebellum,whichusesthis informationtocoordinateskeletalmuscle activity.
Donotcontributetoconscious sensation
Forunconscious proprioception primarilyterminateintheipsilateralcerebellum (not primary somatosensory cortex)
Describe the decussation of the spinocerebellar tracts?
Dorsal spinocerebellar tract = Do not decussate
Ventral ‘’ = decussate TWICE
Cuneocerebellar tract = Do not decussate
Compare the proprioceptive afferents from the different spinocerebellar tracts.
- Dorsalspinocerebellartract:proprioceptive afferentsfromlowertrunkandlowerlimbs, synapse withClarke’scolumncells
- Cuneocerebellar tract:upperlimbsandneck proprioceptiveinformation
- Ventralspinocerebellartract:afferentsfrom lowerlimbsspinalmotorneuronsand interneurons
Which cranial nerves have somatosensory components?
CN 5,7,9,10
Trigeminal
Facial
Glossopharyngeal
Vagus
Describe the 1st, 2nd, 3rd order neuron paths for cranial nerve with somatosensory components.
1st = sensorynucleusofthe cranialnerveofthesameside
2nd = some cross(somedonot)tothe oppositesideinthebrainstem andascendtothethalamus.
3rd = (atVPM) ascendstothesomatosensory cortex.
Compare the sensory information relayed by cranial nerves with somatosensory components.
Trigeminal = Discriminative touch, proprioception (e.g. mastication)
facial (VII), glossopharyngeal (IX), vagus (X) = Pain, temp
Compare the location of 2nd order neurons of cranial nerves with somatosensory components.
Trigeminal = Anterior and posterior trigeminothalamic tract
VII, IX, X = Caudal > relay through spinal trigeminal tract and nucleus (pars caudalis)
Describe the processing of sensory information at the perceptual level of the somatosensory pathways.
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»_space; reaches our awareness
Compare the results of damage to primary sensory neurons vs thalamic projection neurons in spinal cord / brainstem to third order neurons.
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
Define dermatome. What forms it?
- area of skin supplied by right and left dorsal roots of a single spinal segment
- formed by peripheral processes of adjacent spinal nerves overlap
What causes Wallenbergsyndrome?
Wallenbergsyndrome/ PICAsyndrome/ Lateralmedullary syndrome
Blood clot at posterior inferior cerebellar artery
List the 5 nuclei/ tracts affected in wallenberg syndrome and relate to 5 clinical manifestations.
- Contralaterallossofpainand temperaturesensationfromthe body(spinothalamictract)
- Ipsilaterallossofpainand temperaturesensationfromthe face(spinaltrigeminaltractand nucleus)
- Vertigoandnystagmus(vestibular nuclei)
- Lossoftastefromtheipsilateral halfofthetongue(solitarytract andnucleus)
- Hoarsenessanddysphagia, decrease gag reflex (nucleusambiguus orrootsofCN IX,GlossopharyngealandX,Vagus) Specific to PICA
List the 6 nuclei/ tracts affected in AICA syndrome and relate to 6 clinical manifestations.
- Spinal trigeminal nucleus = Loss of pain and temp sensation in ipsilateral face
- Vestibular nuclei = vertigo, nausea, vomiting
- Cochlear nucleus = ipsilateral hearing impairment specific
- Facial nerve = ipsilateral facial paralysis, loss of tast at ant. 2/3 tongue, decrease lacrimation and salivation specific
- Inferior cerebellar peduncle = ataxia, dysmetria
- Sympathetic fibers = ipsilateral horner
Cause of Brown‐Séquard Syndrome and clinical symptoms if i.e. T10 level is affected?
Hemisection/damageofthespinalcord.
hemisectionofspinalcordatT10:
Ipsilaterallossofproprioceptivesensation,two‐pointdiscriminationand ipsilateralmotorparalysisbelowtheleveloflesion (DCML)
Contralaterallossofpainandtemperaturesensationsafewlevelsbelowlesion (SPINOTHALAMIC)
