Lec 14 Somatosensory System 1 and 2 Flashcards
What are exteroreceptors?
receptors that receive stimuli on body surface
What are interoreceptors?
receptors receive stimuli from within body itself
What are proprioceptors?
interoreceptors that receive info about positive of body, head, limbs in space
What types of info about stimulus are encoded by neural code?
- modality
- intensity
- location
- duration
- rate of change
How many type of sensory modality is each axon usually concerned with?
each axon is specific to single type of sensation
What are free nerve endings? what do they sense?
- terminal branches of C fibers or A-delta fibers
- not covered by anything [no myelin]
- enter epidermis as superficially as stratum corneum
- detect painful stimuli, warmth or cold, or mechanical displacement [stretching] of skin
What are hair receptors? What activates them?
- unmyelinated branches that encircle the hair follicle
- multiple for each hair follicle
- terminal axon membrane embedded in the follicle membrane
- activated by hair deflection
What is merkel’s corpuscle? What stimuli do they detect?
- flattened disc formed from terminal axon branch
- associated with modified epidermal merkel cell
- some of merkel cell cytoplasm encloses terminal disk of axon
- detect sustained pressure
What are encapsulated endings?
- terminal axon ends inside tissue capsule
- capsule formed by overlapping processes of several cells [often fibroblast-like cells]
What are pacinian corpuscle
type of encapsulated ending in skin/joints
detect rapid vibrations
What are ruffini endings?
type of encapsulated ending in skin/joints
detect pressure
What do ruffini endings detect?
detect pressure
What do merkel’s corpuscle detect?
detect sustained pressure
What do pacinian corpuscles detect?
detect [rapid] vibrations
What do meissner corpuscles detect?
light touch
What type of encapsulate mechanoreceptor in skeletal muscle? in myotendinous junction?
skeletal: muscle spindle
myotendinous: golgi tendon organ [GTO]
How is intensity of stimulus encoded by mechanoreceptors?
- by frequency of AP and number of fibers recruited
How is location encoded by mechanoreceptors?
by position and size of neuron’s receptive field
How is duration/rate of change determined by mechanoreceptors?
- by discharge characteristics of axon
slowly adapting = will continue to discharge AP as long as stimulus applied
rapidly adapting = discharged during onset, offset, or change
What is difference slowly adapting vs rapidly adapting axons?
slowly adapting = encode for static processes [ex. Meissners]
rapidly adapting = only confer AP at onset or offset of stimulus – when they is a change
What do group 1 muscle fibers innervate?
- group 1 = large diameter axon, heavily myelinated, fast conducting
1a –> primary endings in muscle spindles
1b –> golgi tendon organs
What do group 2 muscle fibers innervate?
- secondary innervation for muscle fibers
Which types of fibers have large diameter axon, heavily myelinated, and fast conducting?
muscle afferents: group 1a, 1b, 2
cutaneous afferents: A-beta
What do group 3 and group 4 do?
pain and temperature in muscle
What is structure of group 3 and A-delta fibers?
thin axon, thin myelin, slow-conducting
What is structure of group 4 and C fibers?
thin axon, unmyelinated, slow-conducting
What are the two type of muscle nociceptors?
Group 3 and Group 4
What is function of alpha-delta/C fibers?
cutaneous pain and temp crude touch
thermoreceptors, nociceptors, small diameter mechano-receptors
many are free nerve endings
Which fibers are part of the medial division of dorsal root? function?
larger afferents
group 1, 2, and A-beta
they sense proprioception and low-threshold touch of cutaneous and and muscle
Which fibers are part of lateral division of dorsal root? function?
smaller afferents
group 3, 4, A-delta, and C
sense pain and temp largely but also inputs from viscera and some high-threshold tactile input
What 3 branches do medial-division fibers branch into when they enter dorsal spinal cord?
- white matter ascending tract [in f.gracilis if below T6, in f. cuneatus if above T6]
- local processing branch:
- – if A-beta: terminates in nucleus proprius [laminae 3/5]
- if group 1/2 in Clarke’s column [laminae VII] or motor neuron pools [laminae 9] - fibers descend caudally in fasiculus interfascicularis/septomarginalis and terminate in nucleus proprius/clarkes/motor neuron pools a few segments below
Where do dorsal column-medial lemniscus ascending fibers terminate?
- ascend via f. gracilis [if below T6] or f. cuneatus [above T6]
- terminate/synapse in nucleus gracilis or cuneatus in medulla
What happens to lateral division fibers when they enter the spinal cord?
- enter spinal cord ventrally
- travel over 2-5 segments and form lissauer’s tract in central/lateral dorsal horn
- terminate on second-order neurons of substantia gelatinosa [lamina 1 and 2]
What two structures make up the dorsal column nuclei of the medulla [DCN]
- gracile nucleus
- cuneate nucelsu
What is the function of fibers in the dorsal-column medial lemniscal [DC-ML] path?
- principal path of info about position, movement, touch, vibration
What are characteristics of DC-ML pathway?
- responsible for somatotopic info
- retain modality and location specificity [little convergence of fibers, receptive field characteristics are transmitted]
- synaptic security [rapid/reliable transmission]
- proportional representation [more important body parts represented more]
What is the path of DC-ML fibers after they reach the medulla?
- synapse in F. gracilis/f. cuneatus of medulla
- 2ndary neurons continue to ascend and cross midline via medial lemniscus tract = “the great sensory decussation” to reach contralateral thalamus
- 2nd synapse in ventral posterolateral nucleus of thalamus
- thalamic fibers project to somatosensory cortex S1, S2 [cortical layer 4]
What is the path of spinothalamic fibers after they enter the spinal cord?
- synapse in substantia gelatinosa in dorsal horn
- secondary nerve fibers cross to the other side of the spinal cord via the anterior white commissure [between central canal and ventral median fissure]
- fibers ascend thus contralaterally from their entrance site in the anterolateral funiculus to the thalamus
- fibers terminate in the thalamus VPL
- ensuing fibers project from thalamus to S1 ,S2 cortical layer 1
What is difference between fibers in the lateral vs ventral spinothalamic tracts?
- lateral spinothalamic tract = pain, temp
- ventral spinothalamic tract is more medial = crude touch
What is difference DC-ML and spinothalamic fiber receptive fields?
- spinothalamic = larger, more overlapping receptive fields
- cruder localization
What are the collaterals in the spinothalamic system?
- spinothalamic fibers have lots of collaterals that end in tectum of midbrain [spinotectal] and reticular formation of mid/pons/med [spinoreticular]
- these collaterals are related to pain mechanisms
What is the trigeminal system?
- somatosensory info from face/head conveyed by V nerve fibers whose cell bodies are in the trigeminal ganglion
- divided into large-diameter vs small-diameter the same as DC-ML vs spinothalamic
What happens to large-diameter fibers in the trigeminal system? function?
- convey touch/pressure/vibration
- terminate in principal trigeminal nucleus in pons on same side = analogous to dorsal column nuclei
- 2nd order neurons cross midline in pons and join ascending medial lemniscus on opposite side
- 2nd synapse in VPM of thalamus
- thalamic fibers project to S1, S2 [layer 4]
What happens to small-diameter fibers in trigeminal system?
- convey pain/temp from face
- enter lateral to pons
- travel caudally to medulla via ipsilateral spinal tract of V
- fibers terminate in spinal nucleus of V
- cross midline and join contralateral ascending spinothalamic tract
- thalamic fiber project to S1, S2
What structure is analogous to lissauer’s tract?
spinal tract of V
What is brown-sequard syndrome?
lateral hemi-section of spinal cord lesion [injury in part but not all of spinal cord]
- below the point of lesion have:
- – contralateral reduced sensation of temp and pain [spinothalamic]
- – ipsilateral reduced 2 point discrimination, vibration, proprioception [dorsal column]
- ipsilateral spastic paralysis, hypertonia below lesion + babinski [cortical spinal, UMN]
- ipsilateral flaccid paralysis, hypotonia, atrophy at the level of the lesion [anterior horn, LMN]
What is sterogenesis? When is it defective?
ability to identify objects placed in hand or to recognize letter or numbers written in hand
defective when lesion of dorsal column
What is tabes dorsalis? What is destroyed in tabes dorsalis? sign?
- due to tertiary syphilis
- dorsal column especially gracile fasiculus destroyed
- illicits rombergs sign [can’t stand upright with eyes closed] = proprioception deficits
- bilateral loss of mechonsensation below leiosn
- no motor deficits
What structure is equivalent to dorsal column nuclei?
principal sensory nucleus of V in pons
What structure is equivalent to substantia gelatinosa?
spinal nucleus of V in medulla
What fibers are responsible for the jaw jerk afferenct and efferent?
afferent: group 1, 2, proprioceptive fibers of CN V enter brainstem
- terminates at motor nucleus of 5
efferent: CN 5 motor nerves from motor nucleus go back and terminate on muscles of mastication
What is syringomyelia?
- cystic enlargement of central spinal canal [most common C8-T1] causes compression of anterior white commissure
- get bilateral loss of pain/temp a few levels above and below the lesion in cape-like distribution
- preservation of fine touch + mechanosensation
What terminates in VPM vs VPL of thalamus?
big fibers of DC-ML terminate in VPL
big fibers of trigeminal mechanosensation terminate in VPM
fibers from spinothalamic/small fibers of trigeminal not as organized
Is the medial lemniscus somatopically organized when it enters thalamus? What about the spinothalamic tracts at the point? how?
- DC-ML organized from lateral to medial LTANF [legs, trunk, arms, neck, face]
- spinothalamic not organized
What is the purpose of the ventral posterior [VP] nucleus of thalamus?
- major somatosensory relay nucleus of thalamus
- only site of termination of medial lemniscus
- major site of termination of spinothalamic fibers
Where does the VP nucleus project to?
S1 [first somoatosensory cortex]
S2 [2nd somatosensory cortex]
Where is S1 located? S2?
S1 = caudal part of central sulcus and postcentral gyrus
S2 = part of parietal operculum [ceiling of lateral fissure]
What are the parts of the VP nucleus of thalamus?
- VPM = medial, receives lemniscal input from contralateral trigeminal nucleus
- VPL = lateral, lemniscal input from non-trigeminal
What is the order from lateral to medial of somatosensory lemniscal representation in the ventral posterior nucleus?
lateral - legs - torso - arms - neck - face [head, face, inside mouth] medial
technically face in VPM, rest is VPL
What parts of body have highest representation of VPL/VPM lemniscal fibers?
- foot, hands, and lips since these have highest density of innervation and thus greatest sensory acuity
What are 3 properties of lemniscal relay neurons in thalamus?
- high synaptic security [reproduce faithful temporal AP pattern]
- modality and place specificity from specific receptor type
- neurons with same place and modality characteristics cluster together [LTANF]
- surround inhibition = activation of one group of neurons is accompanied by simultaneous inhibition of its neighbors
What are the 2 types of neurons in thalamic nuclei?
- relay neurons – relay to mostly cortex, exhibit properties that mirror their input
- instrinsic inhibitory interneurons, use GABA
What are 4 properties of spinothalamic input?
- pain/temp fibers terminate widely and diffusely across VP, not somatotopically organized
- low transmission fidelity —> need intense stimuli to drive AP
- large receptive fields [not as specific location info]
- not modality specific –> can respond to noxious [pain] stimuli, thermal, or both
What are the 4 separate areas of S1 layer 4? How is each area structured?
area 3a, 3b, 1, 2
- within each area there is a contralateral body-map with leg most medial, face/head most lateral, also have disproportionate rep. of feet/hands/lips like in VP
What type of info is received by areas 3a and 2? fiber type?
- proprioceptive info
- group 1a, 1b, 2 fibers
- also spinothalamic input
all contralateral
What type of info is received by areas 1 and 3b? fiber type?
- cutaneous tactile info
- A-beta fibers
- also spinothalamic input
all contralateral
Which of the four S1 areas receive spinothalamic input?
all 4 areas
How is the S1 cortex organized? where does DC-ML and spinothalamic terminate within?
- 6 layers “cortical columns”
- columns arranged by receptive field span/neuron response properties
- information integrated across cortical columns by long-range axons running perpendicular
- DC-ML input terminated in layer 4 across 4 areas
- spinothalamic input distributes widely in layer 1 across all 4 areas
What is function of S2? Where do the fibers in S2 come from?
process info during bimanual manipulation
represents both sides of body
fibers from contralateral body come from : branches of thalamic projection destined for S1 [via internal capsule]
fibers from ipsilateral body come from: contralateral S1 via corpus callosum
What happens in destruction of S2?
loss of interhemispheric transfer of info
can’t discriminate objects on basis of size and texture
Via what structure does VPM/VPL project to S1/S2?
via internal capsule
via corpus callosum from contralateral S1 for S2 ipsilateral info
