8/29 Somatosensory Pathways - Suss Flashcards
sensory neuropathy
negative symptoms
loss of sensation
- analgesia: loss of pain
- anesthesia: loss of touch
“numbness, heaviness, weakness, deadness”
sensory neuropathy
positive symptoms
abnormal sensory phenomenon
- paresthesias: temporary mild pain
- “pins and needles, tingling, burning, prickling”
- neuropathic pain/Central Pain Syndrome : chronic intense pain
- “shooting, stabbing, electric shock-like jolts”
- can’t be treated with conventional analgesics (sometimes anticonvulsants, tricyclic antideps)
encoding of elementary sensory attributes
modality : subsystem for processing diff kinds of stimuli
- touch/vibration
- proprioception
- temperature sense
- pain
intensity : strength of stimulus
- sensory threshold: lowest stimulus strength a subject can detect (i.e. min energy required to generate an AP); determined by sensitivity of receptors
timing : slowly adapting vs. rapidly adapting
location : affected by receptor density, receptive field, inhibitory mechanisms
somatosensory modalities
- modality
- stimulus energy
- receptor class
- receptor cell tyeps
3 major long pathways in CNS
pathway; modality; site of decussation
1. corticospinal tract (CST)
- modality: motor
- site of decussation: pyramids (spino-medullary jx)
2. Dorsal Column-Medial Lemniscus System (DCMLS)
- modality: sensory (vibration, joint position, fine touch)
- site of decussation: lower arcuate fibers (lower medulla)
3. spinothalamic tract (STT)
- modality: sensory (pain, temp, crude touch)
- site of decussation: anterior commissure (spinal cord)
parallel pathways
what are they?
whats the point?
segregation of nerve cell axons that process the distinct stimulus attributes that comprise a particular modality = parallel pathways
why?
compensation! improved reliability: if one breaks down, the other can pick up the load
speed! multiple tracts means signal can get from one place to another quicker
what do the DCMLS and the SCT sense?
what do they have in common?
what distinguishes them?
dorsal column-medial lemniscus system : fine touch, vibration, proprioception
spinothalamic tract : pain, temp, crude touch
similarities:
- both have sensory neurons all over body
- 3 neuron pathway w/ 2 relay points
- primary: dorsal root ganglion neuron
- secondary: CNS
- tertiary: thalamus → cortex
- both cross over to contralateral side
differences:
- slightly diff types of receptors
- slightly diff types of DRG neurons (due to morphology)
sensory threshold
lowest stimulus strength a subject can detect (i.e. min energy required to generate an AP)
determined by sensitivity of receptors
receptors differ in their timing of responses to a stimulus
- slowly adapting = tonic
- detects static qualities of stimulus
- rapidly adapting = phasic
- detects dynamic qualities of stimulus
two point discrimination
lets you know how well your patient can discriminate between sensory stimuli in terms of space
“minimal insterstimulus distance required to perceive two simultaneously applied stimuli as distinct”
- spatial resolution
*higher density of receptors = better resolution
receptive field
region in sensory space within which a specific stimulus elicits the greatest AP response
receptive fields determine resolution
touch in receptive field? increases activity of cortical neuron
touch outside of receptive field? no change in activity
- touch in area JUST OUTSIDE of receptive field?* INHIBITS activity! (lets you distinguish border of stimulus)
- neurons that want to be heard silence their neighbors so that their message is heard
dermatomes
area of skin innervated by a single dorsal root ganglion
- differ among individuals
- display overlap
- axons making up a dorsal root originate from several different peripheral nerves
- individ peripheral nerves contribute axons to adjacent dorasal roots
- overlap more sensitive for touch/vibration than pain/temp → can be easier to distinguish with pain and temp in a physical exam!
THEREFORE: clinically, see 2 sequential dermatomal regions affected in order to perceive it
morphology of cutanous mechanoreceptors
4 types (location, detection)
- Meissner’s corpuscles
- just below hairless skin
- detect surface/motion
- rapidly adapting, low threshold of activation
- test: Qtip, test touch on either side of body
- Merkel cells
- located at tip of epidermal ridge
- useful for edges and indentations
- slowly adapting
- Ruffini corpuscle
- located in dermal layer
- aligned in parallel longitudinally, in line with stretch lines of skin
- detect skin stretch
- Pacinian corpuscle
- deep, subcutaneous layer
- onion-like
- detect vibration
morphology of proprioceptors
1. muscle spindles
- detect muscle length
- consist of intrafusal muscle fibers in capsule (arr. in parallel with extrafusal muscle fibers)
- extrafusals innervated by alpha motor neurons
- intrafusals innervated by gamma motor neurons (resets for load)
- innervated by Ia afferents (rapidly adapting changes) and II afferents (slowly adapting changes)
2. Golgi tendon organ
- detect muscle tension
- arranged in series (not parallel) with extrafusal fibers
- innervated by Ib afferents that intercalate in collagen fibrils of GTO → when muscle contracts, they get activated and send signal to cortex
test: joint position sense
types of sensory neurons within a single DRG
INCOMPLETE
4 classes
- differ in diameter, amt of myelination
sensory fx : receptor type → axon type
- PROPRIOCEPTION : muscle spinder → Ia, II
- TOUCH : Merkel, Meissner, Pacinian, Ruffini → Abeta
- PAIN, TEMP : free nerve endings → Adelta
- PAIN, TEMP, ITCH : free nerve endings (unmyelinated) → C
Pacinian corpuscles
- deep, subcutaneous layer
- onion-like
- detect vibration
trigeminal mechanosensory system
touch, vibration information from face to cortex
mechanosensory receptors from face send info through dorsal roots into pons
FIRST SYNAPSE in principal nucleus of trigeminal complex/chief nucleus
second neuron is decussates over to contralateral side of spinal cord, begins to ascend on trigeminal lemniscus
trigeminal lemniscus travels up through midbrain → SECOND SYNAPSE in thalamus (ventral posterior medial nucleus)
after second synapse, third order projection neuron runs from VPM_thalamus to primary somatosensory cortex!
DCML Pathway part1
touch, vibration and proprioception info from spinal cord to cortex
mechanosensory receptors from body send info through dorsal roots into tracts up to medulla
- lower body : enter at lumbar, travel on gracile tract
- upper body : enter at cervical, travel on cuneate tract
FIRST SYNAPSE on cell bodies in dorsal column of caudal medulla
- gracile tract → gracile nucleus
- cuneate tract → cuneate nucleus
at first synapse, second neuron is decussates via INTERNAL ARCUATE FIBERS over to contralateral side of spinal cord, begins to ascend on medial lemniscus
medial lemniscus travels up through rostral medulla, pons, midbrain → SECOND SYNAPSE in thalamus (ventral, posterior lateral nucleus)
after second synapse, third order projection neuron runs from VPL_thalamus to primary somatosensory cortex!
spinocerebellar tract for prioprioception
involves large diameter neurons
always ipsilateral
-
posterior SCT : lower limbs
- hit Clark’s nucleus in lumbar SC
- cuneocerebellar tract : upper limbs
- anterior SCT : lower limbs (interneurons), decussates twice
- rostral SCT : upper limbs (interneurons)
clinical correlates involving dorsal columns and spinocerebellar tract
Tabes Dorsalis caused by tertiary syphilis infection
- degen of dorsal columns causing:
- impaired sensation and proprioception
- progressive sensory ataxia
Subacute Combined Degeneration caused by Vit B12 or E deficiency
- demyelination of dorsal columns, lateral corticospinal tracts, and spinocerebellar tracts causing
- ataxic gait
- paresthesia
- impaired position/vibration sense
somatosensory portions of thalamus and cortical targets
thalamus:
- VPL (ventral posterior lateral) : lower/upper body
- VPM (ventral posterior medial) : face
primary somatosensory cortex
- comprised of subregions: Brodmann’s areas 3a, 3b, 1, 2
- 2: complex feature detection via sterognosis (orientation) and graphesthesia (direction)
- each has a separate and complete map of contralateral body surface (homunculus pattern)
connections from primary somatosensory cortex
- posterior parietal cortex (areas 5, 7) → motor/premotor cortex for association
- involved in attn
- secondary somatosensory cortex → limbic system (amygdala, hippocampus)
- tactile learning, emmory
nocireceptors and thermoreceptors
A-delta and C afferents both carry pain and temp signals
- A-deltas have a liiiittle myelin and are slightly bigger than C → faster conduction
each thermoreceptive neuron only expresses a single type of temp receptor
- slow adapting
- fire mostly during changes of temp
spinothalamic system for discriminative aspects of pain/temp in body
for pain, temp, crude touch
enter through dorsal root, hit Lissauer’s tract to get onto the level of spinal cord they need to get to, and then penetrate into dorsal horn
FIRST SYNAPSE in dorsal horn
- C fibers : layer 2 of Rexed’s laminae
- A-delta fibers : layers 1, 5 of Rexed’s laminae
after synapse, secondary neuron ASCENDS and DECUSSATES (crosses over over 2 segments) in anterior white commissure
continues to move up via anterolateral system all the way up to VPL nucleus of thalamus where it completes SECOND SYNAPSE
third order neuron goes from VPL to primary somatosensory cortex
trigeminal system for discriminative aspects of pain/temp in face
for pain, temp, crude touch
enter through trigeminal ganglion, travels DOWNWARD through spinal-trigeminal tract and synapses (FIRST SYNAPSE) in caudal medulla in spinal nucleus of trigeminal complex
DECUSSATION occurs in caudal medulla and second neuron continues up through trigemino-thalamic tract → VPM of thalamus
review of DC-MLS
dorsal column-medial lemniscus system
review of STT
spinothalamic tract
review of trigeminal sensory system nuclei/pathways
touch vibration/proprioception
- in through trigeminal ganglion → chief/principal nucleus of CN V
- decussation in pons → ascend in trigeminal lemniscus to VPM → out to somatosensory cortex
pain/temp
- in through trigeminal ganglion → descend through spinal tract of CN V
- decussation in lower medulla → ascend in trigemino-thalamic tract to VPM → out to somatosensory cortex
review of thalamic relays of somatosensory system
VPL vs VPM (info received from..)
VPL receives info from
- medial lemniscus
- spinothalamic tract
VPM receives info from
- trigeminal lemniscus
- trigeminothalamic tract
graphical representation
general rule for associated/dissociated symptoms of
- touch/vibration
- pain/temp
lesion in SPINAL CORD will give you DISSOCIATED touch/vib and pain/temp symptoms
lesion ABOVE MEDULLA will give you ASSOCIATED touch/vib and pain/temp systems
variable: location of decussation!
plasticity
- repeated use/practice can alter the cortical area of representation assigned to a body part (ex. hand)
-
amputation can cause change in cortical representation
- can cause phantom limb and phantom pain sensations!
