Somatosensory system: Flashcards
somatosensory receptors: types
- general somatic afferents (GSA)
- general visceral afferents (GVA)
somatosensory receptors: general somatic afferents
- cutaneous = exteroceptors
- muscles, tendons, joints = proprioceptors
somatosensory receptors: general visceral afferents
- internal organs (viscera) = enteroreceptor
cutaneous receptors: mechanoreceptors
- touch
- pressure
- vibration
- stretch
cutaneous receptors: thermoreceptors
- hot
- cold
cutaneous receptors: nociceptors
- intense mechanical
- intense hot and cold
- chemicals
mechanoreceptors (non-nociceptive): typical features
- low threshold (highly sensitive)
- glutamatergic
- predominantly A-beta afferent nerve fibres (large, myelinated, fast)
hair follicle (peritrichial) receptors (RA1): features
- detects bending of hairs (air movement, touch)
- bending of hair stretches membrane
- opens mechanically gated cation channels in membrane of afferent fibre
- depolarisation –> action potentials
hair follicle (peritrichial) receptors (RA1): non-encapsulated endings
- single process of an afferent sensory fibre, wrapped in spiral around base of hair
hair follicle (peritrichial) receptors (RA1): adapting
rapid adapting
hair follicle (peritrichial) receptors (RA1): receptive field
small receptive field
Meissner’s corpuscle (RA1): features
- glabrous (hairless) skin only (eg. fingertips)
- superficial location
- sheath of ct, packed w pancake-like stack of pear-shaped Schwann cells (teloglia)
- compression causes mechanically gated cation channels in membrane to open
- depolarisation -> action potentials
Meissner’s corpuscle (RA1): good for and adapting
- discriminative touch
- grip
- touch
rapid adapting
Meissner’s corpuscle (RA1): receptive field
- encapsulated capsule
- small receptive field
Pacinian corpuscle (RA2): good for and adapting
- pressure and vibration
- rapid adapting (sustained pressure dissipates)
Pacinian corpuscle (RA2): receptive field
- large receptive field
Pacinian corpuscle (RA2): pressure causes
- mechanically gated Na channels in afferent nerve ending to open-> depolarise cell -> AP
Pacinian corpuscle (RA2): features
- concentric layers of flattened fibroblasts (ct)
- viscous fluid in btw
duplex theory: tactile texture perception - depends on
- spatial cues
- temporal cues
duplex theory: tactile texture perception eg. spatial cue
- size
- shape
duplex theory: tactile texture perception eg. temporal cues
- rate of vibration as object moved across skin
duplex theory: tactile texture perception for fine texture
- spatial cues alone can’t be used in static touch
- must move fingers across surface
duplex theory: tactile texture perception vibration sensitivity from
- high freq vibration receptors (Pacinian corpuscles RA2)
Merkel cell - neurite complex (SA1): features and adapting
- superficial (lips, fingertips)
- discriminative touch and pressure
- edges of objects (shape, curvature) texture
- slow adapting
Merkel cell - neurite complex (SA1): receptive field
- small receptive field
Merkel cell - neurite complex (SA1): mechanism
- afferent fibre terminals (neurites) form disks which are mechanoreceptive
- discs contact epidermal cells (Merkel)
Merkel cell - neurite complex (SA1): merkel cell job
- also mechanoreceptive
- release NT glutamate onto afferent terminals (disks) -> depolarisation -> AP
Ruffini’s corpuscles (SA2): adapting and field
- slow adapting
- large receptive field
Ruffini’s corpuscles (SA2): for and located
- stretching of skin (ligs, tendons)
- joint position (kinaesthesia and proprioception)
- long axis along stretch lines of skin
Ruffini’s corpuscles (SA2): features
- thin, cigar shaped capsule deep in skin
- interior of capsule have ct
- branched terminals of primary afferent sensory fibre lie btw strands
Ruffini’s corpuscles (SA2): mechanism
- stretching compresses mechanoreceptive nerve terminals
- opens cation channels
- depolarises primary afferent -> AP
free nerve endings- mechanoreceptive (non-nociceptive): features
- terminal branches of afferent fibres (Type C, unmyelinated) btw epidermal cells
- membrane distortion opens mechanically gated cation channels
free nerve endings- mechanoreceptive (non-nociceptive): adaptive and receptive field
- slow adapting
small field: pleasant touch
large field: crude ‘non-discriminative’ touch
free nerve endings-general visceral afferents: faetures
- most visceral receptors are also free nerve endings
- mostly subconscious sensory pathways (visceral autonomic reflexes)
free nerve endings-general visceral afferents: types
- mechanoreceptors
- chemoreceptors
- nociceptors
free nerve endings-general visceral afferents: mechanoreceptor eg
mechanoreceptor:
- walls of hollow organs, blood v
- eg. aortic arch
- respond to increase in BP -> vasodilation and lower heart rate
free nerve endings-general visceral afferents: chemoreceptor eg
- blood gases
- blood pH
- trigger compensatory respiratory and cardiovascular changes
free nerve endings-general visceral afferents: nociceptor eg
- signal pain
- excessive distention of an organ
free nerve endings- thermoreceptors (RA2): adapting and receptive field
- rapid adapting
- large receptive
free nerve endings- thermoreceptors (RA2): features
- superficial layers of skin
- transduction = temperature sensitive transient receptor potential (TRP) ion channels
free nerve endings- thermoreceptors (RA2): non-nociceptive thermal sensation
warm: 29-45˚ maximal activity at 45˚
cold: 5-40˚ maximal activity at 25˚
TRP channels and thermoreception: features
- non specific cation channels (Ca, Na)
- different TRP variants for dif temp
- activation cause influx of cations and depolarisation of cell -> increase firing of AP
TRP channels and thermoreception: TRP channel agonists
- chilli and mint
thermoreceptors: features
- rapidly adapting (best when temp changes)
- signal direction of temp change
thermoreceptors: eg. drop in temp
- momentarily silences warm receptors while cold receptors show burst in firing
- vice versa
thermoreceptors: perceived temp
comparing relative activities of warm and cold receptors
- population coding
proprioceptors: list types
- muscle spindle Type Ia
- muscle spindle Type II
- golgi tendon organs
- ruffini’s corpuscles
- pacinian corpuscles
proprioceptors: located in mm
- muscle spindle Ia, II
proprioceptors: located in tendons
- golgi tendon organs
proprioceptors: located in joint capsule
- ruffini’s corpuscle
- pacinian corpuscle
proprioceptors: modality- muscle spindles
Ia: rate of mm stretch
II: degree of mm stretch
proprioceptors: modality- golgi tendon organs
- tendon stretch
proprioceptors: modality- ruffini’s and pacinian corpuscles
- stretch in articular cap
proprioceptors: adaptation slow
- muscle spindle II
- golgi tendon organs
- ruffini’s corpuscles
proprioceptors: adaptation fast
- muscle spindle Ia
- pacinian corpuscles
ascending sensory pathways: generalised circuit has
- 1st, 2nd and 3rd order neurons
ascending sensory pathways: generalised circuit - 1st order neuron
- pseudounipolar
- transmits sensory info from PNS to CNS
- cell body usually in ganglion
- synapse w 2nd order in spinal cord/ brainstem
ascending sensory pathways: generalised circuit - 2nd order neuron
- multipolar neuron
- relay sensory info from spinal cord/ brainstem to thalamus
- also (reticular system, limbic, cerebellum)
ascending sensory pathways: generalised circuit - 3rd order neuron
- multipolar
- relay sensory info from thalamus to cortex where conscious sensory perception occurs
ascending sensory pathways: lateral spinothalamic (ALS) pathway for
body:
- pain
- temp
ascending sensory pathways: anterior spinothalamic (ALS) pathway for
body:
- crude (non-discriminative) touch
- pressure
ascending sensory pathways: dorsal column-medial lemniscal (DCML) for
body
- discriminative touch
- pressure
- kinaesthesia
- conscious proprioception
ascending sensory pathways: face- discriminative touch and pressure, kinaesthesia
- primary trigeminal (pons)
ascending sensory pathways: face- pain, temp, crude touch, pressure
spinal trigeminal (cervical cord)
ascending sensory pathways: pain and pressure in viscera
spinothalamic (ALS)
ascending sensory pathways: subconscious
- most proprioceptive info not processed consciously, directed to cerebellum without passing thalamus
ascending sensory pathways: spinocerebellar, spinohypothalamic for
- subconscious proprioception
- autonomic activation
ascending sensory pathways: subconscious pathway/ target/ function- proprioceptors
- post spinocerebellar
- cerebellum
- coordination of mm movements
- maintain posture
ascending sensory pathways: subconscious pathway/ target/ function- mechanoreceptors
- ant spinocerebellar
- cerebellum
- coordination of mm movements
- maintain posture
ascending sensory pathways: subconscious pathway/ target/ function- nociceptors/ thermoreceptors in spinohypothalamic pathway
- hypothalamus
- autonomic (hormonal etc)
- reflex and emotional aspects of pain
ascending sensory pathways: subconscious pathway/ target/ function- nociceptors/ thermoreceptors in spinoreticular pathway
- reticular formation
- arousal
- alertness
- evading injury
spinothalamic pathway: primary afferents
terminate in spinal cord (dorsal horn)
- nociceptors
- thermoreceptors
- mechanoreceptors (crude touch, pressure)
spinothalamic pathway: 2nd order neurons will
- decussate in spinal cord, travel contralaterally and terminate in
- thalamus (ventral-post complex)
- reticular formation (arousal, alertness)
spinothalamic pathway: 3rd order neurons will terminate in
- primary somatosensory cortex (SI) - discrimination
- limbic cortex (cingulate gyrus, insular cortex) - hypothalamus - affective motivatinal
dorsal column-medial lemniscal pathway: primary afferents
- mechanoreceptors
- proprioceptors
- axons ascend ipsilaterally in dorsal columns and terminate in medulla:
- cuneate nucleus (upper body)
- gracile nucleus (lower body)
- collaterals (reflexes) terminate in spinal cord (dorsal horn)
dorsal column-medial lemniscal pathway: 2nd order
- axons decussate forming medial lemniscus (sensory decussation)
- terminate in thalamus (ventral post complex)
dorsal column-medial lemniscal pathway: 3rd order
- terminate in primary somatosensory cortex (SI) -> discrimination
trigeminothalamic pathway: features
- principal somatosensory nerve of head
- sensory info from face, nose, mouth, teeth, eyes, (excl. retina), dura, blood vessels
trigeminothalamic pathway: branches
- ophthalmic n
- maxillary n
- mandibular n
trigeminothalamic pathway: primary afferents
- converge in trigeminal n
trigeminothalamic pathway: 1st soma
trigeminal ganglion (PNS)
trigeminothalamic pathway: 2nd soma
various nuclei in brainstem and cervical spinal cord
trigeminothalamic pathway: 3rd
thalamus (ventral post)
parietal lobe: somatosensory cortex SI primary
- postcentral gyrus
- somatotopic organisation
- raw somatosensory info from thalamus
- encodes type, intensity, localisation of sensory input
parietal lobe: somatosensory cortex SII secondary
- parietal operculum/ upper lip of lateral sulcus
- somatotopic organisation
- high order somatosensory functions (sensorimotor integration, integration of bilateral sensory info, attention, learning, memory)
sensory homunculus:
- area of cortex devoted to given body region is proportional to sensory input (no. of sensory afferents) from that region vs physical SA
dermatome: define
- sensory region on surface of body that sends afferents via spinal n
parietal lobe: primary somatosensory SI cortex functional organisation
divided into narrow strips -> relate to Brodmann’s areas 3a, 3b, 1, 2
- different sensory receptor type
- each area in SI has separate somatotopic map
parietal lobe: somatosensory cortex columnar organisation
- area 3b: functionally distinct (vertical columns) receive input from SA or RA skin receptors in same body region
parietal lobe: somatosensory cortex hierarchal organisation
- cortical neurons show increase in size of receptive field from area 3b - 1 - 2 - 4
- increase complexity of somatosensory processing and intergration
parietal lobe: somatosensory cortex -post parietal cortex
- somatosensory association cortex areas 5, 7
- receives input from SI and SII
- stereognosis
integration of somatosensory and visual input:
- dorsal ‘where’ stream
- spatial vision
- coordinates motor output via motor cortex
- arm, hand, eye movements
stereognosis:
haptic (tactile) perception
- perceive and recognise object by touch without visual/ auditory info
stereognosis: cues
- size
- shape
- texture
- temp
stereognosis: pathway and diagnostic test for
- DC-ML mostly
- for integrity of parietal lobe: postcentral gyrus, parietal association area
astereognosis:
- Alzheimer’s disease
- less in other forms of dementia
- disturbance of associative links btw tactile info and memory
