PHYSIOLOGY - Somatosensory system and ascending tracts Flashcards
what senses does the DCML pathway transmit
fine touch
proprioception
vibration
what are the 1st order neurones of the DCML
peripheral nerves to medulla oblongata
sensory information enters the spinal cord where
dorsal root ganglion
signals from where travel in the fasciculus cuneatus to the cuneate nucleus in the DCML
upper limb - T6 and above
signals from where travel in the fasciculus gracilis to the gracile nucleus in the DCML
lower limb - below T6
is the fasciculus gracilis or cuneatus more lateral
fasciculus gracilis is medial
fasciculus cuneatus is lateral
where do all fibres in the DCML decussate
medulla
what are the second order neurones in the DCML and where do they travel through
medulla (cuneate or gracile nucleus)
to the thalamus
contralateral medial lemniscus in brainstem
what are the 3rd order neurones in the DCML and what do they travel through
VP nucleus of thalamus to the primary sensory cortex of the brain through internal capsule
what does the spinothalamic tract transmit
pain
temperature
pressure
describe the path of the 1st order neurones of the spinothalamic tract
peripheral nerves, enter spinal cord, ascend 1-2 levels and synapse at tip of dorsal horn (substantia gelatinosa)
describe the path of second order neurones of the spinothalamic tract
substantia gelatinosa to the thalamus
where do fibres of the spinothalamic tract decussate
spinal cord
after decussating the spinothalamic splits to form how many pathways
anterior and lateral
which pathway transmits crude touch and pressure
anterior spinothalamic
what does the lateral spinothalamic tract transmit
pain and temperature
describe 3rd order neurones of the spinothalamic tract
VP nucleus of the thalamus through internal capsule to the primary sensory cortex
what does the spinocerebellar tract control
unconscious info from muscles to ipsilateral cerebellum
what are the 4 tracts of the spinocerebellar tract
posterior
anterior
cuneocerebellar
rostral tract
which tracts transmit info from muscles of upper limb to cerebellum
rostral and cuneocerebellar
which tracts transmit info from muscles of lower limbs to cerebellum
anterior and posterior
which tract decussates twice meaning the signals go to the ipsilateral cerebellum
anterior spinocerebellar
what are the 3 categories of the somatosensory system
exteroceptive - cutaneous
proprioceptive
enteroceptive - internal
what is the cell body location of 1st order sensory neurones
dorsal root ganglia
what has its 1st order neurone cell body in the cranial ganglia
CN V - sensory innervation of the anterior head
sensory 3rd order neurone cell bodys are all located where
thalamus
stimulus opens a anion/cation selective ion channel in peripheral terminal or primary sensory afferent eliciting a depolarising receptor potential
cation
the amplitude of the receptor potential is inversely proportional/proportional to the stimulus intensity
proportional
a supra-threshold receptor potential triggers an all or none action potential conducted by the axon, at a ____ proportional to its amplitude
frequency
what is the relationship between stimulus strength and amplitude of observed and why
non-linear
greater sensitivity to change at low stimulus strength
what is the term for the type of energy that a sensory cell is tuned to respond to
modality - the adequate stimulus
what detects mechanical forces on the skin (touch, pressure, vibration)
skin mechanoreceptors
what detects proprioception / mechanical forces acting on joints and muscles
joint and muscle mechanoreceptors
what detects temperature
cold and warm thermoreceptors
what detects pain
mechanical, thermal and polymodal nociceptors
what detects itch - irritant e.g. chemical on skin or mucous membranes
itch receptors
what mediates fine discriminatory touch
low threshold mechanoreceptors
what mediates cold through to hot temperatures but not extremes
low threshold thermoreceptors
if there is increasing stimulus strength resulting in an increased rate of firing of LRMs and perception of increased intensity, will the qualitative character of the perception e.g. pressure change e.g. to pain
not if only LTMs are activated
what is another name for high threshold units
nociceptors
mechanical nociceptors (HTMs) respond to what
high intensity mechanical stimuli
e.g. distension (stretch) and pressure in inflammation
thermal nociceptors respond to what
extreme degrees of heat - over 45 or under 10-15
chemical nociceptors respond to what
exogenous and endogenous chemical substances in the tissue e.g. in inflammation
give 4 chemicals that chemical nociceptors will respond to
prostaglandins bradykinin serotonin (5-HT) histamine (also K+, H+ and ATP and others)
what do polymodal nociceptors respond to
more than one stimuli
what are the 3 types of adaptation of sensory nerve cells
tonic/static response - slow adapting (SA)
phasic/dynamic response - fast adapting (FA)
very phasic/dynamic response - very fast adapting
what kind of AP will tonic (SA) receptors produce
continuous information to CNS - greater the stimulation intensity the greater the firing rate
what kind of AP will phasic (FA) receptors produce
respond quickly to stimuli but stop responding upon continual stimulation
with FA receptors the number of impulses is proportional to the rate of ____
change of stimulus
VFA receptors respond only to
very fast movement
give an example of an SA receptor
stretch receptors
give an example of a FA receptor
some muscle spindle afferents
give an example of a VFA receptor
pacinian corpuscle
what are the 4 groups of primary sensory afferent fibres
Aa
Ab
Ad
C
which of the 4 groups of primary sensory afferent fibres has the biggest diameter
Aa
which of the 4 groups of primary sensory afferent fibres has the thickest myelination
Aa
which of the 4 groups of primary sensory afferent fibres has the fastest conduction velocity
Aa
which of the 4 groups of primary sensory afferent fibres is unmyelinated
C
nociceptors are tonic/phasic
phasic (FA)
what are the 2 types of pain fibre
Ad
C
how would you describe the receptors found on the ends of the type Ad and C pain fibres
free dendritic nerve endings
thermoreceptors are tonic/phasic
phasic
what kind of fibres respond to warm temperatures
C fibres
what kind of fibres respond to cold temperature
Ad fibres
are cold or warm thermoreceptors more common
cold
what allows adaptation to warm temperature if it remains constant
switch from initial phasic response which changes very quickly based on minute temperature changes
to tonic if it remains constant
what is the term for increased sensitivity to pain
hyperalgesia
primary hyperalgesia is thought to be due to what
nociceptor sensitisation
secondary hyperalgesia is thought to be due to what
central sensitisation
what drugs can induce hyperalgesia
opioids
what is a receptive field
target territory from which a single sensory unit can be excited - area on skin corresponding to the territory of one sensory neurone
how is RF related to innervation density and therefore sensory acuity
inversely related - bigger the RF the less densely innervated and therefore the less sensory acuity of that area
can skin contain overlapping RFs?
yes
name an area with a very small RF and therefore high density of innervation and high acuity
finger tip (2mm)
what is 2 point discrimination
the minimum distance required between two simultaneous stimulations applied to be registered
how is 2 point discrimination clinically tested
applying 2 sharp point stimuli separated by a variable distance at different sites on the body surface
how are cutaneous receptors subdivided - 4 types
small field - type 1 unit
wide field - type 2 unit
SA or FA
SA1 SA2 FA1 FA2
what is the fibre type of free nerve endings
Ad or C
what is the fibre type of follicular nerve endings
Ab or Ad
what is the receptor and parent fibre type of merkel cell neurite complex
Ab
SA1
what is the receptor and parent fibre type of meissner corpuscles
Ab
FA1
what is the receptor and parent fibre type of Ruffini endings
Ab
SA2
what is the receptor and parent fibre type of Pacinian corpuscles
Ab
FA2
what nerve endings are encapsulated
pacinian corpuscles
ruffini endings
meissner corpuscles
which of the following has the smallest RF
pacinian corpuscles
ruffini endings
meissner corpuscles
meissner corpuscles
where are free nerve endings found in the skin
everywhere
merkels discs are tonic/phasic
tonic (SA)
where are merkel discs found in the skin and in what distribution
near border of dermis and epidermis
abundant in skin locations where 2 point discrimination is highest, also in moderate numbers in hairy skin
what do merkel discs detect
pressure
differentiate texture and shape of objects
what can merkel cells develop
high mortality cancer
meissner’s corpuscles are tonic/phasic
phasic (FA)
where are meissner’s corpuscles found in the skin and in what distribution
in dermis, mainly on palms of hands, soles of feet, lips and tongue
are meissner’s corpuscles present in hairy skin
no
what do meissner’s corpuscles detect
fine touch - allow 2PD
what do pacinian corpuscles detect
pressure
pacinian corpuscles are tonic/phasic
phasic (FA)
pacinian corpuscles are the smallest/largest category of mechanoreceptors
largest
where are pacinian corpuscles found
dermis, hypodermis, ligaments, external genitalia
fascia
what is a group of merkel cells called
Iggo dome
An iggo dome is innervated by a single/many myelinated fibre(s)
a single myelinated fibre innervates the merkel cells in an Iggo dome
what do merkel cells express that makes them mechanosensitive
Piezo2 - mechanosensitive cation channel
where are Krause end bulbs found
border of the skin and mucous membranes
Ruffini end-organs are tonic/phasic
tonic (SA)
Ruffini end-organs respond to what
stretch and shearing forces
where are Ruffini end-organs found
dermis and joint capsules
where are muscle spindles and golgi tendon organs
skeletal muscle
what do muscle spindles and golgi tendon organs detect
stretch
how many spinal nerves are there for each section
C1-8 T1-12 L1-5 S1-5 coccygeal
what is a dermatome
area of skin innervated by the left and right dorsal root of a single segment
what nerve innervates the anterior head
CN V
shingles is an infection of what
dorsal root ganglion
what virus causes shingles
reactivation of varicella zoster
what does varicella zoster cause in children
chicken pox
does shingles usually occur in 1 or more ganglions
typically a single ganglion
what does shingles look like
extremely painful, inflamed and blistered skin in the dermatome innervated by that ganglion
what is found in the grey matter of the spinal cord
cell bodies and sensory afferent terminals
what is found in the white matter of the spinal cord
fibre tracts
how many laminae of rexed are there in the grey matter of the spinal cord
10
nociceptors (Ad and C) terminate in which laminae of rexed
I and II (also V for Ad fibres)
visceral nociceptors terminate on laminae I and V but not II
LTMs (AB) terminate in which laminae of rexed
III to VI
proprioceptors (Aa) terminate in which laminae of rexed
VII to IX
give and example of a sensory receptor that uses Aa fibres
proprioceptors of skeletal muscle
give and example of a sensory receptor that uses Ab fibres
mechanoreceptors of skin
give and example of a sensory receptor that uses Ad fibres
pain
temperature
give and example of a sensory receptor that uses C fibres
temperature
pain
itch
what 2 tracts make up the DCML pathway
cuneate (lateral) and gracile tract (medial)
the dorsal columns are further divided into what from lateral to medial
cervical - thoracic - lumbar - sacral
where do 1st order neurones of the DCML synapse for spinal reflexes
deep in the dorsal horn upon 2nd order neurones
what is stereognosis
ability to recognise an object by feeling it
how do you test fine touch
2PD
how do you test conscious proprioception
ask patient to close eyes - can they recognise when body part moved up / down
how can you test vibration sensation
128Hz tuning fork with onset offset reporting from bony prominence
what is lateral inhibition
when one neurone is active it inhibits the activity of its neighbours via inhibitory interneurones to sharpen stimulus
what does lateral inhibition lead to
contrast enhancement
the trigeminal system conveys general somatic information from the anterior head including the …
oral and nasal cavities paranasal sinuses teeth intracranial dura cerebral arteries
where do impulses originate in the trigeminal system
terminals of the 2 trigeminal nerves each of which has 3 divisions
what are the 3 divisions of CN v
v1 - ophthalmic
v2 - maxillary
v3 - mandibular
where are the soma of the sensory neurones of the trigeminal system located
trigeminal sensory ganglion
central terminals of the trigeminal nerve synapse upon second order neurones in the _______ or the _____
chief sensory nucleus or the spinal nucleus
what stimuli go to the chief sensory nucleus
general tactile stimuli
what stimuli go to the spinal nucleus
pain and temperature
second order neurones decussate and project via _____ to the _____ of the _____
the trigeminal lemniscus to the VPM nucleus of the thalamus
3rd order neurones relay information to the cortex via ___
thalamocortical neurones
where is the somatosensory cortex (SI) located
post central gyrus of the parietal cortex immediately posterior to the central culcus
what is the somatosensory cortex adjacent to
posterior parietal cortex (SII)
SI is divided into _____ areas which are
Broadmann areas BA 3a BA 3b BA 1 BA 2
where does the somatosensory cortex receive input from
VP thalamus
which broadman areas recieve most input from VP thalamus
70% - 3a and 3b - initial processing
30% - 1 and 2 - more complex processing
BA 3a receives information from
proprioceptors
BA 3b receives information from
cutaneous mechanoreceptors e.g. merkel cells and meissners corpuscles
3a
BA 1 receives information from
cutaneous mechanoreceptors and 3b
BA 2 receives information from
joint afferents, golgi tendon organs, deep tissues
3a and 3b
how many cell layers does the somatosensory cortex have
6
thalamic inputs to SI terminate mainly on neurones within layer ___ which projects cells ___
4 (IV)
which projects cells towards the surface of the cortex and deeper layers
what extend across the layers of the sensory cortex
vertical columns
each column consists of neurones with similar ____ and ____
inputs and responses
if a finger is amputated what happens to the area of SI representing that finger
it will be utilised by other sensory inputs - cortical representation of adjacent fingers will expand into territory
if the sensory input from a finger increases e.g. violin player what happens to its contralateral cortical representation
expands
e.g. use right finger a lot, part on left side of brain will expand as contralateral
what does the posterior parietal cortex (SII) do
receives and integrates information from SI and other cortical areas - visual, auditory and sub-cortical areas - thalamus - deciphers the deeper meaning of the information
damage to the posterior partial cortex can cause what
bizarre neurological disorders with simple sensory skill intact
e.g. agnosia, astereognosia, hemispatial neglect syndrome
damage to the right parietal cortex resulting in neglect syndrome would present how
patient doesnt believe the left side of the world exists, will even discount left side of body
what order neurones are nociceptors
first
pathological pain is adaptive/maladaptive
maladaptive
Ad fibres mediate ___ ___ pain
C fibres mediate ___ ___ pain
Ad - fast first
C - slow second
Ad fibres respond to what noxious stimuli
thermal and mechanical
C fibres respond to what kind of pain
all noxious stimuli - polymodal
give 3 receptors that are activated by noxious heat
TRPA1
TRPC3
TRPV1
H+ activates what channels
acid sensing ion channels (ASICs)
ATP activates what receptors
P2X and P2Y receptors
bradykinin activates what receptors
B2 receptors
what are peptidergic polymodal nociceptors
subset of C fibres with afferent and efferent functions
what does the afferent limb of peptidergic polymodal nociceptors do
transmits nociceptive information to the CNS via release of glutamate and peptides
what does the efferent limb of peptidergic polymodal nociceptors do
release of pro-inflammatory mediators contributing to neurogenic inflammation
noxious stimulation in the long term increases/decreases spinal excitability
increases - contributes to hyperalgesia and allodynia
what is neurogenic inflammation
peptides such as substance P and CGRP (calcitonin gene-related peptide) released from free nerve endings of peptidergic nociceptor due to tissue damage or inflammatory mediators
what does substance P do
vasodilation and extravasation of plasma proteins
release of histamine from mast cells
sensitises surrounding nociceptors
what does CGRP do
induces vasodilation
what follows on from neurogenic inflammation
primary and secondary hyperalgesia and allodynia
visceral pain tends to be what in nature
dull aching throbbing
visceral afferents from nociceptors follow _____ pathways before reaching the dorsal horn
sympathetic
why can the brain interpret pain coming from the viscera as originating from an area of skin distant to the internal organ
some visceral and skin afferents converge upon the same spinothalamic neurones - all cells with a visceral RF will also have a separate cutaneous RF
where may heart pain e.g. angina be referred
T1 - T5
where may pain from the gall bladder be referred
C4
what causes viscerosomatic pain
when inflammatory exudate from a diseased organ contacts a somatic (body wall) structure
what is an example of viscerosomatic pain
appendicitis may start as diffuse visceral pain in the umbilical region that moves down to the iliac fossa and progresses to sharp viscerosomatic pain
can nociception occur in the absence of pain
yes pain is just awareness of the nociception
where can stomach/pancreas pain be referred
epigastric region
liver can refer pain to
side of neck
diaphragm/lungs can refer pain to
shoulder
pain evoked by activity in nociceptors can be reduced by simultaneous activity in ___
LTMRs (AB)
what theory explains the reduction in pain when AB fibres are stimulated
gate control theory
what are the 2 major pain pathways
spinothalamic
spinoreticular
spinoreticular transmits more ____ pain
spinothalamic more ___ pain
SRT - slow
STT - fast
spino____ tract makes extensive connections with reticular nuclei in brainstem (e.g. PAG, parabrachial nucleus)
spinoreticular
SRT is involved in what
autonomic responses to pain, arousal, emotional responses, fear of pain
what temperate change can we perceive
as small as 0.01 degrees
what lesion can be seen with vit B12 deficiency and tabes dorsalis (complication of syphilis)
DCML lesion
if lesion to DCML is in spinal cord, sensory loss will be contra/ipsilateral
ipsilateral
damage to the spinothalamic tract will have contra/ipsilateral sensory loss
contralateral
