NS 3: Somatic sensation and the Sensory pathway Flashcards
2 main functions of neuronal cell bodies of SC?
- serving local functions of a neuronal segment: local reflexes, sensory functions of neuronal segment to which they belong (dermatomes- area of skin supplied by sensory nerve fibres of a single spinal nerve root), motor functions of neuronal segment to which they belong (myotomes- muscles supplied by motor nerve fibres of a single spinal nerve root).
- relaying sensory information to the brain: ‘ascending axons’ as transmitting information to higher levels of neuraxis.
2 main functions of axonal fibres of SC?
- carry sensory information from surface of body and muscles to brain (ascending tracts)
- carry motor commands from brain to cell bodies of spinal motor neurones (descending tracts.)
how are cell bodies arranged in the grey matter of the SC?
in layers of cell bodies with similar functions, that stretch horizontally, arranged from dorsal to ventral
name given to each of the discrete layers of cell bodies forming the grey matter of the SC?
Rexed laminae
numbered I to X
each lamina is equivalent to a neuronal nucleus
where might cell bodies of axons of white matter be found?
SC
brain
dorsal root ganglion (PNS)
what is the propriospinal system of the SC?
soma axons have cell bodies in the SC but don’t project to the brain, axons instead sent to neighbouring spinal segments to help share info. locally.
define a fasciculus
a clump of axonal fibres of SC which travel together with a common origin or destination
tracts/fasciculi are grouped in anterior, posterior and lateral columns= funiculi
where is the destination of ascending tracts for conscious sensations?
postcentral gyrus- primary sensory cortex in parietal lobe
which tract carries pain fibres from the SC to the thalamus?
spinothalamic tract
what does the corticospinal tract do?
carries motor fibres from cortex dward to spine
how is the gray matter on either side of the SC connected?
by comissures- contain bundles of fibres that allow info flow across midline
what is sensory acuity?
the precision by which a stimulus can be located
what 3 things determine sensory acuity?
lateral inhibition in CNS
2 point discrimination- determined by density of sensory receptors, size of neuronal receptive fields and psychological factors
synaptic convergence- decreases acuity and divergence- amplifies signal
what type of sensation is the dorsal column associated with?
(medial lemniscal tract)
fine touch
conscious proprioception
course of SC?
lies within vertebral canal, extends from foramen magnum to lower border of 1st lumbar vertebra (or upper border of 2nd)
where is cell body of all 1st order neurones of the tracts carrying sensation from the trunk and limbs located?
dorsal root ganglion
where is the 1st point that a stimulus is perceived?
thalamus
where are the cell bodies of 2nd order neurones of the dorsal column located?
nucleus gracilis or nucleus cuneatus
where are cell bodies of 2nd order neurones of anterior and lateral spinothalamic tracts located?
dorsal horn
where are cell bodies of 2nd order neurones of spinocerebellar tracts located?
spinal grey matter
where are cell bodies of 2nd order neurones of cuneocerebellar tracts located?
nucleus cuneatus
where are cell bodies of 3rd order neurones of spinothalamic tracts and dorsal column located?
thalamus
where are cell bodies of 3rd order neurones of spinocerebellar and cuneocerebellar tracts located?
NO cell bodies
function of anterior spinothalamic tract?
crude touch
pressure
function of anterior and posterior spinocerebellar tracts?
unconscious proprioception
function of cuneocerebellar tract?
unconscious proprioception
function of cuneocerebellar tract?
unconscious proprioception
where is there decussation of spinothalamic tracts?
spinal cord
where is there decussation of dorsal column?
medulla
where is there decussation of spinocerebellar tracts?
anterior= spinal cord posterior= none
where does the dorsal column, and spinothalamic tracts terminate?
somatosensory/primary sensory cortex/ postcentral gyrus
where do the spinocerebellar and cuneocerebellar tracts terminate?
cerebellum
define sensory agnosia
a loss of awareness of the affected side e.g. with a lesion of the thalamus.
contrast how many neurones make up the conscious and unconscious sensory pathways?
conscious= 3 unconscious= 2
what is conscious proprioception from?
joints and tendons
why does all sensory input not reach consciousness?
receptors for unconscious proprioception ar muscle spindles and their output isn’t consciously perceived as taken by spinocerebellar tracts to cerebellum rather than to the somatosensory cortex, and signals in cerebellum not perceived consciously.
in which location are lesions commonly associated with sensory agnosia?
parietal lobe, but condition can also come about with tumours of thalamus or internal capsule which interrupt ascending tract fibres making their way to somatosensory cortex of parietal lobe.
where are the cell bodies of the divisions of the trigeminal nerve located?
trigeminal ganglion
and their central processes synapse in trigeminal nucleus in brainstem. 2nd order neurones ascend from there to thalamus, and 3rd order to cerebral cortex.
what 2 factors does accuracy of detecting a touch stimulus on surface of body depend on?
density of receptors and size of their receptive fields
high density of receptors with small receptive fields= touch stimuli can be very accurately localised e.g. fingertips and hands.
in which region of thalamus do 2nd order neurones of dorsal column and spinothalamic tracts end?
ventral posterolateral nucleus
what is a topographical representation?
suggests each region of somatosensory cortex receives an input from a specific body part so a map of the distribution of sense organs on the body surface can be plotted.
how is area of somatosensory cortex occupied by particular surfaces of the body determined?
by their degree of sensory innervation, and NOT their physical size
so hand’s sensory connections occupy a large area of cortex due to hand’s importance in being an organ of touch meaning that it has a large number of cutaneous sensory receptors.
why is sensation lost with a full thickness burn, but may be retained if burn superficial?
superficial= cutaneous sensory receptors can recover, but full thickness- skin destroyed and replaced by scar tissue, and though scar invaded by pain fibres, it has no other sensation.
name given to pattern of sensory loss resulting from lesion in 1 1/2 of SC producing ipsilateral hemiplegia and contralateral pain and temperature sensation defecits?
Brown Sequard syndrome
can occur with hemisection or lateral injury of cord
which receptors for fine touch is the dorsal column associated with and what are their functions?
meissner corpuscles= tactile:shapes/surfaces-stroking
hair follicle receptors= tactile (in hairy skin)-stroking
Pacinian corpuscles= virbrations
Merkel disc= pressure
Ruffini ending= skin stretch
Other than spinothalamic tracts (anterior and lateral) what other tracts are part of the AL system conveying conscious sensation from limbs and body?
spinomesencephalic
spinoreticular tracts
what modalities is the AL system associated with?
pain, temperature, crude touch and pressure
function of lateral spinothalamic tract?
pain
temperature
function of spinoreticular tract?
terminates in reticular formation of brainste,
assoc. with triggering arousal systems- pain signals
function of spinomesencephalic tract?
terminates in midbrain
regulation of pain information
type of receptors for both conscious and unconscious proprioception?
muscle spindles and golgi tendon organs
what do muscle spindles monitor?
muscle length, and limb movement
what do golgi tendon organs monitor?
strength of muscle contraction
what do joint receptors monitor?
stretching of joints, movement
importance of synapses in sensation pathway of ascending tracts?
modulation takes place
divergence- 1st order neurones make contact with 2nd order neurones
nucleus in thalamus for vision?
lateral geniculate
nucleus in thalamus for hearing?
medial geniculate
types of 1st order neurone fibres in dorsal column?
Aalpha- limb position and motion and Abeta (cutaneous)- tactile, pressure and vibration. Both large diameter and myelinated.
Group I and II= MSK
why might touching a patient’s middle finger following a lesion of median nerve at wrist result in patient feeling touch in their thumb?
reinnervation patterns as nerve fibres regrow, axons may not grow towards place where they were before so grow towards middle finger rather than thumb but axons still connected to same region of somatosensory cortex= labelled line representation in topography- also reposible for phantom limb- missing or amputated limb can still be felt as somatosensory cortex still knows where limb located due to labelled line representation.
receptors for pain in AL system?
nociceptors (all free ending afferents- bare terminals of neurone extending into tissue, rather than terminal being associated with a specific sensory structure as with fine touch): mechanical-sharp, thermal-burning, cold/freezing, polymodal-slow, burning pain/ache
thermoreceptors (all free ending): thermal for cold and warm
mechanoreceptors for crude touch: free ending, and Merkel disks- crude touch and pressure
role of periaqueductal grey surrounding cerebral aqueduct in midbrain?
pain modulation
types of 1st order afferent fibres in AL tracts?
Adelta= small diameter myelinated, fast pain and cold
C= even smaller diameter, unmyelinated, slow pain and warmth
Group III and IV= musculoskeletal
explain the reason for referred pain e.g. angina pectoris= shoulders and L arm?
2nd order spinothalamic afferents, part. lamina 5, carrying pain sensation receive convergent inputs from lots of receptors. Pain e.g. from liver, doesn’t usually occur, so brain not prepared to detect this, so e.g. in lamina 5, convergence of afferents from liver with cutaneous ones (where pain expected). So brain doesn’t know which pathway is coming through, and assumed it’s the 1 where pain usually comes from, so with liver, this would be pain in R shoulder.
give an example of a disease which selectively affects the spinothalamic tracts of the SC, and describe its symtpoms
syringomyelia
due to formation of an elongated cavity/syrinx around central canal of SC, expansion causes compression of ST tract fibres which decussate segmentally in mid-line of cord, through the anterior (ventral) white commissure.
dorsal column unaffected, so fine touch and conscious proprioception intact.
BUT results in bilateral pain and temperature sensation loss, due to crossing over of ST tract fibres segmentally.
different categories of general senses?
somatic: tactile- pressure, light touch, vibration
pain
temperature
proprioception
and visceral: from internal organs
special senses?
vision taste hearing balance smell
what might happen if a particular stimulus detected by a specially adapted sensory receptor is very strong?
can causes APs to fire from another modality receptor despitr sensory receptors being modality specific.
Modalities= touch, temp, vibration, pressure light, and these are subdivided into qualities e.g. taste= sweet/sour
different types of sensory receptors of 1st order neurones?
free nerve endings e.g. for temperature, no specialised sensory structure. Bare dendrites.
encapsulated nerve endings e.g. pressure stimulus ( NOT Merkel discs= unencapsulated tactile doiscs) like lamellar corpuscles- deep pressure. Nerve fibres wrapped in glial cells or CT.
with specialised cell e.g. taste receptors.
describe sensory transduction
stimulus evokes change in permeability of receptor membrane at peripheral end of axon to ions, so ion channels open creating a receptor potential with Na+ movement, if particular threshold reached then APs fire- all or nothing, same amplitude no matter stimulus but threshold must be reached for AP to fire.
describe the frequency of APs in response to a strong stimulus
high frequency, and may also activate nearby receptors but stimulus will be weaker here so lower AP frequency.
describe adaptation of different receptors
slowly adapting (tonic) e.g. nociceptors detecting pain in foot and joint receptors- APs continue to fire as long as stimulus lasts for rapidly adapting (phasic) e.g. touch receptors, respond maximally to stimulus straight away, then APs no longer fire, so brief response. May be off signal when stimulus stops.
describe lateral inhibition
inhibitory interneurones activated can stop some 2nd order neurones being stimulated, which helps to increase acuity as helps localise stimulus e.g. in visual pathway- lots of inhibitory interneurones to increase acuity- tested for with Snellen chart- optic nerve.
define 2 point discrimination
Minimal interstimulus distance required
to perceive two simultaneously applied
skin indentations.
what may reduce 2 point discrimination?
fatigue
stroke
example of reduced sensory acuity due to convergence: multiple axons converging on same 2nd order neurone?
referred pain
what may cause thalamic overreaction?
stoke
as lose some inhibitory control
where is the choice to respond to a sensory stimulus taken?
somatosensory cortex
why might 2 point discrimination be lost with epilepsy?
repeated epileptic events cause lesions of sensory cortex
may also cause asterognosis- inability to identify an object with just active touch of hands
define synesthesia
stimulation of 1 sensory pathway leads to involuntary, automatic experiences in another sensory pathway e.g. hearing a particular word is associated with a particular taste. Results from crosswiring in thalamocortical projections, causing senses to be mixed up.
what causes brown-sequard syndrome and how does it present?
lesion involving 1 1/2 of SC ( e.d. SC hemisection)
ipsilateral spastic weakness
ipsilateral loss of fine touch, vibration and conscious proportioception
contralateral loss of pain and temp senses as decussation of anterolateral tracts fibres (spinothalamic tracts) at segmental level of SC via ventral white commissure.
losses occur below level of lesion
through what structure do anterior and lateral spinothalamic tract fibres decussate?
ventral (anterior) white commissure
what presentation would a AL cordotomy produce?
CL loss of pain and temperature sensations in dermatomes below level of lesion
this surgery may be used to relieve pain in patients with terminal cancer
