3.1 somatic sensation and ascending pathways Flashcards
what are the 2 broad catergories of sensation?
general
special
what are the 2 subcatergories of general sensation? how do they differ?
somatic = consciously perceived, localised
visceral = generally not consciously perceived (carotid bodies), poorly localised
when might visceral sensation become somatic?
if there is very strong stimulation of the visceral receptors, can have referred pain in the somatic distribution e.g. appendicitis
what are modalities?
the basic units of sensation. Each modality has a specialised receptor to detect that modality.
what are the 7 different types of sensory modalities
temperature pain pressure / crude touch vibration fine touch proprioception 2-point discrimination
what is proprioception?
spatial awareness
Awareness of where out muscles are in space, occur as muscle spindle fibres report on muscle length
what are the modalities of the spinothalamic system?
temperature
pain
crude touch / pressure
what is 2 point discrimination?
a sensory modality that allows us to resolve two simultaneous stimuli on the skin.
what modalities travel in the dorsal column mediated lemniscus?
vibration
fine touch
2 point discrimination
proprioception
describe the structure of a primary sensory neurone
pseudopolar neurone. has 1 process projecting from the body
how is the information of sensation carried to the spine?
certain sensory modality is detected by a specific receptor
receptor passes on information about modality to 1st order sensory neurone
this is carried down the axon and top the cell body in the dorsal root ganglion
dendrite of the 1st order sensory neurone carries information into the spinal cord.
why does the A.P. frequency along the axon of a 1st order sensory neurone vary?
Strong stimulus to the receptor = high frequency of A.P along axon
weak stimulus of the receptor = low frequency of A.P. running along the axon
why is sensory detection an analogue to digital conversion?
the modality receptor is operated by electrical charge as ions are moving across the membrane
Action potentials are not operated on a gradient and are instead on or off (discrete).
what are the 2 broad types of primary sensory neurones?
- rapidly adapting type
2. slowly adapting type
how do rapidly adapting sensory neurones and slowly adapting sensory neurones differ?
Rapidly adapting = when stimulated by receptor, intially has high frequency of A.P., which gradually reduces over time to a lower frequency of A.P.
slowly adapting receptors = when a stimulus is applied the frequency of the A.P. doesnt tend to decay. stimulus is constantly perceived.
give an example of a rapidly adapting sensory neurone
mechanoreceptors in the skin
give an example of a slowly adapting sensory neurone
nociceptors
what is the receptive field of a sensory neurone?
the area of skin a sensory neurone collects information from
can a primary sensory neurone carry information about multiple modalities?
no as each individual sensory neurone only collects information from one type of receptor
what determines the level of sensory acuity of sensory neurones
the size of the receptive field
the smaller the size of the receptive field the larger the sensory acuity
why are dermatomal boundaries fuzzy?
why is this important clinically?
as the receptive fields of 2 different neurones that are in 2 different dermatomes overlap
This is important clinically as when assessing dermatomes we should touch thr autonomous centre region of the dermatome and not the periphery
what is the somatosensory system?
a system that carries conscious sensation from the body wall ( skin/mucosal surfaces/parietal pleura/parietal peritoneum)
describe the order of neurones in the somatosensory pathway
chain of 3 sensory neurones
- receptor transmits information to 1st order sensory neurone
- cell body of 1st order sensory neurone is in a sensory ganglion (dorsal root ganglion or trigeminal ganglion)
- dendrite of the 1st order neurone projects into the CNS
- synapses onto the body of the 2nd order neurones
- 2nd order neurones crosses the midline in the brainstem or cord
- 2nd order neurone projects onto a third order neurone cell body in the thalamus on the contralateral side of the body to the 1st order neurone
- 3rd order neurone projects up to the cerebral cortex.
- conscious perception of sensation occurs.
what are thalamocortical neurones?
3rd order sensory neurones
cell body is in the thalamus and axon projects up to the primary sensory cortex
what is somatotopy?
topographical representation. essentially adjacent parts of the body are represented by adjacent parts of the CNS
describe the trajectory of sensory pathways of the CNS
short parallel organised pathways running in a topographical manner
why is trajectory of the sensory neurones so ordered?
As a disordered trajectory would be longer and would use more axon and myelin. Therefore far more efficient to have a shorter, organised trajectory
how must the information carried within the sensory system change as it traverses from the ski the the cerebral cortex?
initially in dermatomal pattern. This must be converted to a homoncular pattern to be recognized by the cerebral cortex.
this is the role of the structures in the brainstem of the medulla and in the thalamus
what modalities are carried in the dorsal column medial lemniscus pathway?
proprioception
2 point discrimination
fine touch
vibration
when ascending in the spine, where does the dorsal column medial lemniscus pathway run?
runs in the white matter dorsal to the dorsal horn of the spinal cord
which sensory neurone ascends in the spine in the dorsal column medial lemniscus pathway?
1st order sensory neurone axon
where is the 2nd order neurone in the dorsal column medial lemniscus pathway located?
in the medulla, where it immediately dessicates and crosses the midline.
what is the gracile nucleus?
the most medial 2nd order nucleus. 1st order neurones from the lower half of the body project up to the medulla and synapse onto the gracile nucleus
what is the cuneate nucleus?
2nd order neurones that receive input from the upper half if the body.
1st order neurones from the upper half of the body project onto the cuneate nucleus in the medulla on the ipsilateral side of the body to the receptor
how are axons added to the dorsal column medial lemniscus pathway?
axons that are added as the pathway ascends (so axons from 1st order neurones at a higher level) are added laterally to the pathway.
describe the order of axons in the dorsal column pathway from medial to lateral
medially axons from the lumbar region are located
thoracic axons are medial
cervical axons are lateral
what is the medial lemniscus?
the pathway running from the 2nd order neurones of the dorsal pathway in the medulla ( gracile and cuneate) to the thalamus on the contralateral side. Runs through the brainstem
what modalities are carried in the spinothalamic system?
crude touch / pressure
pain
temperature
where are 2nd order neurones of the spinothalamic pathway situated?
cell body of 2nd order neurones situated approximately at the same level as where the 1st order nucleus enters the CNS. cell body lies within the dorsal horn of the grey matter before the axon immediately dessicates and ascends
in a central cord lesion, which areas of the body are most likely to lose dorsal column modalities first
lumbar regions
in the spinothalamic system, which neurones ascend in the spine?
2nd order neurones
how are axons added to the spinothalamic pathway as you ascend up the cord?
added to the medial aspect of the pathway
within the spinal cord, where does the spinothalamic pathway run?
runs in the white matter infront of the ventral horn on the contralateral side of the receptor.
what is a syringomyelia?
A chronic disease of the spinal cord characterized by the development of fluid-filled cavities and leading to spasticity and sensory disturbances.
which pathways are most likely to be affected in a syringomyelia?
early on would affect the dessicating 2nd order fibres of the spinothalamic tract on both sides of the body.
as the lesion grows it would cause bilateral symptoms
the dorsal column modalities of the lower half of the body and the spinothalamic modalities of the upper half of the body would be lost
what is the fasiculus proprius?
Lissauer’s tract
what is Lissaeur’s tract?
when the axons of 1st order sensory neurones ascend before synapsing on 2nd order sensory neurones within the spinal cord grey matter.
which systems does lissaeurs tract effect?
the spinothalamic system
where is lissaeurs tract in the spinal cord
in the white matter at the tip of the dorsal horn
how many vertebrae levels does lissaeurs tract ascend?
2
what is a hemisection of the spinal cord?
half of the spinal cord segment destroyed
what does a nociceptor detect?
pain modality
what is a c fibre?
a first order sensory neurone that is associated with a nociceptor
slow conducting pain fibre
what are AB fibres?
primary afferent sensory nerve fibres that carry information from mechanoceptors in the skin.
what is the function of AB neurone in pain regulation?
AB neurones are activated by rubbing sensation in the skin.
AB neurones are able to stimulate a particular type of interneurone in the dorsal horn
this inhibitory interneurone can inhibit the 2nd order sensory neurones, especially the C fibres. Effectively switches off pain transmission
what do inhibitory interneurones release in the dorsal honr?
release endorphins
- encephalin
- morphine works here
why during extreme trauma or hypnosis do we not feel any pain?
as we have descending modulation from the brain to inhibit pain transmission at the level of the spinal cord.
describe the descending modulation of pain pathway
- activation of excitatory cortical neurones
- stimulates 2nd neurone in the midbrain within the periaqueductal grey
- these periaqueductal gray neurones project down to the nuclei raphe magnus in the medulla (part of the reticular system)
- nucleus raphe magnus neurones descend down the cord and have an inhibitory affect on the 2nd order sensory neurones of the spinothalamic system (c fibres) by stimulating the inhibitory interneurones
what are the 5 special senses?
taste smell hearing sight balance
what neurones travel through the gacile fasciculus?
the first order sensory neurones of T7 and below
what neurones travel through the cuneate fasciculus?
first order sensory neurones of T6 and above
where do 2nd order neurones of the dorsal column pathway project to?
cross the midline to the contralateral thalamus in the medial lemniscus
where do second order neurones of the spinothalamic pathway dessicate?
at the level of the cell body in the ventral white commissure of the cord and then go on to form the spinothalamic tract
what is brown-sequard syndrome?
the set of signs seen when there is complete hemisection causing destruction of one lateral half of the spinal cord. The signs are:
- ipsilateral complete segmental anaesthesia affecting a single dermatome
- ipsilateral loss of dorsal column modalities below the destroyed segment
- contralateral loss of the spinothalamic modalities at and below the destroyed segment (although level can be up a couple of segments depending on Lissaeurs tract)
