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