Physiology - The Somatosensory System Flashcards
where are receptors for the somatosensory system located?
throughout the body
what are the receptors for the somatosensory system?
peripheral nerve endings and their associated specialisations
what five sensory modalities are mediated by the somatosensory system?
fine touch proprioception temperature pain itch
what five things come under fine discriminatory touch?
light touch pressure vibration flutter stretch
what is another name for mediating fine touch?
mechanosensation
what is another name for mediating temperature?
thermosensation
what is another name for mediating pain?
nociception
what is another name for mediating itch?
pruriception
how many neurones usually make up a somatosensory pathway?
three
where is the first order neurone of a somatosensory pathway found?
the PNS
what type of neurone is the first order order neurone of a somatosensory pathway?
primary sensory afferent
pseudounipolar
where is the cell body of the first order order neurone of a somatosensory pathway found?
dorsal root ganglia if innervating limbs/trunk/posterior head
cranial ganglia if innervating the anterior head
what type of neurone is the second order neurone of a somatosensory pathway?
projection neurone
where is the cell body of the second order neurone of a somatosensory pathway found?
dorsal horn of the spinal cord or a brainstem nuclei
what type of neurone is the third order neurone of a somatosensory pathway?
projection
where is the cell body of the third order order neurone of a somatosensory pathway found?
thalamic nuclei
where are the cell bodies located in the somatosensory cortex?
parietal and posterior parietal cortex
what do sensory neurone terminals do?
transduce a stimulus into electrical activity
what effect does a stimulus have when it reaches a sensory neurone terminal?
opens cation selective ion channels, eliciting a depolarising receptor potential
what determines the amplitude of the sensory receptor potential produced in response to a stimulus?
the intensity of the stimulus
when a sensory receptor potential is produced what does it do?
if over threshold, triggers an AP which then causes the graded release of neurotransmitters onto the second order neurones
what is modality?
the type of stimulus that excites the sensory receptor
what is the adequate stimulus?
the specific type and intensity of energy that excites a primary afferent neurone
what is the stimulus and sensory unit for the modality of touch, pressure and vibration?
stimulus = mechanical forces acting on the skin
unit = skin mechanoreceptors
what is the stimulus and sensory unit for the modality of proprioception?
stimulus = mechanical forces acting on joints and muscles
unit = joint and muscle mechanoreceptors
what is the stimulus and sensory unit for the modality of temperature?
stimulus = heat
unit = thermoreceptors
what is the stimulus and sensory unit for the modality of pain?
stimulus = strong mechanical force or heat
unit = mechanical, thermal and polymodal nociceptors
what is the stimulus and sensory unit for the modality of itch?
stimulus = irritant on the skin/mucous membranes
unit = itch receptors
what does threshold mean?
the intensity of the stimulus required for excitation of the receptor
what do low threshold units respond to?
low intensity/non-damaging stimuli
what do low threshold mechanoreceptors mediate?
fine discriminatory touch
what do low threshold thermoreceptors mediate?
cold through to hot temperatures
what do high threshold units respond to and what is another name for these units?
high intensity potentially damaging stimuli
aka nociceptors
what do high threshold mechanoreceptors (HTMs) respond to?
high intensity mechanical stimuli
aka mechanical nociceptors
what do thermal nociceptors respond to?
extreme degrees of heat or cold
what do chemical nociceptors respond to?
substances in tissue that are harmful
what do polymodal nociceptors respond to?
at least two types of noxious stimuli
what does adaptation rate?
whether a sensory unit discharges AP’s continuously during the stimulus or does it respond preferentially to a changing stimulus
what are the three possible adaptation rates that a sensory unit can have?
slowly adapting (SA) fast adapting (FA) very fast adapting (very FA)
what is another name for the slowly adapting response?
tonic/static response
describe an SA adaptation rate
AP’s are released in relation to when and how the stimulus is present - either step or ramp
describe a step SA adaptation rate
stimulus present at all times at the same amplitude
action potentials released at even intervals
describe a ramp SA adaptation rate
stimulus varies until it reaches a maximum
action potentials released proportionally to the stimulus present - reaches a peak
name a receptor that shows SA adaptation
stretch receptors
describe a FA adaptation response
generates a burst of action potentials and then no more
number of impulses proportional to the rate of change of the stimulus
describe a very FA adaptation response
responds only to very fast movement
produces one AP at the start of the stimulus
what is conduction velocity?
how rapidly the sensory unit conducts AP’s along its axons
what is the receptive field (RF)?
the target territory from which a sensory unit can be excited
what determines the RF?
innervation density - inverse relationship
describe the relationship between RF size and sensory acuity
small RF = high acuity (+ high density of innervation)
large RF = low acuity (+low density of innervation)
what is another name for two point discrimination?
spatial acuity
how can spacial acuity be tested clinically?
applying two sharp point stimuli at two different sites simultaneously
patient reports whether one or two points are felt
what is a dermatome?
the area of skin innervated by the bilateral dorsal roots of a single spinal segment
what forms the dorsal columns?
medial gracile tract = fasiculus gracilis
lateral cuneate tract = fasciculus cuneatus
what do the dorsal and ventral spinocerebellar tracts convey?
unconscious proprioceptive information to the cerebellum
