Physiology - The Somatosensory System

Question 1

where are receptors for the somatosensory system located?

Answer 1

throughout the body

Question 2

what are the receptors for the somatosensory system?

Answer 2

peripheral nerve endings and their associated specialisations

Question 3

what five sensory modalities are mediated by the somatosensory system?

Answer 3

fine touch 
proprioception 
temperature 
pain 
itch

Question 4

what five things come under fine discriminatory touch?

Answer 4

light touch
pressure
vibration 
flutter 
stretch

Question 5

what is another name for mediating fine touch?

Answer 5

mechanosensation

Question 6

what is another name for mediating temperature?

Answer 6

thermosensation

Question 7

what is another name for mediating pain?

Answer 7

nociception

Question 8

what is another name for mediating itch?

Answer 8

pruriception

Question 9

how many neurones usually make up a somatosensory pathway?

Answer 9

three

Question 10

where is the first order neurone of a somatosensory pathway found?

Answer 10

the PNS

Question 11

what type of neurone is the first order order neurone of a somatosensory pathway?

Answer 11

primary sensory afferent

pseudounipolar

Question 12

where is the cell body of the first order order neurone of a somatosensory pathway found?

Answer 12

dorsal root ganglia if innervating limbs/trunk/posterior head

cranial ganglia if innervating the anterior head

Question 13

what type of neurone is the second order neurone of a somatosensory pathway?

Answer 13

projection neurone

Question 14

where is the cell body of the second order neurone of a somatosensory pathway found?

Answer 14

dorsal horn of the spinal cord or a brainstem nuclei

Question 15

what type of neurone is the third order neurone of a somatosensory pathway?

Answer 15

projection

Question 16

where is the cell body of the third order order neurone of a somatosensory pathway found?

Answer 16

thalamic nuclei

Question 17

where are the cell bodies located in the somatosensory cortex?

Answer 17

parietal and posterior parietal cortex

Question 18

what do sensory neurone terminals do?

Answer 18

transduce a stimulus into electrical activity

Question 19

what effect does a stimulus have when it reaches a sensory neurone terminal?

Answer 19

opens cation selective ion channels, eliciting a depolarising receptor potential

Question 20

what determines the amplitude of the sensory receptor potential produced in response to a stimulus?

Answer 20

the intensity of the stimulus

Question 21

when a sensory receptor potential is produced what does it do?

Answer 21

if over threshold, triggers an AP which then causes the graded release of neurotransmitters onto the second order neurones

Question 22

what is modality?

Answer 22

the type of stimulus that excites the sensory receptor

Question 23

what is the adequate stimulus?

Answer 23

the specific type and intensity of energy that excites a primary afferent neurone

Question 24

what is the stimulus and sensory unit for the modality of touch, pressure and vibration?

Answer 24

stimulus = mechanical forces acting on the skin

unit = skin mechanoreceptors

Question 25

what is the stimulus and sensory unit for the modality of proprioception?

Answer 25

stimulus = mechanical forces acting on joints and muscles unit = joint and muscle mechanoreceptors

Question 26

what is the stimulus and sensory unit for the modality of temperature?

Answer 26

stimulus = heat unit = thermoreceptors

Question 27

what is the stimulus and sensory unit for the modality of pain?

Answer 27

stimulus = strong mechanical force or heat unit = mechanical, thermal and polymodal nociceptors

Question 28

what is the stimulus and sensory unit for the modality of itch?

Answer 28

stimulus = irritant on the skin/mucous membranes unit = itch receptors

Question 29

what does threshold mean?

Answer 29

the intensity of the stimulus required for excitation of the receptor

Question 30

what do low threshold units respond to?

Answer 30

low intensity/non-damaging stimuli

Question 31

what do low threshold mechanoreceptors mediate?

Answer 31

fine discriminatory touch

Question 32

what do low threshold thermoreceptors mediate?

Answer 32

cold through to hot temperatures

Question 33

what do high threshold units respond to and what is another name for these units?

Answer 33

high intensity potentially damaging stimuli aka nociceptors

Question 34

what do high threshold mechanoreceptors (HTMs) respond to?

Answer 34

high intensity mechanical stimuli aka mechanical nociceptors

Question 35

what do thermal nociceptors respond to?

Answer 35

extreme degrees of heat or cold

Question 36

what do chemical nociceptors respond to?

Answer 36

substances in tissue that are harmful

Question 37

what do polymodal nociceptors respond to?

Answer 37

at least two types of noxious stimuli

Question 38

what does adaptation rate?

Answer 38

whether a sensory unit discharges AP's continuously during the stimulus or does it respond preferentially to a changing stimulus

Question 39

what are the three possible adaptation rates that a sensory unit can have?

Answer 39

``` slowly adapting (SA) fast adapting (FA) very fast adapting (very FA) ```

Question 40

what is another name for the slowly adapting response?

Answer 40

tonic/static response

Question 41

describe an SA adaptation rate

Answer 41

AP's are released in relation to when and how the stimulus is present - either step or ramp

Question 42

describe a step SA adaptation rate

Answer 42

stimulus present at all times at the same amplitude action potentials released at even intervals

Question 43

describe a ramp SA adaptation rate

Answer 43

stimulus varies until it reaches a maximum action potentials released proportionally to the stimulus present - reaches a peak

Question 44

name a receptor that shows SA adaptation

Answer 44

stretch receptors

Question 45

describe a FA adaptation response

Answer 45

generates a burst of action potentials and then no more number of impulses proportional to the rate of change of the stimulus

Question 46

describe a very FA adaptation response

Answer 46

responds only to very fast movement produces one AP at the start of the stimulus

Question 47

what is conduction velocity?

Answer 47

how rapidly the sensory unit conducts AP's along its axons

Question 48

what is the receptive field (RF)?

Answer 48

the target territory from which a sensory unit can be excited

Question 49

what determines the RF?

Answer 49

innervation density - inverse relationship

Question 50

describe the relationship between RF size and sensory acuity

Answer 50

small RF = high acuity (+ high density of innervation) large RF = low acuity (+low density of innervation)

Question 51

what is another name for two point discrimination?

Answer 51

spatial acuity

Question 52

how can spacial acuity be tested clinically?

Answer 52

applying two sharp point stimuli at two different sites simultaneously patient reports whether one or two points are felt

Question 53

what is a dermatome?

Answer 53

the area of skin innervated by the bilateral dorsal roots of a single spinal segment

Question 54

what forms the dorsal columns?

Answer 54

medial gracile tract = fasiculus gracilis lateral cuneate tract = fasciculus cuneatus

Question 55

what do the dorsal and ventral spinocerebellar tracts convey?

Answer 55

unconscious proprioceptive information to the cerebellum