The Somatosensory System

Question 1

What are the two main categories of sensation?

Answer 1

Somatic sensation and special sensation

Question 2

The somatosensory pathways usually consist of how many neurones?

Answer 2

Three

Question 3

With regards to the first order neurone (i.e. the primary sensory afferent), where will the a) receptor and b) soma be found?

Answer 3

a) in the periphery e.g. the skin b) more central, though still in the PNS e.g. dorsal root ganglion or cranial ganglia

Question 4

What is found in the dorsal root ganglion? Where is this found?

Answer 4

A cluster of first order neurones (i.e. primary sensory afferents) / in the dorsal root of a spinal nerve, in the intervertebral foraminae

Question 5

Complete the following sentence: when a sensory receptor is activated, the information is received by ? - this carries information within a ? to the ? where it connects with the ?

Answer 5

The first order neurone (primary sensory afferent) / peripheral nerve / CNS / second order neurone

Question 6

Complete the following sentence: a projection from a second order neurone carries sensory information to the ?, often via a ? in the ? before reaching the ? for processing

Answer 6

Brain / 3rd order neurone / thalamus / cortex

Question 7

Where are the cell bodies of somatosensory neurones located?

Answer 7

In clusters outside the vertebral column in the dorsal root ganglia

Question 8

The afferent projections from first order sensory neurones enter the spinal cord where?

Answer 8

At the dorsal horns

Question 9

What is the terminal of the 1st order sensory neurone?

Answer 9

The receptor

Question 10

Where would the cell body be found of each of the following neurones in a somatosensory pathway coming from the limbs/trunk or posterior head: a) first order? b) second order? c) third order?

Answer 10

a) dorsal root ganglion b) dorsal horn of spinal cord c) thalamus

Question 11

Where would the cell body be found of each of the following neurones in a somatosensory pathway coming from the anterior head: a) first order? b) second order? c) third order?

Answer 11

a) cranial ganglia b) brainstem nuclei c) thalamus

Question 12

What is the sensory unit?

Answer 12

The whole first order neurone, including the cell body and the receptor

Question 13

Sensory units/primary afferent neurones are especially tuned to respond to a specific type of energy which normally excites them. This is known as what?

Answer 13

Their modality

Question 14

What do low threshold units respond to?

Answer 14

Low intensity, non-damaging stimuli

Question 15

What do high threshold units respond to? What type of receptors are these?

Answer 15

High, noxious stimuli only / nociceptors

Question 16

Thermal nociceptors respond to extremes of heat - what would these be?

Answer 16

< 10 degrees or > 45 degrees

Question 17

Sensory receptors can be what three main types?

Answer 17

Free nerve endings, encapsulated nerve endings or specialised receptors

Question 18

Which type of sensory receptor will detect tactile sensation e.g. touch and pressure?

Answer 18

Mechanoreceptors

Question 19

Which type of sensory receptor will detect temperature changes?

Answer 19

Thermoreceptors

Question 20

Which type of sensory receptor will detect painful stimuli?

Answer 20

Nociceptors

Question 21

Which type of sensory receptor will detect changes in the position and movement of the body?

Answer 21

Proprioceptors

Question 22

The area of skin innervates by the left and right dorsal roots of a single spinal segment is termed what?

Answer 22

A dermatome

Question 23

What is the only part of the body which is not innervated dermatomally?

Answer 23

Anterior face

Question 24

If a single dorsal root is cut, the corresponding dermatome does not lose all sensation. Why is this?

Answer 24

There is a degree of overlap with the region of adjacent dorsal roots

Question 25

Axons from 2nd order sensory neurones form what?

Answer 25

Sensory ascending tracts

Question 26

What are the two main sensory ascending tracts and where do each of them decussate?

Answer 26

DCML - in the medulla / spinothalamic - in the spinal cord

Question 27

Complete the sentence: axons from the ascending spinal cord tracts reach the ? where they communicate with ? which project into the ? where signals give rise to the awareness of the feeling

Answer 27

Thalamus / 3rd order neurones / cortex

Question 28

What is stereognosis? Which ascending spinal cord pathway allows this to be possible?

Answer 28

The ability to recognise an object by feeling it / DCML

Question 29

Complete the following sentence about trigeminal sensation: impulses originate at the terminals of two trigeminal nerves, each of which has ? divisions. The soma of sensory neurones are located in the ?

Answer 29

Three / trigeminal sensory ganglion

Question 30

Central terminals of the trigeminal nerve synapse upon second order neurones where, for general tactile stimuli?

Answer 30

The main sensory nucleus

Question 31

Central terminals of the trigeminal nerve synapse upon second order neurones where, for pain and temperature?

Answer 31

The spinal nucleus

Question 32

Complete the following sentence about trigeminal sensation: central terminals of the trigeminal nerve synapse upon 2nd order neurones in the respective nucleus. These in turn decussate and project via the ? to the ? nucleus of the ?

Answer 32

Trigeminal lemniscus / ventroposteriomedial (VPM) / thalamus

Question 33

Complete the following sentence about trigeminal sensation: 3rd order neurones relay information to the cortex via ?

Answer 33

Thalamocortical neurones

Question 34

In general, where are the somatosensory cortices located?

Answer 34

In the postcentral gyrus

Question 35

Where is the primary somatosensory cortex located?

Answer 35

Immediately posterior to the central sulcus

Question 36

Where is the secondary somatosensory cortex located?

Answer 36

Adjacent to the posterior parietal cortex

Question 37

The primary somatosensory cortex is classically described as consisting of what?

Answer 37

Brodmann areas

Question 38

The primary somatosensory cortex receives input from where?

Answer 38

The ventral posterior nucleus in the thalamus

Question 39

Briefly, describe the order of structures on the somatosensory homunculus from medial to lateral?

Answer 39

Genitals, lower limb, trunk, upper limb, face, mouth viscera

Question 40

How many cell layers are there in the somatosensory cortex?

Answer 40

6

Question 41

What happens if a finger is lost in an accident or amputated?

Answer 41

After months, the area that used to represent that finger will now represent the adjacent fingers

Question 42

What happens if the sensory input from a finger increases?

Answer 42

The area of cortex associated to that finger enlarges

Question 43

What is the function of the secondary somatosensory cortex?

Answer 43

The integrate sensory information with other cortical areas

Question 44

What is agnosia?

Answer 44

You can see a structure but can't tell what it is

Question 45

What is astereognosia?

Answer 45

Can touch an object but can't tell what it is

Question 46

What is hemispatial neglect syndrome?

Answer 46

The patient neglects the contralateral half of their body