3.1.2 Somatosensory II

Question 1

What is the chain of neurones in the somatosensory system?

Answer 1

First order sensory neurones
Second order sensory neurones
Third order neurones

Question 2

What are the features of first order sensory neurones?

Answer 2

• Cell bodies in DRG
• Communicate with a receptor
• Central axon projects ipsilaterally to cell body
• Project onto second order neurones

Question 3

What are the features of second order sensory neurones?

Answer 3

• Cell bodies in spinal cord dorsal horn or medulla
• Decussate
• Project onto third order neurones

Question 4

What are the features of third order neurones?

Answer 4

• Cell bodies in the thalamus
• Project to primary sensory cortex (postcentral gyrus)

Question 5

What is the principle of somatotopy or topographical representation?

Answer 5

Idea that for every point on the surface of the body, an equivalent point can be identified along the sensory pathway

Some exceptions (e.g. face next to hand), adjacent body regions map to adjacent regions of the sensory system

Question 6

Why is there somatotopy?

Answer 6

Minimised the amount of wiring needed to transmit sensory information

Question 7

What happens at the level of the sensory humunculus?

Answer 7

All modalities converge e.g. head area of sensory cortex deals with pain, temperature and vibration etc. all at same time)

Question 8

What is the dorsal column-medial lemniscus system responsible for?

Answer 8

Carrying impulses concerning:
- Light touch
- Vibration
- Two point discrimination
- Proprioception

2 -Two point discrimination
T -light Touch
Vs -Vibration
Please - Proprioception

Question 9

What do first order neurones in the dorsal column-medial lemniscus do?

Answer 9

Ascend ipsilaterally through dorsal columns of the spinal cord

Question 10

What are the two nuclei that first order neurones in the dorsal column medial lemniscus ascend to?

Answer 10

Gracile nucleus
Cuneate nucleus

Question 11

What spinal level is the gracile nucelus responsible for?

Answer 11

First order neurones from T7 and below ascend through gracile fasciculus to the gracile nucleus

Gracile has 7 letters, T7

Question 12

What spinal level is the cuneate nucelus responsible for?

Answer 12

T6 and above ascend through the cuneate fasciculus to the cuneate nucelus

Question 13

What do the second order neurones in the dorsal column medial lemniscus do?

Answer 13

Neurones in the gracile nucleus project to the contralateral thalamus
in the medial lemniscus

Neurones in the cuneate nucleus project to the contralateral
thalamus in the medial lemniscus

Second order decussate

Question 14

What do the third order neurones of the dorsal column medial lemniscus system do?

Answer 14

Thalamic neurones receiving information from the lower
half of body (via gracile nucleus) project to the medial
primary sensory cortex

Thalamic neurones receiving information from upper
half of body (via cuneate nucleus) project to lateral primary sensory cortex

Question 15

What is the topographical organisation of dorsal columns?

Answer 15

Axons from lower parts of body run medially

Axons from more superior segments added laterally to dorsal columns

Medial to lateral

Question 16

Fibre ascension in dorsal column vs spinothalamic

Answer 16

Dorsal medial lemniscus column- lower fibres are medial

Spinothalamic- lower fibres are lateral

Always think about the lower fibres first and use to work out higher ups

Question 17

What is the spinothalamic pathway responsible for?

Answer 17

Carrying impulses concerning:
- Pain
- Temperature
- Crude touch

Spinothalamic= 13 letters 1-3

Ignore 1 letter, then 3 then all but last
S
P - Pain
i
n
o
T - Temperature
h
a
l
a
m
i
C - Crude touch

Question 18

What do the first order neurones of the spinothalamic tract supply?

Answer 18

Project to ipsilateral dorsal cord BUT,

Supplies contralateral half of the body

Question 19

What do first order neurones of the spinothalamic tract do?

Answer 19

Project onto second order neurones in ipsilateral spinal cord dorsal horn in the segment which they enter the dorsal root

Essentially, normal cell body in DRG, but they enter the dorsal horn

Question 20

What do the second order neurones do in the spinothalamic system?

Answer 20

Cell bodies in the dorsal horn

Axons decussate in ventral white commissure of cord then go on to form the spinothalamic tract

Spinothalamic tract projects to thalamus

Question 21

What do third order neurones do in the spinothalamic pathway?

Answer 21

Thalamic neurones receiving information from inferior body project to medial sensory cortex

Vive.

Question 22

What is the topographical organisation of the spinothalamic tract?

Answer 22

Axons from lower body run laterally and axons from superior segments are added medially

Opposite to dorsal column due to decussation of the STT second order neurones at the level of entry of the first order neurones

Question 23

Image showing how inferior nerves pass laterally and superior medially

Spinothalamic tract

Answer 23