S3 - somatosensory system

Question 1

somatic sensation : name the 7 modalites, and the 2 groups they split into, where are their respective locations in the spinal cord ?

Answer 1

Spinothalamic system
• Temperature (thermoreceptors)
• Pain (nociceptors)
• Pressure/crude touch (mechanoreceptors)

 Dorsal column-medial lemniscus system
• Vibration (mechanoreceptors)
• Proprioception
• Fine touch (mechanoreceptors)
• Two point discrimination (mechanoreceptors)

Question 2

outline how a strong signal and a weak signal receptor activaton differ in terms of the singal sent ?

Answer 2

Strong receptor activation causes high frequency of action potentials
in the primary sensory neurone

Weak receptor activation causes a low frequency of action potentials
in the primary sensory neurone

Question 3

outline the difference between rapid and slow adapting receptors, give an example of each.

Answer 3

Rapidly adapting receptors (e.g. mechanoreceptors) respond best to changes in strength of stimulation. However, their frequency of firing diminishes
rapidly after the initial stimulus (i.e. they rapidly adapt).

Slowly adapting receptors (e.g. nociceptors) change their frequency of firing very little after the initial stimulus.

Question 4

receptive fields, outline what they are

define acuity and what two factors of receptive fields is acuity dependent on

Answer 4

Receptive fields
A single primary sensory neurone supplies a given area of skin (it’s receptive
field)

If an area of skin is supplied by sensory neurones with relatively large receptive fields, this area will have low sensory acuity.

If an area of skin is supplied by sensory neurones with relatively small receptive fields, this area will have high sensory acuity

o The overlap of receptive fields of primary sensory neurones from adjacent
dermatomes is one of the reasons why dermatomes can have ‘fuzzy’boundaries

Question 5

outline the concept of going from dermatonal sensation in skin too a homunculs in the brain. via what neurons does this change happen

Answer 5

Information becomes reorganised as we move upwards through the neuraxis, such that at the level of spinal nerves and spinal cord we have a dermatomal
organisation, but at levels of the thalamus and above we have a ‘homuncular’

we have 1 ipsilateral projection neurons- cell body in the drg
2- recptor in the medulla - dessucates - projects to 3 in the thalamus - which relays informaiton to 1 somatosensory cortex.

Question 6

what modalitys are from the dorsal column medial lemniscus pathway ?

draw the pathway and explain it - then check vs lec notes

include - medial or lateral tavel of nerve
2 named neucli and what they supply - where these then relay too in the brain.

Answer 6

The dorsal column-medial lemniscus system (DCML)

o Responsible for carrying impulses concerning light touch, vibration, two point
discrimination and proprioception

o Axons of first order neurones ascend ipsilaterally through the dorsal columns
of the spinal cord

 Those from the lower body (T7 and below) ascend through the gracile fasciculus to the gracile nucleus in the medulla)
 Those from the upper half of the body (T6 and above) ascend through the cuneate fasciculus to the cuneate nucleus in the medulla

Concerning second order neurones of the DCML system:
 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
o Concerning third order neurones of the DCML system:

 Thalamic neurones receiving information ultimately from the lower half of the body (via gracile nucleus) project to the medial part of the
primary sensory cortex
 Thalamic neurones receiving information ultimately from the upper half of the body (via cuneate nucleus) project to the lateral part of the
primary sensory cortex

o Topographical organisation of the dorsal columns
 Axons from the lower parts of the body run most medially
 Axons from progressively superior body segments are added laterally
to the dorsal columns

Question 7

what modalitys are from the spinothelamic pathway ?

draw the pathway and explain it - then check vs lec notes

include - medial or lateral tavel of nerve
2 named neucli and what they supply - where these then relay too in the brain.

Answer 7

The spinothalamic pathway or anterolateral system or spinothalamic tract (STT)

o Responsible for carrying impulses concerning pain, temperature and crude touch

o Axons of first order neurones project to the ipsilateral dorsal cord, but the spinothalamic tract supplies the contralateral half of the body
o Concerning first order neurones of the STT:
 They project onto second order neurones in the ipsilateral spinal cord
dorsal horn in the segment at which they enter the cord through the dorsal root (generally)
o Concerning second order neurones of the STT:
 Their cell bodies are in the dorsal horn
 Their axons decussate in the ventral white commissure of the cord and then go on to form the spinothalamic tract
 The spinothalamic tract projects to the thalamus

o Concerning third order neurones of the STT:

 Thalamic neurones receiving information ultimately from more
inferior parts of the body project to the medial part of the primary sensory cortex

 Thalamic neurones receiving information ultimately from more
superior parts of the body project to the lateral part of the primary sensory cortex

o Topographical organisation of the spinothalamic tract
 Axons from the lower parts of the body run most
laterally/superficially
 Axons from progressively superior body segments are added medially/deeper onto the spinothalamic tract

entry of the first order neurones

Question 8

outline brown sequard syndrome

3 clinical signs

Answer 8

• Brown-Sequard syndrome (limited to sensory features)

o If we consider a complete cord hemisection causing destruction of one lateral half of a single cord segment resulting from trauma or ischaemia, the
following structures will be completely destroyed unilaterally:
 The dorsal horn
 The ventral horn
 All other cord grey matter
 All white matter pathways
 Dorsal and ventral roots

o This will lead to the following signs (making reference to the side of the lesion):
 Ipsilateral complete segmental anaesthesia affecting a single dermatome (due to destruction of dorsal root and dorsal horn)
 Ipsilateral loss of dorsal column modalities below the destroyed
segment
 Contralateral loss of spinothalamic modalities at and below the destroyed segment

Question 9

try and expalin abit of liassuers tract

Answer 9

just have a go at this

Question 10

try outline the descending control of pain

Answer 10

• Descending modulation of pain

o Second order neurones of the spinothalamic system dealing with pain receive nociceptive primary afferents as well as inhibitory interneurones which
contain the endorphin encephalin

o These encephalinergic interneurones can be activated by incoming impulses from mechanoreceptors (hence explaining why rubbing a sore area relieves
the pain)

a fibers carry impulses from mechanoreceptors in the skin
c fibers carry pain

activation of the mechanorecotors can stimulate and inhibtory response of enkephalinergic interneurons - reducing pain by rubbing

the inhintion of pain is caused by acivation of descending analgesics from the PAG and raphe neucleus.