Somatosensory system

Question 1

Dorsal column medial lemniscal pathway

• describe the pathway from periphery to cortex?
• what sensations are involved?
• describe in detail the path of the 3 neurones involved?
• overview of the DCML pathway capabilities? (6)

Answer 1

-nerve fibres enter the spinal cord from from the periphery and split not 2 branches, one branch goes deep and the other ascends the spinal cord to the medulla ipsilaterally i.e. on the same side it enters.
at this point it is still a primary afferent neurone
The first synapse occurs in the brainstem and decussation occurs here i.e. the fibre moves from one side of the nervous column to the other the 2nd order neurone ascends to the thalamus and then synapses with the third which ascends to the somatosensory cortex.

-discriminatory touch, pressure, vibration, conscious proprioception

-1st order neurone (primary afferent)
Enters the dorsal horn and branches forming synapses deep in the dorsal horn, synapses with 2nd order neurones at dorsal column gracile nucleus and cuneate nucleus

2nd order neurone
axons from this dorsal column nuclei cross collectively in the great sensory decussation and ascend in the medial leminiscus to the ventral posteriolateral nucleus of the thalamus

3rd order neurone
thalamic neurones, project to the primary somatosensory cortex via the posterior internal capsule

-Stereognosis 
Vibration detection
Fine touch
Conscious proprioception
weight discrimination

Question 2

Spinothalamic tract

• describe the pathway peripheral to cortex
• sensation involved?

Answer 2

-There are two main parts, the lateral (pain and temp) and the anterior (crude touch, pain, tickle)
Synapse occurs shortly after entry to the spinal cord so the 2nd order neurone ascends the spinal cord.
decussation occurs at all levels of the spinal cord (close to point of entry via Lissauer’s tract) i.e. contralateral ascension.
The 2nd synapse occurs in the thalamus which projects into the somatosensory cortex

Question 3

What is Brown-Séquard syndrome?

Answer 3

damage to one half of the spinal cord resulting in paralysis and loss of proprioception on the ipsilateral side of injury and loss of temp/pain sensation on the contralateral side

Question 4

Somatic organisation of the dorsal columns

• what do the dorsal columns consist of?
• function of the component parts?

Answer 4

-medial fasciculus gracilis and the lateral fasciculus cuneatus

-MFG= sensory input to T6 and below travels here 
LFC= sensory input above T6 travel here

Question 5

Contrast inhibition and lateral inhibition

• what is this?
• How does it work?

Answer 5

-Capacity of an excited neurone to reduce the activity of its neighbours as info conveyed from one neurone to the next in a sensory pathway differences in the activity of adjacent neurones are amplified
Neurones run very close together so need to determine what signals are useful

-Prevent the spread of Aps from excited neurones to neighbouring neurones to increase the sensory in put from a particular neurone

Question 6

Trigeminal system

• supplies what?
• divisions of CNV?
• where is the soma of these nerves located?
• where does the synapse with 2nd order neurones occur and what is their pathway?
• 3rd order neurones rely info to cortex via what?

Answer 6

• general somatic information from the ant head
• trigeminal, mandíbula, maxillary
• trigeminal sensory ganglion

-chief sensory nucleus or spinal nucleus
these then decussate and project to the ventroposteriomedial nucleus of the thalamus

-thalamocortical neurones

Question 7

Somatosensory cortex (SI)

• location?
• what areas is it formed from & what is their function?
• receives input from where?
• no. of cell layers?
• structure?
• there are somatotopic maps present, what is important about these maps?
• What happens to the area of the Supplying an amputated finger?

Answer 7

-post central gyrus of parietal cortex
immediately posterior to central sulcus and adjacent to the posterior parietal cortex

-Brodman areas: BA3a, BA3b, BA1, BA2
BA3a- body position 
BA3b- Touch
BA1- texture discrimination 
BA2- object perception 

• six cell layers, the thalamic inputs terminate mainly on layer IV which then projects cells towards surface and deeper
• there are vertical columns of neurones across the 6 layers, each consist of neurones with similar inputs and responses
• adjacent strips of cortex map the same part of the body in parallel to each other and the maps are mirror images of each other.
• it is utilised by other sensory inputs and might explain phantom limb sensation shows that somatotopic maps are plastic, they can also enlarge if the sensory input increases

Question 8

Posterior parietal cortex

• function?
• what might damage to this cause?

Answer 8

• receieves and integrates information form SI and other cortical areas e.g. visual/auditory and sub cortical areas (thalamus)
• agnosia, astereognosia, hemispatial neglect syndrome) with simple sensory skills remaining intact might occur due to stroke