The Somatosensory System Flashcards
Describe the Dorsal Column - Medial Lemniscal Tract
Answer must include:
- The system the tract is involved in
- The neurons involved
- Where the neurons project
- Where neurons cross over
The dorsal column-medial lemniscal tract is involved in the perception and appreciation of mechanical stimuli as part of the somatosensory system.
Primary somatosensory afferent neurons enter the dorsal horn of the spinal cord and branch.
The most significant branch enters the dorsal column of the spinal cord and ascends to the ipsilateral caudal medulla where it projects onto the dorsal column nuclei. The two nuclei are the medial Gracile nucleus (lower limb) and lateral Cuneate nucleus (upper limb).
The Gracile nucleus and Cunate Nucleus neurons cross (decussate) in the caudal medulla.
These fibres position themselves to begin ascending in the ventral aspect of the medulla known as the medial lemniscus - a white matter tract to the thalamus.
Progressively, the medial lemniscus positions the tracts carrying lower limb information laterally and upper limb medially (this is opposite to within the spinal cord). These tracts innervate the ventral posterolateral nuclei of the thalamus.
Projections from the thalamus innervate the primary somatosensory cortex (S1) in the post central gyrus.
What are the dorsal columns and how are they organised?
The dorsal columns are ascending white matter tracts positioned within the dorsal white matter of the spinal cord.
They transmit the somatosensory information of mechanical stimuli.
Ascending tracts of inferior structures (e.g. lower limb) is positioned medially within the dorsal column and superior structure (upper limb) tracts are located laterally within the dorsal column.
Where is the primary somatosensory cortex (S1) located?
The primary somatosensory cortex is directly posterior to the central sulcus.
It is comprised of four sub regions:
Maps 1,2,3a and 3b
The secondary somatosensory cortex is intimately associated latero-inferiorly to the primary cortex.
What differentiates the secondary somatosensory cortex from the primary?
The secondary somatosensory cortex (S2) has slightly different inputs than the S1
It recieves some thalamic inputs but primarily recieves significant amounts of primary sensory afferents (unlike S1)
Is S1 somatotopically organised? What does this mean?
Yes; S1 is somatotopically organised.
Somatotopy is the point-for-point correspondance of an area of the body to a precific area of the post-central gyrus of the S1 cortex.
Not all regions of the body have equal distribution within the S1 region - the hands, digits and lips have the greatest area dedicated to it; recognising the increased somatosensory information recieved and processed from the these areas.
How are slowly adapting and rapidly adapting inputs of the somatosensory cortex organised?
Within a specific somatotopic area of the S1, slowly and rapidly adapting inputs are segregated into functionally distinct columns
It is unknown why this occurs
What are the four maps of the S1 cortex hypothesised to be specialised to?
There may be some specialised modalities across the differnt maps of the S1 system.
Map 3a:
- No known specialty
Map 3b:
- Main source of input from the ventral posterior complex of the thalamus
Map 1:
- More related to texture
Map 2:
- More related to size/shape
All maps contribute to innervation of the secondary somatosensory cortex to percieve ‘congitive touch’ = what something feels like.
Map 2 additional contributes to the parietal areas in motor and pre-motor control.
What is somatosensory plastcity?
Somatosensory plasticity describes the dynamic way in which topographical areas of the S1 cortical space can change in size/contribution
The S1 space is finite though - if the area dedicated to the hand is increased, something else must decrease in order for this to occur. Generally, a use-it-or-lose-it rule applies - organs that are used more frequently are retained.
Somatosensory plasticity occurs in the event of loss of limb or other disabilities that limit the somatosensory activation of particular topographical areas of S1
How and where this occurs is still unknown
What involvement of somatosensory plasticity occurs when neurons regenerate to incorrect targets following PNS nerve damage?
Unfortunately, somatosensory plasticity does not assist in this situation.
Nerves that regenerate to incorrect sites of innervation signals sensory infromation to the S1 as if it were still in its original place.
Cortical S1 plasticity is unable to compensate and re-think what the periphery has done
Discuss the phenomenon of phantom limbs
The majority of people with amputations experience phantom limbs - it is a normal phenomenon.
These phantom limbs may be whole or partial limbs.
A damaged axon is able to be stimulated/irritated at a point proximal to the injury site - the signal carried is inferred to have come from the original distal innervation: a limb structure no longer there.
If the signals being transmitted are pain/nociception signals - this is phantom pain. This extremely difficult to treat and is highly troubling/debilitating
What is the somatosensory system?
The somatosensory system transmits and analyzes touch or tactile information from external and internal locations on the body and head.
The result of these processes leads to the appreciation of somatic sensations, which can be subdivided into the submodalities of:
- discriminative touch
- flutter-vibration
- proprioception ( position sense)
- crude ( nondiscriminative) touch
- thermal ( hot and cold) sensation; and nociception ( pain).
The following anatomically and functionally discrete pathways transmit these signals:
(1) the dorsal column–medial lemniscal pathway
(2) the trigeminothalamic pathways,
(3) the spinocerebellar pathways, and
(4) the anterolateral system.
Describe the importance of glabrous skin
Most of the body is covered by hairy skin. Particular parts of body skin are specialised for sensation and are covered by hairless glabrous skin
Areas with glabrous skin include the palmar surfaces of the hands and soles of the feet.
Glabrous skin have:
- Skin ridges: ridges (such as finger pads/prints) provide traction and grip as well as being dense in sensation receptors
- Non-hairy skin: have highly concentrated regions pf receptors for sensory transduction
Discuss the location, sensation and adapting status of the four prominant skin mechanoreceptor of the PNS
Receptor Type
Sensation Produced
Adapting Status
Location within skin layers
Merkel Complexes
Touch - Pressure
SA
Dermis
Meissner Corpuscle
Tap
RA
Epidermis
Ruffini Corpuscle
Skin stretch
SA
Dermis
Pacinian Corpuscle
Vibration
RA
Subcutaneous Layer
What is the difference between a slowly adapting and rapidly adapting mechanoreceptor?
Rapidly adapting mechanoreceptors only signal a change in stimulus i.e the onset of a stimulus. It rapidly stops firing signals when a sustained stimulus is present. These include Paccinian and Meissner corpuscles
Slowly adapting mechanoreceptors are capable of encoding the stimulus intensity and duration of a stimulus. They actively signal until the stimulus is no longer present. These include Merkel complexes and Ruffini corpuscles
Discuss the relevance of stimulus threshold for the activation of tactile and nociceptive primary afferent receptors
Stimuli must induce a change in the sensory receptor membrane sufficient enough to reach threshold for action potential propagation to the CNS.
Nociceptors are high threshold receptors - they require significantly higher levels of stimulus to induce notable signalling to the CNS. Nociceptors are pain receptors and largely monitor the internal body environment.
