The Somatosensory System 2 Flashcards
What are the 2 main parts of the somatosensory pathways?
Dorsal column leminscal (DCML) pathway
SpinothalamicTract (STT)
What do both DCML and STT consist of?
3 neurons in series: 1st, 2nd and 3rd order neurones
Where is the cell body of the 3rd order neurone?
THALAMUS
Where does the 3rd order neurone project to?
SOMATOSENSORY CORTEX
What is the DCML pathway responsible for?
Discriminative touch
Pressure
Vibration
Conscious proprioception
What is the STT pathway responsible for?
Pain Thermosensation Crude touch Itch Tickle
Where does the decussation of DCML fibres take place?
At the great sensory decussation of the MEDULLA
2nd order neurones all cross together here
Where does the decussation of STT fibres take place?
At all levels of the spinal cord, close to the point of entry, via Lissauer’s tract.
ie. synapse in the dorsal horn of the spinal cord, meaning that information ascends contralaterally.
Brown - Sequard Syndrome explains?
The differences in the level of decussation of the DCML and STT
What do the dorsal columns consist of?
The medial fasiculus gracilis (gracile tract)
+
The more lateral fasiculus cuneatus (cuneate tract)
Where does sensory input to T6 and below (inc. legs and lower trunk) travel?
In the fasiculus gracilis (which runs the length of the cord)
Where does sensory input to T6 and above (inc. arms and upper trunk) travel?
In the fasiculus cuneatus
DCML is the major route by which?
Touch and proprioceptive information ascends to the cerebral cortex (apart from anterior head)
DCML primary afferent (1st order neurone) enters the dorsal horn and branches to 1 of 2 things. What are these?
1) Synapses deep in the dorsal horn upon second order neurones
2) An immediately ascending component (via the dorsal column fasiculus gracilis and fasiculus cuneatus)
Then to the GN or CN
Where do axons of 2nd order neurones of the dorsal column nuclei cross?
COLLECTIVELY, in the greater sensory decussation
What happens after the axons of the 2nd order neurones of the dorsal column nuclei cross?
They ascend in the medial lemniscus, to the ventral posteriolateral (VPL) nucleus of the thalamus
Where do 3rd order neurons of the dorsal column project to?
Project to the primary somatosensory cortex (S1), via the posterior internal capsule
What conveys sensory information to the head?
The Trigeminal System
Outline the 5 capabilities of the DCML pathway.
- Stereognosis the ability to recognise an object by feeling it.
- Vibration detection.
- Weight discrimination.
- Fine touch ability to recognise exact location of light touch – 2 point discrimination.
- Conscious Proprioception awareness of body position and awareness of body movements (kinaesthesia).
How do you test stereognosis?
Ask subject to identify familiar object by touch alone
What does testing stereognosis evaluate?
Cortical function
How is vibration detection tested?
Clinician places 128Hz tuning fork against a finger tip, top of toe or a bony prominence
How is conscious proprioception tested?
Clinician moves a finger, toe, hand or foot – subject asked (with eyes closed) to report if the structure is being moved up, or down
What produces contrast enhancement?
When information is conveyed from one neurone to the next in a sensory pathway, the differences in the activity of adjacent neurones are amplified
What does the trigeminothalamic pathway provide sensory innveration to?
ANTERIOR HEAD Oral cavity Nasal cavity Paranasal sinuses Teeth Dura Cerebral arteries
What nerve innervates the trigeminal system?
CN V
Where are the soma of sensory neurones of the trigeminal system located?
In trigeminal sensory ganglion
What do 3rd order neurones relay information to the cortex via?
Thalamocortical neurones
Central terminals of the trigeminal nerve synapse upon second order neurones in the chief sensory nucleus (general tactile stimuli), or spinal nucleus (pain, temperature information) which in turn decussate and project (via the trigeminal lemniscus) to the ventroposteriomedial (VPM) nucleus of the thalamus
NOTE
Where is the somatosensory cortex located?
In the POST-CENTRAL GYRUS of the PARIETAL CORTEX, immediately posterior to the CENTRAL SULCUS and adjacent to the POSTERIOR PARIETAL CORTEX
What does the somatosensory cortex consist of?
Brodmann areas (BA) 1, 2, 3a and 3b (the latter being denoted ‘primary somatosensory cortex’ in some texts, S1)
Where does the somatosensory cortex receive input from?
THALAMUS
What do receptive fields of SI neurones form?
An orderly somatotopic map of the body surface on the primary somatosensory cortex.
(revealed by electrical stimulation and neuronal recording techniques)
Scaling the body surface to the volume of SI cortex devoted to the representation of a body part gives rise to the ………?
Homunculus
Reveals the relative volume of cortex devoted to each body part is correlated with the density of sensory input from that part (not its size) and hence sensory acuity and importance of sensory input
TRUE
How many cell layers are there in the somatosensory cortex?
Six - I to VI
Where do thalamic inputs to SI terminate?
On neurones in layer VI
What do neurones in layer IV project to?
Cells towards the surface of the cortex, and also deeper layers
What extends across each of the 6 layers?
Vertical columns (0.3 to 0.5mm diameter) of neurones (about 10,000)
What does each vertical column consist of?
Neurones with similar inputs and responses
Adjacent strips of the cortex map…..
The same parts of the body in parallel to each other
The maps are not identical or mirror images of each other?
FALSE - they are mirror images of each other
What does the posterior parietal cortex do?
Receives and integrates information from SI and other cortical areas (visual, auditory) and sub-cortical areas (thalamus)
What is the posterior parietal cortex important for?
Deciphering the deeper meaning of the information in S 1
What can damage to the posterior parietal cortex result in?
Bizarre neurological disorders (ie. agnosia, astereognosia, hemispatial neglect syndrome) with simple sensory skills remaining intact
What condition occurs as a result of damage to the RIGHT posterior parietal cortex?
Neglect syndrome
What happens to patients with Neglect Syndrome?
Believe that the left side of the world does not exist, and may even disclaim the existence of the left side of their body
