(Transcribed from Dr. Glasser’s lecture, 13 June 2000 by Brian Buschman)
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In sensory systems there are two main forms of sensory stimuli that were discussed in the last lecture, epicritic and protopathic. Epicritic relays carry sensation associated with conscious proprioception, vibratory sense and fine touch. Protopathic relays are associated with diffuse touch and temperature stimulation.
The epicritic relay involves fibers with cell bodies in the DRG that enter and ascend ipsilaterally in either the fasciculus cuneatus or gracillius that will synapse in either the nucleus cuneatus or gracillius. The second order neurons cross in the anterior arcuate fibers and ascend in the medial lemniscus to the VPL of the thalamus. The third order neurons then travel in the posterior limb of the internal capsule on their way to areas 3, 1, 2 (the postcentral gyrus).
The protopathic relay involves DRG fibers that enter the dorsal gray and immediately descend a couple of levels in Lissauer’s tract. After their descent they will synapse in the nucleus proprius, cross and ascend in the anteriolateral spinothalamic tract. When they get to the brainstem the tract becomes the spinal lemniscus and it gives off collaterals as it goes up. These collaterals are the fibers of the spinotectal tract that are hidden within the anteriolateral spinothalamic tract. Like the epicritic relay it will synapse in the VPL of the thalamus on it’s way to the postcentral gyrus.
The regular protopathic relay, as given above, carries fast, diffuse pain. Slow pain is still protopathic but there are five major differences in it’s pathway then in the above mentioned pathway.
1) It synapses in the substancia gelatinosa (substance P is the NT) on it’s way to nucleus proprius.
2) It is a bilateral pathway.
3) It has a diffuse relay with unknown ascending tracts.
4) No slow pain goes directly to the thalamus. It has many synapses in the reticular formation nucli on it’s way.
5) When it gets to the thalamus it uses two different nucli:
a. The MD nucli receives fibers bound for the prefrontal gyrus. (Remember that it also carries fibers involved in the limbic/BG functions.)
b. The anterior nucli passes it’s fibers on to the cingulated gyrus.
If a surgeon cuts the MD nucli the patient still feels the pain but doesn’t care. If he cuts the anterior nucleus the patient does not feel the pain.
Fast pain is carried in A-delta fibers. Remember that these fibers are thin, myelinated and slow. They carry fast pain because they are faster then the slow fibers.
Slow pain is carried in C-fibers. These are the smallest and slowest. They are unmyelinated.
We must not forget that a large part of sensory stimulation is from the face and is carried in the trigeminal nerve. Without this somatosensory systems would be very simple to study but we do have this nerve. There are four nucli associated with the trigeminal nerve which are the chief sensory, motor, spinal and mesencephalic nucli.
The epicritic relay of the trigeminal system enter the CNS as the fibers with cell bodies in the trigeminal ganglion that go directly to the chief sensory ganglia of V. The second order fibers then branch into two tracts. One stays ipsilateral and ascends as the dorsal trigeminal thalamic tract (DTTT) but it’s functional role is unknown. The epicritic fibers cross and ascend as the ventral trigeminal thalamic tract (VTTT). On a given side the DTTT of that side, the contralateral VTTT and protopathic fibers that we will soon discuss ascend to the thalamus in the trigeminal lemniscus to the VPM. It then sends the fibers to the lateral part of 3, 1, 2 near the Sylvain fissure.
The protopathic trigeminal relay enter the pons and descend (just like the spinal protopathics) to synapse in the spinal nucli. It then crosses and ascends with the VTTT to the VPM. Again it becomes part of the trigeminal lemniscus enroute to the VPM.
The third sensory nucleus of the trigeminal nerve is the mesencephalic which houses the cell bodies of the first order neurons of 1A neurons. This is the only place in the body that first order sensory neurons have cell bodies outside of the PNS (i.e. DRG or trigeminal nucleus). It sends it’s fibers to the motor nucleus to function in the 1A reflex and some to the chief sensory nucleus that will ascend with the epicritics to be used for conscious proprioception.
To this point we have studies 7 of the 11 thalamic nucli that we will need to talk about this semester and he assures us that we will see questions related to this little bit of data:
1) CM – Functions in the feedback looks of the BG. Inputs from the GPI and output to the straitum.
2) VL – Receives projections from GPI and sends fibers from:
a. VLP to area 4
b. VLA to area 6
3) VA – Again receives fibers from GPI but sends it’s output to areas 6 and 8.
4) MD – Functions in the middle of the limbic system’s role in BG function and is a relay for slow pain.
5) VPL – Is the relay nucleus between the spinal lemniscus and the postcentral gyrus.
6) VPM – Is the relay between the trigeminal lemniscus and the prefrontal gyrus.
7) Anterior – If a slow pain relay between the spinal lemniscus and the cingulated gyrus.
Conscious proprioception synapses in the cuneate and gracillius nucli.
Unconscious proprioception synapses in:
a) LL – Clark’s column
b) UL – Accessory (external) cuneate nucleus
Now that we have discussed a little more we can see the increased complications associated with a Brown-Sequard legion. It results in:
1) Ipsilateral UMN legion.
2) Ipsilateral epicritic (fine touch, vibratory sensation and conscious proprioception) loss.
3) Contralateral protopathic (diffuse tough, temp and fast pain) loss.
Since slow pain travels bilaterally it will not be lost with a Brown-Sequard legion.
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