(Transcribed from Dr. Glasser’s lecture, 6 and 9 June 2000 by Brian Buschman)
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All of the info that is passed between the brainstem and the cerebellum enters and exits through one of three cerebellar peduncles.
1) Inferior cerebellar peduncle (ICP) or the restiform body at the level of the medulla.
2) Middle cerebellar peduncle (MCP) or the brachium pontis at the level of the pons.
3) Superior cerebellar peduncle (SCP) or the brachium conjunctivum at the level of the midbrain.
Info from the lateral vestibular, oralis caudalis and gigantocellularis (all in the lower brainstem) crosses from contralateral to ipsilateral to enter the cerebellum via the ICP. It then goes to stimulate the vermis and the fastidual nucleus. The vermis will be inhibitatory to the fastidual nucleus and the fastidual nucleus will be stimulatory back on the three motor nucli after crossing back over in the ICP. The part of the ICP that they cross in is known as the juxtarestiform body.
The red nucleus is located in the upper brainstem and works contralaterally. It’s fibers enter the cerebellum via the SCP and cross on the way in. Since the red nucleus is associated with limb movement it’s fibers go to the paravermis and the interposed nucleus (ebolaform nucleus and globose nucleus). Like all parts of the cerebellar cortex the paravermis is inhibitatory on the interposed nucleus and the interposed nucleus is stimulatory on the red nucleus (after leaving and crossing in the ICP).
Feedback from the cerebellar cortex is a little more complex. Motor fibers leave the cerebellar cortex via the corticopontine tract and enter the posterior restiform nucleus. Fibers leave the posterior restiform nucleus and enter the cerebellum via the MCP crossing as they go. These fibers are associated with limb movement so they enter the paravermis and interposed nucleus. They are different from the above examples because they leave on a different peduncle then they entered on, in this case the ICP as they head towards the thalamus. They will go to the VL nucleus of the thalamus and then ascend to area 4.
In this case the corticopontine fibers enter the cerebellum via the MCP as before but go to the lateral lobe and the dentate nucleus to function in coordination and balance. The output then goes back through the VA nucleus of the thalamus rather then the VL because the VA nucleus projects onto the premotor cortex (area 6).
The dentatorubrio thalamic tract is the tract that comes from the dentate nucleus of the lateral cerebellum and goes to the red nucleus ad the thalamus. It’s pathways are intermixed with those of the BG system. There is a net of fibers that leave the GPI on their way to the VL and VA nucli of the thalamus that break into two bundles. The first is the ansa lenticularis which arcs around the end of the internal capsule. The other is the lenticular fasciculus which goes right through the internal capsule. On the other side they join together and receive the dentatorubrio thalamic tract to become the thalamic fasciculus. Part goes to the VA which will end up in area 6 and part to the VL which will end in area 4.
The floccularnodular lobe receives fibers from the vestibular nucleus via the ICP. They then are processed in the lobe which inhibits the red nucleus which stimulates the VL nucleus of the thalamus which is projects to area 6. The floccularnodular lobe is inhibitatory to the red nucleus because all cerebellar cortex lobes are inhibitatory to the associated nucli. Since it does not have an associated deep nucli it will then inhibit the first nucleus it comes to, being the red nucleus.
There are two primary pathways that ascent in the spine:
1) Spinothalamic tract which carries pain, temperature and diffuse touch. It enters the dorsal horn, synapses, crosses to over and ascends in the lateral spinothalamic tract.
2) The other tract is made of the fasciculus gracillius and the fasciculus cuneatus which carry fine touch, vibratory sense and conscious proprioception. They synapse in the nucleus gracillius and nucleus cuneatus. They then cross in the internal accurate fibers and ascend in the medial lemniscuses.
There are four main sensory tracts that enter directly into the cerebellum. They are the tracts for IA and IB fibers from the upper and lower bodies.
|
|
IA (length) |
IB (Tension) |
|
Upper Body |
- Do not synapse - Hitch a ride with the fasciculus cuneatus (ipsilaterally). - Ascend to the cuneate nucleus. - Enter the cerebellum in the cuniocerebellar tract in the ICP. |
We are not really sure about this one. - Ascends in the rostral spinocerebellar tract. - Probably ipsilaterally. - Probably uses the ICP. |
|
Lower Body |
- Synapse on the dorsal column (of Clark). - Ascend in the dorsal spinocerebellar tract (ipsilaterally). - Enter the cerebellum in the ICP. |
This is the strange
exception that is HY for exams. - Synapses in the central boarder cells (lamina VI-VII). - Ascends contralaterally in the ventral spinocerebellar tract. - Enters the cerebellum on the SCP and then crosses in the cerebellum to become ipsilateral again. |
All of them ascend ipsilaterally and enter through the ICP except for the IB fibers of the lower body which run in the ventral spinocerebellar tract. They ascend contralaterally and enter through the SCP.
EVERYTHING that goes to the cerebellum also goes to the ION. There is a massive projection from the cerebellum to the ION in the ICP. It crosses on it’s way to the ION (R. cerebellum to L ION).
|
SCP |
Afferents – ventral spinocerebellar tract |
|
|
Efferents – dentatorubrio thalamic tract |
|
MCP |
Only afferents – pontocerebellar tract |
|
ICP |
Afferents – vestibulocerebellar tract n dorsal spinocerebellar tract n fibers on the fasciculus cuneatus n rostral spinocerebellar tract n ION |
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Efferents – vestibulocerebellar tract n ION
|
As we talked about in histology the cerebellar cortex is made of three cell layers, the molecular, perkinji and the granule. The perkinji cells are lined up with lots of branching dendrites and an axon that is inhibitatory on the deep cerebellar nucli. It is innervated by two types of fibers, climbing fibers and mossy fibers.
Climbing fibers come from the ION and stimulate multiple perkinji cells at multiple points. That will then work to inhibit the associated deep nucli.
The other type of fibers are mossy fibers that come from any input except the ION. They will stimulate the granule cells (we never talked about what the molecular layer does). The granule cells have an axon that will go up in the perkinji cells, branch and turn at right angles. This will create multiple fibers known as parallel fibers that cross and stimulate the perkinji cells. They also stimulate basket cells and satellite cells. These two cell types will then inhibit the same perkinji cells. The net stimulation is zero if both granule and associated basket and satellite cells stimulate the same perkinji cells. The way the system works is that the granule cell will stimulate some perkinji cells that are not inhibited by the other cell types. This is what causes the stimulation and further inhibition of the deep nucli. Granule cells also stimulate golgi cells which are like Renshaw cells in that they turn back on the granule cells and inhibit them.
The deep nucli of the cerebellum have many fibers that are stimulated by climbing fibers and inhibited by perkinji cells. The control of the cerebellum is based on the combination of these two. Output is based on the stimulation of a fiber that is not inhibited by a perkinji cell.
When you learn a new voluntary movement it’s cerebellar input is via a climbing fiber circuit. Once you have it down the input is from a mossy fiber circuit.
1) Motor association (area 6) initiates the motor program. The cerebellum and BG make the program. Area 4 plays out the program.
2) The dentatorubrio thalamic tract begins in the dentate AND interposed nucleus. It exits and crosses in the SCP on it’s way to the red nucleus. All of the fibers go to the red nucleus but only some synapse there while some continue on to the thalamus.
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