Point
3
Intro
When the 1a, 1b, and II axons enter the
spinal cord they dive into the grey matter of the dorsal
horn until they reach its base. An investigator by the name
of Rexed has divided the grey matter of the cord into layers
or laminae, and the base of the dorsal horn is called lamina
VII. Within this lamina at spinal segments C8-L3,
AND ONLY AT THESE LEVELS, there is a very distinctive
cell group called CLARKE'S NUCLEUS OR COLUMN. The
1a, 1b and II fibers terminate on cells in Clarke's
nucleus. From there, cells in Clarke's nucleus send
axons into the IPSILATERAL lateral funiculus where
they are located dorsal and laterally. These axons comprise
the DORSAL SPINOCEREBELLAR TRACT (DSCT). The cells of
origin of this tract lie in the IPSILATERAL Clarke's
nucleus. The pathway passes rostrally in the lateral
funiculus and eventually terminates within the
IPSILATERAL CEREBELLUM. As fibers from cells
in Clarke's column enter the DSCT and ascend, they
are organized such that the most caudal fibers lie
laterally within the DSCT, while the most
rostral (C8) lies medially in the DSCT.
Compare this with the dorsal column system and ALS.
To get into the cerebellum, the DSCT courses
within (is a component of) the inferior cerebellar peduncle
(L., a little foot) or restiform body. Think of a cerebellar
peduncle as a bundle of axons connecting the spinal
cord/brain stem and the overlying cerebellum. There are
three of these peduncles. More on this later in the course!!
It's a crying shame that Clarke's nucleus is not present at every spinal cord level. As I mentioned earlier, it is only present at spinal cord segments C8-L3. So, if a 1a, 1b, or II axon comes into the spinal cord between C8-L3, fine!!! There is a Clarke's nucleus waiting for it and bingo, the fiber dives into the nucleus and the information that it is conveying is relayed to the cerebellum (via the DSCT). However, think about a 1a, 1b or II fiber coming in at spinal level L5. It looks around and there is no Clarke's nucleus to hitch a ride on. What would you do if you were a fiber who wanted to get your information to the cerebellum?? Personally, I would pass rostrally in the fasciculus gracilis (no fasc. cuneatus is here!!) until I got to L3, where there is a Clarke's column, and dive into the nucleus.
What about 1a, 1b and
II fibers associated with dorsal roots above C8? Well, they
enter the cord, don't have a Clarke's nucleus , so they
enter the fasciculus of the upper extremity (fasc. cuneatus)
until they reach the caudal medulla, where they synapse in
the ACCESSORY CUNEATE NUCLEUS. Cells in the accessory
cuneate nucleus project to the IPSI cerebellum via
the inferior cerebellar peduncle (just like cells in
Clarke's column do).
I realize that this is a tremendous amount of information about the DSCT. Some instructors are fearless enough to talk about a ventral spinocerebellar tract. I am not fearless at this point! But you should know that you might hear the term SPINOCEREBELLAR PATHWAYS sometime in your career!