Spinocerebellar tract
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The spinocerebellar tract is a set of axonal fibers originating in the spinal cord and terminating in the ipsilateral cerebellum. This tract conveys information to the cerebellum about limb and joint position (proprioception).
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[edit] Origins of Proprioceptive information
Proprioceptive information is obtained by Golgi tendon organs and muscle spindles.
Golgi tendon organs consist of a fibrous capsule enclosing tendon fasciculi and bare nerve endings that respond to tension in the tendon by causing action potentials in 1β (relatively large, myelinated, quickly conducting) afferent neurones.
muscle spindles fibres are complicated systems of tension monitoring within muscles which result in information being carried via 1α (larger and faster than 1β) neurones (from both nuclear bag fibres and nuclear chain fibres) and II neurones (solely from nuclear chain fibres).
All of these neurones are "first order" or "primary", are sensory (and thus have their cell bodies in the dorsal root ganglion) and pass through layers I-VI of the dorsal horn, to form synapses with "second order" or "secondary" neurones in the layer just beneath the dorsal horn (layer VII)
[edit] Subdivisions of the tract
The tract is divided into:
- dorsal (posterior) spinocerebellar tract
- ventral (anterior) spinocerebellar tract
- spinocuneocerebellar tract
- rostral spinocerebellar tract
Information from muscle spindles in the hind limbs travel via the dorsal tract and golgi tendon organs in the hind limb travel via the ventral tract. Muscle spindle information from forelimbs travel via the spinocuneocerebellar tract and corresponding golgi tendon organ information travels via the rostral spinocerebellar tract.
[edit] Pathway for Dorsal and Spinocuneocerebellar Tracts
In the dorsal tract, the sensory neurones synapse in an area known as Clarke's nucleus or "Clarke's column".
This is a column of relay neurone cell bodies within the medial gray matter within the spinal cord in layer VII (just beneath the dorsal horn), specifically between C8-L3. These neurones then send axons up the spinal cord and form synapses in the accessory (lateral) cuneate nucleus, lateral to the cuneate nucleus in the medulla.
Below L3, relevant neurones pass into the fasciculus gracilis (usually associated with the dorsal column-medial lemniscal system) until L3 where they synapse with Clarke's nucleus (leading to considerable caudal enlargement).
From above C8, neurones enter the fasciculus cuneatus directly and again synapse with neurones in the accessory cuneate nucleus. This pathway is known as the spinocuneocerebellar tract.
The neurones in the accessory cuneate nucleus have axons leading to the ipsilateral cerebellum via the caudal cerebellar peduncle.
[edit] Pathway for Ventral and Rostral Tracts
Some neurones instead form synapses with neurones in layer VII of L4-S3. Most of these fibres cross over to the contralateral lateral funiculus via the anterior white commissure and proceed up the spinal cord to synapse with neurones in the superior cerebellar peduncle. The fibres then often cross over again within the cerebellum to end on the ipsilateral side.
The Rostral Tract is similar but is uncrossed and enters the cerebellum through both the superior and inferior cerebellar peduncles.
