Neuronal Connections in the Cerebellum in Short

Control of movement is largely determined by incoming (afferent) and outgoing (efferent) neural impulses in the cerebellum.

Motor information input travels from the spinal cord, cerebral cortex and vestibular system via mossy fibers.

Feedback regarding movements returns to the cerebellum via the inferior olivary nucleus in the medulla oblongata. This feedback loop allows the brain to coordinate movement.

All outgoing neural impulses from the cerebellum travel via the deep cerebellar and vestibular nuclei. Proper functioning of the neuronal pathway between mossy fibers, granular cells, parallel fibers, climbing fibers and Purkinje cells are thought to be essential for coordinated muscular movement. Glutamate is a neurotransmitter in the excitatory synapses between climbing fibers and Purkinje cells as well as between granular cells and mossy fibers. Disruptions in this system are thought to be involved in a variety of movement disorders.

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Descending Tracts

Descending tracts have three neurons:

1.   1st order neurons (UMN): cell bodies are in the cerebral cortex and other supra spinal areas

2.   2nd order neurons: short and situated in the anterior grey column of the spinal cord

3.  3rd  order neuron (LMN): situated in the anterior grey column and innervate the skeletal muscles through anterior roots of the spinal nerves

Corticospinal tract: rapid, skilled and voluntary movements

1st order neuron

Axons arise from the pyramidal cells of the cerebral  cortex  (situated  in  the  5th   layer),  2/3 from the pre central gyrus and 1/3 from the post central gyrus:

1. 1/3 of fibers arise from the 1stry motor  cortex (Area 4)

2. 1/3 of fibers arise from the 2ndry motor cortex (Area 6)

3. 1/3 of fibers arise from the parietal lobe

(Area 1, 2 and 3).

Descending fibers converge in the corona radiata and  pass  though  the   posterior  limb  of  the internal capsule; organization of fibers within the internal capsule:

1. close to genu (medial): concerned with the cervical parts of the body

2. away from the genu (lateral): concerned with the lower extremity.

The tract then passes through the middle 3/5 of the basis pedunculi of the midbrain; organization of fibers in the midbrain:

1. medially: cervical parts of the body
2. laterally: lower limbs.

When the tract enters the pons, it’s broken into many bundles by the transverse pontocerebellar fibers. In the medulla oblongata, the bundles group together to form the pyramids. At the junction of the MO and the spinal cord, most fibers cross the midline at the decussation of the pyramids and enter the lateral white column of the spinal cord to form the lateral corticospinal tract (LCST). LCST descends length of the spinal cord and terminates in the anterior grey column of all the spinal segments.

The fibers which didn’t cross, descend in the anterior white column of the spinal cord as the anterior corticospinal tract (ACST). Fibers of the ACST eventually cross and terminate in the anterior grey column of the spinal cord segments in the cervical and upper thoracic regions.

2nd order neuron:

It’s an internuncial neuron.

3rd order neuron:

It’s a alpha or gamma motor neuron.

To read more click on this link to the full article: Descending Tracts