Afferent Limb: Signal Input to the Cerebellum
Information regarding body position and muscle state reaches the cerebellum from two main types of receptors. All these afferent pathways ultimately terminate in the cerebellar cortex.
- Proprioceptive Circuit (from muscles and tendons). Primary sensory neurons are located in the dorsal root ganglia. Signals from these neurons are transmitted to secondary neurons residing in the dorsal horns of the spinal cord (including Clarke's column, or nucleus dorsalis). The axons of these secondary neurons form two ascending tracts traveling in the lateral funiculi of the spinal cord:
- Posterior spinocerebellar tract (Flechsig's tract) — an uncrossed pathway (decussation may occur only upon entry into the cerebellum through its peduncles).
- Anterior spinocerebellar tract (Gowers' tract) — undergoes two decussations (one at the spinal cord level, the second in the brainstem), returning the signal to its original side.
- Vestibular Circuit (from the balance organ). Receptors are located in the inner ear. Primary neurons are localized in the vestibular ganglion of the temporal bone (signals travel via cranial nerve VIII). Secondary neurons are represented by cells of the vestibular nuclei of the pons. From here, information is directed via the vestibulocerebellar pathway.
Signal Processing and Neural Loop Formation
Incoming information is analyzed in the cerebellar cortex. Here, complex integration of signals from different loops occurs (theoretically allowing for "collisions"—mutual signal cancellation, the exact mechanism of which is still being studied).
Control is based on an inhibitory effect: the cerebellar cortex sends inhibitory impulses to its deep nuclei (e.g., the dentate nucleus — nucleus dentatus).
From the cerebellar nuclei, efferent signals are distributed to three brainstem structures:
- Downward — to the inferior olivary nucleus (medulla oblongata).
- At the same level — to the vestibular nuclei (pons).
- Upward — to the red nuclei (midbrain).
These connections establish closed loops of excitation circulation necessary for continuous movement correction. The olivocerebellar loop runs from the cerebellar cortex to its nuclei, then to the olivary nuclei, and returns to the cerebellum. The vestibulocerebellar loop functions similarly, relaying through the vestibular nuclei of the pons.
Efferent Limb: Pathways to Effector Organs
The outgoing cerebellar signal does not project directly to muscles. From the deep cerebellar nuclei, impulses are transmitted to intermediate brainstem centers, primarily the red nuclei (nucleus ruber).
The rubrospinal tract (tractus rubrospinalis) originates from the red nuclei. It descends in the lateral funiculi of the spinal cord and terminates on cranial nerve motor nuclei as well as large alpha motor neurons in the ventral horns. This alters the functional state of the motor neurons (activating some and inhibiting others) and coordinates the contraction of extrafusal skeletal muscle fibers.
In addition to the rubrospinal tract, phylogenetically older, shorter pathways are under cerebellar control:
- Olivospinal tract: originates from the inferior olivary nucleus (nucleus olivaris) and runs at the border of the lateral and anterior funiculi of the spinal cord.
- Vestibulospinal tract: originates from the vestibular nuclei of cranial nerve VIII and descends in the anterior funiculi, mediating balance.