Human Anatomy - Kotsan I. Y. 2009

Pathways of the Brain and Spinal Cord
Proprioceptive Pathways of the Cerebellar System

The proprioceptive pathways directed toward the Cerebellum form part of the reflex apparatus that ensures equilibrium and subconscious coordination of movement. They transmit subconscious Muscle-joint Proprioception from the movement apparatus and participate in the Regulation of Muscle tone. Phylogenetically, these pathways are evolutionarily older and are present in all vertebrates. They are subdivided into the anterior spinocerebellar tract (Gowers' tract) and the posterior spinocerebellar tract (Flechsig's tract).

The anterior spinocerebellar tract (tractus spinocerebellaris anterior), or Gowers' tract, is named after the English neurologist W. R. Gowers (1845–1915) and is a two-neuron pathway.

The Cell bodies of the first (receptor) Neurons are located in the respective sensory ganglia of Spinal Nerves. Their peripheral processes—dendrites—travel peripherally within spinal nerves and terminate in proprioceptors located in the periosteum, ligaments, joint capsules, tendons, and Muscles.

Their central processes—axons—enter the Spinal Cord as part of the dorsal roots and proceed to the intermediate zone. Here, some fibers give off collaterals, terminating together with them on neurons of the intermediomedial Nucleus, where the second neurons reside. The aggregate of these second-neuron axons forms the anterior spinocerebellar tract.

Some fibers of this tract decussate, passing segmentally through the white commissure to the contralateral side of the spinal cord, and ascend within the lateral funiculus along its anterolateral periphery. The remaining fibers do not decussate and ascend within the ipsilateral lateral funiculus, also occupying its anterolateral portion.

As it ascends cranially, the anterior spinocerebellar tract thickens due to fibers joining it from superior segments. The fibers then pass along the Medulla Oblongata, lying between the inferior olivary nucleus and the inferior cerebellar peduncle, continue through the pontine tegmentum, and—at the border with the Midbrain—turn sharply dorsally toward the superior medullary velum, where some fibers cross over to the opposite side once again. Via the superior cerebellar peduncles, the fibers of the anterior spinocerebellar tract reach the cortex of the cerebellar vermis.

Because the nerve fibers comprising Gowers' tract undergo decussation twice (in the white commissure of the spinal cord and in the superior medullary velum), subconscious proprioceptive impulses are projected to the cerebellar vermis predominantly from the ipsilateral side of the body.

The posterior spinocerebellar tract (tractus spinocerebellaris posterior), or Flechsig's tract, is named after the German neurologist and histologist P. E. Flechsig and is likewise a two-neuron pathway.

The cell bodies of the first (receptor) neurons are located in the respective spinal ganglia. Their peripheral processes—dendrites—travel to the periphery within spinal nerves and terminate in proprioceptors found in the periosteum, ligaments, joint capsules, tendons, and muscles.

Their central processes—axons—enter the spinal cord via the dorsal roots, where some fibers issue collaterals and terminate alongside them on the neurons of the thoracic nucleus (nucleus thoracicus, or Clarke's Column). These fibers constitute the gangliospinal tract (tractus gangliospinalis). The nerve Cells of the thoracic nucleus serve as the second neurons.

The collective axons of these second neurons form the posterior spinocerebellar tract. Initially, the axons of the second neurons course laterally to reach the posterior region of the ipsilateral lateral funiculus, where they bend at a right angle and ascend, occupying the peripheral zone of the posterior lateral funiculus just dorsal to the anterior spinocerebellar tract.

Subsequently, the fibers ascend into the medulla oblongata, lying between the inferior olivary nucleus and the spinal trigeminal tract. They then pass through the inferior cerebellar peduncles to the cortex of the cerebellar vermis, with the majority of these fibers Crossing Over within the vermian cortex.

Because the fibers of the posterior spinocerebellar tract do not decussate within the spinal cord or medulla oblongata, this pathway is also referred to as the direct or uncrossed cerebellar tract.

The cerebellum Functions as a major proprioceptive center. It receives collateral branches from cortical-directed proprioceptive pathways, while cerebellar proprioceptive pathways also project onward to the Cerebral Cortex.

Thus, proprioceptive pathways are connected to both the CEREBRAL CORTEX AND the cerebellum, with the cortex maintaining ultimate control over the cerebellar functions.

Up to this point, we have examined afferent pathways associated with specific neuronal specializations that transmit particular modalities of specific impulses (tactile, interoceptive, and proprioceptive). Together with pathways originating from the visual, auditory, gustatory, and olfactory Organs, they comprise the so-called specific afferent system. Concurrently, there exists an afferent system represented by the reticular formation, which belongs to non-specific structures.

The reticular formation perceives all types of sensory impulses (painful, thermal, visual, auditory, etc.); however, its neurons lack strict modality specialization—the same neurons process diverse impulses and Relay them to various Brain regions and throughout the cerebral cortex. The reticular formation maintains reciprocal, bidirectional connections with the cerebellum, autonomic subcortical centers, cranial nerve nuclei, the cortex of all cerebral lobes, and the spinal cord. These extensive bidirectional connections enable the reticular formation to fulfill its vital functions: regulating the flow of information transmitted to the Central Nervous system along afferent pathways; facilitating the transmission of afferent impulses to the cerebral cortex and producing diffuse cortical activation; and regulating overall vegetative functions, muscle tone, and more.