Human Anatomy - Kotsan I. Ya. 2009

Pathways of the Brain and Spinal Cord
Proprioceptive Cortical Pathways

The cortical sensory pathways primarily serve to transmit conscious Proprioception from the mechanoreceptors of the Locomotor Apparatus, enabling spatial orientation, awareness of body posture, and the perception of both active and passive movements.

The cortical proprioceptive pathways include the ganglion-bulb-thalamo-cortical tract (tractus gangliobulbothalamocorticalis), which is an ascending, three-neuron pathway responsible for conscious proprioceptive impulses. It consists of three sequentially arranged tracts: the ganglion-bulbar, bulbo-thalamic, and thalamo-cortical. It is also referred to as the posterior spinothalamic tract (tractus spinothalamicus posterior).

The first-order Neurons of this pathway are represented by receptor (pseudounipolar) Cells whose Cell bodies are located in the spinal ganglia. The single process extending from The Cell body immediately divides into two branches: a dendrite and an axon. The dendrites course peripherally as part of the Spinal Nerves and terminate in specialized proprioceptors located in Muscles, tendons, ligaments, fasciae, joint capsules, bones, and periosteum. The axons of these cells enter the Spinal Cord segmentally via the posterior roots (at the posterolateral sulcus) and ascend within the posterior funiculi of the spinal cord, forming the medially positioned fasciculus gracilis (of Goll) and the laterally positioned fasciculus cuneatus (of Burdach).

The fasciculus gracilis conducts deep Muscle and joint proprioception from the lower limbs and the lower half of the trunk on the ipsilateral side, incorporating fibers from the 19 lower spinal ganglia (8 lower thoracic, 5 lumbar, 5 sacral, and 1 coccygeal). The fasciculus cuneatus transmits signals from the upper trunk, neck, and upper limbs, corresponding to the 12 upper spinal ganglia (8 cervical and 4 upper thoracic). Within the posterior funiculi, the fibers of the fasciculi gracilis and cuneatus arising from individual segments are arranged in stratified layers, with the fibers of the sacral spinal ganglia located most medially, adjacent to the posterior septum. As they ascend cranially, each subsequent spinal ROOT contributes a new, more superficially positioned layer, while the length of the fibers decreases in accordance with the rostral-caudal level of the originating spinal ganglia.

The fasciculi gracilis and cuneatus ascend without synapsing in the spinal cord until they reach The Nucleus gracilis and nucleus cuneatus located in the Medulla Oblongata, where they synapse with second-order neurons. Collectively, these first-order neurons constitute the ganglion-bulbar tract (tractus gangliobulbaris).

The axons of the second-order neurons, whose cell bodies reside in the nucleus gracilis and nucleus cuneatus, cross to the contralateral side as internal arcuate fibers, intersecting the midline and forming the sensory decussation (decussatio lemniscorum medialium) in the medulla oblongata. Bending sharply in a vertical direction, these fibers turn superiorly and anteriorly, ascending through the medulla oblongata dorsal to the pyramids and between the olives as the medial lemniscus. Subsequently, passing through the posterior part of the Pons and the cerebral peduncles, they reach the ventral nuclei of the thalamus, where they synapse with third-order neurons. In the pons, the medial lemniscus is joined by the spinal lemniscus (pathways for cutaneous sensation from the neck, trunk, and limbs) and the trigeminal lemniscus, which is formed by Trigeminal nerve fibers carrying cutaneous and proprioceptive sensations from the face. The entire pathway of second-order neurons from the gracile and cuneate nuclei to the thalamus is termed the bulbo-thalamic tract (tractus bulbothalamicus).

A portion of the second-order fibers (posterior external arcuate fibers) originating from the Cells of the gracile and cuneate nuclei passes via the inferior cerebellar peduncle to the ipsilateral cerebellar hemisphere, while others (anterior external arcuate fibers) project to the contralateral cerebellar hemisphere, curving ventrally around the pyramid and olive to enter the contralateral inferior cerebellar peduncle. Through these connections, the Cerebellum participates in the motor coordination mechanism by receiving conscious proprioceptive impulses via multiple routes.

The axons of the third-order neurons, whose cell bodies are located in the ventral nuclei of the thalamus, travel as part of the thalamocortical tract (tractus thalamocorticalis) to the precentral gyrus of the cerebral hemispheres, terminating in synapses on the cells of layer IV of the cortex.

Conscious proprioceptive impulses from the lower limb and the corresponding half of the trunk project to the upper third of the precentral gyrus, those from the upper limb to the middle third, and those from the HEAD to the lower third of the gyrus. In the medulla oblongata, the fibers carrying conscious proprioceptive impulses cross the midline (forming the sensory decussation/medial lemniscus decussation); therefore, the right half of the body is represented in the precentral gyrus of the left cerebral hemisphere, and the left half in the right hemisphere.

Conscious proprioceptive impulses from the Muscles of the head, as well as the capsule and ligaments of the temporomandibular joint, are transmitted via nerve fibers running within the Pathways of the trigeminal, facial, and glossopharyngeal nerves. These pathways also consist of three neurons.

The first-order neurons are located peripherally in sensory ganglia and are represented by pseudounipolar cells. For instance, the trigeminal ganglion is situated on the anterior surface of the petrous part of the Temporal bone near its apex, within the trigeminal impression. The peripheral processes of these ganglion cells, which conduct conscious proprioceptive and tactile impulses, course within the Branches of the trigeminal nerve to terminate in proprioceptors located in the masticatory muscles, the muscles of the floor of the Oral Cavity, and the extraocular muscles. Meanwhile, their central processes (axons), traveling within the sensory root, reach the pontine trigeminal nucleus located in the tegmentum of the pons, where they synapse with second-order neurons.

The axons of the second-order neurons immediately decussate to the contralateral side and ascend to the ventrolateral Nucleus of the thalamus as part of the nucleothalamic tract (tractus nucleothalamicus), where they synapse with third-order neurons.

The axons of the third-order neurons pass through the middle segment of the posterior limb of the internal capsule as part of the thalamocortical tract (tractus thalamocorticalis) and terminate in the Fourth layer of the cortex within the lower third of the precentral gyrus.

Proprioceptive impulses from the Muscles of facial expression are conveyed by the seventh cranial nerve (VII), whereas those from the Tongue and pharyngeal musculature are transmitted via the ninth, tenth, eleventh, and twelfth Cranial Nerves (IX, X, XI, and XII).