Human Anatomy Part 2 - K. A. Dyubenko, A. K. Kolomiytsev, Yu. B. Chaykovsky 2008

Special Part
Nervous system, systema nervosum - Special Part
Pathways of the central nervous system - Ascending pathways (sensory)

Class="center">Proprioceptive Pathways

The fasciculus gracilis and fasciculus cuneatus (Goll's and Burdach's columns, respectively) (Fig. 163) are pathways that conduct conscious deep proprioceptive sensibility, Muscle-joint sense from the limbs and trunk, as well as cutaneous, tactile, and spatial body position awareness. They originate from receptors (Küne's muscle spindles) located in Muscles, tendons, and articular capsules. The peripheral processes transmit impulses to pseudounipolar Cells of the spinal ganglia, where the bodies of the first-order Neurons are situated. Their central processes form the posterior roots, which segmentally enter the posterior funiculus on their own side to form the fasciculus gracilis and fasciculus cuneatus. Rather than terminating in the posterior horns, they ascend to reach their respective nuclei—The Nucleus gracilis and nucleus cuneatus—in the Medulla Oblongata, where The Cell bodies of the second-order neurons are located. The axons of these second-order neurons in the medulla oblongata, at the level of the olives, form the sensory decussation of the medial lemnisci, decussatio (sensitivus) lemniscorum, after which the fibers cross to the contralateral side and, at the border with the Pons, give rise to the sensory medial lemnisci, lemniscus (sensitivus) medialis (BNA)*.

* V. M. Bekhterev considered the lemniscus medialis to be the main lemniscus.

Fig. 163. Proprioceptive pathways. Fasciculus gracilis and cuneatus

Along the course of the medial lemnisci, which pass through the tegmentum of the pons and reach the thalamus, terminating in its posterolateral nucleus (third-order neurons), projection fibers for pain and Temperature Sensation—originating from cells in the posterior horns of the Spinal Cord (tractus spinothalamicus lateralis)—join them from the Midbrain. These fibers run adjacent to the medial lemniscus and terminate in the lateral nuclei of the thalamus, forming the spinal lemniscus, lemniscus spinalis (PNA). Projection fibers of the sensory trigeminal pathway, originating from sensory cells of its nuclei and forming the trigeminal lemniscus, lemniscus trigeminalis (PNA), also join the medial lemniscus. Together, they form the bulbothalamic tract, tractus bulbo-thalamicus. Axons of the third-order neurons depart from the cells of the posterolateral Nucleus of the thalamus, pass through the internal capsule (posterior limb), and reach the cortex of the superior parietal lobule (areas 3–7) and the postcentral gyrus (areas 4–6). Furthermore, dorsal and ventral arcuate fibers, fibrae arcuatae dorsales et ventrales, arise from the cell bodies of the second-order neurons (nuclei gracilis et cuneatus) and convey impulses to the Cerebellum.

Topographically and anatomically, the fasciculus gracilis is situated medially, encompassing 19 caudal segments of the spinal cord (1 coccygeal, 5 sacral, 5 lumbar, and 8 thoracic). The fasciculus cuneatus is located laterally, encompassing 12 cranial segments (upper 4 thoracic and upper 8 cervical).

Damage to these pathways results in a loss of sensibility, leading to sensory and sensitive ataxia.

The posterior spinocerebellar tract, tractus spinocerebellaris dorsalis (posterior) (Flechsig)*, is an ascending projection proprioceptive pathway of the cerebellum (Fig. 164). It transmits sensory impulses from nerve endings in the Skin, muscles, joints, and tendons to the cerebellum. The cell bodies of the first-order neurons reside in the spinal ganglia (pseudounipolar cells). From these cells, axons travel within the posterior (sensory) ROOT to reach the thoracic nucleus, nucleus thoracicus (Clark-Stilling Column), located at the Base of the posterior horn of the spinal cord, which houses the second-order neurons of this pathway. Axons from the thoracic nucleus enter the posterior part of the lateral funiculus of the same side without crossing to the contralateral side; hence, the posterior spinocerebellar tract is uncrossed. The tract then ascends and, via the inferior cerebellar peduncle, reaches the anterior and posterior Regions of the cerebellar vermis cortex, where it terminates. A portion of its fibers decussates exclusively within the vermis.

The anterior spinocerebellar tract, tractus spinocerebellaris ventralis (anterior) (Gowers)**, transmits sensory impulses from nerve endings in the skin, muscles, joints, and tendons to the cerebellum (Fig. 164). It is located in the lateral funiculus of the contralateral side. The cell bodies of the first-order neurons are located in the spinal ganglia (pseudounipolar cells). Their axons reach the intermediate medial nucleus of the spinal cord, which contains the cell bodies of the second-order neurons. Axons of the intermediate medial nucleus cells emerge from the posterior horns, pass through the anterior commissure (first decussation), and enter the lateral funiculi of the opposite side. The pathway then ascends cranially, passing through the medulla oblongata, pons, and midbrain tegmentum to reach the ISTHMUS OF THE rhombencephalon, where it crosses to the opposite side, forming the second decussation. Finally, via the superior cerebellar peduncle, the pathway enters the anterosuperior regions of the cerebellar vermis cortex and terminates there.

* Paul Emil Flechsig (1847–1929) was a neurologist from Leipzig, Germany.

** Sir William Richard Gowers (1845–1915) was an English neurologist.

Thus, the anterior spinocerebellar tract undergoes a double decussation, highlighting its complex structural Organization and the projection of proprioceptive sensibility back to the side of origin.

Both the anterior and posterior spinocerebellar tracts conduct unconscious impulses to the cerebellum, which coordinates movements, maintains equilibrium, and ensures proper posture. Lesions of these pathways result in dynamic and static ataxia* on the ipsilateral side, manifested by impaired coordination during walking and limb movements.

Fig. 164. Posterior and anterior spinocerebellar tracts. Tractus spinocerebellaris posterior et anterior

* Ataxia: disorders of movement coordination.

Exteroceptive Pathways

The lateral spinothalamic tract, tractus spinothalamicus lateralis (PNA), is a projection afferent pathway running within the lateral funiculus (Fig. 165 A). It serves as the primary pathway whose fibers conduct impulses from cutaneous receptors sensitive to temperature and pain. The pathway originates from receptors (exteroceptors—Krause end bulbs and Ruffini corpuscles) located in the skin of the neck, trunk, and limbs, excluding the skin and structures of the facial region. These receptors perceive pain and temperature and transmit impulses via peripheral processes to the cell bodies of the first-order neurons, which reside in the spinal ganglion (pseudounipolar cells). The central processes of these pseudounipolar cells travel within the posterior roots to reach the posterior horn of the spinal cord, where the cell body of the second-order neuron is located in the nucleus proprius. Axons of the second-order neurons cross to the contralateral side via the anterior gray commissure and enter the White matter OF the spinal cord (lateral funiculi) to form the lateral spinothalamic tract. The pathway then ascends cranially, passing through the medulla oblongata, pons, and cerebral peduncle tegmentum, and enters the thalamus as part of the medial lemniscus, terminating in the posterolateral nucleus. Here lie the cell bodies of the third-order neurons; their axons pass through the internal capsule (middle portion of the posterior limb) and, via the corona radiata, reach the Cerebral Cortex (postcentral gyrus, areas 1, 2, 3) and the superior parietal lobule (areas 5–7). The fibers of the lateral spinothalamic tract maintain connections with pre-thalamic structures, specifically the medial reticular formation, ventrolateral formation, midbrain periaqueductal gray, superior colliculi, and the central and ventrolateral thalamic nuclei (J. Schadé, D. Ford, 1976).

It should be noted that temperature and pain sensations are transmitted via ascending pathways located on the side contralateral to the stimulus, whereas tactile pathways are bilaterally represented in the spinal cord.

In hemisection of the spinal cord, pain and temperature sensitivity are lost on the opposite side of the body below the level of the lesion—a crucial consideration in neurology. Simultaneously, conscious deep sensibility is lost on the ipsilateral side due to involvement of the pathways of Goll and Burdach.

The anterior spinothalamic tract, tractus spinothalamicus centralis (anterior) (PNA), is a projection afferent pathway running in the anterior funiculus (Fig. 165 A). It conducts tactile and pressure sensations. The receptors for this pathway are located in the skin and mucous membranes (Meissner's and Merkel's corpuscles, as well as Vater-Pacini corpuscles); they perceive and transmit impulses via peripheral processes to the first-order neuron cell bodies located in the spinal ganglion (pseudounipolar cells). The central processes of these pseudounipolar cells reach the posterior horn of the spinal cord (second-order neurons), where they synapse with posterior horn neurons after ascending 2 to 15 segments within the spinal cord. Axons of the second-order neurons cross to the opposite side via the anterior gray commissure and enter the anterior funiculi of the spinal cord.

According to E. P. Kononova (1959), this decussation does not occur immediately at the entry level of the respective posterior root into the spinal cord, but rather 2–3 segments higher, which must be taken into account in clinical neurology. Consequently, unilateral damage to this pathway causes cutaneous sensory deficits on the contralateral side below the level of the lesion.

Fig. 165. Exteroceptive pathways.

A – lateral and anterior spinothalamic tracts; B – trigeminal lemniscus

Fig. 166. Pyramidal Tracts.

A – lateral corticospinal (pyramidal) tract and anterior corticospinal (pyramidal) tract; B – corticonuclear tract

Further ahead, the pathway courses cranially, passing through the medulla oblongata, pons, and midbrain, to reach the ventrolateral posterolateral nucleus of the thalamus, which houses the cell bodies of the third-order neurons. The fibers of the third-order neurons course through the posterior limb of the internal capsule and, as part of the corona radiata, reach layer IV of the cerebral cortex (postcentral gyrus).

Sensory pathway of the Trigeminal nerve. Trigeminal lemniscus (Fig. 165 B).

The pathway originates from tactile, pain, and temperature receptors of the skin of the face and HEAD (excluding the occipital region), as well as the structures of the Orbit and Oral Cavity. Impulses travel via its dendrites to the trigeminal ganglion (first-order neuron). The axons of the trigeminal ganglion cells form the sensory root of the trigeminal nerve, which reaches the dorsal region of the pons and divides into ascending and descending branches. Impulses from tactile and proprioceptive receptors travel along the ascending branch, whereas pain and temperature impulses travel along the descending branch. The second-order neuron is located in the mesencephalic nucleus of the trigeminal nerve (nucleus mesencephalicus nervi trigemini). Its axons cross to the contralateral side to form the trigeminal lemniscus (lemniscus trigeminalis*), which ascends to the Diencephalon (the ventroposterior nucleus of the thalamus, serving as the third-order neuron). The axons of the thalamic cells pass through the posterior third of the posterior limb of the internal capsule and project to the cortex of the lower part of the postcentral gyrus (internal granular layer).

The descending fibers of the sensory root of the trigeminal nerve descend into the spinal cord to form the spinal tract of the trigeminal nerve (tractus spinalis nervi trigemini). This bundle descends to the fourth cervical segment (IV) and terminates in the substantia gelatinosa, which Functions as the sensory nucleus of this nerve (nucleus spinalis nervi trigemini). The Nerve Cells of this nucleus act as the second-order neurons for the pain and temperature pathways. The axons of the second-order neurons decussate in the pons, medulla oblongata, and spinal cord, travel through the reticular Formation of the medulla and pons, join the medial lemniscus, and terminate in the anterior nucleus of the thalamus (third-order neuron). The axons of the third-order neurons, running within the thalamocortical pathway, reach the postcentral gyrus (areas 3, 1, 2) and the limbic cortex (cingulate gyrus).

* A 14.1.05.310. The trigeminal lemniscus is a bundle of axons originating from all sensory nuclei of the trigeminal nerve. These axons form the posterior (dorsal, crossed) and anterior (ventral, uncrossed) pathways (trigeminothalamic tracts).